A method of mass spectrometry comprising: providing a group of different ions derived from chemical compounds in the same class intermixed with other ions that are not derived from chemical compounds in said same class, wherein at least some of said other ions have the same charge state as at least some of said ions derived from chemical compounds in said same class, wherein said class of chemical compounds corresponds to compounds that share one or more common functional group or share a common moiety, and wherein said different ions have different mass to charge ratios and ion mobilities; and isolating the group of different ions derived from chemical compounds in said same class from the other ions that are not derived from the chemical compounds in said same class by: (i) obtaining a known relationship, or determining a relationship between the mass to charge ratios and the ion mobilities of said different ions derived from chemical compounds in said same class; (ii) temporally separating the ions according to their ion mobility in an ion mobility separator; and then (iii) mass filtering the resulting separated ions according to mass to charge ratio with a mass filter, wherein the mass to charge ratios transmitted by the mass filter are varied with time as a function of the ion mobilities received at the mass filter from the ion mobility separator and according to said relationship such that said different ions derived from chemical compounds in said same class are transmitted by the mass filter and said other ions not derived from the chemical compounds in said same class are not transmitted by the mass filter; wherein said method further comprises removing the temporal separation of ions after they have been separated and mass filtered, and then mass analysing the ions or ions derived therefrom. providing a group of different ions derived from chemical compounds in the same class intermixed with other ions that are not derived from chemical compounds in said same class, wherein at least some of said other ions have the same charge state as at least some of said ions derived from chemical compounds in said same class, wherein said class of chemical compounds corresponds to compounds that share one or more common functional group or share a common moiety, and wherein said different ions have different mass to charge ratios and ion mobilities; and isolating the group of different ions derived from chemical compounds in said same class from the other ions that are not derived from the chemical compounds in said same class by: (i) obtaining a known relationship, or determining a relationship between the mass to charge ratios and the ion mobilities of said different ions derived from chemical compounds in said same class; (ii) temporally separating the ions according to their ion mobility in an ion mobility separator; and then (iii) mass filtering the resulting separated ions according to mass to charge ratio with a mass filter, wherein the mass to charge ratios transmitted by the mass filter are varied with time as a function of the ion mobilities received at the mass filter from the ion mobility separator and according to said relationship such that said different ions derived from chemical compounds in said same class are transmitted by the mass filter and said other ions not derived from the chemical compounds in said same class are not transmitted by the mass filter; (i) obtaining a known relationship, or determining a relationship between the mass to charge ratios and the ion mobilities of said different ions derived from chemical compounds in said same class; (ii) temporally separating the ions according to their ion mobility in an ion mobility separator; and then (iii) mass filtering the resulting separated ions according to mass to charge ratio with a mass filter, wherein the mass to charge ratios transmitted by the mass filter are varied with time as a function of the ion mobilities received at the mass filter from the ion mobility separator and according to said relationship such that said different ions derived from chemical compounds in said same class are transmitted by the mass filter and said other ions not derived from the chemical compounds in said same class are not transmitted by the mass filter; wherein said method further comprises removing the temporal separation of ions after they have been separated and mass filtered, and then mass analysing the ions or ions derived therefrom.