The ratio of the maximum ion current recorded at a mass m to the ion current arising from the same species recorded at an adjacent mass (m ± 1).
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 1554 (https://doi.org/10.1351/pac199163101541)