Ion modification An ion mobility spectrometer (100) comprising a sample inlet (108) comprising an aperture arranged to allow a sample of gaseous fluid to flow from an ambient pressure region to a low pressure region of the ion mobility spectrometer to be ionized; a controller (200) arranged to control gas pressure in the low pressure region to be lower than ambient pressure; and an ion modifier (126, 127, 202) configured to modify ions in the low pressure region, wherein the ions are obtained from the sample of gas.
