Patent ID: 8253095

Claim:
A method for dividing ions in a gas according to their mobility, comprising: forming a supersonic gas jet, having gas molecules with substantially equal velocities, by directing the gas through a Laval nozzle into an evacuated chamber; forming a field barrier, having a substantially uniform height across a cross-section of the supersonic gas jet, by respectively applying potentials U 2 , U 3 and U 4 to an arrangement of three apertured diaphragms R 2 , R 3 and R 4 , which are respectively separated by distances d 2 and d 3 , where (U 4 −U 3 )/(U 3 −U 2 )=d 3 /d 2 ; and directing the ions in the supersonic gas jet against the field barrier, where ions with a mobility less than a mobility threshold are pushed over the field barrier, and where ions with a mobility higher than the mobility threshold are held back by the field barrier.