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I've written m/z 300 - 1100 Thomson in my paper, but the reviewer said that the Th is already deprecated. So in this case, should I write m/z 300 - 1100 Da? or something else?

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    $\begingroup$ Generally I see it written as a dimensionless number, and occaisonally as dalton. Not dalton per anything, so calling it dalton is somewhat incorrect, but I have seen a few published papers on mass spectroscopy written as sch. $\endgroup$
    – iammax
    Jul 11, 2017 at 14:10

2 Answers 2

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TL;DR: According to current IUPAC recommendations, $m/z$ is an abbreviation for a dimensionless quantity. Use of thompson unit is indeed currently discouraged.


There is an overview provided in Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013) [1, p. 1516] (reference numbers were updated):

Labeling of mass spectra

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 dimensionless quantity obtained by dividing the mass number of an ion by its charge number [2].

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 $\ce{^{16}O},$ or to the isotope-averaged mass of an oxygen atom, or to the mass of a single atom of $\ce{^{12}C}.$

Gold Book entry:

mass-to-charge ratio, $m/z$

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 $\ce{C7H7^{2+}}$, $m/z$ equals $45.5$.

For historical purposes the aforementioned letter to the editor regarding the thompson unit [4]:

The ‘Thompson’. 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. […] 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 evolution of mass spectrometry. Using standard rules for abbreviation, we have $\pu{1 Th} ≡ \pu{1 u/\text{ atomic charge}}.$ For example, the molecular weight of the peptide myoglobin (isotopic average molecular weight $\pu{16950 u})$ can be deduced from measurement […] of a peak at $\pu{998.0 Th},$ provided it is known that the ion bears $17$ charges. If this suggestion is accepted, other simplifications ensue. For example, the benzoate anion, mass $\pu{121 u}$ 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!


* $\pu{1 u} = m(\ce{^{12}C})/12 = \pu{1.660540E-27 kg}$
The unified atomic mass unit, $\pu{u},$ is also known by the name Dalton and the symbol $\pu{Da}.$

References

  1. Murray, K. K.; Boyd, R. K.; Eberlin, M. N.; Langley, G. J.; Li, L.; Naito, Y. Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013). Pure and Applied Chemistry 2013, 85 (7), 1515–1609. DOI: 10.1351/PAC-REC-06-04-06. (Free Access)
  2. IUPAC. Compendium of Chemical Terminology (the “Gold Book”). Online Version (2019-) Created by S. J. Chalk, 2nd ed.; McNaught, A. D., Wilkinson, A., Eds.; IUPAC Recommendations; Blackwell Scientific Publications: Oxford, 1997.
  3. IUPAC. Compendium of Analytical Nomenclature (the “Orange Book”), 3rd ed.; J. Inczédy, T. Lengyel, A. M. Ure, Eds.; IUPAC Recommendations; Blackwell Science: Oxford, 1998. https://media.iupac.org/publications/analytical_compendium/
  4. Cooks, R. G.; Rockwood, A. L. The Thomson-A Suggested Unit for Mass Spectroscopists. Rapid Communications in Mass Spectrometry 1991, 5 (2), 93–93. DOI: 10.1002/rcm.1290050210.
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  • $\begingroup$ Of course, the IUPAC books are the description of chemistry. No book should explain anything more clearly than those books. $\endgroup$ Jul 11, 2017 at 15:52
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    $\begingroup$ Oh, but @Loong is the IUPAC here :D He even informed them about mistake in one of "Books" :) $\endgroup$
    – Mithoron
    Jul 11, 2017 at 16:10
  • $\begingroup$ This answer is wrong. m/z is not dimensionless and does not involve mass "number". $\endgroup$
    – Curt F.
    Apr 3, 2018 at 2:44
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    $\begingroup$ According to IUPAC, $m/z$ is a ratio of "mass number" to "charge number". Also according to IUPAC, "mass number" is the number of nucleons (protons + neutrons) in a nucleus. According to IUPAC, saying that the $\ce{H3O+}$ ion has an $m/z$ of 19.01786 is wrong, because 19.01786 is the ratio of the mass, not the mass number of the ion in Da to the charge in elementary units. Unfortunately for IUPAC but fortunately for chemistry, few working analytical chemists use their silly rules. $\endgroup$
    – Curt F.
    Feb 3, 2022 at 3:13
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The m/z ratio is not dimensionless. Mass to charge ratio should have a unit, since mass has an IUPAC unit (kg) and charge has an IUPAC unit (coulomb). To me, Thomson is the convenient unit for biochemists, as we often use Dalton for molecular weight instead of kg. Thus, I felt thankful that it was proposed. If we don't like or don't adopt the Thomson unit for any reason, we'll have to propose a new unit or spell it out as Dalton/charge#.

Until the mass spec community proposes a different unit, I will continue to use the Thomson unit because it is the only available unit out there that is convenient for biochemists. The change in mass (delta mass) would lead to a change in m/z. Without a unit, how do we refer to the delta m/z? For a charge state of 4, the delta m/z between isotopes would be 0.25 Th. Without the unit, how do you communicate it effectively? Now that charge detection is evolving, we'll have m/z in the tens of thousands. Will we say 80,000 m/z? Or would we say 80 "k-m/z"? I would say "80 kTh" would be a much more effective communication. Similar to charge detection, MALDI also gives signals in the thousands. When we have mass spectra with tens of thousands in the m/z axis, it would be cumbersome to label the axis with all the zeros spelled out (such as 130000, 140000, 150000, etc...) if we simply label the axis as m/z. The better way to communicate would be using the kilo pre-fix just like any other unit, label our axis with 130, 140, 150,... and call the unit as kTh. If my reviewer doesn't let me use Thomson, I can imagine myself calling the axix "m/z (x10e3) to work around the issue. But kTh would be scientifically correct as m/z does have units, and it would be a more effective communications.

I think the Thomson unit is not widely adopted because many professors don't teach it in schools. If you work in the academia, please make sure to teach your students about the unit of m/z. Unlike absorbance or pH, m/z is NOT unitless. When we teach Thomson in school, it'll be adopted similar to ampere (A), Celsius (°C), hertz (Hz), joule (J), newton (N), volt (V), watt (W). I think it's not widely "taught" because m/z is probably not taught until grad school, unlike other physical properties. If you are a reviewer, please have an open mind to see the author's perspective if they write the unit Thomson. If you don't like Thomson for any reason, then please propose another unit and influence the mass spec community in that direction. But we can't say m/z is unitless, nor it is Dalton, or amu, or any other neutral mass unit. Many thanks!

To answer your question specifically, if you can't convince your reviewer about the unit Thomson, you probably have to spell it out such as Da/charge#, kDa/charge#, or use "m/z" as if it is the unit, such as 12345 m/z, or 15x10e3 m/z. It doesn't make sense to me, but I do understand that we are obligated to address our reviewers' requests. Good luck!

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  • $\begingroup$ The answer is not easily readable, the use of the unit $\pu{1 Th} = \pu{1 Da}/\pu{e}$, based on Wikipedia's article apparently did not find wide adoption: «The thomson is not an SI unit, nor has it been defined by IUPAC. Since 2013, the thomson is deprecated by IUPAC (Definitions of Terms Relating to Mass Spectrometry). Since 2014, Rapid Communications in Mass Spectrometry regards the thomson as a "term that should be avoided in mass spectrometry publications."» $\endgroup$
    – Buttonwood
    Feb 29 at 21:27

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