Ion mobility spectrometry of explosives, the stability of gas phase ions, and prospectives for future explosive trace detectors

The ionization of explosives vapors, principally the formation of chloride adducts and their drift swarms in an electric field, commonly at ambient pressure, is central to the performance of explosive trace detectors (ETDs) based on ion mobility spectrometry (IMS). Although these ions have near thermal energies, decomposition can occur at temperatures in the range 100–180 °C, which brackets the operating temperatures of drift tubes used for commercial ETDs. Consequently, a range of spectral patterns can be observed in IMS-based ETDs operating at a fixed temperature including: a Cl − or NO 3 − peak formed from decomposition in the ion source; NO 3 − adducts from secondary reactions in the ion source; baseline distortions caused by ion decomposition during swarm transit through the drift region; hydrogen abstraction; and the unaltered adduct. Studies of the enthalpies of ion decomposition may guide designs and operational parameters of future ETDs having reduced drift region temperatures to promote the lifetimes of chloride adducts and with ions intentionally decomposed in tandem mobility instruments to introduce a new level of selectivity of response over chemical noise.