Positive Reactant Ion Chemistry for Analytical, High Temperature Ion Mobility Spectrometry (IMS): Effects of Electric Field of the Drift Tube and Moisture, Temperature, and Flow of the Drift Gas

Abstract

Mobility spectra for reactant ions were used to assess the importance of several instrumental parameters with a high temperature drift tube in the positive polarity. Moisture, temperature and electric field strength altered the mobility spectra for reactant ions. Since the reactant ions are central to the response of an ion mobility spectrometer to a vapor sample, changes in reactant ion identities may alter IMS response qualitatively and quantitatively. In anticipation of such investigations, reactant ion peak (RIP) profiles were examined in detail. Between 5 to 1500 ppm of water in the drift gas, the RIP underwent gradual changes consistent with increases in hydration of the RIP ions which are NH4 + ·(H2O)n, NO + ·(H2O)n, and H + ·(H2O)n. In contrast, the RIP profiles from 1500 to 7800 ppm suggested wholly new ion identities which are unassigned, pending IMS/mass spectrometry studies. Increases in temperature caused the RIP to change as expected for a declustering of the hydrated ions. The dependence of the RIP on electric field was unexpected and was attributed to the residence time of ions in the drift tube. Apparently, new ions appeared with the RIP through slow reactions occurring on the millisecond time scale when ion drift times exceeded 5-10 ms at 250°C. The implications of these influence for creating standard databases are presently unknown and need to be considered for interlaboratory comparisons of spectra. Nonetheless, the sensitivity of the RIP toward these parameters provides a ready diagnostic of drift tube performance.