A differential mobility spectrometer comprising: at least two electrodes (7,8) that are substantially similar above and below a plane of symmetry and a gas stream with embedded ions, said gas stream having an axis in said plane of symmetry, wherein the potential difference between said at least two electrodes is a sum of: an output of a square wave generator that generates a constant reference voltage to which is added a train of rectangular pulses for which the up and down times differ, said output comprising positive and negative rectangular pulses generated by a high voltage switch having different durations and amplitudes, said output being applied to one of said at least two electrodes (7,8) directly and not by using a transformer, an output of a saw tooth generator (9, 43) that slowly varies between a first adjustable voltage and a second adjustable voltage applied to the other of said at least two electrodes (7,8), wherein a voltage pulse amplitude and a repetition rate of said output of the square wave generator are adjustable, and wherein the duration of said rectangular pulses in the output of the square wave generator is adjustable independently, and further wherein rise times and fall times of said square wave pulses are short compared with pulse durations thereof. at least two electrodes (7,8) that are substantially similar above and below a plane of symmetry and a gas stream with embedded ions, said gas stream having an axis in said plane of symmetry, wherein the potential difference between said at least two electrodes is a sum of: an output of a square wave generator that generates a constant reference voltage to which is added a train of rectangular pulses for which the up and down times differ, said output comprising positive and negative rectangular pulses generated by a high voltage switch having different durations and amplitudes, said output being applied to one of said at least two electrodes (7,8) directly and not by using a transformer, an output of a saw tooth generator (9, 43) that slowly varies between a first adjustable voltage and a second adjustable voltage applied to the other of said at least two electrodes (7,8), wherein a voltage pulse amplitude and a repetition rate of said output of the square wave generator are adjustable, and wherein the duration of said rectangular pulses in the output of the square wave generator is adjustable independently, and further wherein rise times and fall times of said square wave pulses are short compared with pulse durations thereof. an output of a square wave generator that generates a constant reference voltage to which is added a train of rectangular pulses for which the up and down times differ, said output comprising positive and negative rectangular pulses generated by a high voltage switch having different durations and amplitudes, said output being applied to one of said at least two electrodes (7,8) directly and not by using a transformer, an output of a saw tooth generator (9, 43) that slowly varies between a first adjustable voltage and a second adjustable voltage applied to the other of said at least two electrodes (7,8), wherein a voltage pulse amplitude and a repetition rate of said output of the square wave generator are adjustable, and wherein the duration of said rectangular pulses in the output of the square wave generator is adjustable independently, and further wherein rise times and fall times of said square wave pulses are short compared with pulse durations thereof.