Laser Desorption of Explosives Traces with Low Vapors Pressure

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Laser Desorption of Explosives Traces with Low Vapors Pressure

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Physics Procedia 71 (2015) 207 – 211

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18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015,
18-20 February 2015
Laser desorption of explosives traces with low vapors pressure
A.E. Akmalov*, A.A. Chistyakov, G.E. Kotkovskii
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31,115409, Moscow, Russia

In this work comparison of the desorption effectiveness of picosecond and nanosecond laser sources (λ = 266, 532 nm) were carried out to investigate the possibility of creating a non-contact sampling device for detectors of explosives on the principles of ion mobility spectrometry (IMS) and field asymmetric ion mobility spectrometry (FAIMS). The results of mass spectrometric studies of TNT (2,4,6-Trinitrotoluene), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (1,3,5-Trinitro-1,3,5triazacyclohexane) laser desorption from a quartz substrate are presented. It is shown that the most effective laser source is a Nd:YAG3+ laser (λ = 266 nm; E = 1 mJ; τ = 5-10 ns; q = 108 W/cm2). The typical desorbed mass is 2 ng for RDX, 4-6 ng for TNT and 0.02 ng HMX per single laser pulse. The results obtained make it possible to create a non-contact portable laser sampling device operating in frequency mode with high efficiency. ©©22001155TTheheAAutuhtohros.rsP.uPbulibshliesdhebdy EbylseEvliseervBie.Vr .BT.hVis. is an open access article under the CC BY-NC-ND license (Phtetepr:-//rcerveiaetiwveucnomdemr orenssp.oorgn/sliibceilnitsyeso/bfyt-hnec-nNd/a4t.0io/)n. al R esearch N uclear U nivers ity M E P hI (M oscow E ngineering PPeehry-rsevicieswIunnsdteitruretesp)onsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) .
Keywords: desorption; laser; pulse radiation; explosives; explosives desorption; noncontact methods; mass-spectrometry.
1. Introduction
To date there are some types of devices for sample input that are usually used in the equipment based on IMS and FAIMS techniques for detection of traces of explosives with low vapors pressure. The operation principles of such devices are based on evaporation of explosives from special napkin after wiping a suspicious surface (the achieved

* Corresponding author. Tel.: +7-495-788-56-99 (ext. 8554). E-mail address: [email protected]

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.370


A.E. Akmalov et al. / Physics Procedia 71 (2015) 207 – 211

detection threshold is 10-100 pg). There are a lot of negative factors associated with napkins: problem of complete transfer of a substance to a
napkin, too prolonged heating (up to 10 seconds), need for consumables, etc. In this regard, an approach associated with use of laser radiation for contactless inspection of objects by initiating laser desorption seems to be promising. Ultraviolet laser beam of nanosecond duration effectively evaporates (desorbs) explosives from wide class of surfaces, and a time profile of the concentration of vaporized explosives does not exceed one second [Akmalov et al. (2013)].
The purpose of this work is to compare different modes of laser irradiation to assess the effectiveness of desorption, desorbed mass, to study the possibility of creating a non-contact sampling device.
2. Theoretical basis
Various mechanisms are known to initiate laser desorption. Resonant absorption of a photon by an adsorbed molecule can cause direct photo-desorption of a molecule [Laznev (1990)]. Another possible process is a nonthermal desorption associated with transfer of the energy of electron excitation to the adsorbed molecule from a substrate when it absorbs radiation [Laznev (1990)]. A mechanism of local heating is also possible for thermal desorption. In this case an adsorbed molecule can be considered as a point source of heat waves after relaxation of energy of laser excitation to a substrate.
In practice the thermal desorption process is the most often realized [Laznev (1990), Bechtel (1975), de Boer (1962)]. In this case, the thermal effect of light causes heating of a surface sufficient to excite the desorption process. Value of heating temperature is determined by the parameters of laser radiation (intensity, duration, energy) and the properties of the irradiated material (absorption coefficient, thermal conductivity, specific heat).
Currently, Nd:YAG3+ -, CO2- and N2- lasers are commonly used to initiate desorption of traces of explosives from surfaces of various objects [ Morgan et al. (1999), Badjagbo et al. (2012), Fain and Lin (1989), Smith et al. (2009), Ehlert et al. (2013), Brady et al. (2010), Huang et al. (1987), Ehlert et al. (2013), Nguyen et al. (2004)]. According to some authors, the best laser source seems to be a CO2 - laser. However, making a portable device using this laser is not possible. The use of Nd:YAG3+ laser (λ = 266, 532 nm) could help to provide high sensitivity and small dimensions of a contactless sampling device.
3. Experimental setup and technique
Lasers based on YAG:Nd3+ were selected as a source of laser radiation in this study. The parameters of lasers used are:
wavelength λ = 266 nm, pulse duration t = 6 ns, pulse energy E = 1 mJ, pulse repetition rate f = 10 Hz λ = 532 nm, t = 10 ns, E = 5 mJ, f = 10 Hz λ = 266 nm, t = 0.3 ns, E = 0.1 mJ, f = 100 Hz λ = 532 nm, t = 0.4 ns, E = 0.3 mJ, f = 100 Hz
Laser intensity was changed in the range of 107-109 W/cm2 for all sources used. For the quantitative determination of the sample mass desorbed by laser radiation the calibrated quadrupole mass spectrometer Shimadzu GCMS-GP2010 with a direct sample introduction device and ionization by electron impact was used. The laser-vacuum module for sample direct input (Fig. 1), that provides the possibility of laser action on a sample near the ion source of the mass spectrometer was developed for current research.

A.E. Akmalov et al. / Physics Procedia 71 (2015) 207 – 211


Fig. 1. Laser-vacuum direct input module.
The experiment is as follows: the prepared sample is placed on the end of the module, and then the module via a special gateway is installed in the mass spectrometer. The laser beam passes through the optical windows and hits the back side of sample through the transparent substrate. The formed desorption products arrive to the ion source and then are analyzed by the mass-spectrometer.
We used TNT (2,4,6-trinitrotoluene), HMX (octogen, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (cyclonite, 1,3,5-Trinitro-1,3,5-triazacyclohexane) as samples. Solutions of explosives in acetonitrile with concentrations 10-5 g/μL and volume V = 1 μL were placed on a clean quartz substrate to form a spot area S = 0.06 cm2, and then dried in 3 minutes.
For all studied explosives there is an intense absorption in the spectral range Δλ = 200-280 nm [Wynn et al. (2008)]. In the visible range (380-730 nm) absorption is significantly weaker. Typical peak absorption cross section σ is ~ 10-17 cm2. All the samples were made as optically thin layer for the wavelength used.
4. Results and discussion
Figure 2 shows the mass-spectrometric data corresponding to a series of single laser pulse action on the TNT sample with wavelength λ = 266 nm and intensity of 107 W/cm2. The ion current registered is a curve with peaks each of which corresponds to some quantity of desorbed explosives. The total rise of the curve is associated with desorption of a sample to vacuum.
Fig.2. Results of action of nanosecond pulses with λ = 266 nm and intensity of 1,5 × 107 W/cm2 on TNT a) the ion current for m/z = 210 (principle fragmentation mass in the mass spectrum, b) fragment of the ion current curve


A.E. Akmalov et al. / Physics Procedia 71 (2015) 207 – 211

The results obtained for TNT and RDX desorption from a quartz substrate are very similar. The desorbed mass is 1-2 ng for RDX and 4.6 ng for TNT per each laser pulse.
It should be noted that the irradiation by a series of pulses leads to an increase of TNT, RDX desorbed mass several times until samples total desorption.
The results of action by a single nanosecond laser pulses on HMX sample differ significantly from the results obtained for TNT and RDX (Fig. 3a). In this case, no desorption HMX is observed. The only result of the laser action is the process associated with heating of a sample with the final stage of NO and NO2 formation. Desorption (evaporation) in vacuum is completely absent for HMX, unlike TNT and RDX.



Fig.3. Results of action of nanosecond pulses with λ = 266 nm and intensity of 6 × 108 W/cm2 on HMX. Ion current curve for m/z = 46 (corresponds to NO2), (b) ion current after action by a series of 50-nanosecond laser pulses with λ = 266 nm and intensity of 3 × 109 W/cm2 on

The results are changed absolutely in the case of frequency irradiation. So, irradiation with frequency 10 Hz at 5
seconds gives rise to the desorption of HMX molecules. The yield of NO2 is also registered. In this case, the heating of a sample from each laser pulse is summed, providing a surface temperature sufficient to desorb HMX.
Results of laser action with λ = 532 nm on the studied samples are similar to the results for λ = 266 nm. The desorbed TNT and RDX mass per pulse is approximately equal to the mass under laser irradiation with λ = 266 nm (Table 1). The absorption cross section of the studied substances at λ = 532 nm is considerably smaller than for λ =
266 nm (Figure 1a). Perhaps the impurities contained in the sample play a significant role in this process. For picosecond laser source with λ = 266 nm the amount of desorbed substance is significantly lower, despite that
the irradiation was carried out for 5 seconds (Table 1). For the picosecond laser with wavelength λ = 532 nm for all intensities there is no desorption signal for all the
test samples of explosives.
Table 1 shows the mass of desorbed samples under various modes of laser irradiation.

Table1. Mass of explosives desorbed by laser irradiation (* - for one laser pulse, ** - 5 second laser irradiation)

Wavelength (nm) Pulse duration (ns)

TNT (ng)

RDX (ng)






















A.E. Akmalov et al. / Physics Procedia 71 (2015) 207 – 211


The results show that the desorbed mass for nanosecond laser pulse exceeds essentially any others. The frequency mode of laser irradiation significantly increases the efficiency of desorption. This occurs due to the intense heating of the substrate surface that leads to desorption of the substance even between laser pulses.
In this work the comparative tests of the desorption effectiveness of laser sources with wavelengths of 266, 532 nm and picosecond and nanosecond pulse duration were carried out to investigate the possibility of creating a non-contact sampling device. It was shown the most effective laser source is a Nd:YAG3+ laser with λ = 266, 532 nm; E = 1 mJ; τ = 5-10 ns; q = 108 W/cm2. The results obtained make it possible to create a non-contact portable laser sampling device operating in frequency mode with high efficiency.
Akmalov, A., Bogdanov, A., Kotkovskii, G. et al., 2013. A laser desorption ion mobility increment spectrometer for detection of ultralow concentrations of nitrocompounds. Instruments and experimental techniques, 56, 309-316.
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