Fast Switching of CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> Reactant Ions in Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Explosives Detection

Abstract

Ion mobility spectrometry (IMS) has become the most deployed technique for on-site detection of trace explosives, and the reactant ions generated in the ionization source are tightly related to the performances of IMS. Combination of multiform reactant ions would provide more information and is in favor of correct identification of explosives. Fast switchable CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> reactant ions were realized in a dopant-assisted negative photoionization ion mobility spectrometer (DANP-IMS). The switching could be achieved in less than 2 s by simply changing the gas flow direction. Up to 88% of the total reactant ions were CO₃^–(H₂O)_<i>n</i> in the bidirectional mode, and 89% of that were O₂^–(H₂O)_<i>n</i> in the unidirectional mode. The characteristics of combination of CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> were demonstrated by the detection of explosives, including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For TNT, RDX, and BP, product ions with different reduced mobility values (<i>K</i>₀) were observed with CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i>, respectively, which is a benefit for the accurate identification. For ANFO, the same product ions with <i>K</i>₀ of 2.07 cm² V^–1 s^–1 were generated, but improved peak-to-peak resolution as well as sensitivity were achieved with CO₃^–(H₂O)_<i>n</i>. Moreover, an improved peak-to-peak resolution was also obtained for BP with CO₃^–(H₂O)_<i>n</i>, while the better sensitivity was obtained with O₂^–(H₂O)_<i>n</i>