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.
Sources:
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)