1. A method of determining an empirical formula of an analyte ion from a measured mass spectrum including a main peak and one or more isotope peaks, the method comprising: a) comparing a relative isotopic intensity of the measured isotope peak to a calculated relative isotopic intensity of an isotopic ion of a proposed empirical formula; and b) comparing a relative mass defect of the measured isotope peak to a calculated relative mass defect of the isotopic ion of the proposed empirical formula; and c) identifying the proposed empirical formula as a potential candidate for the analyte ion based on the comparisons of a) and b).