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CAD and CID

Resolution and resolving power controversy

QUOTED TEXT FROM IUPAC RECOMMENDATIONS 2013
The IUPAC definition of resolution in mass spectrometry expresses this value as m/Δm, where m is the mass of the ion of interest and Δm is the peak width (peak width definition) or the spacing between two equal intensity peaks with a valley between them no more than 10 % of their height (10 % valley definition) [1]. Resolving power in mass spectrometry is defined as the ability of an instrument or measurement procedure to distinguish between two peaks at m/z values differing by a small amount and expressed as the peak width in mass units [2]. Mass resolving power is defined separately as m/Δm in a manner similar to that given above for mass resolution [3]. These definitions of mass resolving power and resolving power in mass spectrometry are contradictory, the former is expressed as a dimensionless ratio and the latter as a mass. The definitions for resolution in mass spectrometry and resolving power in mass spectrometry come from Todd's 1991 recommendations [4], and the definition for mass resolving power comes from Beynon's 1978 recommendations [5]. Beynon's work contains no definition for mass resolution.

Alternative definitions for resolution and resolving power in mass spectrometry have been proposed [6][7]. It has been suggested that resolution be given by Δm and resolving power by m/Δm; however, these definitions are not widely used.

The majority of the mass spectrometry community uses resolution as defined by IUPAC. The term resolving power is not widely used as a synonym for resolution. In this document, the IUPAC definition of resolution in mass spectrometry remains in place. The definition of resolving power has been adapted from the current IUPAC definition of mass resolving power.

From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013.

Books defining resolution and/or resolving power

Books using resolution is m/Δm

Mass Spectrometry and its Applications to Organic Chemistry
J. H. Beynon, Elsevier, 1960
p. 51 "The terms 'resolution' and 'resolving power' have been used a great deal in the above discussion. It has been assumed that the doublet is 'resolved' when its constituent ion species are 'separated' and that the difficult of separation or 'resolving power' necessary to separate the adjacent mass peaks is given by M/ΔM."
Mass Spectrometry - Organic Chemical Applications
Klaus Biemann, McGraw-Hill, 1962
(p. 13) the term resolution is used in different ways - Throughout this book resolution will be considered as M/ΔM
Lasers and Mass Spectrometry
By David M. Lubman, Oxford University Press US, 1990, ISBN 0195059298
Interpretation of Mass Spectra
Fred W. McLafferty, Turecek, University Science Books, 1993, Language: English, ISBN 0935702253
Mass Spectrometry: Clinical and Biomedical Applications
By Dominic M. Desiderio, Springer, 1993, ISBN 0306442612
Practical Organic Mass Spectrometry: A Guide for Chemical and Biochemical Analysis
J. R. Chapman, Wiley_Default, 1995, ISBN 047195831X
Mass Spectrometry for Chemists and Biochemists
Robert Alexander Walker Johnstone, M. E. Rose, Cambridge University Press, 1996, ISBN 0521424976
Introduction to Mass Spectrometry
By J. Throck Watson, Lippincott-Raven, 1997, ISBN 0397516886
Ionization Methods in Organic Mass Spectrometry
By Alison E. Ashcroft, Royal Society of Chemistry (Great Britain), Royal Society of Chemistry, 1997, ISBN 0854045708
Accelerator Mass Spectrometry: Ultrasensitive Analysis for Global Science
Claudio Tuniz, John R. Bird, Gregory F. Herzog, David Fink, CRC Press, 1998, ISBN 0849345383
Mass Spectrometry in Biology & Medicine
By A. L. Burlingame, Steven A. Carr, Michael A. Baldwin, Humana Press, 1999, ISBN 0896037991
Mass Spectrometry and Genomic Analysis
J. Nicholas Housby, Springer, 2001, ISBN 0792371739
Mass Spectrometry Basics
Christopher G. Herbert, Robert Alexander Walker Johnstone, CRC Press, 2002, ISBN 0849313546
Liquid Chromatography Mass Spectrometry: An Introduction Robert E. Ardrey
Wiley, 2003, ISBN 0471498017
Mass Spectrometry: A Textbook
Jurgen H. Gross, Springer, 2004, ISBN 3540407391
Quadrupole Ion Trap Mass Spectrometry
By Raymond E. March, John F. Todd, Wiley-IEEE, 2005, ISBN 0471717975
The Expanding Role of Mass Spectrometry in Biotechnology
Gary Siuzdak, McC Pr, 2006, ISBN 0974245127
Quantitative Applications of Mass Spectrometry
Pietro Traldi, Franco Magno, Irma Lavagnini, Roberta Seraglia, Wiley, 2006, ISBN 0470025166
Assigning Structures to Ions in Mass Spectrometry
John L. Holmes, Christiane Aubry, Paul M. Mayer, CRC, 2006, ISBN 0849319501
Mass Spectrometry: Principles and Applications
Edmond de Hoffmann, Vincent Stroobant, Wiley-Interscience, 2007
ISBN 047003310X
Mass Spectrometry: Principles and Applications
Edmond de Hoffmann, Jean Charette, Vincent Stroobant, Wiley, 1996, ISBN 0471966975
p 287: "Resolution: the ratio of m/δm where m and m+δm are the mass numbers of the two ions that yield neighboring peaks with a valley depth of x% of the weakest peak's intensity."
Quantitative Proteomics by Mass Spectrometry (Methods in Molecular Biology)
Salvatore Sechi, Humana Press, 2007, ISBN 1588295710
Computational Methods for Mass Spectrometry Proteomics
Ingvar Eidhammer, Kristian Flikka, Lennart Martens, Svein-Ole Mikalsen, Wiley-Interscience, 2008, ISBN 0470512970

Books that use resolution is Δm

Mass Spectrometry Desk Reference
David Sparkman, Global View, 2006, ISBN 0966081390
"Incorrect: resolution - when defined in the same way as resolving power. Resolution is the inverse of resolving power and expressed as ΔM at M."
Introduction to Mass Spectrometry: Instrumentation, Applications, and Strategies for Data Interpretation
J. Throck Watson, O. David Sparkman, Wiley, 2007, Language: English, ISBN 0470516348
Fundamentals of Contemporary Mass Spectrometry
Chhabil Dass, 2007, ISBN 0471682292
p. 68: "[mass resolution] is the inverse of resolving power (RP), given as RP=m/Δm"
Proteomics in Practice: A Guide to Successful Experimental Design
Reiner Westermeier, Tom Naven, Hans-Rudolf H??pker, Wiley, 2008, ISBN 3527319417

Early manuscripts defining resolution and/or resolving power

F. Aston, Bakerian lecture. "A new mass-spectrograph and the whole number rule", Proc. R. Soc. London, 1927. http://dx.doi.org/10.1098/rspa.1927.0106
"Its resolving power was sufficient to separate mass lines differing by about 1 in 130 and its accuracy of measurement was about 1 in 1000. […] It was finally decided that the increase of resolution could best be obtained by doubling the angles of electric and magnetic deflection, and sharpening the lines by the use of finer slits placed further apart, in addition special methods were considered for the necessary increase of accuracy in measurement. After numerous setbacks all these objects have been successfully carried out. The new instrument has five times the resolving power of the old one, far more than sufficient to separate the mass lines of the heaviest element known. Its accuracy is 1 in 10,000 which is just sufficient to give rough first order values of the divergences from whole numbers."
F.W. Aston, "Atoms and their Packing Fractions", Nature, 120 (1927) 956-959. http://dx.doi.org/10.1038/120956a0
"the resolution of the mass lines of the heavier elements […] resolving power was suflicient to separate mass lines differing by about 1 in 130, and its accuracy of measurement was about 1 in 1000. […] new instrument has five times the resolving power of the old one, far more than sufficient to separate the mass lines of the heaviest element known. Its accuracy is 1 in 10,000, […]"
W. Bleakney, "A New Method of Positive Ray Analysis and Its Application to the Measurement of Ionization Potentials in Mercury Vapor," Phys. Rev., 34 (1929) 157-160. http://dx.doi.org/10.1103/PhysRev.34.157
"While the resolving power of the analyzer is not particularly high, yet it has proved to be excellent for the purposes for which it was designed."
F.W. Aston, The Isotopic Constitution and Atomic Weight of Lead from Different Sources, Proceedings of the Royal Society of London Series a-Containing Papers of a Mathematical and Physical Character, 140 (1933) 535-543. http://dx.doi.org/10.1098/rspa.1933.0087
"a view to increasing resolving power […] Increased accuracy has been obtained, but full advantage cannot be taken of it until higher resolution is available on account of the inevitable error involved in measuring the distance between lines not of the same intensity"
A.J. Dempster, New Methods in Mass Spectroscopy, Proceedings of the American Philosophical Society, 75 (1935) 755-767. http://dx.doi.org/
"The main limitation to increased accuracy in mass determinations is in the comparatively small resolving power of the mass spectrographs hitherto used. On page 78 of "Mass Spectra and Isotopes," Aston says: "The resolving power is sufficient to separate lines differing by I in 6oo, . . . since the lines are irregularly curved and change in shape as one moves from one end of the spectrum to the other, it is impossible to assign positions to them relative to the fiducial spot with sufficient accuracy to approach the figure of 1 in 10,000 aimed at. This can only be done by measuring the distance between lines of approximately the same intensity and therefore the same shape, when they are quite […] The spectra reproduced by Bainbridge 1 show a resolving power of approximately 1 in 200, that is, the image produced by the atoms of one element is so broad that the value obtained for the weight, if one side of the image is observed, differs by 1 in 200 from the weight obtained if the other side is used. Of course, the center is measured but some of the mass determinations given by Dr. Bainbridge involve estimating the center of the image with an accuracy of one hundredth of the width of the image. While the progress made by Dr. Aston and Dr. Bainbridge has been most re- markable, it is permissible to hope that an increase in sharpness of the images with a corresponding increase in resolving power would give a still greater precision in atomic mass determinations.close together. The accuracy of 1 in 1O,OOO estimated by Dr. Aston implies the judging of the centers, […] As explained in the introduction, this is primarily a problem of increased resolution with greater sharpness of the ion images. […] The resolving power with this comparatively wide slit is I in 1OOO."

Other IUPAC definitions of resolution

Gold Book

GOLD BOOK DEFINITION

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997).

Style guide

http://goldbook.iupac.org/R05319.html

resolution (in optical spectroscopy)

Wavenumber, wavelength or frequency difference of two still distinguishable lines in a spectrum.

Source: Green Book, 2nd ed., p. 31


IUPAC Gold Book
Index of Gold Book Terms


Gold Book

GOLD BOOK DEFINITION

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997).

Style guide
http://goldbook.iupac.org/P04465.html

peak resolution, Rs (in chromatography)

The separation of two peaks in terms of their average peak width at base (t R2 > t R1):

R s = t R2 ? t R1 w b1 + w b2 2 = 2 ( t R2 ? t R1 ) w b1 + w b2

In the case of two adjacent peaks it may be assumed that w b1 ? w b2, and thus, the width of the second peak may be substituted for the average value:

R s = t R2 ? t R1 w b2

Source: PAC, 1993, 65, 819 (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 847

Orange Book, p. 108

IUPAC Gold Book
Index of Gold Book Terms


Gold Book

GOLD BOOK DEFINITION

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997).

Style guide

resolution (in gas chromatography)

http://goldbook.iupac.org/R05317.html

A characteristic of the separation of two adjacent peaks. It may be expressed according to the equation:

RAB = 2(|dR(B)-dR(A)|)/(|wB+ wA|)

where RAB is the resolution, dR (A) and dR (B) are the retention distances (time or volume) of each eluted component A and B, and wA and wB are the respective widths of each peak at its base.

PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2211

IUPAC Gold Book
Index of Gold Book Terms


Gold Book

GOLD BOOK DEFINITION

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997).

Style guide
http://goldbook.iupac.org/E02113.html

energy resolution (in radiochemistry)

A measurement, at given energy, of the smallest difference between the energies of two particles or photons capable of being distinguished by a radiation spectrometer.

Source: PAC, 1994, 66, 2513 (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2519

IUPAC Gold Book
Index of Gold Book Terms


Resolution and resolving power terminology in mass spectrometry

ASMS 2022
Poster MP 113
Kermit K Murray

Premise

Nomenclature inconsistencies and conflicts can best be resolved through a detailed understanding of the origin and development of terms. The goal of this project is to investigate the origins and use as well as prior and current definitions of resolution and resolving power in order to make informed recommendations on the controversial and in some cases conflicting terminology.

Current definitions

In mass spectrometry, two peaks in a mass spectrum are resolved if they are distinguishable as separate. The degree to which the peaks are resolved can be quantified using the peak width or the separation between two peaks and is represented by Δ(m/z) where m/z is the mass-to-charge ratio. For singly charged ions, this can be expressed as Δm or, in older publications, as ΔM. The smallest value of Δm for which peaks are resolved is the limit of resolution. There are two general methods to determine Δm: peak width and valley:

Peak width: Δm is the peak width at a specified fraction of the peak height, for example at 50% Δm is the full width at half maximum

Valley: Δm is the separation between two equal height peaks that produces a valley a specified fraction of the height, for example 10%.

The 10% valley Δm is comparable to the 5% peak height Δm and approximately half that obtained from the FWHM. There are three general interpretations of the definitions of resolution and resolving power:

  • the terms are equivalent and represented by m/Δm (Meyerson 1975, Murray 2013)
  • resolution is m/Δm and resolving power is Δm (Price 1991, Todd 1991)

Historical use

Prior to the Second World War, the term resolving power, defined as M/ΔM, was used almost exclusively. Resolution was used as a binary variable or as the limit of resolution. In the second half of the 20th century, the two terms were increasingly used interchangeably.

F.W. Aston used resolution as a binary variable and resolving power as a quantitative measure, for example, “the instrument will resolve beams of different masses if the change in ϕ for change of mass is greater than the geometrical spread, and the greater ϕ for a given mass and given spread the greater the resolving power” (Aston 1922). In his book Mass Spectra and Isotopes, Aston defines resolving power as M/ΔM (Aston 1933).

A. J. Dempster defined limit of resolution as Δm/m (Dempster 1918) and, like Aston, often used the construct “one in [mass]” for resolving power, as in “resolving power with this comparatively wide slit is 1 in 1000” (Dempster 1935).

K. T. Bainbridge stated that “resolving power is defined as the ratio M/ΔM for complete separation of two lines and so is more stringent than the optical definition” (Bainbridge 1936).

J. Mattauch defined resolving power as M/ΔM and resolution as ΔM/M (Mattauch 1936)

W. Bleakney used the term resolving power in a 1929 publication (Bleakney 1929) but defined resolution as m/Δm in a 1949 publication (Mariner 1949).

A. O. Nier used both resolving power (Nier 1936) as well as resolution (Nier 1960).

J. H. Beynon in his textbook Mass Spectrometry and its Applications to Organic Chemistry writes “’resolution’ and ‘resolving power’ have been used a great deal in the above discussion. It has been assumed that the doublet is ‘resolved’ when its constituent ion species are ‘separated’ and that the difficult of separation or ‘resolving power’ necessary to separate the adjacent mass peaks is given by M/ΔM” (Beynon 1960)

K. Biemann in his textbook Mass Spectrometry: Organic Chemical Applications, states that “the term resolution is used in different ways – Throughout this book resolution will be considered as M/ΔM” (Biemann 1962).

ASMS Definitions

Subcommittee 10 on Definitions and Terms of ASTM Committee E-14 on Mass Spectrometry was established in 1970 and presented a compendia of terms at the 1974 ASMS meeting (Meyerson 1975). The ASMS Nomenclature Committee presented a list of terms at the 1982 ASMS meeting in Honolulu (Cameron 1982) and terms assembled by the ASMS Measurements and Standards Committee were published in 1991 (Price 1991) which closely paralleled the contemporary IUPAC recommendations (Todd 1991).

IUPAC Definitions

There have been four IUPAC recommendations for mass spectrometry terminology in the past five decades produced by the IUPAC Analytical Chemistry Division Commission on Analytical Nomenclature (Robertson 1974), the IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy (Beynon 1978), the IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy Subcommittee on Mass Spectroscopy (Todd 1991), and the IUPAC Physical and Biophysical Chemistry Division (Murray 2013). The IUPAC Compendium of Chemical Terminology “Gold Book” gives definitions of resolution (valley and width) from Todd 1991 and gives two conflicting definitions for resolving power, one from Todd 1991 (also Robertson 1974) that defines resolving power as Δm and one from Beynon 1978 that defines resolving power as m/Δm.

Recommendations

Terminology recommendations for resolution and resolving power must take into account the current interchangeable use of the terms as well as the longstanding use of resolving power as m/Δm. It is the opinion of the author that resolution should be used as a binary variable, resolving power defined as m/Δm be encouraged, and limit of resolution defined as Δm/m be used where necessary.

Resolution: The use of resolution as a quantitative measure is discouraged: use resolving power or limit of resolution as appropriate.

Resolving power: The observed m/z value divided by the smallest difference Δ(m/z) for two peaks that can be separated: (m/z)/Δ(m/z).

Limit of resolution: The smallest difference Δ(m/z) for two peaks that can be separated divided by m/z: Δ(m/z)/(m/z).

The recommendations above are those of the author who hopes that these concepts will be considered when developing the next list of terminology.

References

Aston, F.W.: Some problems of the mass-spectrograph. Philos. Mag. 43, 514 (1922)

Aston, F.W.: Mass Spectra and Isotopes, Arnold, London, (1933).

Bainbridge, K.T., Jordan, E.B.: Mass Spectrum Analysis. Phys. Rev. 50, 282 (1936)

Biemann, K: Mass Spectrometry: Organic Chemical Applications, McGraw-Hill, New York (1962).

Bleakney, W.: A New Method of Positive Ray Analysis and Its Application to the Measurement of Ionization Potentials in Mercury Vapor. Phys. Rev. 34, 157 (1929)

Beynon, J.H.: Recommendations for Symbolism and Nomenclature for Mass Spectroscopy. Pure Appl. Chem. 50, 65 (1978)

Beynon, J.H. Mass Spectrometry and its Applications to Organic Chemistry, Elsevier, (1960)

Cameron, D.: ASMS Nomenclature Committee Workshop. Annual Conference on Mass Spectrometry and Allied Topics Abstracts. 30, 901 (1982).

Dempster, A.J.: A new method of positive ray analysis. Phys. Rev. 11, 316 (1918)

Dempster, A.J.: New Methods in Mass Spectroscopy. Proc, Am. Phil. Soc. 75, 755 (1935)

Mariner, T., Bleakney, W.: A large mass spectrometer employing crossed electric and magnetic fields. Rev. Sci. Instrum. 20, 297 (1949)

Meyerson, S.: Definitions and terms in mass spectrometry. Biomed. Mass Spectrom. 2, 59 (1975)

Mattauch, J.: A Double-Focusing Mass Spectrograph and the Masses of N15 and 018. Phys. Rev. 50, 617 (1936)

Murray, K.K., Boyd, R.K., Eberlin, M.N., Langley, G.J., Li, L., Naito, Y.: Definitions of terms relating to mass spectrometry, Pure. Appl. Chem. 85, 1515-1609 (2013)

Nier, A.O.: A Mass-Spectrographic Study of the Isotopes of Argon, Potassium, Rubidium, Zinc and Cadmium. Phys. Rev. 50, 1041 (1936)

Nier, A.O.: Small General Purpose Double Focusing Mass Spectrometer. Rev. Sci. Instrum. 31, 1127 (1960)

Price, P.: Standard definitions of terms relating to mass spectrometry. J. Am. Soc. Mass Spectrom. 2, 336 (1991)

Robertson, A.J.B.: Recommendations for Nomenclature of Mass Spectrometry. Pure Appl. Chem. 37, 469 (1974)

Todd, J.F.J.: Recommendations for Nomenclature and Symbolism for Mass-Spectroscopy. Pure. Appl. Chem. 63, 1541 (1991)



QUOTED TEXT FROM IUPAC RECOMMENDATIONS 2013
The terms collision-induced dissociation (CID) and collisionally activated dissociation (CAD) are both recommended by IUPAC [8] and are used interchangeably in recent literature. They are listed as synonyms in this document.
From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013.

Daughter ion and related terms

QUOTED TEXT FROM IUPAC RECOMMENDATIONS 2013
The anthropomorphic terms for ions involved in fragmentation reactions, for example, daughter ion, have fallen into disuse after strong sentiments against the use of the term were voiced two decades ago [9][10]. The term product ion is recommended in place of daughter ion and precursor ion in place of parent ion. The use of nth-generation product ion is recommended in place of granddaughter ion and similar terms.
From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013.

Mass defect

Mass defect in mass spectrometry and nuclear physics

Mass defect (mass spectrometry)
The difference between the exact mass and the nearest integer mass
Mass defect (physics)
The difference between the mass of a composite particle and the sum of the masses of its parts

Links

Land, A. Neutrons in the Nucleus. I. Phys. Rev. 43, 620-623 (1933).
http://dx.doi.org/10.1103/PhysRev.43.620
Carlson (1960); High Resolution Mass Spectrometry. Interpretation of Spectra of Petroleum Fractions
http://dx.doi.org/10.1021/ac60167a032
Kendrick (1963); A Mass Scale Based on CH2= 14.0000 for High Resolution Mass Spectrometry of Organic Compounds.
http://dx.doi.org/10.1021/ac60206a048
Hughey (2001); Kendrick Mass Defect Spectrum:? A Compact Visual Analysis for Ultrahigh-Resolution Broadband Mass Spectra
http://dx.doi.org/10.1021/ac010560w
Zhang (2003); A software filter to remove interference ions from drug metabolites in accurate mass liquid chromatography/mass spectrometric analyses
http://dx.doi.org/10.1002/jms.521
Hall, M.P., Ashrafi, S., Obegi, I., Petesch, R., Peterson, J.N., Schneider, L.V. Mass defect tags for biomolecular mass spectrometry. J. Mass Spectrom. 38, 809-816 (2003).
http://dx.doi.org/10.1002/jms.493
Zhang (2009); Mass defect filter technique and its applications to drug metabolite identification by high-resolution mass spectrometry
http://dx.doi.org/10.1002/jms.1610
Sleno (2012); The use of mass defect in modern mass spectrometry
http://dx.doi.org/10.1002/jms.2953
Pourshahian (2017); Mass Defect from Nuclear Physics to Mass Spectral Analysis
http://dx.doi.org/10.1007/s13361-017-1741-9

m/z issues

QUOTED TEXT FROM IUPAC RECOMMENDATIONS 2013
The labeling of the x-axis of a mass spectrum engendered the most discussion during the creation of this document; however, in spite of a general desire for a better way to label the x-axis of mass spectra, there was no broad consensus for any of the proposed changes. Therefore, this document continues the use of the definitions of the Gold Book [11] and the similar definitions in the Orange Book [12]. The Gold Book recommendation is for the use of m/z as an abbreviation for mass-to-charge ratio, a dimension- less quantity obtained by dividing the mass number of an ion by its charge number [13].

The thomson unit, defined as the quotient of mass in units of u and the number of charges (z), was proposed nearly two decades ago [14], but has not been widely adopted and is therefore not recommended. Labeling the x-axis of a mass spectrum with any unit of mass such as dalton (Da), atomic mass unit (amu), or unified atomic mass unit (u) is strongly discouraged due to the confusion that would result when reporting spectra of multiply charged ions. The quantity plotted on the x-axis of a mass spectrum is a function of both the mass and charge of the ion. Furthermore, the use of amu in place of u is strongly discouraged in all cases; it has been used to denote atomic masses measured relative to the mass of a single atom of 16O, or to the isotope-averaged mass of an oxygen atom, or to the mass of a single atom of 12C

From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013.


The 39th ASMS Conference on Mass Spectrometry and Allied Topics [ 1991 / pp 1770-1771 ]

Nomenclature for Mass-to-Charge Ratio

A Workshop Sponsored by the Measurements and Standards Committee

The Measurements and Standards Committee sponsored a workshop on nomenclature for mass-to-charge ratio. After a welcome by Michael Bowers, Alan Rockwood gave a short presentation. This was followed by an open discussion. At the end of the workshop, participants returned an informal survey which has been forwarded to the Measurements and Standards Committee for further consideration.

It was proposed that mass spectrometrists adopt a unit for mass-to-charge ratio to be called a thomson (in honor of J. J. Thomson) or some other suitable name. The 12C+ ion would have a mass-to-charge ratio of 11.9994514198 thomsons and the 12C- ion would have a mass-to-charge ratio of 12.0005485802 thomsons. Thus, the thomson would have a value of 1.0364272 x 108 kilograms/coulomb with the polarity of the ion included in the scale. A proposed abbreviation for the thomson was also discussed, and it was pointed out that the originally suggested abbreviation, Th, conflicts with the abbreviation of thorium so something like Tn might be a better choice.

Supporters of the nomenclature proposal generally felt that present nomenclature has encouraged (or at least failed to discourage) imprecise usage, particularly the use of terms related to mass when mass-to-charge ratio is meant. In part this may be attributed to the lack of a convenient name for a unit of mass-to-charge ratio. Imprecise usage may lead to faulty communication or worse, particularly when one is dealing with multiply charged ions. In the past this imprecision has not been a great problem because multiply charged ions were relatively uncommon, but with the advent of techniques capable of producing tens or even hundreds of charges on an individual ion the distinction between mass and mass-to-charge ratio must be more strictly maintained. For example, according to one anecdote given at the workshop, the confusion between charge and charge-to-mass had led to false conclusions about the upper molecular weight limit of a particular analyzer when used with electrospray ion sources. It was argued that defining and naming the thomson as an explicit unit of mass-to-charge ratio would help enforce a strict distinction between mass and mass to charge ratio and lead to clearer and more concise communication. A second reason for supporting the proposal is that given the central role of mass-to-charge ratio in the field of mass spectrometry as the quantity actually measured in mass spectrometers, it makes sense that the unit for this quantity be given a convenient name.

Opponents of the proposal countered that m/z is already a very clear, well defined and convenient terminology. It was also pointed out that the "thomson" is not self defining, that like the hertz, the units are not explicitly conveyed by the name. This could confuse some readers, particularly the uninitiated. (Whether m/z is a self defining unit was not discussed, but at least it has become understandable through wide and long usage). It was also pointed out that imprecision is not inherent in present nomenclature and that by a combination of present nomenclature and careful language one can write and speak without ambiguity, although at times some extra wordiness might be required.

Both sides presented good reasons for their respective positions, and this seems to have been reflected in the vote. Forty-seven response forms were returned with 57% in favor of the proposal, 34% opposed, and 9% uncommitted. (Not counted in the balloting were letters of support from two editors, and the moderator (ALR) clearly advocated the proposal but didn't vote.) Regardless of the positions on the proposal itself, there was widespread agreement that imprecise communication is sometimes a problem and that mass spectrometrists should be encouraged to avoid incorrect or ambiguous usage.

A surprising part of the workshop was a widespread disagreement on the correct dimensionality of m/z. Three opinions were expressed. The view of the workshop leader going into the workshop was that m/z represents mass-to-charge ratio so the proper dimensionality would be mass divided by charge. This would be the correct dimensionality to use In the equations of motion for a charged particle in a mass spectrometer and it would be analogous to the usage in tables of fundamental constants in which (for example) the charge-to-mass ratio of the proton (e/mp) is given as 9.5788309 x 107 coulombs per kilogram. This is also closely related to the terminology of "grams per equivalent" and "equivalent weight" from electrochemistry. A second more popular view was that m is a mass but z Is a pure number (being charge number, not charge), so m/z would have dimensionality of mass. A third view held that both m and z are dimensioniess so m/z is a dimensionless number. The official definition of m/zis that it is a dimensioniess number that is proportional to the charge-to-mass ratio. In this respect it somewhat resembles other dimensionless numbers such as reduced parameters from thermodynamics (e.g., reduced temperatures) and dimensioniess groups from engineering (e.g., Reynolds numbers). Calling m/z the mass-to-charge ratio is a bit of convenient linguistic shorthand that Is not strictly correct. (A subtle difference between the thomson and m/z then would be that the thomson has the dimensionality of mass/charge while m/z is dimensionless, although in magnitude the two are identical.) This widespread disagreement on the correct meaning of such a widely used symbol as m/z indicates a possible need (or opportunity) for an effort in education or self education. The nomenclature summary that will soon appear in J. Amer. Soc. Mass Spec, is a significant effort in this direction, and it should be read by all mass spectrometrists.

In one of the lighter moments of the workshop, it was pointed out that a natural form to plot electrospray mass spectra would be intensity versus charge-to-mass ratio (rather than the mass-to.charge ratio) resulting in almost evenly spaced peaks. The unit of charge-to-mass ratio could be called a nosmoht which is thomson spelled backwards. (This is analogous to the unit of inverse resistance, the mho, which is ohm spelled backwards.) However, support for the nosmoht appeared to be minimal.

Submitted by:

Alan L. Rockwood Battelle, Pacific Northwest Laboratory

Reference: R. G. Cooks and A. L. Rockwood, Rapid Commun. Mass Spectrom., 5, 93 (1991).http://dx.doi.org/10.1002/rcm.1290050210

Multiple reaction monitoring is not deprecated

Please note that the term multiple reaction monitoring is not deprecated in the IUPAC "Standard definitions of terms relating to mass spectrometry" Pure Appl. Chem., 2013, 85, 1515. There is no small amount of confusion regarding this fact due in part to the seven years that elapsed between the posting of the unreviewed 2006 draft of the document (still linked as "provisional recommendations" on the IUPAC website) and the publication of the peer reviewed document in 2013. Comments from reviewers during the peer review process led to a revision of the definition to what is now indicated on the multiple reaction monitoring page of this wiki. In several publications between 2006 and 2012, the draft definition was cited (e.g. http://dx.doi.org/10.1038/msb.2008.61, http://dx.doi.org/10.1002/pmic.200800577, http://dx.doi.org/10.1039/c0mb00159g, http://dx.doi.org/10.1002/pmic.201200042), inadvertently leading to further confusion. Again, please note that the IUPAC recommendation for multiple reaction monitoring is the one indicated in http://dx.doi.org/10.1351/PAC-REC-06-04-06.

Summary of reaction monitoring definitions

Term Acronym Definition Diagram Reference
Selected ion monitoring SIM Operation of a mass spectrometer in which the abundances of ions of one or more specific m/z values are recorded rather than the entire mass spectrum.      . Gold Book
Selected reaction monitoring SRM Data acquired from one or more specific product ions corresponding to m/z selected precursor ions recorded via two or more stages of mass spectrometry.
Note 1: Selected reaction monitoring in multiple-stage mass spectrometry is known as consecutive reaction monitoring.
Note 2: Selected reaction monitoring applied to multiple product ions from one or more precursor ions is known as multiple reaction monitoring.
SRM.jpg
de Hoffmann. J. Mass Spectrom. 31, 129 (1996).
Consecutive reaction monitoring CRM Multiple-stage mass spectrometry experiment with three or more stages of m/z separation in which products of sequential fragmentation or bimolecular reactions are selected for detection.
CRM.jpg
Tomer, Guenat, Deterding. Anal. Chem. 60, 2232 (1988).
Multiple reaction monitoring MRM Application of selected reaction monitoring to multiple product ions from one or more precursor ions.
Note: This term should not be confused with consecutive reaction monitoring, which involves the serial application of three or more stages of selected reaction monitoring.
MRM-Single.jpg
Roepstorff, Fohlman. Biomed. Mass Spectrom. 11, 601 (1984).

Nominal mass

Related definitions

Other definitions

Mallet and Down ISBN 0470027614
"The mass of a molecule or ion calculated using the integral masses of the most abundant isotopes of each element present"
Sparkman ISBN 0966081390
"The integer mass of the most abundant naturally occurring stable isotope of an element ... the nominal mass of an element is equal to the mass number of the most abundant stable isotope of an element"
de Hoffmann ISBN 0470033118
"The nominal mass is calculated using the mass of the predominant isotope of each element rounded to the nearest integer value that corresponds to the mass number ..."
Watson and Sparkman ISBN 0470516348
"The nominal mass of an element is the integer mass of its most abundant stable isotope ... the nominal mass of a molecule, radical, or ion is the sum of the nominal masses of all the atoms of its constituent elements." (common organic elements this is the lowest but not always)
Gross ISBN 3642423469
"the nominal mass of an element is defined as the integer mass of its most abundant naturally occurring stable isotope ... the nominal mass of an ion is the sum of the nominal masses of its constituent elements."

References

IUPAC reaction monitoring terms

Term Acronym Definition Diagram Reference
Selected ion monitoring SIM Operation of a mass spectrometer in which the abundances of ions of one or more specific m/z values are recorded rather than the entire mass spectrum.      . Gold Book
Selected reaction monitoring SRM Data acquired from one or more specific product ions corresponding to m/z selected precursor ions recorded via two or more stages of mass spectrometry.
Note 1: Selected reaction monitoring in multiple-stage mass spectrometry is known as consecutive reaction monitoring.
Note 2: Selected reaction monitoring applied to multiple product ions from one or more precursor ions is known as multiple reaction monitoring.
SRM.jpg
de Hoffmann. J. Mass Spectrom. 31, 129 (1996).
Consecutive reaction monitoring CRM Multiple-stage mass spectrometry experiment with three or more stages of m/z separation in which products of sequential fragmentation or bimolecular reactions are selected for detection.
CRM.jpg
Tomer, Guenat, Deterding. Anal. Chem. 60, 2232 (1988).
Multiple reaction monitoring MRM Application of selected reaction monitoring to multiple product ions from one or more precursor ions.
Note: This term should not be confused with consecutive reaction monitoring, which involves the serial application of three or more stages of selected reaction monitoring.
MRM-Single.jpg
Roepstorff, Fohlman. Biomed. Mass Spectrom. 11, 601 (1984).

Slashes or hyphens for combined methods

There is a great deal of confusion on the use of slashes, hyphens, spaces, or no spaces to indicate the combination of techniques, particularly when acronyms and abbreviations are used. The Chicago Manual of Style tends to favor hyphens due to the ambiguity of the slash, which has connotations of "and/or" in many instances. The ACS Style Guide makes no specific recommendations but gives examples of slashes, hyphens, spaces and no spaces in examples. The American Institute of Physics Style Manual makes no specific recommendation but contains no examples of the slash usage. David Sparkman calls for separate connotations of the slash and hyphen with the former separating techniques and the latter instruments. Rapid Communications in Mass Spectrometry has called for a slash to separate combined methods and a hyphen to highlight a particular component such as the ionization method (Sparkman instead suggests a space to separate the ionization method). The Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013) suggests the use of the hyphen but indicates that the slash can also be used.

QUOTED TEXT FROM IUPAC RECOMMENDATIONS 2013
The hyphen, or alternatively the slash (forward stroke), can be used to indicate combined methods such as gas chromatography separation combined with mass spectrometry detection. Thus, the above combination can be written as gas chromatography-mass spectrometry or alternatively as gas chromatography/mass spectrometry. The corresponding abbreviations are GC-MS or GC/MS. The first use of a hyphen to indicate the combination of a separation method with mass spectrometry was in the early 1960s [15], and the use of a slash separator was in the 1970s [16]. The term hyphenated techniques was coined in 1980 [17]. Currently, hyphens and slashes are used interchangeably [18]. The journal Rapid Communications in Mass Spectrometry has in the past recommended that the combination of two analytical techniques be designated by a slash (Conventions adopted by RCM in Advice to Authors. Rapid Commun. Mass Spectrom. 17, Issue 1 (2003)). A recent Journal of Chromatography glossary also favors this usage [19]. IUPAC recommends that hyphens be used to describe variants of separation techniques, for example, gas-liquid chromatography and pyrolysis-gas chromatography [20]. The authors of this document are evenly split in their preference for hyphen or slash. For consistency with the prior recommendations, we use the hyphen for combined techniques but note that the slash can be used interchangeably.
From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013.

Other recommendations are given below.

Chicago Manual of Style

See http://www.chicagomanualofstyle.org/

The 16th edition of the Chicago Manual of Style indicates that slashes are most commonly used to indicate alternatives in the "and/or" formulation, for example "Hercules/Heracles."(CMOS 6.104) The CMOS also indicates that the slash is occasionally use to indicate "and" as in "Jekyll/Hyde." The "per" and "divided" by meanings are also noted.

The CMOS big table of hyphenation rules states that two nouns indicating two functions (the first noun doesn't modify the second) are hyphenated in both the noun and adjective forms.(CMOS 7.85)

American Chemical Society Style Guide

Chapter 10 of the ACS Style Guide[21] discusses editorial style including the use of hyphens and abbreviations.

Specific rules for combined methods are not given, but there are several examples in a list of abbreviations use space, no space, hyphen, en-dash, or slash. Surprisingly, neither GC-MS nor LC-MS are given in the list. Hyphen proponents will point to CE-MS, but slash advocates will point to CP/MAS.

Specific examples are: capillary electrophoresis mass spectrometry is abbreviated CE-MS, but cross-polarization/magic-angle spinning is abbreviated CP/MAS, but also CP-MAS, CP-MAS, CPMAS, and CP MAS are also indicated. Other examples are fast atom bombardment mass spectrometry (FABMS), Fourier transform ion cyclotron resonance (FTICR), Fourier transform infrared (FTIR, FT/IR, FT-IR, and FT IR), glow discharge mass spectrometry (GDMS), high-resolution mass spectrometry (HRMS), isotope dilution mass spectrometry (IDMS), isotopic ratio mass spectrometry (IRMS), laser desorption mass spectrometry (LDMS), matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS and MALDI-TOF MS), plasma desorption mass spectrometry (PDMS), pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), time-of-flight mass spectrometry (TOFMS TOF MS), triple-quadrupole mass spectrometry (TQMS).

American Institute of Physics Style Manual

The AIP style manual uses the hyphen exclusively for combined terms.[22]

Mass Spectrometry Desk Reference

David Sparkman in his Mass Spectrometry Desk Reference recommends the use of the slash to indicate the combination of techniques and the hyphen to indicate the combination of instruments. Thus

Gas chromatography/mass spectrometry (GC/MS)
Gas chromatograph-mass spectrometer (GC-MS)

similarly

time-of-flight mass spectrometry (TOFMS)
time-of-flight mass spectrometer (TOF-MS)

Ionization methods are set apart by a space, for example

electron ionization time-of-flight mass spectrometry (EI TOFMS)

Rapid Communications in Mass Spectrometry

The journal Rapid Communications in Mass Spectrometry has in the past given instructions to authors on combined techniques. For example, from the July 12, 2009 RCM:

The Rapid Communications in Mass Spectrometry author guidelines state

"A single analytical technique, or a type of instrument, is abbreviated without hyphens. Thus, TOFMS, FTICRMS."
"A hyphen is used when highlighting a particular component or feature of an instrument or technique. Thus, MALDI-TOFMS, ESI-MS/MS. When 2 or more different analytical techniques are coupled in tandem, this is represented by a solidus placed between the abbreviations for the techniques. Thus we write Py/GC/EI-MS, CZE/TOFMS."

Do Not Use Trademark Symbols ® or ™ in Scientific Writing

ACS Style Guide: "Avoid using trademarks and brand names of equipment and reagents. [...] In ACS publications, do not use trademark (™) and registered trademark (®) symbols."
JACS: "It is not necessary to use the trademark, registered trademark, or service mark symbol to ensure legal protection for the trademark."
Chicago Manual of Style : "In publications that are not advertising or sales materials, all that is necessary is to use the proper spelling and capitalization of the name of the product."

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