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The distinctive features of laser ionization source in field asymmetric ion mobility spectrometry method are studied in this paper. A YAG:Nd 3+ nanosecond laser (λ = 266 nm, τpulse = 6 ns) with variable pulse energy E pulse = 700 – 2500 μJ and frequency ν = 10 – 20 Hz was used as a source of laser radiation. Pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT) cyclotrimethylenetrinitramine (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), explosives were investigated with use of field asymmetric ion mobility spectrometry (FAIMS) method. Ion spectra were recorded by with separating fields 8 – 12 kV/cm. Nickel radioactive isotope 63Ni was used as reference ionization source for explosive molecules.
Non-linear ion mobility spectra of each substance ionized with UV laser radiation and radioactive 63Ni were compared and reasons for similarities and distinctive features are discussed. For peaks of explosives and reactant-ion peaks the dependences of their positions along compensating voltage axis on the magnitude of separating voltage (separating electric field) were measured for all the ion sources and substances.
All the experiments were carried out under controlled ambient temperature and relative humidity (t=25℃, RH 30%). Humidity was supported the same inside and outside the gas system to minimize the influence of water cluster formation on the results obtained. Explosives vapors generators were made with 10mg samples of chemically pure explosives. RDX, HMX and PETN were heated (RDX 50℃, HMX 50℃, PETN 45℃) to increase the concentration of ions.
It was shown that the behavior of the peaks of the explosives at laser excitation is different from the behavior at radioactive source for each of the substances. This indicates the presence of an additional ionization mechanism under laser radiation along with the traditional one. Spectra of reactant ion peaks under radioactive ionization also show difference in ion formation for each substance. Behavior of reactant-ion peaks of each of the substances with laser ion source shows nearly perfect coincidence. This fact can demonstrate well controlled experimental conditions and further confirms the difference in ionization mechanisms for laser and nickel radioactive ion source.
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