Thomson

From Mass Spec Terms
IUPAC RECOMMENDATIONS 2013
Thomson , Th
This term is deprecated.

See m/z.

Related Term(s):
Reference(s):

R. G. Cooks, A. L. Rockwood. Rapid Commun. Mass Spectrom. 5, 93 (1991).

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

Index of Recommended Terms

 


Article

<QUOTE> R. G. Cooks, A. L. Rockwood. Rapid Commun. Mass Spectrom. 5, 93 (1991).

"To the Editor-in-Chief

Sir,

The 'Thomson'. A Suggested Unit for Mass Spectroscopists

Recently, there has been a rapid increase in experiments in which multiply charged ions are generated in mass spectrometers.1,2 This welcome development has permitted the measurement of molecular weights of compounds having masses in the tens and even hundreds of kilodalton. These advances exacerbate the problems caused by a common imprecision, namely, the terms mass measurement, mass range, etc are often used imprecisely when mass-to-charge ratio measurement, mass-to-charge range, etc. are intended. The implicit assumption that the charge state of ions is unity is no longer valid when very-high-charge states are so easily produced.

With this letter come two suggestions. First, that the longer but more exact term be used whenever it is intended. Second, that a unit of mass-to-charge ratio be adopted. After all, this is the quantity which all mass spectrometers measure; mass is a derived quantity requiring an independent measurement or knowledge of charge. Such a unit would be defined as the quotient of mass, in units of u* and the number of charges, z. The number of charges could be positive or negative, depending on the sign of the charge. The name Thomson suggests itself in view of J. J. Thomson's contributions to measurement of this quantity and his preeminent role in the evolutjon of mass spectrometry. Using standard rules for abbreviation, we have 1 Th= 1 u/ atomic charge. For example, the molecular weight of the peptide myoglobin (isotopic average molecular weight 16 950 u) can be deduced from measurement' of a peak at 998.0 Th, provided it is known that the ion bears 17 charges. If this suggestion is accepted, other simplifications ensue. For example, the benzoate anion, mass 121 u and charge -1 atomic units, is -121 Thomson not m/z 121. The latter is actually the mass-to-charge ratio of the corresponding (unstable) cation!

We hope these suggestions are useful and thank Professors John F. J. Todd and I. M. Mills for their comments on them. One of us (ALR) would like to acknowledge support by the Molecular Science Research Center of Pacific Northwest Laboratory. Pacific Northwest Laboratory is operated by Battelle Memorial Institute for the Department of Energy under contract DE-AC06-76RLO 1830.

Sincerely,

R. Graham Cooks
Chemistry Department Purdue University
West Lafayette
IN 47907
USA
Alan L. Rockwood
Chemical Sciences Department
Battelle Pacific North West Laboratory
PO Box 999
Richland WA 99352
USA

7 January 1991

References

1. S. F. Wong, C. K. Meng and J.B. Fenn, J. Phys. Chem. 92, 546 (1988).

2. R. D. Smith, C. J. Barinaga and H. R. Udseth, Anal. Chem. 60, 1948 (1988).

3. T. R. Covey, R. F. Bonner, B. I. Shushan and J. D. Henion, Rapid Commun. Mass Spectrom. 2, 249 (1988).

Editor's Note: if other mass spectroscopists have any differing views the Editor-in-Chief would be pleased to receive them. If not, it would seem sensible to take up these suggestions."

</QUOTE> R. G. Cooks, A. L. Rockwood. Rapid Commun. Mass Spectrom. 5, 93 (1991).

Workshop

1991 ASMS Workshop on the thomson unit

The 39th ASMS Conference on Mass Spectrometry and Allied Topics 1991; pp 1770-1771.[1]

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



Past definitions and discussion regarding m/z

The 2013 IUPAC recommendations retain the use of m/z as the x-axis of a mass spectrum which has been in place since the early 1970s (see ASMS 1974 and Beynon 1978). Note that m is taken as the mass in u (per Price 1991; McLafferty 1993) rather than mass number (per Todd 1991; Todd 1995).

2013 IUPAC Comment

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 [2] and the similar definitions in the Orange Book [3]. 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 [4].

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 [5], 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.

ASMS 1997 Terms and Definitions Poster

The ASMS 1997 definition is similar to the McLafferty 1993 definition.

Mass-to-charge ratio (m/z)
Daltons/electronic charge.
Note from a reader: on Thomson - the fluid dynamics people have already used that one; it is listed in the CRC Handbook and IUPAC documents. ASMS should be doing things in addition to or clarifying points mentioned (or not) in IUPAC. However, we should be cautious about doing anything that actually opposes or conflicts with IUPAC documents.

Gold Book

The IUPAC Gold Book uses the mass number definition of Todd 1991. The IUPAC Orange Book definition is simply "m/z ratio".

http://goldbook.iupac.org/M03752.html

The abbreviation m/z is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple or the elementary (electronic) charge, e. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C7H72+, m/z equals 45.5..

Source: PAC, 1991, 63, 1541 (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1544

Todd 1995; Todd 1991

Todd uses the mass number definition of Beynon 1978.

m/z
This abbreviation is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple of the elementary (electronic) charge, e. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C7H72+, m/z = 45.5.

. . .

The number of charges carried by an ion should be indicated by the symbol z. The ratio of the mass number of an ion to the number of charges carried (commonly referred to as the mass-to-charge ratio) should be written m1/z, m2/z, etc. m/e should not be used to indicate this ratio, e (italic) being reserved for the charge upon the electron and e (Roman) for the electron itself when it appears in an equation.

McLafferty 1993

McLafferty uses mass rather than mass number and notes the proposed thomson unit (see ASMS 1991).

m/z
The mass of the ion in daltons divided by its charge (usually unity), a Thomson; m/e has also been used.

Price 1991

Price 1991 definition uses mass rather than mass number usage.

m/z
An abbreviation used to denote the dimensionless quantity formed by dividing the mass of an ion by the number of charges carried by the ion. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple of the electronic charge, e-. The abbreviation m/e, therefore, is not recommended. Thus, for example, for the ion C7H72+, m/z = 45.5.

ASMS 1981

The 1981 ASMS meeting continues the mass number usage. ASMS Nomenclature Committee Workshops, Minneapolis, 1981 [6]

m/z
This abbreviation is used to denote the dimensionless quantity formed by dividing the mass number of an ion by the number of charges carried by the ion. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple of the electronic charge, e-. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C7H72+, m/z = 45.5.

Beynon 1978

Beynon 1978 is the first published recommendation for m/z as opposed to m/e.

An acronym, abbreviation or invented jargon should only be used after a full explanation of its meaning has been given in the text.

. . .

The only exceptions, relating to mass spectroscopy, should be the following few commonly accepted initials that may be used freely and without amplification:

. . .

m/z meaning mass-to-charge ratio

. . .

The number of charges carried by an ion should be indicated by the symbol z. The ratio of the mass number of an ion to the number of charges carried (commonly referred to as the mass-to-charge ratio) should be written m1/z, m2/z, etc. m/e should not be used to indicate this ratio, e being reserved for the charge upon the electron and e- for the electron itself when it appears in an equation.

ASMS 1974

The 1974 ASMS meeting marks the m/e to m/z changeover.

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 [7]

. . .

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.

Retrieved from "https://kermitmurray.com/msterms/index.php?title=Thomson&oldid=21103"