Definitions and Terms in Mass Spectrometry
A progress report from ASTM Committee E-14 Subcommittee 10
Printed in the 1974 ASMS Proceedings
Subcommi t Ll.'!.' 10 on Definitions nnd 'rcrms of AS I'M Committee I>1^ on Mass Spectrometry was L-stnbl islu-d in 1970 nt tlu' i;i>;lit eenth Annual Conference on Mass Spectrometry nnd AlHed 'I'opLcs in Snn I'rnncisco, Hie ob ject i vi.- wns to "nssumc rcsponsibi Vi ty for a lon>;-tiTm, de 1 iljern ti', and comprehensive studv of tho words, plirnses, terms, numbers, nnd .'Specific InnKi'nK^' of mass spectrometry" and to formulate recommendations that might be K^n^'rally acceptable for standard usage. Clenn Cook, tho subcommittee chairman, nami-d Sy .Meyerson, Dan Oblas, and .lerry McCIeary ciiairmcn of task groups designated as orj^anic, inorganic, and instrumental, respectively. To broaden the base of participation bevond the formal task-group memberships, Meyerson and McCIeary convened a meeting of i nteres ted persons at the 1970 Triennial International Conference on Mass Spectrometry in Brussels. 'I'wenty-nine of the 35 to ^0 persons in attendance, most of them from Western lluropean countries, signifii-d nn interest in working with one or more of tlie task groups. A dialog conducted since tlu'n among membi^rs and correspondents of the organic task group has eliciti.'d input from 26 persons in addition to the chairman, McCIeary prepared a list of terms in nc-ed of precise definition, intended to serve as a starting point for a similar dialog among pt.'opl(' wiili instrumental interests, but the hoped-for response did not ma ter i a 1 i/.e. '[he inorganic task group lost its chairman by resignation in (.'arly 1973 and it han not functioned, iiecause of a change in job responsibilities. Cook resigned as chairman of the subcommittee, also in 1973, and a replacement for him has not bi'cn found. Partly b..>cause of the lack of a chairman, the work of the subcommittee has been in abeyanci.' for over a year. In the meantime, lUPAC has es tab I islu'd a Mass Spectrometry Snbcomnii ss ion of ttio Commission on >tolecular Structure and Spectroscopy, wliich in turn is part of the Division of Physical Chemistry, ['his subcommiss ion, under John iicynon's cliairmanship, has undertaken a review of the terminology of mass spectrometry and has requested access to the body of opinion accumulated by AS'l'M \-:-\h Subcommittee 10. In view of all the circumstances, ASTM Committee l-'.-lh at its 1974 business meeting recognir.ed 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.
5. Resolution, resolving power.
5. Resolution^ resolving power. These terms, commonly used interchangeably, are usually defined as M/AM or AM/M at a value of M such that the minimum output signal in the valley betrween 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 resolucion at mass M be defined as the peak width W at M. For the A.V.S, proposals, see their proposed tentative standard(7).
5.1. 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.II. One vote in favour of the proposed A.V.S. definition.
5.Ill, 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 s a y , 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 attempting 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 Che A.V.S. Committee report where "Resolution" is the peak width, W or iiM, in mass units obtained under the given circumstances and "Resolving Power" is M/AM. If AM is measured as the intercept drawn at 107, peak height we should obcain resolving powers very similar to the figures quoted (incorrectly) by instrument manufacturers as the "resolution" of their instruments using the lO'/, valley definition. It would be interesting to hear the comments of instrument makers on this subject. An insCrument 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/AM" and paragraph 4.2.1 has one measure W or AM at 107, of the peak height. So, 1 see no major differences between their definition and the 107, 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. Ue haven't heard much on how this is progressing.
5.VII. I think the A.V.S, committee proposals are 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 measured 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 d e c i s i o n as to whether this can be done within the required accuracy is easier to make from the 107, 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, 1 suggest holding this question aside in vour comnittee until the A.V.S. and ASMS Committee VIII come up with their proposals on this question.