Abstract: An IMS system has a cell (1, 101) with an inlet (2) at one end by which gas or vapour to be analysed can be supplied to the cell. A selective barrier (6) in the cell allows selected molecules to pass into an ionisation region (7) and the ions produced then pass into a drift region (9) via a gate (8). The ions drift along the cell (1, 101) to a collector (11) and provide an electrical output. A system (40, 140) supplies cleaned, dried air to the opposite end (30) of the cell (1, 101) to flow along it against the flow of the ions. The system (40) includes a doped molecular sieve (41), which continuously adds a first reagent to the air for combination with selected ions in the cell. The system (40) also has additional reservoirs (42, 43, 44) of further, different reagents, which are switched selectively to supply to the air in addition to the first reagent when indicated by the cell output, such as to combine with an interferent ion.