A method of quantifying the amount of at least two analytes A1 and A2 involving (a) adding at least one salt (S) to at least a portion (P1) of the sample comprising the at least two analytes A1 and A2, (b) ionizing at least a portion of the sample according to (a) thereby forming an analyte flow comprising the analytes A1 and A2 in ionized form, (c) separating the ionized analytes A1 and A2 from each other by using at least one ion mobility separator (124), wherein the analyte flow according to (b) at least partially passes through the ion mobility separator (124), and (d) quantifying the amount of the separated ionized analytes obtained according to (c), wherein A1 is a pharmaceutically active compound C or derivative thereof and A2 is a metabolite or stereoisomer of C.