Dual Ambient Sampling Hollow Cathode Discharge Ionization-Mass\nSpectrometry System: An Approach for Detecting Explosives and Drugs\nof Abuse at Trace to Ultratrace Levels

Detection of illicit\ncompounds like explosives and drugs of abuse\nat trace levels is crucial to provide public security and health safety.\nA dual ambient sampling system hollow cathode discharge (HCD) ion\nsource was developed to investigate its performance. Here, trinitrotoluene\n(TNT), trinitrobenzene (TNB), hexamethylene triperoxide diamine (HMTD),\nand triacetone triperoxide (TATP) as explosives and methamphetamine\n(MA) as drugs of abuse were taken as model compounds. Two sample inlets,\ninlet-1 and inlet-2, are available for ambient sampling. In negative\nion mode, N<sub>2</sub> and air HCD plasmas are confined close to\ninlet-1, but in positive ion mode, they are confined close to inlet-2.\nSpecial design of the ion source makes it feasible to generate multiple\nions from a single analyte, which assists in understanding the gas\nphase ionization mechanism. In negative ion mode, both TNT and TNB\ngave radical ions, [M]<sup>−•</sup>, as major ions for\nN<sub>2</sub> HCD plasma as they were introduced via inlet-1 or inlet-2.\nTNB gave radical ions for air and N<sub>2</sub> HCD plasmas, while\nTNT exhibited adduct ions, [TNT–H]<sup>−</sup>, by using\nair HCD plasma. In positive ion mode, HMTD gave [HMTD + H]<sup>+</sup> <i>m</i>/<i>z</i> 209 ions, while TATP only\nproduced adduct ions with ammonia, [TATP + NH<sub>4</sub>]<sup>+</sup> <i>m</i>/<i>z</i> 240. Regardless of ion source\ninlet, MA showed protonated molecule ions, [MA + H]<sup>+</sup> <i>m</i>/<i>z</i> 150. As analytes were introduced via\ninlet-1, the stability of the HCD background ion signal reduced, leading\nto a decrease in sensitivity. Unlike that in negative ion mode, introduction\nof ambient air in positive ion mode enhanced the sensitivity of the\nair HCD ion source through the formation of hydronium ions, which\ngave protonated molecule ions. Ionization mechanisms are also discussed.