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

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 [4], 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