A reactant is produced from a gas or liquid chromatograph by producing reactant ions, reacting the reactant ions with molecules of at least one component of the effluent from the chromatograph to produce secondary ions, subjecting the ions to a drift field between electrodes and detecting at least some of the secondary ions. The reacting and subjecting steps are carried out in a chamber whose pressure is maintained at a level sufficient to ensure that the mean free path of ions is very small compared with the dimensions of the chamber. The ions thus reach the constant mass-dependent statistical terminal velocity in the field.
