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

Detection of illicit compounds like explosives and drugs of abuse at trace levels is crucial to provide public security and health safety. A dual ambient sampling system hollow cathode discharge (HCD) ion source was developed to investigate its performance. Here, trinitrotoluene (TNT), trinitrobenzene (TNB), hexamethylene triperoxide diamine (HMTD), and triacetone triperoxide (TATP) as explosives and methamphetamine (MA) as drugs of abuse were taken as model compounds. Two sample inlets, inlet-1 and inlet-2, are available for ambient sampling. In negative ion mode, N₂ and air HCD plasmas are confined close to inlet-1, but in positive ion mode, they are confined close to inlet-2. Special design of the ion source makes it feasible to generate multiple ions from a single analyte, which assists in understanding the gas phase ionization mechanism. In negative ion mode, both TNT and TNB gave radical ions, [M]^-•, as major ions for N₂ HCD plasma as they were introduced via inlet-1 or inlet-2. TNB gave radical ions for air and N₂ HCD plasmas, while TNT exhibited adduct ions, [TNT-H]^-, by using air HCD plasma. In positive ion mode, HMTD gave [HMTD + H]⁺ <i>m</i>/<i>z</i> 209 ions, while TATP only produced adduct ions with ammonia, [TATP + NH₄]⁺ <i>m</i>/<i>z</i> 240. Regardless of ion source inlet, MA showed protonated molecule ions, [MA + H]⁺ <i>m</i>/<i>z</i> 150. As analytes were introduced via inlet-1, the stability of the HCD background ion signal reduced, leading to a decrease in sensitivity. Unlike that in negative ion mode, introduction of ambient air in positive ion mode enhanced the sensitivity of the air HCD ion source through the formation of hydronium ions, which gave protonated molecule ions. Ionization mechanisms are also discussed.