Cooks 1991
<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 The implicit assumption that the charge state of ions is unity is no longer valid when very-high-charge states are so easily produced.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).
