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In this paper we study the influence of the carrier and drift gas composition on ionization processes taking place inside drift chamber of field asymmetric ion mobility spectrometer with laser ionization. Solid state nanosecond laser of YAG:Nd 3+ type with fourth harmonic unit (λ = 266 nm, τpulse = 6 ns, E pulse = 700 – 2500 μJ, ν = 10 – 20 Hz) was used for negative ion generation. In this study we experimentally discover the features of laser ionization of four nitro-compounds: cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT) explosives.
Drift and sample carrier gas were prepared by mixing purified air with different amounts of water vapor and organic dopants. Ion mobility increments were calculated after calibration of field asymmetric ion mobility spectrometer (FAIMS) based on published data for TNT and Iodine and measured alternating separation field waveform. The experimental setup also included drift time ion mobility spectrometer (IMS) which was used to verify linear ion mobility spectra to supplement ion mobility increment values, obtained by FAIMS.
Previous studies of laser ionization with optimization of intensity and pulse repetition rates gave LOD values well below 10−15 g/cm3: 3 × 10−15 g/cm3 for RDX, 8 × 10−15 g/cm3 for PETN and less than 3 × 10−15 g/cm3 for HMX. Common ideas about ionization mechanisms of nitro-based explosives propose that indirect processes with ion-molecular reactions substantially contribute to negative ion formation as well as resonant enhanced multi photon ionization (REMPI) direct processes. Ionization process starts with electron generation by organic impurities in atmospheric air. These organic compounds have low ionization energy and require less than two photons to ionize.
Current research involves doping air sample with such substances as: toluene acetone, naphthalene and chloroform at different UV irradiation modes. Such compounds can act as electron source for rising TNT and RDX ion signal levels above background. Such selectivity enhancement can be a step on the way to achieving even lower detection limits to sense trace explosive vapor concentrations in real conditions.
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