Thomson

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

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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 The implicit assumption that the charge state of ions is unity is no longer valid when very-high-charge states are so easily produced.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

• 1 u = m(12C)/12 = 1.660540 x 10^-27 kg The unified atomic mass unit, u, is also known by the name Dalton and the symbol Da.

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