Mass/charge Ratio

Orange Book Entry

m/z ratio.

Gold Book Entry

The abbreviation m/z is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple or the elementary (electronic) charge, e. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C₇H₇²⁺, m/z equals 45.5..

Proposed New Entry

The mass/charge ratio is a physical property that is measured by mass spectrometers.

The symbol for the physical quantity mass/charge is m/q. The former m/z is based on a missconception and should no longer be used.

The SI unit of the physical quantity m/q is kilogram/coulomb (kg/C). In mass spectrometry it is more common to use the Atomic Mass Unit u and the atomic unit for charge e. This means the unit for mass/charge is:

[m/q] = u/e

u is sometimes called Dalton (Da). u/e is sometimes called Thomson (Th). Hence, a mass spectrum x-axis should be labeled as either of the below:

• m/q (u/e)
• m/q (Da/e)
• m/q (Th)