mass-to-charge ratio, m z in mass spectrometry

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.

Source:

PAC, 1991, 63, 1541 (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1544