Artem E. Akmalov

A laser desorption ion-mobility increment spectrometer for detection of ultralow concentrations of nitro compounds

A compact instrument that implements for the first time a method for analyzing substances, in which laser desorption of analyzed molecules and laser ionization of an air sample are combined with the ionmobility increment spectroscopy, is described. Pulse radiation of the fourth harmonic of a portable (2.6 kg) GSGG: Cr3+: Nd3+ laser (λ = 266 nm) is used. The detection limit of the developed laser desorption spectrometer of the ion-mobility increment is 40 pg for trinitrotoluene (TNT), and the linear dynamic range for TNT is 0.1–20.0 ng. The results from detection of nitro compounds are presented: trinitrotoluene, cyclotrimethylene trinitramine (RDX), and octogen (HMX). It is shown that laser desorption of nitro compounds from a metal is accompanied by their decomposition on the surface and emission of fragments. An ion signal is obtained for nitro compounds that were ionized outside the spectrometer.

An excimer-based FAIMS detector for detection of ultra-low concentration of explosives

A non-contact method for analyzing of explosives traces from surfaces was developed. The method is based on the laser desorption of analyzed molecules from the surveyed surfaces followed by the laser ionization of air sample combined with the field asymmetric ion mobility spectrometry (FAIMS). The pulsed radiation of the fourth harmonic of a portable GSGG: Cr3+ :Nd3+ laser (λ = 266 nm) is used. The laser desorption FAIMS analyzer have been developed. The detection limit of the analyzer equals 40 pg for TNT. The results of detection of trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX) are presented. It is shown that laser desorption of nitro-compounds from metals is accompanied by their surface decomposition. A method for detecting and analyzing of small concentrations of explosives in air based on the laser ionization and the FAIMS was developed. The method includes a highly efficient multipass optical scheme of the intracavity fourthharmonic generation of pulsed laser radiation (λ = 266 nm) and the field asymmetric ion mobility (FAIM) spectrometer disposed within a resonator. The ions formation and detection proceed inside a resonant cavity. The laser ion source based on the multi-passage of radiation at λ = 266 nm through the ionization region was elaborated. On the basis of the method the laser FAIMS analyzer has been created. The analyzer provides efficient detection of low concentrations of nitro-compounds in air and shows a detection limit of 10-14 - 10–15 g/cm3 both for RDX and TNT.

Registration and spectral identification of THz images in reflected or transmitted light

The paper presents the results of application of terahertz radiation for detection of traces of explosives on surfaces of objects in reflected light. The process of detection and identification of explosives is based on a recording of interferograms of reflected radiation in spectral range of 0.5 -2.5 THz with help of a Michelson interferometer. The reverse Fourier processing lets to obtain reflection or transmittance spectra and images of objects. Spectral ranges for imaging are chosen by an operator. An installation elaborated for this purpose is described. Specific features of reflection spectra of some organic substances are determined.

Parameters of laser ionization of explosives for ion mobility spectrometry

The possibility of optimizing the parameters of laser radiation for increasing the efficiency of ionization of vapors of explosives in a laser field asymmetric ion mobility spectrometer (FAIMS) was investigated. The dependences of TNT ion signal on intensity of laser radiation, pulse energy and repetition frequency of laser pulses were obtained. Optimum value of the intensity of laser radiation for ionization in air was experimentally obtained - 3•107 W/cm2. It is shown that defocusing of laser radiation and expansion of the ionization region increase the TNT signal if the radiation intensity remains at the optimum level. It is shown that repetition rate should be such that the interval between pulses is shorter than the time of diffusion of non-ionized molecules into the laser irradiation region. A scheme for a multi-pass laser beam through the ion source of a spectrometer was proposed. The scheme will effectively use laser radiation and improve the ionization efficiency of nitro aromatic molecules.

Using dopants to increase the sensitivity of a laser field asymmetric ion mobility spectrometer for detection of explosives

One of the possible ways to increase sensitivity of ion mobility spectrometers for explosives detection is the use of dopants - substances enhancing ion formation. The specificity of laser ionization in ion mobility spectrometry with regard to possible dopants - ion-formation enhancers - remains still unclear when detecting nitro compounds. The purpose of this work was to study the opportunities to increase sensitivity of detection of explosives at the presence of dopants under laser ionization of sample vapors. Trinitrotoluene (TNT) was used as an explosive. Toluene, naphthalene and chloroform were chosen as dopants, as they differ in the magnitude of ionization potential and in the value of electron affinity. The laser radiation had a wavelength of λ=266 nm and a repetition rate of 10 Hz. It was found that the use of toluene at laser intensities (0.04÷3).107 W/cm2 leads to an increase of TNT ion signal by 50%. The use of naphthalene leads to an increase of TNT ion signal by 15%. It was found that chloroform reduces the efficiency of formation of negative ions of TNT due to the capture of free electrons. This is caused by its high electron affinity and the impossibility of two-step ionization mechanism of chloroform for the laser intensities used.

Laser ion mobility spectrometry in the detection of ultra-low quantities of explosives

The paper presents the results of research and development of ion mobility spectrometer with laser ion sources for detecting ultra-low quantities of explosives in air. Particular attention is paid to the mechanisms of the formation of negative ions. It is shown that laser ionization with respect to ion mobility spectrometry provides an increase of sensitivity and selectivity of detection. The ion mobility spectra of the most common nitro explosives are investigated. The detection threshold for laser ion mobility spectrometer was reached at the level of 10−14 g/cm3 (for trinitrotoluene).

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.

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 vortex sampling system for remote contactless survey of surfaces by laser-based field asymmetrical ion mobility spectrometer

The ways for increasing the distance of non-contact sampling up to 40 cm for a field asymmetric ion mobility (FAIM) spectrometer are formulated and implemented by the use of laser desorption and active shaper of the vortex flow. Numerical modeling of air sampling flows was made and the sampling device for a laser–based FAIM spectrometer on the basis of high speed rotating impeller, located coaxial with the ion source, was designed. The dependence of trinitrotoluene vapors signal on the rotational speed and the optimization of the value of the sampling flow were obtained. The effective distance of sampling is increased up to 28 cm for trinitrotoluene vapors detection by a FAIM spectrometer with a rotating impeller. The distance is raised up to 40 cm using laser irradiation of traces of explosives. It is shown that efficient desorption of low-volatile explosives is achieved at laser intensity 107 W / cm2 , wavelength λ=266 nm, pulse energy about 1mJ and pulse frequency not less than 10 Hz under ambient conditions. The ways of optimization of internal gas flows of a FAIM spectrometer for the work at increased sampling distances are discussed.

Effectiveness of laser sources for contactless sampling of explosives

A mass-spectrometric study of photo processes initiated by ultraviolet (UV) laser radiation in explosives adsorbed on metal and dielectric substrates has been performed. A calibrated quadrupole mass spectrometer was used to determine a value of activation energy of desorption and a quantity of explosives desorbed by laser radiation. A special vacuumoptical module was elaborated and integrated into a vacuum mass-spectrometric system to focus the laser beam on a sample. It has been shown that the action of nanosecond laser radiation set at q= 107 - 108 W/cm2, λ=266 nm on adsorbed layers of molecules of trinitrotoluene (TNT ) and pentaerytritoltetranitrate (PETN) leads not only to an effective desorption, but also to the non-equilibrium dissociation of molecules with the formation of nitrogen oxide NO. The cyclotrimethylenetrinitramine (RDX) dissociation products are observed only at high laser intensities (q> 109 W/cm2) thus indicating the thermal nature of dissociation, whereas desorption of RDX is observed even at q> 107 W/cm2 from all substrates. Desorption is not observed for cyclotetramethylenetetranitramine (HMX) under single pulse action: the dissociation products NO and NO2 are registered only, whereas irradiation at 10Hz is quite effective for HMX desorption. The results clearly demonstrate a high efficiency of nanosecond laser radiation with λ = 266 nm, q ~ 107 - 108 W/cm2, Epulse= 1mJ for desorption of molecules of explosives from various surfaces.