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Contrary to what the name implies, mass spectrometers do not measure the mass of an ion, they measure another physical quantity, the mass/charge ratio. This simple fact is the source of much confusion.
Using the correct SI units, the mass/charge ratio sould be indicated in kg/C (kilograms/Coulombs). In mass spectrometry, however, this system of units is hardly ever used. Instead the mass is indicated in the Atomic Mass Unit u and the charge is indicated in multiples of the elementary charge e.
Hence the correct labling of "mass spectra" would be m/q (u/e) where m is the symbol for mass, q the symbol for charge, u the unit of mass in atomic mass units (also called Dalton, Da), and e the unit of charge in elementary charge units. This notation, however, is hardly ever used. The reason is that one could argue that e is not an elementary charge unit but an elementary charge constant. The difference seems artificial, however, whereas the atomic mass unit u is accepted by the CPGM, the elementary charge seems to lack such an acceptance. This is rather surprising considering that the elementary charge is a natural fundamental unit, whereas the atomic mass unit is defined somewhat arbitrarily. Also, the elementary charge unit is often used not only by mass spectrometrists but by many (other) physicist.
As a way out of this confusion, many mass spectrometrists do not use m/q. Instead they use a somewhat artificial unit m/z where z is considered a unitless property indicating the number of elemental charges e missing in a molecule. This questionable approach, however, leads to the problem that the unit of m/z becomes the same as the unit for a mass m, thereby creating even more confusion.
To avoid this, some people go on and declare also m as a unitless property, whereby the m/z also becomes unitless. Following this approach, however, one has to wonder why not all measurments (length, time, speed, ...) are declared unitless.
In order to find a way out of the mess, it was proposed to introduce a new unit by 1 u/e = 1 Th (Thomson - see Cooks & Rockwood, Rapid Commun. Mass Spectrom. 5, 93, 1991 and Sparkman, Mass Spec Desk Reference p. 27).
According to this convention, mass spectra x axis should be labled m/q (Th). However, it will take years to change the commonly used m/z into m/q and to introduce the still not well known unit Th.
Since Th is derived from the elemental charge unit e, this unit should get official status. Therefore I propose installing a new unit for charge that is equal to the elementary charge. A good name would be Millikan (Mi).
- [q] = Mi