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==Definitions and Terms in Mass Spectrometry==
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A progress report from ASTM Committee E-14 Subcommittee 10
Progress Report from ATSM Committee E-14 Subcommittee 10, presented at the Twenty-Second Annual Conference on Mass Spectrometry and Allied Topics, Philadelphia, Pennsylvania, May 19-24, 1974. pp. 545-561 [https://www.asms.org/docs/default-source/proceedings-archive/1974_asms_22nd_conference.pdf?sfvrsn=a7f279c3_2]
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Printed in the [https://www.asms.org/docs/default-source/proceedings-archive/1974_asms_22nd_conference.pdf?sfvrsn=a7f279c3_2 1974 ASMS Proceedings]
<big><big>Definitions and Terms in Mass Spectrometry</big></big>
 
<big>A progress report from ASTM Committee E-14 Subcommittee 10</big>
 
__TOC__
 
Subcommittee 10 on Definitions and Terms of ASTM Committee E-14 on Mass Spectrometry was established in 1970 at the Eighteenth Annual Conference on Mass Spectrometry and Alied Topics in San Francisco. The objective was to "assume responsibility for a long-term, deliberate, and comprehensive study of the words, phrases, terms, numbers, and scientific language of mass spectrometry" and to formulate recommendations that might be generally acceptable for standard usage. Glenn Cook, the subcommittee chairman, named Sy Meyerson, Dan Oblas, and Jerry McCleary chairmen of task groups designated as organic, inorganic, and instrumental, respectively. To broaden the base of participation beyond the formal task-group memberships, Meyerson and McCleary convened a meeting of interested persons at the 1970 Triennial International Conference on Mass Spectrometry in Brussels. Twenty-nine of the 35 to 40 persons in attendance, most of them from Western European countries, signified an interest in working with one or more of the task groups. A dialog conducted since then among members and correspondents of the organic task group has elicited input from 26 persons in addition to the chairman, McCleary prepared a list of terms in need of precise definition, intended to serve as a starting point for a similar dialog among people with instrumental interests, but the hoped-for response did not materialize. The inorganic task group lost its chairman by resignation in Early 1973 and it has not functioned, Because of a change in job responsibilities, Cook resigned as chairman of the subcommittee, also in 1973, and a replacement for him has not been found. Partly because of the lack of a chairman, the work of the subcommittee has been in abeyance for over a year. In the meantime, IUPAC has established a Mass Spectrometry Subcomission of the Commission on Molecular Structure and Spectroscopy, which in turn is part of the Division of Physical Chemistry. This subcommission, under John Beynon's chairmanship, has undertaken a review of the terminology of mass spectrometry and has requested access to the body of opinion accumulated by ASTM E-14 Subcommittee 10. In view of all the circumstances, ASTM Committee E-14 at its 1974 business meeting recognized that Subcommittee 10 is no longer active and agreed to transmit its collected deliberations to the lUPAC Mass Spectrometry Subcommission for incorporation in its study of terminology.
 
At that that same business meeting, Bob Kiser, a member of Subcommittee 10's organic task group, observed that participation in the dialog had been highly valuable to him by virtue of directing his attention to and stimulating his thinking on controversial questions. This reaction, corroborated by other task group members present led to a recommendation for publication of an informal report of Subcommittee 10 in its present state, incomplete though it is, before transmittal to the IUPAC group. Accordingly, in in the hope of reaching as wide as audience of mass spectrometrists as possible, arrangements were made for publication in the appropriate journals. Readers are invited to send comments, further questions, and any pertinent thoughts to Meyerson, who will then forward the dialog and all feedback so received, to the IUPAC subcommission.
 
The charge to the task group was couched as follows: The prime objective of our efforts is clarity -- first, to mass spectrometrists and second, to chemists and physicists generally. Conciseness is desirable, but distinctly secondary to clarity. We will want to make a conscious effort to avoid specialized jargon as well as as ambiguous or misleading terminology and notation. The fact that a term is firmly established and has attained widespread usage is often cited as a compelling reason for retaining it even though it is undesirable on other grounds. I would suggest that such widespread usage need be no more than marginally relevant to our considerations.
 
A framework for discussion was delineated, at an early stage of the study, in terms of twelve problem areas, each of which contains one or more troublesome terms and/or symbols. Each problem area is defined below and followed by the pertinent responses, which are reported verbatim to better convey the flavor of the exchanges. The problem areas are identified by Arabic numerals 1 to 12, and the replies by Roman numerals I to X, some of which came from individuals and others from groups of two or more persons who responded jointly. Some of the letters did not comment on all twelve problem areas. All the comments received in response to an April 1972 mailing to all participants are reported here. I have added a few remarks of my own, which are identified by the initials S.M. Literature references are collected at the end of the report. Item 13 comprises a series of suggestions, all dealing with nomenclature related to metastable ions, received in June 1972 from John Beynon. In part, these suggestions overlap matters that had already been considered in correspondence with all the participants in the dialog. Most recently, all of the material presented here under items 1 to 13, inclusive, was circulated to the formal task-group membership. Their comments are not included here but will be reported to the IUPAC subcommission. Item 14 comprises a list of 19 terms, all pertinent to instrument performance specifications, selected by McCleary as being particularly in need of clear definition.
 
==Mass, mass-to-charge ratio, m/e, etc. ==
The major problem here, which has troubled many workers, lies in the symbol "[[m/e]]." As one of our group has stated the matter, in most of chemistry and physics, "m" means the mass in grams and "e" is the charge on the electron (or the electron itself when it is part of an equation). The use of 'M' for [[molecular mass]] in [[atomic mass unit]]s and that of "[[z]]" for the number of charges on an ion are established and unambiguous in physics and chcmistry(1,2). Thus, "M/z" would appear to be the preferred notation.
 
A minor problem centers about the term used to denote this same quantity. Not too many years back, the established term was "[[specific mass]]"(3), which perhaps merits revival. Such revival would not be in accord with the lUPAC recommended usage of the word "specific," preceding the name of an extensive physical quantity, to mean "divided by mass"(4). On the other hand, it would not be the only exception to this recommendation. For example, "[[specific ionization]]" is a well established term denoting the number of ion pairs produced per unit of distance along the track of an ionizing particle(5,6).
 
Incidentally, the lUPAC-recommended symbol for atomic mass units is "u" rather than "[[amu]]"(4).
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1.1.
Since m/e is such a well-established term, I think it should not be abandoned, especially since the meaning of m and e in this context is clear, After all, m has quite a number of meanings. M/z would apply only to M<sup>+</sup> what about the other "m/e" values?
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1.II.
M/z is acceptable.
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1.III.
Very good. I agree that specific mass might well be an acceptable method of expressing mass to charge ratio.
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1.IV.
"z" would appear to be preferable to e as the number of charges on the ion but if M is used for the mass of any ion (as is done in the A.V.S. Standard)(7) it will conflict with the definition of M for the molecular ion. One could, of course, call the molecular ion P (molecular parent ion or primary ionised species) but the use of M is well established. If M is the molecular ion we must use [[m/z]] for the mass to charge ratio of ions other than M and ignore the fact that m is usually mass in grams. I do not like referring to an ion of mass m but can see no way out of it other than using M<sup>*</sup>, M<sup>[bar]</sup> or some other horrible device for the molecular ion. M is already used for the apparent mass of a metastable ion and M signifies an average).
Mass units should be in line with IUPAC using "u", i.e. the loss of 28 u.
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1.V.
In order to say what we mean and have general scientific understanding, I favor "M/z" for mass-to-charge ratio and "µ" for atomic mass unit.
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1.VII.
'M/z' indeed appears to be the preferred notation. It would not lead to great difficulties for those familiar with the symbol m/e. For atomic mass units 'u' is also preferable. To denote M/z, the term "specific mass", although not recommended by the IUPAC, is still better than "[[mass over charge]]" or "[[mass-charge ratio]]".
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1.IX.
I whole-heartedly approve of the notation M/Z for mass-to-charge ratio. However, with respect to the term "specific mass", I cannot show any enthusiasm. I feel that the additional length of the term "mass-to-charge ratio" is worthwhile inasmuch as there is now a distinct difference between it and mass. The most important thing in this regard is for journals and referees to insist on mass-to-charge ratio for mass spectral scales and mass for whenever they mean mass. The term "specific mass" will probably be subjected to the same sloppy writing habits as its predecessor, but will not have any of the advantages in clarity.
 
==Metastable, metastable peak, metastable ion.==
 
The three terms listed above are often used interchangeably--a practice that strikes me and several of my correspondents as highly unfortunate. "[[Metastable]]" is an adjective and its use as a noun is undesirable. The [[metastable ion]] is not detected; it has decomposed in a metastable transition or metastable dissociation to the ionic species that is in fact detected in the spectrum. The resultant peak itself is, of course, not metastable. Nonetheless, I invite your consideration of the term "[[stable peak]]" as a colloquial but widely understood name for a peak attributed to the product of a [[metastable transition]]. May I suggest revival of the term "[[apparent mass]]" to locate the metastable peak in the spectrum.
 
The use of the asterisk in decomposition schemes to indicate a supporting metastable peak might well be questioned. Except to refer to a footnote, the assignment of any specific significance to an asterisk is highly arbitrary, and to the uninitiated such a symbol can serve only as a source of confusion. Related symbols--*2, *2, and **-- suggested by correspondents to identify metastable peaks detected by various methods, or derived from transitions in various [[field-free region]]s, would, of course, reduce the number of words required. But, again, they appear highly arbitrary and they would be incomprehensible to potential readers who are not members of the club. Some of us have adopted the practice of drawing solid arrows in a decomposition to denote processes supported by metastable peaks and broken arrows for processes not so supported; a statement specifying the symbolism assures understanding by the reader.
 
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2.I. I am in favor of retaining "metastable peak" as a terminus technicus and I think it would be beneficial to discourage at the same time the terms '1 transition signal", "metastable transition", "diffuse peak" etc. which are rather confusing to anybody "not from the club".
 
If one settles form m<sup>*</sup> and m<sup>**</sup> to describe transitions in the first and second field free region, this would. avoid unnecessary words. I don't think that the asterisk would be confusing. If it indicates an excited state, nobody thinks of a footnote, either. Metastable peaks are something so common in mass spectrometry that anybody with the faintest idea about it will know the symbol and those not from the community would not know what to do with the descriptive sentence, either. And if one is to state what solid and broken arrows mean one can do the same with m<sup>*</sup> and m<sup>**</sup>
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2.II. Conflicting opinions
a In favour of "metastable peak" instead of "apparent mass", since there is nothing
apparent about the position of the peak on the mass scale, which could be confusing.
 
b Not in favour of "metastable peak", and use "apparent mass" at present. However,
strongly in favour of the term "[[transition signal]]"(8) to replace "metastable peak".
Some concise nomenclature is needed to differentiate between signals due to transitions
in different regions of the flight path and which would also give a concise description
of the regions in which daughter ions are produced by consecutive metastable decompositions. Suggest something like e<sub>𝛕</sub> to denote decomposition before the [[ESA]], e<sub>𝛕</sub> before
magnet and c<sub>𝛕</sub> decomposition before the collector. 𝛕<sup>e</sup><sub>m</sub> could then denote further decomposition in the pre-magnetic field-free region of a daughter ion produced in the pre-ESA region and transmitted through the analyser by the "defocusing" technique (this latter being an abhorrent term!)
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2. III. I don't think the term "apparent mass" need be revived since some of us have used it consistently for years, and still do so. There is however a need for a symbol for this term. In the past this has been (m/e)*, (M/q)*, or simply m* (Beynon, Reed, OMS). Despite your disclaimer of the use of asterisks for anything except footnotes, the use of the asterisk to designate the apparent mass of the metastable peak is well established. The use of (M/z)* is sometimes necessary because some metastable peaks are the result of dissociations of multiply charged ions. Also in seminars or teaching the need to write (or derive) the generalized equation for the apparent mass of a generalized metastable transition
 
[[File:Generalized metastable transition 1.jpg|400 px|center]]
 
in which case
 
[[File:Generalized metastable transition 2.jpg|400 px|center]]
 
I object to your use of "m" (sect.12) as the apparent mass of the metastable peak. Firstly, "m" is reserved for the mass in grams and its use would require. explanation if the above equations were written
 
[[File:Generalized metastable transition 3.jpg|400 px|center]]
 
As it is, if the first time the apparent mass is used in a paper, (M/z)* can be defined
as the apparent mass by the use of 3 words, i.e., (M/z)*, the apparent mass, ----.
 
S.M. The intent of my suggestion was to use m to identify metastable peaks in tabular listings of spectra, in accord with usage in the API Catalog(9), and perhaps in conjunction with the apparent mass in stating the assignment of a metastable peak to a particular transition, e.g.,
 
:::31.9 m      58<sup>+</sup> → 43<sup>+</sup> + 15
 
The use of min this sense would be restricted to these two situations, thus minimizing the likelihood of ambiguity.
 
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2.IV. Presumably all ions in a mass spectrometer are to some extent metastable and hence the term metastable peak is the preferred term, being the peak formed by a meta.stable decomposition, the "metastable peak" being at an "apparent mass of ... ".
 
I find the use of an asterisk for a decomposition supported by a metastable peak as being very useful and I do not see that it is any more objectionable to include a statement to this effect than it is to specify the symbolism of solid and broken arrows. I do think that to include the measured apparent mass of the metastable might be useful as I sometimes wonder about the reliability of bland statements that a decomposition is "supported by a metastable" (unspecified); one can often find several decompositions that would fit,
 
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2. V. I prefer "[[metastable peak at apparent mass]] XX".
 
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2.VII. In the circumstances perhaps the term "metastable peak" is the best choice. The word metastable should only be used as an adjective ( -ion, -transition, etc.). The term "apparent mass" is appropriate; m (in italics?) being used as its symbol, and being followed by a value in numbers having two decimals ('m' xx.xx).
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2.IX. One thing that has been omitted is reference to "mass spectra" obtained where only the neutral fragments from metastable processes are detected. While few people are doing this sort of thing at the present, it may become a popular technique.
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2.X. If the usage of m* for the presence of a metastable transition is retained, I would prefer the use of an asterisk over or under an arrow to denote the presence of a metastable peak. A statement specifying the symbolism will assure that the reader understands. Use of broken arrows adds a new symbolism, whereas use of * comes directly from m<sup>*</sup>. I don't like *l, *2, **, etc. I would keep the use of the asterisk limited. I consider it an advantage that the asterisk is not used otherwise in chemistry.
 
==Abundance, intensity, relative intensity, base peak. ==
"[[Abundance]]" and "[[intensity]]" are often used interchangeably. The dictionary definitions of the words would be more in accord with the use of "abundance" of ions and "intensity" of peaks.
 
"[[Peak height]]" or "intensity " may be expressed in any arbitrary units, such as ion current or mm measured on an analog record. "[[Relative intensity]]" implies normalization to some scale, and there is nothing inherently better about any one scale than others. Selection of the scale is a matter of arbitrary choice by the spectroscopist in light of his particular problem. He may prefer to express relative intensity as per cent of total ionization from initial specific mass XX; the commonly used and, I think, fairly clear symbol for such values is % XX. Otherwise, he may define the intensity scale by assigning a value of 100.0 to the molecular-ion peak, to the most intense peak, or to any other peak that suits his convenience. Twenty to 25 years ago, we used the term "[[base peak]]" for the peak so selected as the base for the scale of relative intensities (10). In current usage, the term has come to mean the most intense peak in the spectrum as well as the peak assigned a relative intensity of 100.0. This choice of intensity scale derives, I think, from its use in the API Catalog(9), and it seems an eminently good choice for cataloging purposes. For other purposes, such a choice may have no advantage over other possibilities or, worse, as in the comparison of spectra of various isotopic species of a compound(11), it may tend to obscure significant relationships and to interfere with interpretation of the data. I suggest a return to the earlier, less restricted sense of "base peak," which may or may not coincide with the strongest or most intense peak.
 
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3.II. Strongly favour "base peak" to be retained, applying only to the most intense peak in the spectrum.
 
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3.III. Agree on "abundance " and "intensity".
In regard to base peak, I agree that authors should have the privilege of using any base peak convenient for their purpose, with the proviso that the base peak used should always be specified. If in a table, the base peak, if not obvious, should be given in a footnote to the table. Nothing is more infuriating than to have to search text to understand a table.
 
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3.IV. Though I have always thought of the base peak as being the most intense ion, I agree that the broader definition is preferable. This would particularly assist in the analysis of mixtures where two "base peaks " may be required for different components, in order that spectra may be compared with standards. For most purposes the most in.tense peak would be chosen as base peak --do we need another name for the most intense peak?
 
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3. V. Let's use "base peak" as the peak selected as the base for the "relative intensity " scale.
 
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3.VII. The terms "abundance of ions " and "intensity of peaks" are to be preferred. %&Sigma;xx to be used to express relative intensity as per cent of total ionization from initial specific mass xx; the lower limit (M/z = 12 in organic mass spectrometry?) has to be defined.
The term "base peak" is to be assigned a relative intensity of 100.0, without necessarily referring to the most intense peak in 'the mass spectrum. For cataloging purposes the base peak should be the most intense peak.
 
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3.IX. I do not like the symbol &Sigma;xx for total ionization between the mass-to-charge ratios i and j. I feel that the term &Sigma;I<sub>M/Z <sub>M/Z=i,j</sub></sub>  is much more explicit and that this spectrum should be referred to as a "[[normalized mass spectrum]]".
 
==Recording of solely metastable peaks. ==
Instrumental procedures that eliminate normal peaks and yield spectra consisting solely of metastable peaks are widely referred to as "metastable defocusing. " It is the ion beams responsible for normal peaks that are defocused, however; those giving rise to metastable peaks might be said to be refocused. I $commend for your consideration the expression proposed by Beynon et al., "[[scanning in the metastable mode]]"(12).
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4.I. If we allow "metastable peak" , why not simply "[[metastable scan]]"?
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4. II.
a "Scanning in the metastable mode", .. is wide open to criticism, but not as inaccurate as "[[metastable defocusing]]"
 
b See 2b One can now employ the description "e<sub>&tau;</sub> scanning".
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4.III. Your suggestion of "scanning in the metastable mode" is good.
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4.IV. Metastable defocusing is rather a misnomer and Beynon's term of "Scanning in the metastable mode" is preferable. This would not cover the use of metastable detectors of the [[Daly detector|type developed by Daly et al.]] which are essentially used in the normal mode but they could be covered by "measured with a metastable detector (reference)". 
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4.V, I will also go with Beynon on this.
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4. VI. "Scanning in the metastable mode" leaves something to be desired because there are several ways of doing this --the instruments being designed for [[IKES]] work with the reversed electric and magnetic sectors point to a new "[[metastable mode]]," for example.
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4.
VII. "Scanning in the metastable mode" appears to be by far the best term, but it might be too long to use it in a paper, especially as this technique is rapidly becoming common practice. I cannot find a better alternative; "metastable scan" and "[[metastable defocusing]]" should be avoided.
 
==Resolution, resolving power.==
These terms, commonly used interchangeably, are usually defined as M/&Delta;M or &Delta;M/M at a value of M such that the minimum output signal in the valley between two peaks of equal height equals 10%, or some other specified per cent, of the peak height. On the other hand, the American Vacuum Society's Standards Committee found such a definition inadequate for their needs; Dr. Nerken argues that it is applicable to magnetic instruments only, and is not relevant to quadrupole and time-of-flight instruments. The  A.V.S. committee proposes that resolving power at mass M be defined as M/W, the ratio of M to peak width W, and that resolution at mass M be defined as the peak width W at M. For the A.V.S. proposals, see their proposed tentative standard(7).
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5.I. think there will be always different definitions for different purposes. It might be helpful to give a list which one should be used when and it should be emphasized that the definition used should be specified.
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5.Il. One vote in favour of the proposed A.V.S. definition.
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5.III. No matter what decision is made in regard to the definition of resolution, the manufacturers are going to use a system that makes their instruments appear better than they are. It is apparent we will need industry participation in this, especially from the quadrupole people who are notoriously optimistic in citing their resolution.
 
Also shouldn't there be an intensity factor in defining resolution? On all machines it is possible to increase the resolution at the expense of sensitivity. It seems the figures used by some manufacturers are the limits at zero sensitivity.
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5.IV. As you say, resolution and resolving power are commonly used interchangeably but that does not mean that they are interchangeable.
 
'Resolution' is the result obtained under a certain set of circumstances when attempt ing to differentiate between species, while
 
'Resolving Power' is the ability of a system to resolve, or differentiate, between different species. Hence, in mass spectrometry we arrive at the definitions given in the A.V.S. Committee report where "Resolution" is the peak width, W or &Delta;M, in mass units obtained under the given circumstances and "Resolving Power" is M/&Delta;M. If &Delta;M is measured as the intercept drawn at 10% peak height we should obtain resolving powers very similar to the figures quoted (incorrectly) by instrument manufacturers as the "resolution" of their instruments using the 10% valley definition. It would be interesting to hear the comments of instrument makers on this subject. An instrument of high resolving power will give a result of low resolution if it is knocked while scanning!
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5.V. The A.V.S. proposal, paragraph 3.16.4, suggests "resolving Power = M/W = M/&Delta;M" and paragraph 4.2.1 has one measure W or &Delta;M at lO% of the peak height. So, I see no major differences between their definition and the 10% valley definition except verbosity and the use of equations and figures. Let's stay with the simple stated definition.
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5 and 6.VI. We think that any working definition of resolution and sensitivity will
have to require simultaneous definitions as the A.V.S. proposal seems to indicate.
This is going to be especially important for high-resolution instruments. Presumably
Don DeJongh's committee is working on definitions of standards for resolution and sensitivity, from the aspect of mixtures for testing instruments. We haven't heard much
on how this is progressing.
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5.VII. I think the A.V.S. committee proposals arc better than the definitions used so
far. For comparing instrument performances it might be advisable that resolution (resolving power) for the instrument considered is given at certain specific masses, including the upper mass limit for the instrument.
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5.IX. The definition of resolution, as proposed by A.V.S., i.e., peak width as mea.sured at the base line, may be sufficiently good for triangular or trapezoidal peaks but bears little relationship to the real world. For most cases [where] we are concerned with measuring M/Z's and their relative intensities, the decision as to whether this can be done within the required accuracy is easier to make from the 10% valley definition rather than from the width definition.
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5.X. I think definitions of resolution and sensitivity should be based on procedure used to determine them. Thus I suggest holding this question aside in your committee until the A.V.S. and ASMS Committee VIII come up with their proposals on this question.
 
==Sensitivity. ==
The A.V.S, proposed tentative standard(7) contains a definition of sensitivity for use in quantitative gas analysis. Somewhat different definitions would seem called for in quantitative analysis of heavier materials, in high-resolution mass spectrometry, in directly coupled gas chrom/mass spectrometer operation, and perhaps for other special contexts. However, I found no mention of sensitivity in any of your letters. May I have your thoughts?
 
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6.I. It seems that cholesterol is now widely used by manufacturers, but since it may lose water thermally it may not be the best choice. The drawbacks of cortisol proposed by Spiteller have been discussed recently in OMS For organic work some compound of MW 400-800 of fairly low volatility should be chosen. The best choice would be some aromatic compound which shows little fragmentation.
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6.II
 
a A.V.S. definition of Sensitivity is bad, since noise can be so much a function of the amplifier, or detector, etc. For gases, sensitivity should be defined as ions collected per atom or molecule entering the ion source. Also, sensitivity definition is needed for spark source and surface ionization mass spectrometry. This is not so easy as operatlng conditions must be quoted, e.g., filament material and temperature. It should relate sample consumed to sample collected.
b For organic work, a definition stating sample flow rate in g sec-1 to produce a base peak signal of X times noise or perhaps flow rate in g sec-1 to give base peak signal of amps at Z resolving power. Each type of mass spectrometry needs a separate definition!
 
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6.III. In regard to sensitivity, please do not go to the quadrupole people's standard of "amperes per Torr" In the first place Torr where? It certainly has no meaning in a differentially pumped ion source, nor in any system with a leak to the ion source in which pressures are measured in the sample volume. Ion gauges in the vacuum system are notoriously poor indicators of ion source pressure in systems with relatively open ion sources. The old system of citing sensitivity in divisions per micron and also citing a standard substance in divisions per micron under the same conditions is not bad and perhaps should be continued. The relative sensitivity thus becomes fairly independent of operating parameters unless fragment peaks with high initial kinetic energy are used or one of the peaks has a high ion-source temperature coefficient.
I think one must recognize that it ls difficult, except in the most qualitative sense, to compare patterns or sensitivities between different types of instruments or even the same instrument with different types of detectors such as [https://www.asms.org/docs/history-posters/cec-103c.pdf|CEC 21-103]'s with [[Faraday cup]] detectors or [[multiplier detector]]s. As the detector systems proliferate, each with its own characteristic discrimination in regard to mass or ion energy, this problem becomes more and more difficult to solve.
 
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6.V. Sensitivity should be defined in terms of ion current per unit of partial pressure in the reservoir of a gas at a specific mass. For example: 4 x 10-<sup>14</sup> amperes per micron at mass 78 for benzene.
 
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6.VII. In my opinion the A.V.S. proposed tentative standard definition is good for use in quantitative gas analysis. The specified sensitivity should, as Dr. Nerken states, always be interrelated to the resolution. For high resolution mass spectrometers sensitivity and resolving power might be connected by using the 131 and 132 isotopes of xenon, which are roughly equally abundant (21.2 and 26.9 per cent natural abundance, respectively) The sensitivity for xenon might be related in a proper manner to that of neon or nitrogen.
 
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6.IX. A.V.S.'s definition of sensitivity in amperes/torr acceptable with certain reservations: The sensitivity should be for a Faraday cup (or 1st dynode of a multiplier) and gain due to the multiplier may be given separately. Alternately, the sensitivity may be given in terms of counts/sec/torr which is, of course, easily converted to amperes/torr (if the M/Z is known). Time of flight instruments cause a few problems, but even in this case counts/sec/torr for a given M/Z is probably OK. With G.C. M.S. combinations there are more problems due to more variables. I would think in this case that the only meaningful sensitivity would be, in terms of a single M/Z, as coulombs/ng or counts/ng.
 
==Name reactions, name processes. ==
Name reactions have long been in vogue in organic chemistry and, to a lesser extent in other areas of chemistry and physics. Much of the literature of organic chemistry is quite incomprehensible to a nonspecialist without one of the several available guides to name reactions. We in mass spectrometry have been fortunate so far in having avoided a proliferation of such terms.
 
An unfortunate recent example is "[[Audier's rule]]"(l3), evidently referring to what has long been known as "[[Stevenson's rule]]" and completely ignoring the matter of priority, which is not always as clearcut as in this case. Even the term "Stevenson's rule" has been widely misstated and misused. Despite the extra words that would be required, we would be better off if every author wishing to make use of this generalization were required to state it explicitly, giving appropriate credit via literature citations. The technique dealt with in point 4 above has been referred to as the "[[Barber-Elliott Major defocusing technique]]"(14) and the peaks obtained by this procedure as "[[Jennings signal]]s"(15). Further comment by me would be gratuitous.
 
The one such term that has been most widely used and abused is, of course, the ve "-known "Mclafferty rearrangement. " Despite the considerable and well recognized contributions of Mclafferty to the clarification of this group of processes, the question of priority in this case is not easily brushed aside; consider, for example, the 1954 and 1957 publications of Nicholson(l6) and Manning(l7), respectively. The term, first used, I believe, by Spiteller, and popularized by Djerassi, has in the course of time been attached to the widest variety of processes involving migration originally of hydrogen and subsequently of numerous other atoms and groups as well. The original term has given rise to the "[[McLafferty plus one rearrangement|McLafferty plus one]], " "[[McLafferty plus 14 ion|McLafferty plus 14]]," "[[protonated McLafferty rearrangement|protonated McLafferty]], "[[double McLafferty rearrangement|double McLafferty]], "[[super-McLafferty rearrangement|super-Mclafferty]], "[[quasi-McLafferty rearrangement|quasi-Mclafferty]], and "[[complementary McLafferty rearrangement]]s. I can see two possible options for us in the circumstances: either recommend that the term be dropped completely, or define it narrowly and precisely and discourage the use of derivative terms.
 
----
7.I. agree that name reactions should be kept to a minimum. I am in favor of retain.ing the terms "Stevenson's rule in its original meaning and "Mclafferty rearrangement
defined as shown below because they are fairly widely known, describe phenomena of wide occurrence and would require rather lengthy sentences for a description of what they imply. For any new names to be used as termini technici the last two criteria should be applied rigorously (it would make no sense to call cleavage XY's cleavage).
[[File:McLafferty ASMS 1974.jpg|center|400px]]
 
----
7.II. Drop the lot!
 
----
7.III. I agree that name reactions are anathema and should be discouraged if not banned Your criticism is excellent.
 
----
7.IV. I think there should be a much greater restriction by editors and referees on the use of name processes. They are meaningless jargon to many people and are often used to save authors having to say what they_ really mean. The "McLafferty " has almost become "any rearrangement involving a hetero atom". Either let us drop it altogether or define it rigidly as "a fragmentation involving a site specific transfer of a g-hydrogen to a hetero atom with cleavage of the a bond". Please let us have no more extensions of McLafferty--I see that Budzikiewicz et al. refer to a "McLafferty ion " and a "McLafferty peak".
 
----
7.V. I am against the use of name reactions or processes. But if authors insist in their use, they should define, give an example, and give a reference on their first use {OTFU) of it.
 
----
7.VI. "Stevenson's Rule " does not have much to do with ion intensities at 70 or 80 volts as a function of relative appearance potentials. It deals mostly with the possi.bility of excited states in the neutral when the fragment with the higher ionization potential of the two is actually formed. [One of us] once asked McLafferty how this could be construed as the basis for what is commonly called ''Stevenson's rule" McLafferty wrote to Stevenson to check, and [our] recollection is that Stevenson disclaimed author.ship for this version of the " rule." Hence we refer to the statement without an author.ship, at this point, and would recommend that this be made a formal proposal. Audier seems to be the only person who has presumed to include the statement in a codification of empirical observations, except of course that many people have used it as they needed it prior to his review. But we hardly think that this requires re-naming the rule after him, even if the first name is questionable.
S.M. Correspondent VI is absolutely right, and I offer profuse apologies for doing exactly what I complained about--misstating and misusing "Stevenson's rule. " For the relationship referred to above, pertinent literature references are cited below as 18-22.
 
----
7.Vll. I agree that use of name reactions (processes) should be discouraged as much as possible. As to the term "McLafferty rearrangement " I think its use should be restrict.ed to those cases where a hydrogen bound to an atom in a position Y with respect to a double bond migrates to that bond through a six membered cyclic transition state, prior to (further) fragmentation (Type Hin Biemann 1 s classification of fragmentation pro.cesses (23)). Perhaps the matter of priority might be avoided by using a term like "Y.hydrogen rearrangement "
 
----
7.Vlll. Perhaps one suggestion that could be made is that the loss of 15 mass units from a species could be called "a McLafferty minus 13 simple cleavage."
 
----
7.IX. While I agree with you that "name" reactions should be abolished if only because there is only one name and a hundred variations there is a separate set of names in mass spectrometry:
 
[[Dempster-type mass spectrometer ]]
 
[[Herzog shields]]
 
[[Mattauch-Herzog-type M.S.]]
 
[[Johnson & Nier type MS]]
 
[[Allen-type multiplier]]
 
etc.
 
There are, of course, many others (a good question to ask at this point is whether the Dempster type referred to is single or double focussing), It is not always the case that an instrument bears its inventor's name, i.e., "time of flight " , quadrupole, mono.pole, 120 magnetic sector, etc., so that perhaps we should not treat this area any differently to the name reactions.
 
----
7.X. I too don't approve of the use of name reactions. It may be too late to prevent further misuse of the term "McLafferty rearrangement"! It certainly would help if Djerassi stopped using the term. You don't give your opinions of "[[Nier geometry]]" , "[[Mattauch-Herzog geometry]]", etc. A novice could spend a lifetime finding the meanings of some terms of this type.
 
==Presentation of spectra ==
Until some ten years ago, mass spectra were nearly always reported in tabular form. Since then, bar-chart presentation has come into wide favor. Each of these modes has advantages and disadvantages, and which is preferable depends on the kind of information an author is trying to convey, the point he is trying to make. This choice deserves more attention than it has had from the authors of much recent literature and, of course, from editors and referees. On the other hand, some authors have, within the past few years, taken to reporting mass spectra in paragraph form. I would suggest that this practice is an abomination. The reader can do nothing with such data without first transcribing them to some other format. I would like to see us do whatever we can to discourage such a practice.
 
----
8. I. Agree.
 
----
8.II. Unanimous agreement here. Tabulated data is essential, with line diagrams (expensive to produce) as optional extras. Press hard for exclusion of spectra"
"paragraph
 
----
8.III. I agree that paragraph presentation of spectra is intolerable.
 
----
8.IV. I find it difficult to choose between bar charts and tabular data as both are so useful under different circumstances. The use of paragraph form is certainly to be strongly discouraged; I presume that it arose through the listing of I.R. peaks in this manner in organic chemical journals. In such journals, where mass spectrometry is only a means of identification, the alternative would, I fear, be to have the data left out completely; paragraph form is at least data even if inconveniently presented.
 
----
8.V. prefer the tabular form for presentation of entire spectra, but an author should
be free to make his point in comparison of specific spectral features in any convenient form.
 
----
8.VII. I too feel that mass spectra should be presented either as a bar-chart or in tabular form, and certainly not in paragraph form.
 
----
8.IX. I like matrix spectra 14 M/Z's across and half masses and metastable peaks placed elsewhere.
 
----
8.X. I think that the use of paragraph form for reporting spectra has been forced on us by some journals. This form takes less space than others. If one la afraid that the editor will refuse to allow inclusion of bis data, he presents it as concisely as possible, I say that it is better to have it in paragraph form than not at all.
 
==&alpha;-Cleavage, &beta;-cleavage, etc. ==
Nomenclature to identify bonds and atoms in terms of their position relative to a specific atom, functional group, localized odd electron, or charge center in a molecule continues to be a source of ambiguity. Budzlkiewicz et al.(24) state, " ... we refer to '&alpha;-cleavage' as fission of a bond originating at an which is adjacent to the one assumed to bear the charge; the definition of &beta;, &gamma; . . . cleavage then follows automatically. The following process would thus be described as 'a-fission of a ketone with expulsion of an alkyl radical'
 
[[File:Alpha fission.jpg|400px|center]]
 
One of my correspondents suggests that, "Instead of referring to the ' ... atom assumed to bear the charge ... ' perhaps 'heteroatom' would be better," and he continues: "The reference to &alpha;, &beta;, &gamma;-cleavage in this fashion leaves no Greek letter for the less common R<sub>2</sub>N+R cleavage but I suppose this could be referred to simply as N-C (or N-0, etc.) cleavage.
 
"A problem comes up when aromatic rings or double bonds are involved, however. How should we describe cleavages such as:
 
[[File:Aromatic cleavage.jpg|center|400 px]]
 
Is the bond broken a or b? It is tempting to call it [[&alpha;-cleavage]] since the electronic stabilization of the ion is about the same as in the case of cleavage next to heteroatoms, But calling this bond the &alpha;-bond flies in the face of all my organic training. As I see it, it is more reasonable to call these forms benzylic and allylic cleavage and forget the relationship to heteroatom electron-stabilized &alpha;-cleavage.
 
It also seems reasonable to call R-C=O-R acyl or aroyl cleavage by organic chemistry analogy, The problem is that there is no analogy for R-C-OH-R (&alpha;-cleavage) in organic chemistry and it seems reasonable here to refer to such processes as alcohol cleavage, ether cleavage, amine cleavage, thiol cleavage etc. In some ways I even prefer this to &alpha;-cleavage."
 
How do the rest of you view this matter?
 
----
9.I. In my opinion the term &alpha;-cleavage should be restricted to compounds with hetero atoms (ketones ethers, amines) where the meaning can be defined clearly (R-CO-R'-> RCO<sup>+</sup> , R-O-CH<sub>2</sub>-R -> R-O<sup>+</sup>=CH<sub>2</sub>) since they all follow the same mechanism. One could adopt the
terms "allylic" and "benzylic" cleavage for carbon systems. To introduce terms as amine or thiol cleavage would rather confuse matters since there are several "thiol cleavages" so that somebody not initiated would hardly know which one was referred to.
 
----
9.II. Agreement again. Avoid using &alpha;, &beta;, &gamma;, ..... wherever possible.
 
----
9.III. I am not sure how organic chemists solve the problem of bond designation in complicated systems as my use of this has been limited to simple systems where there is little, if any opportunity for the designation to be misinterpreted. Whatever system is used should certainly not be subject to misinterpretation, and the bond specified should be clear.
 
----
9.IV. I find this one of the most confusing areas and would certainly welcome the dropping of terms &alpha;-cleavage, &beta;-cleavage, etc. because, as your correspondent points out, it can conflict with organic practice. A direct identification of the bond naming the atoms which it joins is far preferable and here present practice in nomenclature would require the use, where possible, of atom numbering, e.g., the C2-C3 bond.
 
One field that appears particularly confusing is the uee of &alpha; and &beta; for the locaation of substituents in rings and for positions of bonds, e.g. (25), we have loss of the &alpha;-substituent in alkyl pyrroles but &beta;-cleavage to give loss of a methyl group in ethyl pyrroles (presumably either &alpha; or &beta; ethyl pyrroles), Is &beta;-cleavage &beta; to the ring or the double bond? --certainly not to the hetero atom in &beta;-pyrroles. In other words is loss of a &beta;-substituent &alpha;-cleavage? This is, I feel, another reason for dropping the use of Greek letters.
 
----
9.V. I think a writer should state "cleavage &alpha;, to the ring", or "cleavage &beta; to the carbonyl group", etc. unless he clearly defines his shortened nomenclature.
 
----
9.VI. We feel that to call benzylic cleavage &alpha;-cleavage is to open a can of worms. Consider the reaction of alkylbenzenes in which hydrogen is transferred back to the aromatic portion and a section of the chain is lost as an olefin. This has been a cleavage with &gamma; hydrogen transfer for years. We cannot break the same bond in benzylic cleavage and call it a cleavage! But if we rename the rearrangement and call it a cleavage with a hydrogen transfer, then the relationship to the common fragmentation of carbonyl compounds, etc., becomes clouded. It seems to us that the least inconsistent approach here is to leave benzylic cleavage as a cleavage, or, as your correspondent suggests, not to give it any Greek letter.
 
To introduce [[acyl cleavage]], [[aroyl cleavage]], and terms like these seems poor since it is not clear whether the acyl fragment is being formed or cleaved in the process described --actually it sounds more like the latter to the uninitiated. We could clear up the matter with new terms--"[[acylogenesis]]" --but this seems unnecessarily cabalistic. Terms like [[alcohol cleavage]], [[ether cleavage]], and so forth are hopelessly vague --and, further, they give no indication whatsoever that the processes are similar.
 
----
9.VII. I agree with one of your correspondents that the definition given by Budzikiewicz et al.(24) is to be amended as follows: &alpha;-cleavage is fission of a bond originating at an atom which is adjacent to a heteroatom, etc. " There is an ambiguity possible
in molecules having more than one heteroatom, and therefore I would suggest to use the term &alpha;-cleavage only when the author specifies the original position chosen.
 
As an example, loss of H. from carbon atom 2 in the molecular ion of imidazole can be described as &alpha;-cleavage with respect to both heteroatoms (case A); loss of H. from C-4(5), however is &beta;-cleavage with respect to nitrogen atom 1 and &beta; with respect to nitrogen atom 3 or, if one wishes, &alpha; with respect to N-1 and &beta; with respect to N-3.
 
[[File:ASMS 1974 imidazole reaction.jpg|center|500px]]
 
The suggestions given by your correspondent are good, I think. The advantage of using terms like benzylic, allylic, alcohol cleavage, etc. is that such terms are readily understandable to all organic chemists.
 
----
9.X. The terminology in regard to cleavages must conform with the nomenclature of organic chemistry. Photochemists call cleavage of the bond between the carbonyl-carbon atom and the adjacent carbon atom "&alpha;-cleavage". I also can accept the terms "benzyllic"
and "allylic" cleavage. For ketones and aldehydes, "&alpha;-cleavage" and "[[&beta;-cleavage with &beta;-hydrogen transfer]]" seem to be understandable to organic chemists. For alcohols,
amines, etc ., one could say "C-X bond cleavage", &alpha;,&beta;-bond cleavage", etc.
 
==Peak, ion, mass.==
The current literature furnishes a growing number of horrible examples of the completely interchangeable use of the words "[[peak]]," "[[ion]]," and even "[[mass]]." One might have hoped that editors and referees would have discouraged such a loose practice.
 
10.I. Agree.
 
10.II. Agreed.
 
10.III. Agree -- the clear usage of terms is, however, the primary responsibility of the author, and secondarily the referee. I have had bad experiences when editors assume the responsibility of changing technical terms in a paper.
 
10.IV. It is up to editors and referees to correct the misuse of peak, ion and mass. The correct usages are fairly obvious.
 
10.V. I favor clear, unambiguous terms, but most writers avoid repetition like the plague, leading to undesirable interchanges. Understanding by the reader should be the aim of expository writing.
 
== Molecular, molecule, and parent ion and peak==
"[[Molecule-ion]] can perhaps be discarded out-of-hand in view of the similarity to "[[ion-molecule]]," an adjective describing a reaction between an ion and a neutral molecule. "Parent" may denote either the ion formed by loss of one (or more than one, if stated explicitly) electron from the original molecule -- precisely the definition of "molecular ion 11 in the O.M.S. Editorial Review on Nomenclature(26) -- or the decomposing ion in any reaction step under consideration, in which the charge-retaining product is referred to as a "daughter ion." The possible ambiguity has evidently caused some concern. On the other hand, the meaning is usually clear from the context. How serious a problem is this?
 
11.I. I think the term "parent" should be used for any decomposing ion and not be restricted to M+.
 
11.II. I think we should press hard for the use of M+ to refer to the ionised molecule, since "parent/daughter'' decompositions can confuse the use of the word "parent". Parent must then be used as the ion giving rise to a decomposition sequence.
 
11.III. I am glad you refer to "ion-molecule" as an adjective only, i.e., describing a type of reaction. The use of "ion-molecule" as a noun, i.e., the product of an [[ion molecule reaction]], should be discouraged.
 
In regard to the use of "daughter ion" as the product of fragmentation, trouble may arise in the second fragmentation step (are these granddaughter ions?). I suggest the use of primary fragment ions, secondary fragment ions, etc. This latter terminology has a long history of usage and is clear. In parent-daughter relationships even in metastable ions one gets into problems, if as you suggest, "parent" is reserved for the molecular ion. What then do you call the relationship when the primary fragment of a fast fragmentation process is a metastable ion. The "parent" as far as the metastable dissociation is concerned is the fragment ion, not the molecular ion. This would be clear if the terms parent and daughter were reserved for [[metastable transition]]s, and fast processes were called fragmentations.
 
11 and 12.IV. See paragraph 1. One further thought, should the term "parent ion" be used for the case where a molecular ion does not appear but the spectrum derives from a, say, M-18 ion (loss of water)? It is probably better to use the term "[[M-l8 ion]]" and keep the terms parent and daughter ions for the precursor and product ions of a decomposition.
 
11.V. Although I prefer molecular ion or parent peak, I do not feel that these terms are a serious problem.
 
11.VI. [One of us] recently reviewed a paper where the authors wanted to call the most intense peak in the spectrum (which was a fragment peak) the parent peak!
 
11.VII. Indeed the term "molecule-ion" should not be used. The term "[[molecular ion]]" is to be used according to the definition given in the OMS Editorial Review on Nomenclature(26).
 
The term "parent" might be used only in the sense of "parent peak", meaning the peak of the molecular ion in a mass spectrum.
 
In the sense of a decomposing ion in a reaction step, perhaps the term "precursor" or "[[precursor ion]]" might be better.
 
==Abbreviations. ==
A list of standard abbreviations would probably be in order. As starters, consider: M (to identify the molecular ion), m (following the apparent mass, to denote a metastable peak), u (atomic mass units), eV (electron volts, to accord with accepted usage in other areas (4,5,6)), [[EI]], [[FI]], [[CI]], [[QET]].
----
12. I. M<sup>+</sup> (not M) for the molecular ion; m<sup>*</sup> (vide supra); [[IP]], [[AP]] for ionisation and appearance potential.
----
12.II. I sec a problem of conflict here. If we keep M for molecular ion as in 11, what about m for metastable? In any computer application one cannot differentiate between upper/lower case letters. May I make a suggestion --stick with the asterisk to denote a metastable peak, both in decomposition schemes and in tabulated data.
----
12.111. I have already commented on the use of "m" for a mass in atomic units. Any table of abbreviations is going to be subject to overlaps, but the overlap should be with units or designations of variables in different fields where there is no possibility of conflict in equations or in derivations. This is obviously a difficult thing to do and requires a complete list and critical evaluation for possible conflicts with established abbreviations.
----
12.IV. I offer the following list of terms and definitions:
 
;M<sup>+</sup>, M<sup>-</sup>
:[[molecular ion]]
 
;M
:[[molecular weight]] of the molecular ion.
 
;M<sup>+</sup>, M<sup>-</sup>
:ion other than the molecular ion.
 
;m
:molecular weight of m+ or m-.
 
;z
:number of charges on an ion.
 
;[[m/z]]
:[[mass to charge ratio]] of ion of mass m.
 
;u
:atomic mass unit.
 
;Δm
:peak width of an ion of mass m measured in mass units at a base line of ten percent of peak height; resolution.
;m/Δm, M/ΔM 
:[[resolving power]]
;m<sup>*</sup>
:[[apparent mass]] of a metastable peak.
;eV
:electron volts.
 
;[[EI]]
:[[electron bombardment ionisation]].
 
;[[FI]]
:[[field ionisation]].
 
;[[CI]]
:[[chemical ionisation]].
 
;[[EC]]
:[[electron capture]] or [[negative ion mass spectrometry]].
 
;[[QET]]
:[[Quasi equilibrium theory]].
 
;* (26.1)
:Symbol to denote that a metastable has been found for this decomposition at an apparent mass of 26.1.
----
12.V Undefined abbreviations should not be used at all unless they are indeed accepted widely by interested disciplines. But an author should be allowed to define his abbreviations for that paper OTFU.
----
12.VI. Other standard abbreviations: [[ICR]], [[IKES]]?
----
12.IX. It would be useful if abbreviations were defined once in every article, the only exceptions being those which are in common usage in physics and chemistry.
 
<big>'''Additional comments and questions'''</big>
 
VI. Another point: should the Society support the effort to supplant other systems of units by the SI units, as the British Chemical Society is doing? (We have some reservations about expressing ionization potentials in joules or attojoules, and pressures in milli-or micropascals, but perhaps the question ought to be addressed.)
----
IX. In the interest of compatibility with the rest of physical science, S.I. units should be used; where other units are used they should be defined in terms of S.I. units.
 
S.M. In re the reporting of metastable peaks in tabulated spectra, note the practice of Archives of Mass Spectral Data of reporting apparent masses of metastable peaks to one decimal place, but of other peaks to the nearest whole number. This accords with a common practice by many workers, with one additional item, the use of fractions rather than decimals in reporting nonintegral M/z values of doubly and triply charged ions.
 
==Some thoughts on nomenclature in the field of metastable ions and some suggestions of terms to be encouraged and of others to be discouraged. ==
 
a. The meaning is perfectly clear when one speaks of a metastable ion and this term should be reserved for an ion that reacts unimolecularly during its passage through a mass spectrometer. It should not under any circumstances be used to describe the product ions resulting from the fragmentation of a metastable ion.
 
b. It seems to me equally clear when one talks of a metastable transition. This is a transition undergone by a metastable ion.
 
c. As in the case of any other ionic reaction, it is perfectly proper to speak about the [[parent ion|parent]] and [[daughter ion]]s of the transition.
 
d. It has become customary to use the symbol ''m''<sup>*</sup> to represent the ''[[apparent mass]]'', i.e., the position on the mass scale where the peak due to arrival of the daughter ions of a metastable transition is observed. It is customary to write ''m''<sup>*</sup> = ''m''<sub>2</sub><sup>2</sup>/''m''<sub>1</sub> and I suggest that the use of ''m''<sup>*</sup>, ''m''<sub>1</sub>, and ''m''<sub>2</sub> to represent the apparent mass, the mass of the metastable ion and the mass of the daughter ion should be confirmed.
 
e. The term ''metastable peak'' has been used for a long time and I suggest that the use of this as an acceptable shorthand should be confirmed, quotation marks no longer being required. The term ''metastable'' (used as a noun) could be used interchangeably with ''metastable peak''.
 
f. For transitions in which the neutral fragment is represented, or in the general case in which there are two daughters of the transition, the term ''m''<sub>3</sub> should be used for the second daughter. This would be so in cases such as:
 
::m<sub>1</sub><sup>+</sup> -> m<sub>2</sub><sup>+</sup> + m<sub>3</sub>
 
or
 
::m<sub>1</sub><sup>++</sup> -> m<sub>2</sub><sup>+</sup> + m<sub>3</sub><sup>+</sup>
All of this would help to establish an easy understanding of the mathematical equations used in this field.
 
g. Use of the word "metastable" in such forms as " ... in metastable work" instead of " ... in work on metastable ions" should be forbidden.
 
h. Use of the symbol E to describe the voltage applied to an electric sector and of V to describe the accelerating voltage should be encouraged.
 
i. The region immediately preceding the electric sector should be called the ''[[first field-free region]]''; the region in front of the magnetic sector should be called ''[[second field-free region]]''. This is particularly important if we are to avoid confusion now that instruments are beginning to appear in which the ion beam passes through the magnetic sector before the electric sector.
 
j. The slit that follows the electric sector is usually called the &beta;-slit, whether or not it is a collector slit. I suggest the slit that follows the magnet (whether or not it is a collector slit) be called the &gamma;-slit.
 
k. There is a well-known method of observing metastable peaks due to transitions occurring in the first field-free region that involves scanning the accelerating voltage at fixed electric sector voltage (due to Futrell and to Barber and Elliott). Many authors call this 11 defocusing the ion beam." This shows a complete misunderstanding of the process; the focusing is not affected in any respect, and the focal length of the sector remains fixed. The main beam is merely deflected and a new beam focused at the same point. I suggest that some other term be approved for this method. "Defocus" (and also "Refocus" that is used from time to time) should not be allowed under any circumstances. We currently use accelerating voltage scan method to describe what is done.
 
1. When a collision gas is used to promote ionic reactions, we speak of "collision-induced dissociations" or CID's. The peaks resulting from CID's should be called "CID peaks." Although their position on the mass scale is very close to the position at
which metastable peaks appear, they should not be, called "[[collision-induced metastable]]s."
 
m. Various names are used to describe reactions in which charges are transferred when a collision gas is introduced. Thus the processes
 
::X<sup>++</sup> + Y -> X<sup>+</sup> + Y<sup>+</sup>
 
or
 
::X<sup>+++</sup> + Y -> X<sup>++</sup> + Y<sup>+</sup>
 
would be called "[[charge exchange]]" or "[[ionization of the collision gas by the ion beam]]"
 
Reactions such as:
 
::X<sup>+</sup> + Y -> X<sup>++</sup> + Y + e
 
or
 
::X<sup>+</sup> + Y -> X<sup>++</sup> + Y<sup>+</sup> + 2e
 
are called "[[charge stripping]]".  
 
I suggest that we should have a nomenclature to cover all possible reactions, and
believe the one proposed (?) and used by Hasted to be ideal. He would refer to the above reactions by the number of charges on the reactants and products' as 20/11; 30/21; 10/20 and 10/21 reactions, respectively. This is, in my opinion, a rapidly developing field and one in which it is important to establish a standard nomenclature before too many people start their own.
n. There are a number of symbols that we are currently using. We hope that they will turn out to be those adopted as standard, but in any event, some of them are:
 
::&epsilon;<sub>0</sub> = [[Activation energy]]
 
::&epsilon;<sup>r</sup><sub>0</sub> = [[Reverse activation energy]]
 
::&epsilon;<sup>&Dagger;</sup> = Non-fixed or internal energy of the reactant ion
 
::c* = Non-fixed or internal energy of the activated complex.
 
::T = Kinetic energy released in a reaction
 
::Q = Kinetic energy converted to internal energy in a collision
 
::Q' = Kinetic energy lost by an ion in a collision.
 
o. As a post-script may I say that we are strongly in favor of the use of a dot to indicate an odd electron, of M<sup>+</sup> for a molecular ion and of P<sup>+</sup> as the symbol for a parent ion (i.e. the parent, as opposed to the daughter for any reaction, not just those involving the molecular ion).
 
==Terms pertinent to instrument performance specifications. ==
a. [[Mass range ]]
 
b. [[Sensitivity ]]
 
:i. Basic (See note 1)
:ii. Dynamic (See note 2)
:iii. Minimum detectable
:iv. [[Abundance sensitivity]]
 
c. [[Resolving power]] ([[Resolution]])
 
d. [[Interference]] (General)
 
:i. [[Gas sensitivity]]
 
:ii. [[Memory]]
:iii. [[Background]]
 
e. [[Test mixture]] or [[standard sample]]
 
f. [[Discrimination]]
 
g. [[Scan speed]]
 
h. [[Signal-to-noise ratio]]
 
i. [[Mass measurement]]
 
j. [[Accuracy of mixture analyses]]
 
k. [[Decoupled or defocused operation]]
 
l. [[Detector response]]
 
'In re' terms b, c, d, and e, see reference 3.
 
Note 1: This should be basic to the instrument. That is, ion current per unit of sample at some resolving power (e.g., amps/~g/sec, etc.)
 
Note 2: This is meant to define a total system specification, including scan speed, amplifier response and resolution.
 
For the members of Subcormiittee 10 and all who participated in our deliberations.
 
::::Seymour Meyerson
::::Research Department
::::Standard Oil Company(Indiana)
::::Box 400
::::Naperville, Illinois 60540, U.S.A.
 
::::June 21, 1974
 
==References==
 
1. "Handbook for Authors," American Chemical Society Publication, Washington, D.C., 1st Ed. (1967), pp. 39-40.
 
2. "Symbols, Units and Nomenclature in Physics," lnternat. Union of Pure and Appl. Phys., UNESCO, Document U.l.P. 9 (S.U.N. 61-44), 1961, pp. 13-14.[https://books.google.com/books/about/Symbols_units_and_nomenclature_in_physic.html?id=Qld0zwEACAAJ]
 
3. Recommended Practices E304-E68 and B137-68, 1969 Yearbook of the American Society for Testing and Materials.
 
4. "Manual of Symbols and Terminology for Physicochemical Quantities and Units," issued by IUPAC Division of Physical Chemistry Commission on Symbols, Terminology and Unite, Butterworths, London, 1970.[{{doi}}10.1351/pac197021010001 ]
 
5. "Radiological Health Handbook," Rev. ed., U. S. Department of Health, Education, and Welfare, Public Health Service, 1970.[{{doi}}https://doi.org/10.2172/4708654]
 
6. G.D. Chase and J. L. Rabinowitz, "Principles of Radioisotope Methodology," 3rd ed., Burgess Publishing Company, Minneapolis, Minn., 1967.
 
7. "Calibration of Gas Analyzers of the Mass Spectrometer Type," American Vacuum Society Proposed Tentative Standard 2.3, Draft No. 9 (Amended)II, June, 1971.
 
8. E. Tajima and J. Seibl, Int. J. Mass Spectrom. Ion Phys., 3, 245 (1969). [{{doi}}10.1016/0020-7381(69)85008-4]
 
9. "Catalog of Selected Mass Spectral Data," American Petroleum Institute Research Project 44, Thermodynamics Research Center, Texas A&M University, College Station, Texas.
 
10. "Mass Spectrometer Computing Manual," [[wikipedia:Consolidated Engineering Corporation|Consolidated Engineering]] Corp., March, 1951, p. i
 
11. See paragraph 3 in book review by S. Meyerson, J. Amer. Chem. Soc., ''88'' 2081 (1966).[{{doi}}10.1021/ja00961a064]
 
12. J. H. Beynon, J. A. Hopkinson, and G. R. Lester, Int. J. Mass Spectrom. Ion Phys.(1969) ''2'', 291. [{{doi}}10.1016/0020-7381(69)80025-2]
 
13. R. G. Cooks and A. G. Varvoglis, Org. Mass Spectrom., ''5'' 687 (1971).[{{doi}}10.1002/oms.1210050607]
 
14. F. W. McLafferty, D. J. McAdoo, J. S. Smith, and R. Kornfeld, 'J. Amer. Chem. Soc.', ''93'' 3720 (1971).[{{doi}}10.1021/ja00744a028]
 
15. Personal conversation; not in print, happily.
 
16. A. J. C. Nicholson, 'Trans. Faraday Soc.', 2Q_, 1067 (1954).[{{doi}}10.1039/TF9545001067]
 
17. P. P. Manning, 'J. Amer. Chem. Soc.', ''79'', 5151 (1957).[{{doi}}10.1021/ja01576a018]
 
18. M. B. Wallenstein, A. L. Wahrhaftig, H. M. Rosenstock, and H. Eyring, in "Symposium on Radiobiology," J. J. Nickson, Ed., Wiley, New York, 1952, p. 70.
 
19. S. Meyerson and J. D. McCollum, 'Advan. Anal. Chem. Instrumentation', ''2'' 179 (1963).
 
20. M. M. Bursey and E. S. Wolfe, 'Org. Mass Spectrom.', ''1'' 543 (1968).[{{doi}}10.1002/oms.1210010408]
 
21. H. E. Audier, Org. Mass Spectrom., ''2'' 283 (1969).[{{doi}}/10.1002/oms.1210010408]
 
22. A. G. Harrison, C. D. Finney, and J. A. Sherk, Org. Mass Spectrom., ''5'' 1313 (1971).[{{doi}}10.1002/oms.1210051109]
 
23. K. Biemann, "Mass Spectrometry. Organic Chemical Applications," McGraw-Hill, New York, 1962, pp. 119ff.
 
24. H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, Inc., San Francisco, Calif., 1967, pp. 2-3.
 
25. H. Budzikiewicz, C. Djerassi and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, Inc., San Francisco, Calif., 1967, pp. 597-598.
 
26. "Editorial Review on Nomenclature," Org. Mass Spectrom., ''2'' 249 (1969). [{{doi}}10.1002/oms.1210020302]
 
[[category:reference]]

Latest revision as of 11:43, 15 March 2024

Progress Report from ATSM Committee E-14 Subcommittee 10, presented at the Twenty-Second Annual Conference on Mass Spectrometry and Allied Topics, Philadelphia, Pennsylvania, May 19-24, 1974. pp. 545-561 [1]

Definitions and Terms in Mass Spectrometry

A progress report from ASTM Committee E-14 Subcommittee 10

Subcommittee 10 on Definitions and Terms of ASTM Committee E-14 on Mass Spectrometry was established in 1970 at the Eighteenth Annual Conference on Mass Spectrometry and Alied Topics in San Francisco. The objective was to "assume responsibility for a long-term, deliberate, and comprehensive study of the words, phrases, terms, numbers, and scientific language of mass spectrometry" and to formulate recommendations that might be generally acceptable for standard usage. Glenn Cook, the subcommittee chairman, named Sy Meyerson, Dan Oblas, and Jerry McCleary chairmen of task groups designated as organic, inorganic, and instrumental, respectively. To broaden the base of participation beyond the formal task-group memberships, Meyerson and McCleary convened a meeting of interested persons at the 1970 Triennial International Conference on Mass Spectrometry in Brussels. Twenty-nine of the 35 to 40 persons in attendance, most of them from Western European countries, signified an interest in working with one or more of the task groups. A dialog conducted since then among members and correspondents of the organic task group has elicited input from 26 persons in addition to the chairman, McCleary prepared a list of terms in need of precise definition, intended to serve as a starting point for a similar dialog among people with instrumental interests, but the hoped-for response did not materialize. The inorganic task group lost its chairman by resignation in Early 1973 and it has not functioned, Because of a change in job responsibilities, Cook resigned as chairman of the subcommittee, also in 1973, and a replacement for him has not been found. Partly because of the lack of a chairman, the work of the subcommittee has been in abeyance for over a year. In the meantime, IUPAC has established a Mass Spectrometry Subcomission of the Commission on Molecular Structure and Spectroscopy, which in turn is part of the Division of Physical Chemistry. This subcommission, under John Beynon's chairmanship, has undertaken a review of the terminology of mass spectrometry and has requested access to the body of opinion accumulated by ASTM E-14 Subcommittee 10. In view of all the circumstances, ASTM Committee E-14 at its 1974 business meeting recognized that Subcommittee 10 is no longer active and agreed to transmit its collected deliberations to the lUPAC Mass Spectrometry Subcommission for incorporation in its study of terminology.

At that that same business meeting, Bob Kiser, a member of Subcommittee 10's organic task group, observed that participation in the dialog had been highly valuable to him by virtue of directing his attention to and stimulating his thinking on controversial questions. This reaction, corroborated by other task group members present led to a recommendation for publication of an informal report of Subcommittee 10 in its present state, incomplete though it is, before transmittal to the IUPAC group. Accordingly, in in the hope of reaching as wide as audience of mass spectrometrists as possible, arrangements were made for publication in the appropriate journals. Readers are invited to send comments, further questions, and any pertinent thoughts to Meyerson, who will then forward the dialog and all feedback so received, to the IUPAC subcommission.

The charge to the task group was couched as follows: The prime objective of our efforts is clarity -- first, to mass spectrometrists and second, to chemists and physicists generally. Conciseness is desirable, but distinctly secondary to clarity. We will want to make a conscious effort to avoid specialized jargon as well as as ambiguous or misleading terminology and notation. The fact that a term is firmly established and has attained widespread usage is often cited as a compelling reason for retaining it even though it is undesirable on other grounds. I would suggest that such widespread usage need be no more than marginally relevant to our considerations.

A framework for discussion was delineated, at an early stage of the study, in terms of twelve problem areas, each of which contains one or more troublesome terms and/or symbols. Each problem area is defined below and followed by the pertinent responses, which are reported verbatim to better convey the flavor of the exchanges. The problem areas are identified by Arabic numerals 1 to 12, and the replies by Roman numerals I to X, some of which came from individuals and others from groups of two or more persons who responded jointly. Some of the letters did not comment on all twelve problem areas. All the comments received in response to an April 1972 mailing to all participants are reported here. I have added a few remarks of my own, which are identified by the initials S.M. Literature references are collected at the end of the report. Item 13 comprises a series of suggestions, all dealing with nomenclature related to metastable ions, received in June 1972 from John Beynon. In part, these suggestions overlap matters that had already been considered in correspondence with all the participants in the dialog. Most recently, all of the material presented here under items 1 to 13, inclusive, was circulated to the formal task-group membership. Their comments are not included here but will be reported to the IUPAC subcommission. Item 14 comprises a list of 19 terms, all pertinent to instrument performance specifications, selected by McCleary as being particularly in need of clear definition.

Mass, mass-to-charge ratio, m/e, etc.

The major problem here, which has troubled many workers, lies in the symbol "m/e." As one of our group has stated the matter, in most of chemistry and physics, "m" means the mass in grams and "e" is the charge on the electron (or the electron itself when it is part of an equation). The use of 'M' for molecular mass in atomic mass units and that of "z" for the number of charges on an ion are established and unambiguous in physics and chcmistry(1,2). Thus, "M/z" would appear to be the preferred notation.

A minor problem centers about the term used to denote this same quantity. Not too many years back, the established term was "specific mass"(3), which perhaps merits revival. Such revival would not be in accord with the lUPAC recommended usage of the word "specific," preceding the name of an extensive physical quantity, to mean "divided by mass"(4). On the other hand, it would not be the only exception to this recommendation. For example, "specific ionization" is a well established term denoting the number of ion pairs produced per unit of distance along the track of an ionizing particle(5,6).

Incidentally, the lUPAC-recommended symbol for atomic mass units is "u" rather than "amu"(4).


1.1. Since m/e is such a well-established term, I think it should not be abandoned, especially since the meaning of m and e in this context is clear, After all, m has quite a number of meanings. M/z would apply only to M+ what about the other "m/e" values?


1.II. M/z is acceptable.


1.III. Very good. I agree that specific mass might well be an acceptable method of expressing mass to charge ratio.


1.IV. "z" would appear to be preferable to e as the number of charges on the ion but if M is used for the mass of any ion (as is done in the A.V.S. Standard)(7) it will conflict with the definition of M for the molecular ion. One could, of course, call the molecular ion P (molecular parent ion or primary ionised species) but the use of M is well established. If M is the molecular ion we must use m/z for the mass to charge ratio of ions other than M and ignore the fact that m is usually mass in grams. I do not like referring to an ion of mass m but can see no way out of it other than using M*, M[bar] or some other horrible device for the molecular ion. M is already used for the apparent mass of a metastable ion and M signifies an average). Mass units should be in line with IUPAC using "u", i.e. the loss of 28 u.


1.V. In order to say what we mean and have general scientific understanding, I favor "M/z" for mass-to-charge ratio and "µ" for atomic mass unit.


1.VII. 'M/z' indeed appears to be the preferred notation. It would not lead to great difficulties for those familiar with the symbol m/e. For atomic mass units 'u' is also preferable. To denote M/z, the term "specific mass", although not recommended by the IUPAC, is still better than "mass over charge" or "mass-charge ratio".


1.IX. I whole-heartedly approve of the notation M/Z for mass-to-charge ratio. However, with respect to the term "specific mass", I cannot show any enthusiasm. I feel that the additional length of the term "mass-to-charge ratio" is worthwhile inasmuch as there is now a distinct difference between it and mass. The most important thing in this regard is for journals and referees to insist on mass-to-charge ratio for mass spectral scales and mass for whenever they mean mass. The term "specific mass" will probably be subjected to the same sloppy writing habits as its predecessor, but will not have any of the advantages in clarity.

Metastable, metastable peak, metastable ion.

The three terms listed above are often used interchangeably--a practice that strikes me and several of my correspondents as highly unfortunate. "Metastable" is an adjective and its use as a noun is undesirable. The metastable ion is not detected; it has decomposed in a metastable transition or metastable dissociation to the ionic species that is in fact detected in the spectrum. The resultant peak itself is, of course, not metastable. Nonetheless, I invite your consideration of the term "stable peak" as a colloquial but widely understood name for a peak attributed to the product of a metastable transition. May I suggest revival of the term "apparent mass" to locate the metastable peak in the spectrum.

The use of the asterisk in decomposition schemes to indicate a supporting metastable peak might well be questioned. Except to refer to a footnote, the assignment of any specific significance to an asterisk is highly arbitrary, and to the uninitiated such a symbol can serve only as a source of confusion. Related symbols--*2, *2, and **-- suggested by correspondents to identify metastable peaks detected by various methods, or derived from transitions in various field-free regions, would, of course, reduce the number of words required. But, again, they appear highly arbitrary and they would be incomprehensible to potential readers who are not members of the club. Some of us have adopted the practice of drawing solid arrows in a decomposition to denote processes supported by metastable peaks and broken arrows for processes not so supported; a statement specifying the symbolism assures understanding by the reader.


2.I. I am in favor of retaining "metastable peak" as a terminus technicus and I think it would be beneficial to discourage at the same time the terms '1 transition signal", "metastable transition", "diffuse peak" etc. which are rather confusing to anybody "not from the club".

If one settles form m* and m** to describe transitions in the first and second field free region, this would. avoid unnecessary words. I don't think that the asterisk would be confusing. If it indicates an excited state, nobody thinks of a footnote, either. Metastable peaks are something so common in mass spectrometry that anybody with the faintest idea about it will know the symbol and those not from the community would not know what to do with the descriptive sentence, either. And if one is to state what solid and broken arrows mean one can do the same with m* and m**


2.II. Conflicting opinions a In favour of "metastable peak" instead of "apparent mass", since there is nothing apparent about the position of the peak on the mass scale, which could be confusing.

b Not in favour of "metastable peak", and use "apparent mass" at present. However, strongly in favour of the term "transition signal"(8) to replace "metastable peak". Some concise nomenclature is needed to differentiate between signals due to transitions in different regions of the flight path and which would also give a concise description of the regions in which daughter ions are produced by consecutive metastable decompositions. Suggest something like e𝛕 to denote decomposition before the ESA, e𝛕 before magnet and c𝛕 decomposition before the collector. 𝛕em could then denote further decomposition in the pre-magnetic field-free region of a daughter ion produced in the pre-ESA region and transmitted through the analyser by the "defocusing" technique (this latter being an abhorrent term!)


2. III. I don't think the term "apparent mass" need be revived since some of us have used it consistently for years, and still do so. There is however a need for a symbol for this term. In the past this has been (m/e)*, (M/q)*, or simply m* (Beynon, Reed, OMS). Despite your disclaimer of the use of asterisks for anything except footnotes, the use of the asterisk to designate the apparent mass of the metastable peak is well established. The use of (M/z)* is sometimes necessary because some metastable peaks are the result of dissociations of multiply charged ions. Also in seminars or teaching the need to write (or derive) the generalized equation for the apparent mass of a generalized metastable transition

Generalized metastable transition 1.jpg

in which case

Generalized metastable transition 2.jpg

I object to your use of "m" (sect.12) as the apparent mass of the metastable peak. Firstly, "m" is reserved for the mass in grams and its use would require. explanation if the above equations were written

Generalized metastable transition 3.jpg

As it is, if the first time the apparent mass is used in a paper, (M/z)* can be defined as the apparent mass by the use of 3 words, i.e., (M/z)*, the apparent mass, ----.

S.M. The intent of my suggestion was to use m to identify metastable peaks in tabular listings of spectra, in accord with usage in the API Catalog(9), and perhaps in conjunction with the apparent mass in stating the assignment of a metastable peak to a particular transition, e.g.,

31.9 m 58+ → 43+ + 15

The use of min this sense would be restricted to these two situations, thus minimizing the likelihood of ambiguity.


2.IV. Presumably all ions in a mass spectrometer are to some extent metastable and hence the term metastable peak is the preferred term, being the peak formed by a meta.stable decomposition, the "metastable peak" being at an "apparent mass of ... ".

I find the use of an asterisk for a decomposition supported by a metastable peak as being very useful and I do not see that it is any more objectionable to include a statement to this effect than it is to specify the symbolism of solid and broken arrows. I do think that to include the measured apparent mass of the metastable might be useful as I sometimes wonder about the reliability of bland statements that a decomposition is "supported by a metastable" (unspecified); one can often find several decompositions that would fit,


2. V. I prefer "metastable peak at apparent mass XX".


2.VII. In the circumstances perhaps the term "metastable peak" is the best choice. The word metastable should only be used as an adjective ( -ion, -transition, etc.). The term "apparent mass" is appropriate; m (in italics?) being used as its symbol, and being followed by a value in numbers having two decimals ('m' xx.xx).


2.IX. One thing that has been omitted is reference to "mass spectra" obtained where only the neutral fragments from metastable processes are detected. While few people are doing this sort of thing at the present, it may become a popular technique.


2.X. If the usage of m* for the presence of a metastable transition is retained, I would prefer the use of an asterisk over or under an arrow to denote the presence of a metastable peak. A statement specifying the symbolism will assure that the reader understands. Use of broken arrows adds a new symbolism, whereas use of * comes directly from m*. I don't like *l, *2, **, etc. I would keep the use of the asterisk limited. I consider it an advantage that the asterisk is not used otherwise in chemistry.

Abundance, intensity, relative intensity, base peak.

"Abundance" and "intensity" are often used interchangeably. The dictionary definitions of the words would be more in accord with the use of "abundance" of ions and "intensity" of peaks.

"Peak height" or "intensity " may be expressed in any arbitrary units, such as ion current or mm measured on an analog record. "Relative intensity" implies normalization to some scale, and there is nothing inherently better about any one scale than others. Selection of the scale is a matter of arbitrary choice by the spectroscopist in light of his particular problem. He may prefer to express relative intensity as per cent of total ionization from initial specific mass XX; the commonly used and, I think, fairly clear symbol for such values is % XX. Otherwise, he may define the intensity scale by assigning a value of 100.0 to the molecular-ion peak, to the most intense peak, or to any other peak that suits his convenience. Twenty to 25 years ago, we used the term "base peak" for the peak so selected as the base for the scale of relative intensities (10). In current usage, the term has come to mean the most intense peak in the spectrum as well as the peak assigned a relative intensity of 100.0. This choice of intensity scale derives, I think, from its use in the API Catalog(9), and it seems an eminently good choice for cataloging purposes. For other purposes, such a choice may have no advantage over other possibilities or, worse, as in the comparison of spectra of various isotopic species of a compound(11), it may tend to obscure significant relationships and to interfere with interpretation of the data. I suggest a return to the earlier, less restricted sense of "base peak," which may or may not coincide with the strongest or most intense peak.


3.II. Strongly favour "base peak" to be retained, applying only to the most intense peak in the spectrum.


3.III. Agree on "abundance " and "intensity". In regard to base peak, I agree that authors should have the privilege of using any base peak convenient for their purpose, with the proviso that the base peak used should always be specified. If in a table, the base peak, if not obvious, should be given in a footnote to the table. Nothing is more infuriating than to have to search text to understand a table.


3.IV. Though I have always thought of the base peak as being the most intense ion, I agree that the broader definition is preferable. This would particularly assist in the analysis of mixtures where two "base peaks " may be required for different components, in order that spectra may be compared with standards. For most purposes the most in.tense peak would be chosen as base peak --do we need another name for the most intense peak?


3. V. Let's use "base peak" as the peak selected as the base for the "relative intensity " scale.


3.VII. The terms "abundance of ions " and "intensity of peaks" are to be preferred. %Σxx to be used to express relative intensity as per cent of total ionization from initial specific mass xx; the lower limit (M/z = 12 in organic mass spectrometry?) has to be defined.

The term "base peak" is to be assigned a relative intensity of 100.0, without necessarily referring to the most intense peak in 'the mass spectrum. For cataloging purposes the base peak should be the most intense peak.


3.IX. I do not like the symbol Σxx for total ionization between the mass-to-charge ratios i and j. I feel that the term ΣIM/Z M/Z=i,j is much more explicit and that this spectrum should be referred to as a "normalized mass spectrum".

Recording of solely metastable peaks.

Instrumental procedures that eliminate normal peaks and yield spectra consisting solely of metastable peaks are widely referred to as "metastable defocusing. " It is the ion beams responsible for normal peaks that are defocused, however; those giving rise to metastable peaks might be said to be refocused. I $commend for your consideration the expression proposed by Beynon et al., "scanning in the metastable mode"(12).


4.I. If we allow "metastable peak" , why not simply "metastable scan"?


4. II. a "Scanning in the metastable mode", .. is wide open to criticism, but not as inaccurate as "metastable defocusing"

b See 2b One can now employ the description "eτ scanning".


4.III. Your suggestion of "scanning in the metastable mode" is good.


4.IV. Metastable defocusing is rather a misnomer and Beynon's term of "Scanning in the metastable mode" is preferable. This would not cover the use of metastable detectors of the type developed by Daly et al. which are essentially used in the normal mode but they could be covered by "measured with a metastable detector (reference)".


4.V, I will also go with Beynon on this.


4. VI. "Scanning in the metastable mode" leaves something to be desired because there are several ways of doing this --the instruments being designed for IKES work with the reversed electric and magnetic sectors point to a new "metastable mode," for example.


4. VII. "Scanning in the metastable mode" appears to be by far the best term, but it might be too long to use it in a paper, especially as this technique is rapidly becoming common practice. I cannot find a better alternative; "metastable scan" and "metastable defocusing" should be avoided.

Resolution, resolving power.

These terms, commonly used interchangeably, are usually defined as M/ΔM or ΔM/M at a value of M such that the minimum output signal in the valley between two peaks of equal height equals 10%, or some other specified per cent, of the peak height. On the other hand, the American Vacuum Society's Standards Committee found such a definition inadequate for their needs; Dr. Nerken argues that it is applicable to magnetic instruments only, and is not relevant to quadrupole and time-of-flight instruments. The A.V.S. committee proposes that resolving power at mass M be defined as M/W, the ratio of M to peak width W, and that resolution at mass M be defined as the peak width W at M. For the A.V.S. proposals, see their proposed tentative standard(7).


5.I. think there will be always different definitions for different purposes. It might be helpful to give a list which one should be used when and it should be emphasized that the definition used should be specified.


5.Il. One vote in favour of the proposed A.V.S. definition.


5.III. No matter what decision is made in regard to the definition of resolution, the manufacturers are going to use a system that makes their instruments appear better than they are. It is apparent we will need industry participation in this, especially from the quadrupole people who are notoriously optimistic in citing their resolution.

Also shouldn't there be an intensity factor in defining resolution? On all machines it is possible to increase the resolution at the expense of sensitivity. It seems the figures used by some manufacturers are the limits at zero sensitivity.


5.IV. As you say, resolution and resolving power are commonly used interchangeably but that does not mean that they are interchangeable.

'Resolution' is the result obtained under a certain set of circumstances when attempt ing to differentiate between species, while

'Resolving Power' is the ability of a system to resolve, or differentiate, between different species. Hence, in mass spectrometry we arrive at the definitions given in the A.V.S. Committee report where "Resolution" is the peak width, W or ΔM, in mass units obtained under the given circumstances and "Resolving Power" is M/ΔM. If ΔM is measured as the intercept drawn at 10% peak height we should obtain resolving powers very similar to the figures quoted (incorrectly) by instrument manufacturers as the "resolution" of their instruments using the 10% valley definition. It would be interesting to hear the comments of instrument makers on this subject. An instrument of high resolving power will give a result of low resolution if it is knocked while scanning!


5.V. The A.V.S. proposal, paragraph 3.16.4, suggests "resolving Power = M/W = M/ΔM" and paragraph 4.2.1 has one measure W or ΔM at lO% of the peak height. So, I see no major differences between their definition and the 10% valley definition except verbosity and the use of equations and figures. Let's stay with the simple stated definition.


5 and 6.VI. We think that any working definition of resolution and sensitivity will have to require simultaneous definitions as the A.V.S. proposal seems to indicate. This is going to be especially important for high-resolution instruments. Presumably Don DeJongh's committee is working on definitions of standards for resolution and sensitivity, from the aspect of mixtures for testing instruments. We haven't heard much on how this is progressing.


5.VII. I think the A.V.S. committee proposals arc better than the definitions used so far. For comparing instrument performances it might be advisable that resolution (resolving power) for the instrument considered is given at certain specific masses, including the upper mass limit for the instrument.


5.IX. The definition of resolution, as proposed by A.V.S., i.e., peak width as mea.sured at the base line, may be sufficiently good for triangular or trapezoidal peaks but bears little relationship to the real world. For most cases [where] we are concerned with measuring M/Z's and their relative intensities, the decision as to whether this can be done within the required accuracy is easier to make from the 10% valley definition rather than from the width definition.


5.X. I think definitions of resolution and sensitivity should be based on procedure used to determine them. Thus I suggest holding this question aside in your committee until the A.V.S. and ASMS Committee VIII come up with their proposals on this question.

Sensitivity.

The A.V.S, proposed tentative standard(7) contains a definition of sensitivity for use in quantitative gas analysis. Somewhat different definitions would seem called for in quantitative analysis of heavier materials, in high-resolution mass spectrometry, in directly coupled gas chrom/mass spectrometer operation, and perhaps for other special contexts. However, I found no mention of sensitivity in any of your letters. May I have your thoughts?


6.I. It seems that cholesterol is now widely used by manufacturers, but since it may lose water thermally it may not be the best choice. The drawbacks of cortisol proposed by Spiteller have been discussed recently in OMS For organic work some compound of MW 400-800 of fairly low volatility should be chosen. The best choice would be some aromatic compound which shows little fragmentation.


6.II

a A.V.S. definition of Sensitivity is bad, since noise can be so much a function of the amplifier, or detector, etc. For gases, sensitivity should be defined as ions collected per atom or molecule entering the ion source. Also, sensitivity definition is needed for spark source and surface ionization mass spectrometry. This is not so easy as operatlng conditions must be quoted, e.g., filament material and temperature. It should relate sample consumed to sample collected.

b For organic work, a definition stating sample flow rate in g sec-1 to produce a base peak signal of X times noise or perhaps flow rate in g sec-1 to give base peak signal of amps at Z resolving power. Each type of mass spectrometry needs a separate definition!


6.III. In regard to sensitivity, please do not go to the quadrupole people's standard of "amperes per Torr" In the first place Torr where? It certainly has no meaning in a differentially pumped ion source, nor in any system with a leak to the ion source in which pressures are measured in the sample volume. Ion gauges in the vacuum system are notoriously poor indicators of ion source pressure in systems with relatively open ion sources. The old system of citing sensitivity in divisions per micron and also citing a standard substance in divisions per micron under the same conditions is not bad and perhaps should be continued. The relative sensitivity thus becomes fairly independent of operating parameters unless fragment peaks with high initial kinetic energy are used or one of the peaks has a high ion-source temperature coefficient.

I think one must recognize that it ls difficult, except in the most qualitative sense, to compare patterns or sensitivities between different types of instruments or even the same instrument with different types of detectors such as 21-103's with Faraday cup detectors or multiplier detectors. As the detector systems proliferate, each with its own characteristic discrimination in regard to mass or ion energy, this problem becomes more and more difficult to solve.


6.V. Sensitivity should be defined in terms of ion current per unit of partial pressure in the reservoir of a gas at a specific mass. For example: 4 x 10-14 amperes per micron at mass 78 for benzene.


6.VII. In my opinion the A.V.S. proposed tentative standard definition is good for use in quantitative gas analysis. The specified sensitivity should, as Dr. Nerken states, always be interrelated to the resolution. For high resolution mass spectrometers sensitivity and resolving power might be connected by using the 131 and 132 isotopes of xenon, which are roughly equally abundant (21.2 and 26.9 per cent natural abundance, respectively) The sensitivity for xenon might be related in a proper manner to that of neon or nitrogen.


6.IX. A.V.S.'s definition of sensitivity in amperes/torr acceptable with certain reservations: The sensitivity should be for a Faraday cup (or 1st dynode of a multiplier) and gain due to the multiplier may be given separately. Alternately, the sensitivity may be given in terms of counts/sec/torr which is, of course, easily converted to amperes/torr (if the M/Z is known). Time of flight instruments cause a few problems, but even in this case counts/sec/torr for a given M/Z is probably OK. With G.C. M.S. combinations there are more problems due to more variables. I would think in this case that the only meaningful sensitivity would be, in terms of a single M/Z, as coulombs/ng or counts/ng.

Name reactions, name processes.

Name reactions have long been in vogue in organic chemistry and, to a lesser extent in other areas of chemistry and physics. Much of the literature of organic chemistry is quite incomprehensible to a nonspecialist without one of the several available guides to name reactions. We in mass spectrometry have been fortunate so far in having avoided a proliferation of such terms.

An unfortunate recent example is "Audier's rule"(l3), evidently referring to what has long been known as "Stevenson's rule" and completely ignoring the matter of priority, which is not always as clearcut as in this case. Even the term "Stevenson's rule" has been widely misstated and misused. Despite the extra words that would be required, we would be better off if every author wishing to make use of this generalization were required to state it explicitly, giving appropriate credit via literature citations. The technique dealt with in point 4 above has been referred to as the "Barber-Elliott Major defocusing technique"(14) and the peaks obtained by this procedure as "Jennings signals"(15). Further comment by me would be gratuitous.

The one such term that has been most widely used and abused is, of course, the ve "-known "Mclafferty rearrangement. " Despite the considerable and well recognized contributions of Mclafferty to the clarification of this group of processes, the question of priority in this case is not easily brushed aside; consider, for example, the 1954 and 1957 publications of Nicholson(l6) and Manning(l7), respectively. The term, first used, I believe, by Spiteller, and popularized by Djerassi, has in the course of time been attached to the widest variety of processes involving migration originally of hydrogen and subsequently of numerous other atoms and groups as well. The original term has given rise to the "McLafferty plus one, " "McLafferty plus 14," "protonated McLafferty, "double McLafferty, "super-Mclafferty, "quasi-Mclafferty, and "complementary McLafferty rearrangements. I can see two possible options for us in the circumstances: either recommend that the term be dropped completely, or define it narrowly and precisely and discourage the use of derivative terms.


7.I. agree that name reactions should be kept to a minimum. I am in favor of retain.ing the terms "Stevenson's rule in its original meaning and "Mclafferty rearrangement defined as shown below because they are fairly widely known, describe phenomena of wide occurrence and would require rather lengthy sentences for a description of what they imply. For any new names to be used as termini technici the last two criteria should be applied rigorously (it would make no sense to call cleavage XY's cleavage).

McLafferty ASMS 1974.jpg

7.II. Drop the lot!


7.III. I agree that name reactions are anathema and should be discouraged if not banned Your criticism is excellent.


7.IV. I think there should be a much greater restriction by editors and referees on the use of name processes. They are meaningless jargon to many people and are often used to save authors having to say what they_ really mean. The "McLafferty " has almost become "any rearrangement involving a hetero atom". Either let us drop it altogether or define it rigidly as "a fragmentation involving a site specific transfer of a g-hydrogen to a hetero atom with cleavage of the a bond". Please let us have no more extensions of McLafferty--I see that Budzikiewicz et al. refer to a "McLafferty ion " and a "McLafferty peak".


7.V. I am against the use of name reactions or processes. But if authors insist in their use, they should define, give an example, and give a reference on their first use {OTFU) of it.


7.VI. "Stevenson's Rule " does not have much to do with ion intensities at 70 or 80 volts as a function of relative appearance potentials. It deals mostly with the possi.bility of excited states in the neutral when the fragment with the higher ionization potential of the two is actually formed. [One of us] once asked McLafferty how this could be construed as the basis for what is commonly called Stevenson's rule" McLafferty wrote to Stevenson to check, and [our] recollection is that Stevenson disclaimed author.ship for this version of the " rule." Hence we refer to the statement without an author.ship, at this point, and would recommend that this be made a formal proposal. Audier seems to be the only person who has presumed to include the statement in a codification of empirical observations, except of course that many people have used it as they needed it prior to his review. But we hardly think that this requires re-naming the rule after him, even if the first name is questionable. S.M. Correspondent VI is absolutely right, and I offer profuse apologies for doing exactly what I complained about--misstating and misusing "Stevenson's rule. " For the relationship referred to above, pertinent literature references are cited below as 18-22.


7.Vll. I agree that use of name reactions (processes) should be discouraged as much as possible. As to the term "McLafferty rearrangement " I think its use should be restrict.ed to those cases where a hydrogen bound to an atom in a position Y with respect to a double bond migrates to that bond through a six membered cyclic transition state, prior to (further) fragmentation (Type Hin Biemann 1 s classification of fragmentation pro.cesses (23)). Perhaps the matter of priority might be avoided by using a term like "Y.hydrogen rearrangement "


7.Vlll. Perhaps one suggestion that could be made is that the loss of 15 mass units from a species could be called "a McLafferty minus 13 simple cleavage."


7.IX. While I agree with you that "name" reactions should be abolished if only because there is only one name and a hundred variations there is a separate set of names in mass spectrometry:

Dempster-type mass spectrometer

Herzog shields

Mattauch-Herzog-type M.S.

Johnson & Nier type MS

Allen-type multiplier

etc.

There are, of course, many others (a good question to ask at this point is whether the Dempster type referred to is single or double focussing), It is not always the case that an instrument bears its inventor's name, i.e., "time of flight " , quadrupole, mono.pole, 120 magnetic sector, etc., so that perhaps we should not treat this area any differently to the name reactions.


7.X. I too don't approve of the use of name reactions. It may be too late to prevent further misuse of the term "McLafferty rearrangement"! It certainly would help if Djerassi stopped using the term. You don't give your opinions of "Nier geometry" , "Mattauch-Herzog geometry", etc. A novice could spend a lifetime finding the meanings of some terms of this type.

Presentation of spectra

Until some ten years ago, mass spectra were nearly always reported in tabular form. Since then, bar-chart presentation has come into wide favor. Each of these modes has advantages and disadvantages, and which is preferable depends on the kind of information an author is trying to convey, the point he is trying to make. This choice deserves more attention than it has had from the authors of much recent literature and, of course, from editors and referees. On the other hand, some authors have, within the past few years, taken to reporting mass spectra in paragraph form. I would suggest that this practice is an abomination. The reader can do nothing with such data without first transcribing them to some other format. I would like to see us do whatever we can to discourage such a practice.


8. I. Agree.


8.II. Unanimous agreement here. Tabulated data is essential, with line diagrams (expensive to produce) as optional extras. Press hard for exclusion of spectra" "paragraph


8.III. I agree that paragraph presentation of spectra is intolerable.


8.IV. I find it difficult to choose between bar charts and tabular data as both are so useful under different circumstances. The use of paragraph form is certainly to be strongly discouraged; I presume that it arose through the listing of I.R. peaks in this manner in organic chemical journals. In such journals, where mass spectrometry is only a means of identification, the alternative would, I fear, be to have the data left out completely; paragraph form is at least data even if inconveniently presented.


8.V. prefer the tabular form for presentation of entire spectra, but an author should be free to make his point in comparison of specific spectral features in any convenient form.


8.VII. I too feel that mass spectra should be presented either as a bar-chart or in tabular form, and certainly not in paragraph form.


8.IX. I like matrix spectra 14 M/Z's across and half masses and metastable peaks placed elsewhere.


8.X. I think that the use of paragraph form for reporting spectra has been forced on us by some journals. This form takes less space than others. If one la afraid that the editor will refuse to allow inclusion of bis data, he presents it as concisely as possible, I say that it is better to have it in paragraph form than not at all.

α-Cleavage, β-cleavage, etc.

Nomenclature to identify bonds and atoms in terms of their position relative to a specific atom, functional group, localized odd electron, or charge center in a molecule continues to be a source of ambiguity. Budzlkiewicz et al.(24) state, " ... we refer to 'α-cleavage' as fission of a bond originating at an which is adjacent to the one assumed to bear the charge; the definition of β, γ . . . cleavage then follows automatically. The following process would thus be described as 'a-fission of a ketone with expulsion of an alkyl radical'

Alpha fission.jpg

One of my correspondents suggests that, "Instead of referring to the ' ... atom assumed to bear the charge ... ' perhaps 'heteroatom' would be better," and he continues: "The reference to α, β, γ-cleavage in this fashion leaves no Greek letter for the less common R2N+R cleavage but I suppose this could be referred to simply as N-C (or N-0, etc.) cleavage.

"A problem comes up when aromatic rings or double bonds are involved, however. How should we describe cleavages such as:

Aromatic cleavage.jpg

Is the bond broken a or b? It is tempting to call it α-cleavage since the electronic stabilization of the ion is about the same as in the case of cleavage next to heteroatoms, But calling this bond the α-bond flies in the face of all my organic training. As I see it, it is more reasonable to call these forms benzylic and allylic cleavage and forget the relationship to heteroatom electron-stabilized α-cleavage.

It also seems reasonable to call R-C=O-R acyl or aroyl cleavage by organic chemistry analogy, The problem is that there is no analogy for R-C-OH-R (α-cleavage) in organic chemistry and it seems reasonable here to refer to such processes as alcohol cleavage, ether cleavage, amine cleavage, thiol cleavage etc. In some ways I even prefer this to α-cleavage."

How do the rest of you view this matter?


9.I. In my opinion the term α-cleavage should be restricted to compounds with hetero atoms (ketones ethers, amines) where the meaning can be defined clearly (R-CO-R'-> RCO+ , R-O-CH2-R -> R-O+=CH2) since they all follow the same mechanism. One could adopt the terms "allylic" and "benzylic" cleavage for carbon systems. To introduce terms as amine or thiol cleavage would rather confuse matters since there are several "thiol cleavages" so that somebody not initiated would hardly know which one was referred to.


9.II. Agreement again. Avoid using α, β, γ, ..... wherever possible.


9.III. I am not sure how organic chemists solve the problem of bond designation in complicated systems as my use of this has been limited to simple systems where there is little, if any opportunity for the designation to be misinterpreted. Whatever system is used should certainly not be subject to misinterpretation, and the bond specified should be clear.


9.IV. I find this one of the most confusing areas and would certainly welcome the dropping of terms α-cleavage, β-cleavage, etc. because, as your correspondent points out, it can conflict with organic practice. A direct identification of the bond naming the atoms which it joins is far preferable and here present practice in nomenclature would require the use, where possible, of atom numbering, e.g., the C2-C3 bond.

One field that appears particularly confusing is the uee of α and β for the locaation of substituents in rings and for positions of bonds, e.g. (25), we have loss of the α-substituent in alkyl pyrroles but β-cleavage to give loss of a methyl group in ethyl pyrroles (presumably either α or β ethyl pyrroles), Is β-cleavage β to the ring or the double bond? --certainly not to the hetero atom in β-pyrroles. In other words is loss of a β-substituent α-cleavage? This is, I feel, another reason for dropping the use of Greek letters.


9.V. I think a writer should state "cleavage α, to the ring", or "cleavage β to the carbonyl group", etc. unless he clearly defines his shortened nomenclature.


9.VI. We feel that to call benzylic cleavage α-cleavage is to open a can of worms. Consider the reaction of alkylbenzenes in which hydrogen is transferred back to the aromatic portion and a section of the chain is lost as an olefin. This has been a cleavage with γ hydrogen transfer for years. We cannot break the same bond in benzylic cleavage and call it a cleavage! But if we rename the rearrangement and call it a cleavage with a hydrogen transfer, then the relationship to the common fragmentation of carbonyl compounds, etc., becomes clouded. It seems to us that the least inconsistent approach here is to leave benzylic cleavage as a cleavage, or, as your correspondent suggests, not to give it any Greek letter.

To introduce acyl cleavage, aroyl cleavage, and terms like these seems poor since it is not clear whether the acyl fragment is being formed or cleaved in the process described --actually it sounds more like the latter to the uninitiated. We could clear up the matter with new terms--"acylogenesis" --but this seems unnecessarily cabalistic. Terms like alcohol cleavage, ether cleavage, and so forth are hopelessly vague --and, further, they give no indication whatsoever that the processes are similar.


9.VII. I agree with one of your correspondents that the definition given by Budzikiewicz et al.(24) is to be amended as follows: α-cleavage is fission of a bond originating at an atom which is adjacent to a heteroatom, etc. " There is an ambiguity possible in molecules having more than one heteroatom, and therefore I would suggest to use the term α-cleavage only when the author specifies the original position chosen.

As an example, loss of H. from carbon atom 2 in the molecular ion of imidazole can be described as α-cleavage with respect to both heteroatoms (case A); loss of H. from C-4(5), however is β-cleavage with respect to nitrogen atom 1 and β with respect to nitrogen atom 3 or, if one wishes, α with respect to N-1 and β with respect to N-3.

ASMS 1974 imidazole reaction.jpg

The suggestions given by your correspondent are good, I think. The advantage of using terms like benzylic, allylic, alcohol cleavage, etc. is that such terms are readily understandable to all organic chemists.


9.X. The terminology in regard to cleavages must conform with the nomenclature of organic chemistry. Photochemists call cleavage of the bond between the carbonyl-carbon atom and the adjacent carbon atom "α-cleavage". I also can accept the terms "benzyllic" and "allylic" cleavage. For ketones and aldehydes, "α-cleavage" and "β-cleavage with β-hydrogen transfer" seem to be understandable to organic chemists. For alcohols, amines, etc ., one could say "C-X bond cleavage", α,β-bond cleavage", etc.

Peak, ion, mass.

The current literature furnishes a growing number of horrible examples of the completely interchangeable use of the words "peak," "ion," and even "mass." One might have hoped that editors and referees would have discouraged such a loose practice.

10.I. Agree.

10.II. Agreed.

10.III. Agree -- the clear usage of terms is, however, the primary responsibility of the author, and secondarily the referee. I have had bad experiences when editors assume the responsibility of changing technical terms in a paper.

10.IV. It is up to editors and referees to correct the misuse of peak, ion and mass. The correct usages are fairly obvious.

10.V. I favor clear, unambiguous terms, but most writers avoid repetition like the plague, leading to undesirable interchanges. Understanding by the reader should be the aim of expository writing.

Molecular, molecule, and parent ion and peak

"Molecule-ion can perhaps be discarded out-of-hand in view of the similarity to "ion-molecule," an adjective describing a reaction between an ion and a neutral molecule. "Parent" may denote either the ion formed by loss of one (or more than one, if stated explicitly) electron from the original molecule -- precisely the definition of "molecular ion 11 in the O.M.S. Editorial Review on Nomenclature(26) -- or the decomposing ion in any reaction step under consideration, in which the charge-retaining product is referred to as a "daughter ion." The possible ambiguity has evidently caused some concern. On the other hand, the meaning is usually clear from the context. How serious a problem is this?

11.I. I think the term "parent" should be used for any decomposing ion and not be restricted to M+.

11.II. I think we should press hard for the use of M+ to refer to the ionised molecule, since "parent/daughter decompositions can confuse the use of the word "parent". Parent must then be used as the ion giving rise to a decomposition sequence.

11.III. I am glad you refer to "ion-molecule" as an adjective only, i.e., describing a type of reaction. The use of "ion-molecule" as a noun, i.e., the product of an ion molecule reaction, should be discouraged.

In regard to the use of "daughter ion" as the product of fragmentation, trouble may arise in the second fragmentation step (are these granddaughter ions?). I suggest the use of primary fragment ions, secondary fragment ions, etc. This latter terminology has a long history of usage and is clear. In parent-daughter relationships even in metastable ions one gets into problems, if as you suggest, "parent" is reserved for the molecular ion. What then do you call the relationship when the primary fragment of a fast fragmentation process is a metastable ion. The "parent" as far as the metastable dissociation is concerned is the fragment ion, not the molecular ion. This would be clear if the terms parent and daughter were reserved for metastable transitions, and fast processes were called fragmentations.

11 and 12.IV. See paragraph 1. One further thought, should the term "parent ion" be used for the case where a molecular ion does not appear but the spectrum derives from a, say, M-18 ion (loss of water)? It is probably better to use the term "M-l8 ion" and keep the terms parent and daughter ions for the precursor and product ions of a decomposition.

11.V. Although I prefer molecular ion or parent peak, I do not feel that these terms are a serious problem.

11.VI. [One of us] recently reviewed a paper where the authors wanted to call the most intense peak in the spectrum (which was a fragment peak) the parent peak!

11.VII. Indeed the term "molecule-ion" should not be used. The term "molecular ion" is to be used according to the definition given in the OMS Editorial Review on Nomenclature(26).

The term "parent" might be used only in the sense of "parent peak", meaning the peak of the molecular ion in a mass spectrum.

In the sense of a decomposing ion in a reaction step, perhaps the term "precursor" or "precursor ion" might be better.

Abbreviations.

A list of standard abbreviations would probably be in order. As starters, consider: M (to identify the molecular ion), m (following the apparent mass, to denote a metastable peak), u (atomic mass units), eV (electron volts, to accord with accepted usage in other areas (4,5,6)), EI, FI, CI, QET.


12. I. M+ (not M) for the molecular ion; m* (vide supra); IP, AP for ionisation and appearance potential.


12.II. I sec a problem of conflict here. If we keep M for molecular ion as in 11, what about m for metastable? In any computer application one cannot differentiate between upper/lower case letters. May I make a suggestion --stick with the asterisk to denote a metastable peak, both in decomposition schemes and in tabulated data.


12.111. I have already commented on the use of "m" for a mass in atomic units. Any table of abbreviations is going to be subject to overlaps, but the overlap should be with units or designations of variables in different fields where there is no possibility of conflict in equations or in derivations. This is obviously a difficult thing to do and requires a complete list and critical evaluation for possible conflicts with established abbreviations.


12.IV. I offer the following list of terms and definitions:

M+, M-
molecular ion
M
molecular weight of the molecular ion.
M+, M-
ion other than the molecular ion.
m
molecular weight of m+ or m-.
z
number of charges on an ion.
m/z
mass to charge ratio of ion of mass m.
u
atomic mass unit.
Δm
peak width of an ion of mass m measured in mass units at a base line of ten percent of peak height; resolution.
m/Δm, M/ΔM
resolving power
m*
apparent mass of a metastable peak.
eV
electron volts.
EI
electron bombardment ionisation.
FI
field ionisation.
CI
chemical ionisation.
EC
electron capture or negative ion mass spectrometry.
QET
Quasi equilibrium theory.
  • (26.1)
Symbol to denote that a metastable has been found for this decomposition at an apparent mass of 26.1.

12.V Undefined abbreviations should not be used at all unless they are indeed accepted widely by interested disciplines. But an author should be allowed to define his abbreviations for that paper OTFU.


12.VI. Other standard abbreviations: ICR, IKES?


12.IX. It would be useful if abbreviations were defined once in every article, the only exceptions being those which are in common usage in physics and chemistry.

Additional comments and questions

VI. Another point: should the Society support the effort to supplant other systems of units by the SI units, as the British Chemical Society is doing? (We have some reservations about expressing ionization potentials in joules or attojoules, and pressures in milli-or micropascals, but perhaps the question ought to be addressed.)


IX. In the interest of compatibility with the rest of physical science, S.I. units should be used; where other units are used they should be defined in terms of S.I. units.

S.M. In re the reporting of metastable peaks in tabulated spectra, note the practice of Archives of Mass Spectral Data of reporting apparent masses of metastable peaks to one decimal place, but of other peaks to the nearest whole number. This accords with a common practice by many workers, with one additional item, the use of fractions rather than decimals in reporting nonintegral M/z values of doubly and triply charged ions.

Some thoughts on nomenclature in the field of metastable ions and some suggestions of terms to be encouraged and of others to be discouraged.

a. The meaning is perfectly clear when one speaks of a metastable ion and this term should be reserved for an ion that reacts unimolecularly during its passage through a mass spectrometer. It should not under any circumstances be used to describe the product ions resulting from the fragmentation of a metastable ion.

b. It seems to me equally clear when one talks of a metastable transition. This is a transition undergone by a metastable ion.

c. As in the case of any other ionic reaction, it is perfectly proper to speak about the parent and daughter ions of the transition.

d. It has become customary to use the symbol m* to represent the apparent mass, i.e., the position on the mass scale where the peak due to arrival of the daughter ions of a metastable transition is observed. It is customary to write m* = m22/m1 and I suggest that the use of m*, m1, and m2 to represent the apparent mass, the mass of the metastable ion and the mass of the daughter ion should be confirmed.

e. The term metastable peak has been used for a long time and I suggest that the use of this as an acceptable shorthand should be confirmed, quotation marks no longer being required. The term metastable (used as a noun) could be used interchangeably with metastable peak.

f. For transitions in which the neutral fragment is represented, or in the general case in which there are two daughters of the transition, the term m3 should be used for the second daughter. This would be so in cases such as:

m1+ -> m2+ + m3

or

m1++ -> m2+ + m3+

All of this would help to establish an easy understanding of the mathematical equations used in this field.

g. Use of the word "metastable" in such forms as " ... in metastable work" instead of " ... in work on metastable ions" should be forbidden.

h. Use of the symbol E to describe the voltage applied to an electric sector and of V to describe the accelerating voltage should be encouraged.

i. The region immediately preceding the electric sector should be called the first field-free region; the region in front of the magnetic sector should be called second field-free region. This is particularly important if we are to avoid confusion now that instruments are beginning to appear in which the ion beam passes through the magnetic sector before the electric sector.

j. The slit that follows the electric sector is usually called the β-slit, whether or not it is a collector slit. I suggest the slit that follows the magnet (whether or not it is a collector slit) be called the γ-slit.

k. There is a well-known method of observing metastable peaks due to transitions occurring in the first field-free region that involves scanning the accelerating voltage at fixed electric sector voltage (due to Futrell and to Barber and Elliott). Many authors call this 11 defocusing the ion beam." This shows a complete misunderstanding of the process; the focusing is not affected in any respect, and the focal length of the sector remains fixed. The main beam is merely deflected and a new beam focused at the same point. I suggest that some other term be approved for this method. "Defocus" (and also "Refocus" that is used from time to time) should not be allowed under any circumstances. We currently use accelerating voltage scan method to describe what is done.

1. When a collision gas is used to promote ionic reactions, we speak of "collision-induced dissociations" or CID's. The peaks resulting from CID's should be called "CID peaks." Although their position on the mass scale is very close to the position at which metastable peaks appear, they should not be, called "collision-induced metastables."

m. Various names are used to describe reactions in which charges are transferred when a collision gas is introduced. Thus the processes

X++ + Y -> X+ + Y+

or

X+++ + Y -> X++ + Y+

would be called "charge exchange" or "ionization of the collision gas by the ion beam"

Reactions such as:

X+ + Y -> X++ + Y + e

or

X+ + Y -> X++ + Y+ + 2e

are called "charge stripping".

I suggest that we should have a nomenclature to cover all possible reactions, and believe the one proposed (?) and used by Hasted to be ideal. He would refer to the above reactions by the number of charges on the reactants and products' as 20/11; 30/21; 10/20 and 10/21 reactions, respectively. This is, in my opinion, a rapidly developing field and one in which it is important to establish a standard nomenclature before too many people start their own.

n. There are a number of symbols that we are currently using. We hope that they will turn out to be those adopted as standard, but in any event, some of them are:

ε0 = Activation energy
εr0 = Reverse activation energy
ε = Non-fixed or internal energy of the reactant ion
c* = Non-fixed or internal energy of the activated complex.
T = Kinetic energy released in a reaction
Q = Kinetic energy converted to internal energy in a collision
Q' = Kinetic energy lost by an ion in a collision.

o. As a post-script may I say that we are strongly in favor of the use of a dot to indicate an odd electron, of M+ for a molecular ion and of P+ as the symbol for a parent ion (i.e. the parent, as opposed to the daughter for any reaction, not just those involving the molecular ion).

Terms pertinent to instrument performance specifications.

a. Mass range

b. Sensitivity

i. Basic (See note 1)
ii. Dynamic (See note 2)
iii. Minimum detectable
iv. Abundance sensitivity

c. Resolving power (Resolution)

d. Interference (General)

i. Gas sensitivity
ii. Memory
iii. Background

e. Test mixture or standard sample

f. Discrimination

g. Scan speed

h. Signal-to-noise ratio

i. Mass measurement

j. Accuracy of mixture analyses

k. Decoupled or defocused operation

l. Detector response

'In re' terms b, c, d, and e, see reference 3.

Note 1: This should be basic to the instrument. That is, ion current per unit of sample at some resolving power (e.g., amps/~g/sec, etc.)

Note 2: This is meant to define a total system specification, including scan speed, amplifier response and resolution.

For the members of Subcormiittee 10 and all who participated in our deliberations.

Seymour Meyerson
Research Department
Standard Oil Company(Indiana)
Box 400
Naperville, Illinois 60540, U.S.A.
June 21, 1974

References

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2. "Symbols, Units and Nomenclature in Physics," lnternat. Union of Pure and Appl. Phys., UNESCO, Document U.l.P. 9 (S.U.N. 61-44), 1961, pp. 13-14.[2]

3. Recommended Practices E304-E68 and B137-68, 1969 Yearbook of the American Society for Testing and Materials.

4. "Manual of Symbols and Terminology for Physicochemical Quantities and Units," issued by IUPAC Division of Physical Chemistry Commission on Symbols, Terminology and Unite, Butterworths, London, 1970.[3]

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9. "Catalog of Selected Mass Spectral Data," American Petroleum Institute Research Project 44, Thermodynamics Research Center, Texas A&M University, College Station, Texas.

10. "Mass Spectrometer Computing Manual," Consolidated Engineering Corp., March, 1951, p. i

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12. J. H. Beynon, J. A. Hopkinson, and G. R. Lester, Int. J. Mass Spectrom. Ion Phys.(1969) 2, 291. [7]

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15. Personal conversation; not in print, happily.

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24. H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, Inc., San Francisco, Calif., 1967, pp. 2-3.

25. H. Budzikiewicz, C. Djerassi and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, Inc., San Francisco, Calif., 1967, pp. 597-598.

26. "Editorial Review on Nomenclature," Org. Mass Spectrom., 2 249 (1969). [15]