Secondary electrospray ionization-ion mobility spectrometry for explosive vapor detection

The unique capability of secondary electrospray ionization (SESI) as a nonradioactive ionization source to detect analytes in both liquid and gaseous samples was evaluated using aqueous solutions of three common military explosives: cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), nitroglycerin (NG) and pentaerythritol tetranitrate (PETN). The adducts formed between the compounds and their respective dissociation product, (RDXNO2-)-N-., NG(.)NO(3)(-), and (PETNNO3-)-N-., gave the most intense signal for the individual compound but were more sensitive to temperature than other species. These autoadducts were identified as (RDXNO2-)-N-., NG(.)NO(3)(-), and (PETNNO3-)-N-. and had maximum signal intensity at 137, 100, and 125 degreesC, respectively. The reduced mobility values of the three compounds were constant over the temperature range from 75 to 225 degreesC. The signal-to-noise ratios for RDX, NG, and PETN at 50 mg L-1 in methanol-water were 340, 270, and 170, respectively, with a nominal noise of 8 +/- 2 pA. In addition to the investigation of autoadduct formation, the concept of doping the ionization source with nonvolatile adduct-forming agents was investigated and described for the first time. The SESI-IMS detection limit for RDX was 116 mug L-1 in the presence of a traditional volatile chloride dopant and 5.30 mug L-1 in the presence of a nonvolatile nitrate dopant. In addition to a lower detection limit, the nitrate dopant also produced a greater response sensitivity and a higher limit of linearity than did the traditional volatile chloride dopant.