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The thomson (symbol: Th) is a unit that has appeared infrequently in scientific literature relating to the field of mass spectrometry as a unit of mass-to-charge ratio. The unit was proposed by R. Graham Cooks and Alan L. Rockwood[1] naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.
| thomson | |
|---|---|
| Unit of | Mass-to-charge ratio |
| Symbol | Th |
| Named after | J. J. Thomson |
The thomson is defined as[2]
where Da is the symbol for the unit dalton (also called the unified atomic mass unit, symbol u), and e is the elementary charge, which is the unit of electric charge in the system of atomic units.
For example, the ion C7H72+ has a mass of 91 Da. Its charge number is +2, and hence its charge is 2e. The ion will be observed at 45.5 Th in a mass spectrum.
The thomson allows for negative values for negatively charged ions. For example, the benzoate anion would be observed at −121 Th since the charge is −e.
The thomson has been used by some mass spectrometrists, for example Alexander Makarov—the inventor of the Orbitrap—in a scientific poster,[3] and a 2015 presentation.[4] Other uses of the thomson include papers,[5][6] and (notably) one book.[2] The journal Rapid Communications in Mass Spectrometry (in which the original article appeared) states that "the thomson (Th) may be used for such purposes as a unit of mass-to-charge ratio although it is not currently approved by IUPAP or IUPAC."[7] Even so, the term has been called "controversial" by RCM's former Editor-in Chief[8] (in a review the Hoffman text cited above[2]). The book, Mass Spectrometry Desk Reference, argues against the use of the thomson.[9] However, the editor-in-chief of the Journal of the Mass Spectrometry Society of Japan has written an editorial in support of the thomson unit.[10]
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).[11][12] Since 2014, Rapid Communications in Mass Spectrometry regards the thomson as a "term that should be avoided in mass spectrometry publications".[13]
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