Thomson (unit)

thomson
Unit of	Mass-to-charge ratio
Symbol	Th
Named after	J. 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 naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.

Definition

The thomson is defined as

$1~\mathrm{Th} = 1~\frac{\mathrm{u}}{e} = 1~\frac{\mathrm{Da}}{e} = 1.036426 \times 10^{-8}\,\mathrm{kg\,C^{-1}}$

where u represents the unified atomic mass unit, Da represents the unit dalton, and e represents the elementary charge which is the electric charge unit in the atomic unit system.

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 not at m/z 121, but at −121 Th since the charge is −e.

Use

The thomson has been used by some mass spectrometrists, for example Alexander Makarov—the inventor of the Orbitrap—in a scientific poster, papers, and (notably) one book. 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." Even so, the term has been called "controversial" by RCM's former Editor-in Chief (in a review the Hoffman text cited above). The book, Mass Spectrometry Desk Reference, argues against the use of the thomson. 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.

The thomson is not an SI unit, nor has it been defined by IUPAC.