Interference in ion mobility spectrometric detection of common explosives by lactic acid of human sweat

Common explosives such as TNT, RDX, and PETN as well as lactic acid generate negative ions in ion mobility spectrometry (IMS). Lactic acid (LA), a major component of human sweat, can interfere with the IMS detection of explosives. In addition to lactate, [LA - H]^- oligomeric ions, specifically [2LA - H₂O - H]^-, [3LA - 2H₂O - H]^-, [4LA - 3H₂O - H]^-, and [5LA - 4H₂O - H]^-, are generated by ionization. In IMS, interference occurs when an ion has a reduced ion mobility (K_o) similar to that of the target compound, complicating detection. TNT, RDX, and PETN ions were detected as [TNT - H]^-, [RDX + NO₂]^-, and [PETN + NO₃]^-, respectively. The [2LA - H₂O - H]^-, [3LA - 2H₂O - H]^-, and [5LA - 4H₂O - H]^- ions exhibited K_o values similar to those of the [TNT - H]^-, [RDX + NO₂]^-, and [PETN + NO₃]^- ions, respectively, despite substantial differences in their m/z values. This similarity in K_o values complicates the detection of trace explosives in the presence of LA. The interference of LA in the IMS detection of explosives was effectively explained by the minimum cross section (CS_min) obtained from the energy-minimized ionic structure. The CS_min values of the product ions demonstrated a close relationship with the K_o values, with the [TNT - H]^-, [RDX + NO₂]^-, and [PETN + NO₃]^- ions displaying similar CS_min values to the [2LA - H₂O - H]^-, [3LA - 2H₂O - H]^-, and [5LA - 4H₂O - H]^- ions, respectively.