The thomson (symbol: Th) is a unit that has appeared infrequently in scientific literature relating to the field of

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

as a unit of

mass-to-charge ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity Ratio, relating the ''mass'' (quantity of matter) and the ''electric charge'' of a given particle, expressed in Physical unit, units of kilograms per coulomb (kg/C). It is most widely ...

. The unit was proposed by R. Graham Cooks and Alan L. Rockwood naming it in honour of J. J. Thomson who measured the

of electrons and ions.

Definition

The thomson is defined as :

1~\mathrm = 1~\frac \approx 1.036426 \times 10^\,\mathrm

where Da is the symbol for the unit dalton (also called the unified atomic mass unit, symbol u), and ''e'' is the

elementary charge The elementary charge, usually denoted by , is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single electron, ...

, which is the unit of

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

in the system of

atomic units The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested ...

. For example, the ion C₇H₇²⁺ has a mass of 91 Da. Its charge number is +2, and hence its charge is 2''e''. 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 Benzoic acid () is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. The benzoyl group is often abbreviated "Bz" (not to be confused with "Bn," which ...

anion would be observed 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 mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel-like electrode and a coaxial inner spindle-like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions i ...

—in a scientific poster, and a 2015 presentation. Other uses of the thomson include 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 The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

, nor has it been defined by

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

. Since 2013, the thomson is deprecated by IUPAC (Definitions of Terms Relating to Mass Spectrometry). Since 2014, ''Rapid Communications in Mass Spectrometry'' regards the thomson as a "term that should be avoided in mass spectrometry publications".

References

{{reflist Units of measurement Mass spectrometry