The ability to distinguish between ions differing in the quotient mass/charge by a small increment. It may be characterized by giving the peak width, measured in mass units, expressed as a function of mass, for at least two points on the peak, specifically at fifty percent and at five percent of the maximum peak height.
Orange Book, 2nd ed., p. 203 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1978, 50, 65. 'Recommendations for Symbolism and Nomenclature for Mass Spectroscopy' on page 72 (https://doi.org/10.1351/pac197850010065)
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 1554 (https://doi.org/10.1351/pac199163101541)