Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Thomson (unit)

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 follows.


 * $$ 1~\mathrm{Th} \equiv 1~\frac{\mathrm{u}}{e} \equiv 1~\frac{\mathrm{Da}}{e} $$

where u represents the 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, for the ion C7H$2+ 7$ has an exact mass of 91.0 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.

An interesting part of their proposal allowed for negative values for negatively charged ions. For example, the benzoate anion would be observed at m/z 121, but at −121 Th since the charge is −e.

Unfortunately, the article proposing the unit of the thomson contains an ambiguity relating to the specification of charge. In one place the article refers to "charge number", but in another place the article specifies charge in atomic charge units: "Using standard rules for abbreviation, we have 1 Th = 1 u / atomic charge." Or in other words the units of the thomson are units of mass u (unified atomic mass units) divided by units of charge e (atomic or elementary charge). This unfortunate ambiguity may have contributed to the controversy over the unit. The ambiguity about specification of charge does not affect the numerical value assigned to the mass-to-charge ratio of an ion, but instead relates to the dimensionality to be associated with the quantity. Clarification of the original intent of the authors has not appeared in the literature, although in private communications Rockwood states that the intended dimensionality was mass/charge with the specific units being unified atomic mass units per elementary charge. It is also worth mentioning that the m/z as it is currently defined by a IUPAC body (as unitless) is not compatible with either interpretations discussed above.

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 editor-in-chief of the Journal of the Mass Spectrometry Society of Japan has written an editorial in support of the thomson unit.

In his book, Mass Spectrometry Desk Reference, Sparkman argues strongly against the use of the thomson. However, his arguments were against a dimensionless unit because of the possible confusion with the Thomson number in fluid dynamics, Thomson scattering, and the Thomson coefficient (the latter named after Lord Kelvin). He seems not to have realized that the unit "thomson" is not dimensionless but actually of dimension mass/charge and that therefore the possibility of confusion is minimal.

The thomson is not an SI unit, nor is it currently accepted by IUPAC; however, it can be argued that the thomson complies better to the international standards about quantities and units as described in ISO 31 and the IUPAC green book than the "unitless" m/z that is widely used for labeling mass spectra.