A method of mass analysis comprising: generating first and second sample ions (14, 15) from at least two ion sources (12, 13), wherein the first sample ions are of opposite polarity to the second sample ions; sending first and second sample ions (14, 15) from the first and second ion sources (12, 13) through first and second ion inlets (26, 28), respectively, of a differential mobility spectrometer: the differential mobility spectrometer including at least three filter electrodes defining a first ion flow path for the first sample ions and a second ion flow path for the second sample ions, one of the at least three filter electrodes being commonly shared between the first and second ion flow paths, the filter electrodes generating asymmetric electric fields for passing selected portions of the sample ions based on the mobility characteristics of the sample ions; providing a voltage source for providing RF and DC voltages to at least one of the filter electrodes to generate the asymmetric electric fields; providing an ion outlet for outputting the selected portion of the sample ions; providing a mass spectrometer for receiving some or all of the selected portion of the sample ions; performing a differential mobility spectrometer separation of the first and second sample ions (14, 15) in the first and second ion flow paths (20, 22), respectively; and performing ion-ion reactions between the separated first and second sample ions (14, 15). generating first and second sample ions (14, 15) from at least two ion sources (12, 13), wherein the first sample ions are of opposite polarity to the second sample ions; sending first and second sample ions (14, 15) from the first and second ion sources (12, 13) through first and second ion inlets (26, 28), respectively, of a differential mobility spectrometer: the differential mobility spectrometer including at least three filter electrodes defining a first ion flow path for the first sample ions and a second ion flow path for the second sample ions, one of the at least three filter electrodes being commonly shared between the first and second ion flow paths, the filter electrodes generating asymmetric electric fields for passing selected portions of the sample ions based on the mobility characteristics of the sample ions; the differential mobility spectrometer including at least three filter electrodes defining a first ion flow path for the first sample ions and a second ion flow path for the second sample ions, one of the at least three filter electrodes being commonly shared between the first and second ion flow paths, the filter electrodes generating asymmetric electric fields for passing selected portions of the sample ions based on the mobility characteristics of the sample ions; providing a voltage source for providing RF and DC voltages to at least one of the filter electrodes to generate the asymmetric electric fields; providing an ion outlet for outputting the selected portion of the sample ions; providing a mass spectrometer for receiving some or all of the selected portion of the sample ions; performing a differential mobility spectrometer separation of the first and second sample ions (14, 15) in the first and second ion flow paths (20, 22), respectively; and performing ion-ion reactions between the separated first and second sample ions (14, 15).