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 use ...

as a unit of

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

. The unit was proposed by Cooks and Rockwood naming it in honour of

J. J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that ...

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 Dalton may refer to: Science * Dalton (crater), a lunar crater * Dalton (program), chemistry software * Dalton (unit) (Da), the atomic mass unit * John Dalton, chemist, physicist and meteorologist Entertainment * Dalton (Buffyverse), minor cha ...

(also called the

unified atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

, symbol u), and ''e'' is the

elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...

which is the

unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (alb ...

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

in the system of

Hartree atomic units The Hartree atomic units are a system of natural units of measurement which is especially convenient for atomic physics and computational chemistry calculations. They are named after the physicist Douglas Hartree. By definition, the following four ...

. 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. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...

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 is ...

—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 ''Rapid Communications in Mass Spectrometry'' (''RCM'') is a biweekly peer-reviewed scientific journal published since 1987 by John Wiley & Sons. It covers research on all aspects of mass spectrometry. According to the ''Journal Citation Reports'' ...

'' (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, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

, 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