In situ trace detection of peroxide explosives by desorption electrospray ionization and desorption atmospheric pressure chemical ionization

Abstract

Desorption electrospray ionization (DESI) mass spectrometry is used for the rapid (<5 s), selective, and sensitive detection of trace amounts of the peroxide-based explosives, hexamethylene triperoxide diamine (HMTD), tetracetone tetraperoxide (TrATrP), and triacetone triperoxide (TATP), directly from ambient surfaces without any sample preparation. The analytes are observed as the alkali metal ion complexes. Remarkably, collision-induced dissociation (CID) of the HMTD, TATP, and TrATrP complexes with Na(+), K(+), and Li(+) occurs with retention of the metal, a process triggered by an unusual homolytic cleavage of the peroxide bond, forming a distonic ion. This is followed by elimination of a fragment of 30 mass units, shown to be the expected neutral molecule, formaldehyde, in the case of HMTD, but shown by isotopic labeling experiments to be ethane in the cases of TATP and TrATrP. Density functional theory (DFT) calculations support the suggested fragmentation mechanisms for the complexes. Binding energies of Na+ of 40.2 and 33.1 kcal/mol were calculated for TATP-Na(+) and HMTD-Na(+) complexes, suggesting a strong interaction between the peroxide groups and the sodium ion. Increased selectivity is obtained either by MS/MS or by doping the spray solvent with additives that produce the lithium and potassium complexes of TATP, HMTD, and TrATrP. Addition of dopants into the solvent spray increased the signal intensity by an order of magnitude. When pure alcohol or aqueous hydrogen peroxide was used as the spray solvent, the (HMTD + Na)+ complex was able to bind a molecule of alcohol (methanol or ethanol) or hydrogen peroxide, providing additional characteristic ions to increase the selectivity of analysis. DESI also allowed the rapid detection of peroxide explosives in complex matrixes such as diesel fuel and lubricants using single or multiple cation additives (Na(+), K(+), and Li(+), and NH4(+)) in the spray solvent. Low-nanogram detection limits were achieved for HMTD, TrATrP, and TATP in these complex matrixes. The DESI response was linear over 3 orders of magnitude for HMTD and TATP on paper surfaces (1-5000 ng), and quantification of both peroxide explosives from paper gave precisions (RSD) of less than 3%. The use of pure water and compressed air as the DESI spray solution and nebulizing gas, respectively, showed similar ionization efficiencies to those obtained using methanol/water mixtures and nitrogen gas (the typical choices). An alternative ambient method, desorption atmospheric pressure chemical ionization (DAPCI), was also used to detect trace amounts of HMTD and TATP in air by complexation with gas-phase ammonium ions (NH4(+)) generated by atmospheric pressure ammonia ionization.