Patent ID: 8884219

Claim:
A method of operating a differential mobility spectrometer, said differential mobility spectrometer including an ionization chamber, a filter channel and a detection region, said method comprising: introducing a sample to be analyzed into said ionization chamber so as to produce analyte ions; subjecting said analyte ions in said filter channel to a time-varying electric field, wherein said time-varying electric field has a longitudinal field component drawing said analyte ions from the ionization chamber through said filter channel into said detection region and a transversal field component, said transversal field component being a superposition of an asymmetrically oscillating transversal field causing said analyte ions to move to and fro in transversal direction and a compensation field for selecting a species of analyte ions by substantially canceling an average transversal velocity of said selected species; collecting analyte ions of said selected species in said detection region; and generating a detection signal responsive to a number of analyte ions collected as a function of said compensating field; wherein said longitudinal field component is non-zero on average and comprises a sequence of pulses of alternating polarity, a polarity of a first pulse of said sequence being such that the analyte ions are drawn towards the detection region while being subjected to said first pulse, and wherein said longitudinal field component oscillates in longitudinal direction so that said analyte ions are imparted on average a non-zero longitudinal velocity in direction of said detection region while on a time scale shorter than a time required for said analyte ions to travel from said ionization chamber to said detection region they are caused to move to and fro in longitudinal direction while they are subjected to said asymmetrically oscillating transversal field in said filter channel, said longitudinal field component oscillating in longitudinal direction resulting for said analyte ions in a virtual increase in length of said filter channel.