Busch 2001

• K. L. Busch, "Units in Mass Spectrometry" Spectroscopy, 16, 28, (2001)

". . . We begin with the units for the x axis in the mass spectrum, which have evolved over the years. The current accepted unit is m/z . . . In this nomenclature, m represents the mass of the ion, and z represents the charge charge. It has been argued that m/z is a symbol, and should therefore be set in italics (1). However, this is not a uniform typographical practice, and there are those that would debate at length the difference between a symbol and a unit. More to the point, although the unit m/z is universally accepted, it is awkward in that a lowercase m is the SI unit for meter. The correct unit for mass is u, the unified atomic mass unit. A more correct unit for use on the x axis of a mass spectrum would therefore be u/z. Mass spectrometrists simply choose not to use it. The usage M/z has appeared intermittently, where M is presumably derived from the usage of M . to represent the molecular ion of a compound of molecular mass M. A unit in common use before about 1980 was m/e, and this unit appears as the x-axis unit label in many of the earlier classic books on MS. The lower case e, however, is the term for the charge on an electron (1.602 x 10^-19 C) rather than the number of such charges, which is designated z . . ."