M/z

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Background

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.

m/z

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



Talk:Mass/Charge Ratio (Th, never use Da, no units?. An ion of m/z or an ion at m/z?)