Analysis of Explosives Using the SICRIT Ionization Source Based on a Dielectric Barrier Discharge Coupled to a Linear Ion Trap Mass Spectrometer

Rationale This paper evaluates the use of a plasma source coupled to an LTQ mass spectrometer for the analysis of explosive residues from soil samples, and in the process examines the advantages and limitations of this new ionization source. Methods Samples were solvent extracted, centrifuged, filtered, and injected into the Solid‐Phase Microextraction (SPME)‐like device that is coupled to the plasma ion source. The ion source is directly coupled to the orifice of the mass spectrometer, which allows for all ions generated to enter the mass spectrometer. Using the nitrate ion formed in the background and the introduction of ammonium chloride solution, the adduct formations with eight explosives (ETN, HMX, PETN, R‐Salt, RDX, tetryl, MHN, and NG) were monitored. Results Reagent ion formation under nitrogen and chloride conditions was studied to determine the optimal conditions for generating adducts with explosive analytes. Extracted soil samples with spiked explosives concentrations ranging from 5 to 1600 ng/mL were analyzed and monitored for the relevant nitrate and chloride adduct masses. For the formation of nitrate adducts, humidified nitrogen at 1500 V was determined to be better than dry nitrogen. For the formation of chloride adducts, 500 mM ammonium chloride at 1500 V was determined to be optimal. Conclusions The highest sensitivity can be achieved by directly coupling the plasma source to the orifice of the LTQ mass spectrometer. A maximum of 2 μL of sample can be introduced in this configuration, but the potential exists to modify the technique for larger volumes.