Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 71 (2015) 293 – 297
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
A method of highly sensitive detecting of explosives on the basis of FAIMS analyzer with laser ion source A.A. Chistyakova*, G.E. Kotkovskiia, I.P. Oduloa, E.M. Spitsynb, A.V.Shestakovb a
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 31, Kashirskoe shosse,115409, Moscow, Russia b Stelmakh Research Institute “Polyus”, Vvedenskogo Ul., 3, Moscow, 117342, Russia
Abstract In this work comparison of the desorption effectiveness of picosecond and nanosecond laser sources (Ȝ=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 (Ȝ = 266 nm; E = 1 mJ; IJ = 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. © by Elsevier B.V. by ThisElsevier is an open access article under the CC BY-NC-ND license © 2015 2015Published The Authors. Published B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)
Physics Institute) .
Keywords: laser; explosives; ions; ionization; mobility; detection; picosecond.
* Corresponding author. Tel.: +7-495-788-56-99, ext. 8554 E-mail address:
[email protected]
1875-3892 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.329
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1. Introduction Today the problem of warning of the acts of terrorism with the use of explosives is extremely actual. Using of competently disguised explosive devices by terrorists leads to a large number of victims and causes serious material losses. Therefore experts from different countries work on creation of the technical means allowing to define the existence of explosive devices until their application. The problem can be solved, along with other methods, with the help of gas-analytical techniques: gas chromatography, mass spectrometry and spectrometry of ion mobility (IMS)/field, asymmetric ion mobility spectrometry (FAIMS) [Blanchard et al.(1989), Cohen et al. (1984), Gorshkov et al. (1982), Buryakov et al. (1991), Sysoev et al.(2013)]. The detection threshold of modern detectors based on the IMS or FAIMS reaches 10-14 g/cm3. Analysis is carried out through sampling of air with target molecules and lasts some seconds. Portability and high sensitivity makes FAIMS a perspective method for creation of a portable detector of midget concentration of vapors of explosives at atmospheric pressure. One of devices created recently on the FAIMS technology is the laser detector of vapors of explosives [Akmalov et al.(2013), Chistyakov et al. (2014)]. This detector has a nanosecond GSGG:Cr3+:Nd3+ laser as a source of ions and possesses both low detection threshold and small time of the analysis (1-2 sec). In (Chistyakov et al. (2014)) perspective results on increase in sensitivity of the laser detector due to using of the picosecond laser are received. In this work the research on further modification of laser FAIMS method for increase in its sensitivity by using of other picosecond laser source, upgrade of physical parameters of an ion source and also optimization of influence of other factors was conducted. 2. Theory Ultraviolet (UV) laser radiation is an effective source for resonance-enhanced multiphoton ionization (REMPI) of nitroaromatic and some other compounds due to their large absorption cross-sections in the range ¨Ȝ=220-270 nm [Abakumov et al.(1977), Abakumov et al. (1982), Chistyakov (1998), Tonnies et al. (2001), Ledingham et al.(1997)]. Formation of positive ions happens through a multistage process including excited singlet and triplet levels. Negative ions of nitroaromatic molecules according to [Chistyakov et al. (2010)] are formed generally through ionization of organic air impurities at high laser intensities through the multiphoton ionization mechanism. The cross-section of this process is several orders of magnitude lower than that for REMPI. The emerging electrons give rise to complex reactions between target molecules, water, oxygen, etc., thus creating negative or deprotonated ions of target molecules or their derivatives. Processes of laser ionization of explosives belonging to a class of nitrate esters or amine derivatives have similarities with ionization of nitroaromatic molecules. The use of laser ion sources in designing portable IMS or FAIMS-based detectors of explosives has been problematic because of mass, size of UV lasers and high energy consumption so far. Recent progress in laser engineering has led to development of the laser IMS and FAIMS techniques. The FAIMS technique separates ions according to their mobility component depending on the field intensity. The FAIMS operating principle is described in details in [Buryakov (2005) and Buryakov et al. (1993)]. 3. Experiment and results The basic element defining sensitivity of laser FAIMS detector of explosives is the ion source - a part in which ions of an air sample are formed under action of laser radiation. After emergence ions are transported by electric field to the separation chamber (Figure 1).
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Fig. 1.Aschematic diagram of the laser FAIM desorption spectrometer: (1) laser, (2) laser beam, (3) ion collector of an electrometric amplifier, (4) cylindrical separation chamber of the FAIM spectrometer, (5) inner electrode of the separation chamber of the FAIM spectrometer, (6) drift ring, (7) laser ion source, (8) ions, (9) air sampling.
As the laser method of ionization demands high radiation intensities, pulse lasers are used as sources of radiation. So, in work [7-8] the UV laser with pulse width of 6 ns and repetition rate of 10 Hz was applied. The use of a picosecond laser with pulse duration 300 ps and adjustable frequency of 20-300 Hz demands the analysis of processes happening in the ion source for optimization of the laser operation mode. In fig. 2 the dependences of amplitude of ion current of trinitrotoluene (TNT) vapors on frequency received at various energy of laser pulse are presented. It can be seen that saturation of a signal in fig.2b occurs at the average power 30 ȝJ * 150Hz=4,5 mW that is significantly lower than the value 150 ȝJ * 100Hz=15 mW for dependence in fig.2a where saturation is still absent. From this it follows that the amplitude of ion current and the dependence of ion signal on frequency is determined not so much by the energy characteristics of the picosecond laser as its temporal parameters. The main processes happening in the ion source and possessing time frames of an order of laser pulses (0,01s) period were considered. These include: • Transport of a clot of ions from ion source to the separation chamber under the action of an electrostatic field • Updating the incoming air sample from atmosphere • Diffusion of not ionized molecules to the space of a laser beam from the periphery
1,2 1,0
0,8 0,6 0,4
Pulse energy 150 μJ
0,2 0,0
Amplitude (r.u.)
Amplitude (r.u.)
1,0
0,8 0,6 0,4
Pulse energy 30 μJ
0,2 0,0
20
40 60 80 Pulse frequency (Hz)
a)
100
50
100 150 200 250 Pulse frequency (Hz)
300
b)
Fig. 2. The amplitudes of TNT ion signals in dependence on frequency of laser pulses (a picosecond laser) at constant pulse energy.
The linear coefficient of ion mobility for TNT ions equals K0=1,54 ɫm2/(VÂs). At the value of electric field E=75 V/cm speed of ions is v1=K0ÂEป115 ɫm/s. With a length of ion source L=4 cm, one can receive a time of transport of ions from the ion source: t1=L/v1=0,035 s. In most cases the speed of an air sample flow is, and the corresponding time of sample updating in the ion source is t2=L/v2=0.13 s. The Einstein's approximation equation was used for calculation of diffusion of molecules. At the minimum distance of the diffusion equal to a half of the diameter of a laser beam x=0.05ɫm, and coefficient of diffusion D=0.054 ɫm2/s we receive diffusion time t3=0,023 s. For easy comparison one can construct the temporal diagram of processes:
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࢚࢙ ൌ ǡ ࢙
Laser pulse period
࢙
࢚ ൌ ǡ ࢙
TNT diffusion to ionization space
nanosecond picosecond
࢚ ൌ ǡ ࢙
Transport of ions from the ion source
࢚ ൌ ǡ ࢙
Full updating of air sample
࢚ ൌ ǡ ࢙ 0,01
0,02
0,03
0,04
0,05
0,06 t, s
Fig.3. Temporal diagram of the physical processes in the ion source.
From comparison of t0ps and t3 it is clear that with frequencies up to 100 Hz the time of diffusion of sample molecules to a laser beam coincides with the period of laser pulses that allows to explain the linear growth of a signal with frequency increase. With frequencies more than 100 Hz diffusive processes do not allow to replace sample completely during the period between pulses that conducts saturation of a signal. Thus there is a summation of a signal from each laser pulse as ions do not have time to be removed from the ion source (t1>t0ps). When using a nanosecond laser the continuous ion flow is absent as each of ion clots formed have enough time to move from the ion source to the separating chamber (t1> t0ps) substantially much larger sampling rates are required, in the absence of which the main role in the formation of a signal the diffusion processes start to play, as it was discussed above. It’s also important to notice that in the case of a picosecond laser the pulse energy is insufficient to ionize all the sample molecules located in the laser beam per a single pulse. The next laser pulse will ionize as non-ionized previously molecules as diffusing from the outside. Therefore, at the increasing the length of a sample which is exposed to a laser beam, the ion current should rise. Indeed, the experimental verification of this assumption confirmed the feasibility of increasing the length of the ion source in the case of a picosecond laser as an ionization source (Fig.4). In the case of a nanosecond laser the pulse energy is great enough, and a single laser pulse can ionize all the sample molecules located in a laser beam. Therefore the growth of ion current is possible only due to diffusion of molecules from the outside at increase in the length of an ion source. Herewith, as it was already noticed above, the registration mode of ions in the separation chamber and in the electrometric detector is not optimal because of pulse character of arriving ion clots. 1,1 1,0
Amplitude (r.u.)
296
0,9 0,8 0,7 0,6 0,5 4
5
6 7 8 9 Ion source length (cm)
10
Fig. 4. Amplitudes of ion signals of TNT vapors in dependence on ion source length.
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On the basis of the carried-out analysis and experiments the application of a picosecond laser at the optimum parameters corresponding to the most effective ionization of explosives (TNT) was investigated with use of the FAIMS detector (fig.5 a, b). ď
Picosecond laser Nanosecond laser
1,0
Amplitude (r.u.)
Ă
0,8 0,6 0,4 0,2 0,0 5
10 15 20 25 Compensating voltage (V)
30
Fig. 5. (a) picosecond laser; ion signals of TNT vapors in dependence on compensating voltage.
It can be seen that the amplitude of a the signal is 2.5-3 times higher for a picosecond laser compared to nanosecond. Comparison with a detection limit determined in (Akmalov et al.(2013), Chistyakov et al. (2014)) may help to assess the achieved detection threshold as 1·10-15g/cm3.
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