Secondary Electrospray Ionization-Ion Mobility Spectrometry for Explosive Vapor Detection
M. Tam and H. H. Hill, Jr.
Abstract: 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, RDX‚NO2-, NG‚NO3-, and PETN‚NO3-, gave the most intense signal for the individual compound but were more sensitive to temperature than other species. These autoadducts were identified as RDX‚NO2-, NG‚NO3-, and PETN‚NO3- and had maximum signal intensity at 137, 100, and 125 °C, respectively. The reduced mobility values of the three compounds were constant over the temperature range from 75 to 225 °C. 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 µg L-1 in the presence of a traditional volatile chloride dopant and 5.30 µg 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.