Detection of explosives in vapor phase by field asymmetric ion mobility spectrometry with dopant-assisted laser ionization.

Detection of low-volatile explosives in concentrations below 10-14 g/cm3 is a great challenge for portable ion mobility spectrometers (IMS) and field asymmetric IMS (FAIMS). We study the capabilities of FAIMS detector with ultraviolet laser ionization combined with organic additives (dopants) toluene and 1-methylnaphtalene to sense nitro-explosives: trinitrotoluene (TNT) and low-volatile cyclonite (RDX) and nitropentaerythritol (PETN). Differential mobility coefficients were measured for target ion peaks of TNT, RDX and PETN. Presence of dopants in the sample results in multiple growth of ion yield at laser intensities lower than 2 × 107 W/cm2. Limits of detection with dopant-assisted laser ionization were determined: 4.7 × 10-16 g/cm3 for RDX and 9.8 × 10-15 g/cm3 for PETN. Obtained results propose a way to further improve sensitivity of detectors along with improvement of portability of current laser-based FAIMS prototypes by using less powerful and smaller lasers.

Laser-stimulated desorption of organic molecules from surfaces, as a method of increasing the efficiency of ion mobility spectrometry analysis.

Application of laser-induced desorption was investigated as a method of increasing the efficiency of gas phase analyzers on principles of field asymmetric ion mobility spectrometry. Mass spectrometric data of investigations of laser desorption of pentaerythritoltetranitrate molecules and cyclotetramethylenetetranitramine molecules from quartz substrate under vacuum were obtained. Laser sources a Nd3+:YAG with nanosecond pulse duration (λ = 532 nm) and a continuous wave diode laser (λ = 440 nm) were used. It was shown that both laser sources have different desorption abilities. This is expressed in various time of appearance of desorbed products that is caused by different heating mechanisms of surface layer. The desorbed quantity under action of both laser sources exceeds the detection threshold for all modern gas phase analyzers. It should be noted that despite the presence of surface dissociation of explosives under laser radiation, the quantity of nondissociated molecules is large enough for detection by ion mobility and field asymmetric ion mobility spectrometers. The optimal parameters of laser radiation for effective removal (evaporation) molecules of low-volatile compounds from surfaces are defined. The conclusion about preferable use of a Nd3+:YAG laser for increasing the detection ability of detectors based on ion mobility spectrometry was made.

Active sampling system for gas-phase analyzers.

The approaches for increasing a contact-free sampling distance up to 40 cm for a field asymmetric ion mobility spectrometer were investigated and implemented by use both the vortex flow made by a rotating impeller and the laser desorption of traces of low volatile explosives. The sampling device for a laser-based field asymmetric ion mobility spectrometer including a high-speed rotating impeller was designed and built with help of computer simulation of vortex and analytical flows. The dependence of a signal of trinitrotoluene vapors on a rotational speed of an impeller was obtained. The optimization of analytical flow was performed. The effective sampling distance is increased up to 28 cm for trinitrotoulene vapors detection by a field asymmetric ion mobility spectrometer equipped with a rotating impeller. The distance can be increased up to 40 cm using laser irradiation of objects with traces of explosives. It was shown that under ambient conditions the efficient desorption of low-volatile explosives is achieved at laser intensity 107 W/cm2, wavelength λ = 266 nm, pulse energy about 1 mJ and pulse frequency not less than 10 Hz.