Abstract: The present invention relates to an atmospheric pressure quadrupole mobility spectrometer for detecting and identifying trace amounts of various molecular species present in a gas. The invention includes an outer housing comprising forward and rear sections which define forward and rear chambers. The gas under analysis is caused to flow into the inlet part of the forward section where ionizers mounted around the circumference of the forward chamber convert a portion of the molecular species present in the gas into stable molecular ions suitable for analysis. Towards the center portion of the forward chamber a carrier gas is mixed with gas and ions so formed. Four parallel, cylindrical electrodes symmetrically positioned about the longitudinal axis of the rear chamber are connected in opposing pairs to a bipolar power supply which supplies a predetermined repetitive voltage waveform to the electrodes thereby creating a generally hyperbolic electric field between the electrodes.