Automated control and optimisation of ion mobility spectrometry responses using a sheath-flow inlet

The stability and speed of the operation of sheath-flow inlet high temperature ion mobility spectrometer were studied over sampled mass fluxes in the range 0-50 ng s⁻¹ for dichloromethane and ethyl acetate. The response to step-changes in sheath-flow in the region of 10 cm min⁻¹ stabilised within 1.4 s, although, the recovery of a response on shutting down the sheath-flow could be longer if adsorptive memory effects had built up within the inlet at high concentrations. The effect of mass flux as opposed to sampled concentration was highlighted and the importance of the role of mixing in the reaction region in controlling ion spectrometric responses emphasised. The incorporation of a sheath-flow inlet into an automated feed-back control system was demonstrated with the system observed to maintain, without operator intervention, a linear response over an order of magnitude increase in the mass flux compared to the same instrument without the sheath-flow inlet fitted. The overall maximum sensitivity of the system was not significantly altered, and the drift time was not affected while the precision of the responses to test atmospheres was comparable to other ion mobility spectrometric systems with a RSD in the responses of less than 7%.