Unit ofMass-to-charge ratio
Named afterJ. J. Thomson

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 Cooks and Rockwood[1] naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.


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 Hartree 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]


  1. ^ Cooks, R. G.; A. L. Rockwood (1991). "The 'Thomson'. A suggested unit for mass spectroscopists". Rapid Communications in Mass Spectrometry. 5 (2): 93.
  2. ^ a b c Stroobant, Vincent; de Hoffmann, Edmond de; Charette, Jean Joseph (1996). Mass spectrometry: principles and applications. New York: Wiley. ISBN 978-0-471-96696-8.
  3. ^ The Orbitrap: a novel high-performance electrostatic trap (ASMS)
  4. ^ Orbitrap Instrumentation: The First Decade and Beyond on YouTube
  5. ^ Pakenham, G; Lango, J; Buonarati, M; Morin, D; Buckpitt, A (2002). "Urinary naphthalene mercapturates as biomarkers of exposure and stereoselectivity of naphthalene epoxidation". Drug Metab. Dispos. 30 (3): 247–53. doi:10.1124/dmd.30.3.247. PMID 11854141.
  6. ^ Mengel-Jørgensen J, Kirpekar F (2002). "Detection of pseudouridine and other modifications in tRNA by cyanoethylation and MALDI mass spectrometry". Nucleic Acids Res. 30 (23): 135e–135. doi:10.1093/nar/gnf135. PMC 137990. PMID 12466567.
  7. ^ "Rapid Communications in Mass Spectrometry Instructions to Authors". Wiley Interscience. Retrieved 2007-12-03.[dead link]
  8. ^ Boyd, Robert K. (4 December 1998). "Book Review: Mass Spectrometry: Principles and Applications. E. de Hoffman, J. Charette and W. Stroobant. Wiley, Chichester 1996. ISBN 0-471-96697-5". Rapid Communications in Mass Spectrometry. 11 (8): 948. doi:10.1002/(SICI)1097-0231(199705)11:8<948::AID-RCM2033>3.0.CO;2-I.
  9. ^ Sparkman, O. David (2000). Mass spectrometry desk reference. Pittsburgh: Global View Pub. ISBN 978-0-9660813-2-9.
  10. ^ Yoshino, Ken-Ichi (2007). "Comments on Abscissa Labeling of Mass Spectra". Journal of the Mass Spectrometry Society of Japan. 55 (1): 51–61. doi:10.5702/massspec.55.51.
  11. ^ Murray, Kermit K.; Boyd, Robert K.; Eberlin, Marcos N.; Langley, G. John; Li, Liang; Naito, Yasuhide (2013). "Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)". Pure and Applied Chemistry. 85 (7): 1515–1609. doi:10.1351/PAC-REC-06-04-06.
  12. ^ "Thomson - MS Terms Wiki".
  13. ^ Volmer, Dietrich A. (2014). "Terms and acronyms that should be avoided in mass spectrometry publications". Rapid Communications in Mass Spectrometry. 28 (17): 1853–1854. Bibcode:2014RCMS...28.1853V. doi:10.1002/rcm.6979. PMID 25088128.