A gas-cylinder-free plasma desorption/ionization system was developed to realize a mobile on-site analytical device for detection of
chemical warfare agents (CWAs). In this system, the plasma source was directly connected to the inlet of a mass spectrometer. The plasma can be generated with ambient air, which is drawn into the discharge region by negative pressure in the mass spectrometer. High-power density pulsed plasma of 100 kW could be generated by using a microhollow cathode and a laboratory-built high-intensity pulsed
power supply (pulse width: 10-20 μs; repetition frequency: 50 Hz). CWAs were desorbed and protonated in the enclosed space adjacent to the plasma source. Protonated sample molecules were introduced to the mass spectrometer by airflow through the discharge region. To evaluate the analytical performance of this device,
helium and air plasma were directly irradiated to CWAs in the gas-cylinder-free plasma desorption/ionization system and the protonated molecules were analyzed by using an ion-trap mass spectrometer. A
blister agent (
nitrogen mustard 3) and nerve
gases [cyclohexylsarin (GF),
tabun (GA), and O-ethyl S-2-N,N-diisopropylaminoethyl
methylphosphonothiolate (
VX)] in
solution in
n-hexane were applied to the
Teflon rod and used as test samples, after
solvent evaporation. As a result, protonated molecules of CWAs were successfully observed as the characteristic ion peaks at m/z 204, 181, 163, and 268, respectively. In air plasma, the limits of detection were estimated to be 22, 20, 4.8, and 1.0 pmol, respectively, which were lower than those obtained with
helium plasma. To achieve quantitative analysis, calibration curves were made by using CWA stimulant dipinacolyl
methylphosphonate as an internal standard; straight correlation lines (R(2) = 0.9998) of the peak intensity ratios (target per internal standard) were obtained. Remarkably, GA and GF gave protonated dimer
ions, and the ratios of the protonated dimer
ions to the protonated monomers increased with the amount of GA and GF applied.