Identification of organophosphate nerve agents by the DMS detector

Identification of organophosphate nerve agents by the DMS detector

Sensors and Actuators B 213 (2015) 368–374 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevie...

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Sensors and Actuators B 213 (2015) 368–374

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage:

Identification of organophosphate nerve agents by the DMS detector ∗ ´ Mirosław Maziejuk, Michał Ceremuga, Monika Szyposzynska , Tomasz Sikora, Anna Zalewska Military Institute of Chemistry and Radiometry, al. gen. A. Chru´sciela 105, 00-910 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 19 November 2014 Received in revised form 16 February 2015 Accepted 17 February 2015 Available online 3 March 2015

a b s t r a c t Ion mobility spectrometry (IMS) is an appropriate technique to detect and identify chemical warfare agents (CWAs), in particular nerve agents. In many cases, the challenge is to correctly distinguish between nerve agents such as sarin, soman, tabun and VX. This paper presents the identification of CWAs by PRS-1W, a detector that uses IMS combined with differential mobility spectrometry (DMS). © 2015 Elsevier B.V. All rights reserved.

Keywords: Differential mobility spectrometry (DMS) Ion mobility spectrometry (IMS) Organophosphate nerve agents Chemical warfare detector

1. Introduction Contamination with hazardous chemicals is a serious threat to the environment and human health. Measures to minimize the effects of these contaminants depend on their rapid detection and identification. Identification of the gaseous substances is crucial in the process of reacting to chemical contamination. Ion mobility spectrometry (IMS) allows rapid responses to hazardous chemicals [1,2]. This technique assesses the mobility of ionized molecules in a carrier gas under the influence of an electric field. Identification of chemical substances is possible because of the differences in mobility created by ionization. Gas ionization is usually achieved with ␣or ␤-radioactivity. The most commonly used ionization source for IMS is the ␤-emitter 63 Ni. In classical IMS, ionized molecules are accelerated along the electric field undergoing collisions with neutral molecules of the carrier gas under atmospheric pressure (Fig. 1). After colliding, the ionized molecules reach a constant speed known as the drift velocity d , which is proportional to E, the intensity of the electric field [V cm−1 ] [3,4]:

vd = E · K where K is ion mobility [cm2 V−1 s−1 ].


The velocity at which ions pass through the drift tube depends on the ion’s mass, charge, size, and the type of drift gas. After transformation, Eq. (1) is replaced by the relationship [6]: K= 0925-4005/© 2015 Elsevier B.V. All rights reserved.



L2 L/td = td · V V/L


where td – drift time [s], L – drift region length [cm], V – the potential impact of the dosing grid [V]. Currently, IMS is often used for the detection of CWAs [5,7,8], drugs [9–11], toxic industrial chemicals [12], and explosives [13,14]. A variation on IMS is differential mobility spectrometry (DMS) [6,15,16]. The techniques differ from one another because the direction of gas flow is parallel to the electric field in IMS, whereas it is perpendicular in DMS. In DMS, ions are separated according to differences in their mobility under an electric field of low and high intensity at atmospheric pressure. With the use of a low-intensity electric field, as in classical IMS, ion mobility is not dependent on the magnitude of the electric field, whereas in DMS, ion mobility is affected by the low-high electric field intensity. The nature of these changes depends on the type of ion, as well as the mass, shape and effective temperature of ions, and ion mobility is expressed as the sum of the ion kinetic energy and thermal energy. The relationship between ion mobility and the intensity of the electric field is expressed by the following Eq. (3) [17]: K

∗ Corresponding author. Tel.: +48 22 516 99 33. ´ E-mail address: [email protected] (M. Szyposzynska).



= K0 · 1 + ˛

 E  N


where K0 is reduced ion mobility [cm2 V−1 s−1 ], E/N is the electric field in Townsend units (1 Td = 10−17 V cm2 ), which is known as

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Fig. 1. Scheme of ion flow through an ion mobility spectrometer [5].

the normalized molecular density, and ˛(E/N) is the normalized function describing electric field mobility dependence. The spectrometer used in DMS is constructed from electrode plates of opposing charge that are connected to a high voltage [18–20]. Movement of ions in the chamber of the DMS spectrometer is shown in Fig. 2. The electric field is applied perpendicular to the direction of flow of the carrier gas. This causes ion oscillation between the electrodes transversely with respect to the flow of the carrier gas. DMS spectrometers are more selective, enabling separation of gaseous substances that cannot be separated by classical IMS spectrometers, even those with high resolution. DMS can simultaneously detect both positive and negative ions whereas IMS can only detect one or the other. These features make DMS most suitable for detecting and monitoring targeted chemicals in complex environmental samples. Further, its gives less false positives. DMS can process ions in gases in milliseconds at ambient conditions [21–24]. In classical IMS, the most important descriptive parameter is the time needed to travel the length of the drift region (time-of-flight), and in the case of DMS, the major parameter is the offset value of the electric field added to the high-frequency, high-voltage electric field that is generated in the electrode of the DMS spectrometer. There are few publications on the detection of CWA by DMS spectrometer. For example Ross et al. described usage of dopants in high field asymmetric waveform spectrometry for analysis of tabun [25,26]. DMS as a pre-filter in hybrid techniques such as GCDMS, DMS-IMS and DMS-MS have also been reported. Kolakowski et al. [27] described analysis of chemical warfare agents in food

products by DMS-MS. Burakov [28] described a combined GC-DMS system for the detection of chemical warfare agents (sarin, mustard gas and lewisite). However, these are laboratory solutions. In our study the IMS is used as an ion filter. 2. Material and methods 2.1. Sample preparation The analytes were tabun (GA), sarin (GB), soman (GD) and VX, which were synthesized at Wojskowy Instytut Chemii i Radiometrii (WIChiR; Warsaw, Poland), in the laboratory of chemical synthesis, covered by the Convention on the Prohibition of Chemical Weapons. WIChiR has the authority to work with toxic warfare agents under Concession No. B-007/2004. The concentration of sarin, soman, tabun and VX were 20 ␮g/m3 . The analyzed substance is aspirated under continuous suction by a pump (the flow of approx. 3 l/min). Gas molecules ionization occurs in the specific region under the influence of radioactive source 63 Ni. Created ions are subjected to the preselection in the IMS spectrometer due to their mobility. Then the analyzed gas is carried to the DMS detector, wherein under the influence of the high frequency strong electric field the ions are separated. 2.2. Chemical warfare detector PRS-1W The tests were performed using the PRS-1W detector (Fig. 3). PRS-1W is a novel device that uses a combination of IMS and DMS.

Fig. 2. Scheme of ion flow through a differential mobility spectrometer [16].


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Fig. 4. Compensation voltage as a function of high-voltage separation of monomers of organophosphate nerve agents.

3. Results and discussions 3.1. Identification of chemical compounds based on the value of the compensation voltage

Fig. 3. Chemical warfare detector PRS-1W.

The chamber of the PRS-1W detector contains thick-film technology composed of ceramic substrates, and this concept is protected by an international patent [29]. Measurements were carried out using air as the carrier gas at a relative humidity of <1%, with a chamber entrance temperature of 318 K (45 ◦ C), chamber temperature of 323 K (50 ◦ C), and H+ (H2 O)2 as the reactant ion. Detection of organophosphate substances for humidity greater than 1% is not a major problem. With increasing of humidity the sensitivity is reduced and with humidity level approx. 70% amplitude of the peak is decreased by approx. 20–40% but significantly changes the position of the peak and therefore is necessary to make corrections by humidity, which is determined individually for each substance. In general, measuring systems contains a semipermeable membrane, but with this solution the sensitivity drops to 50 ␮g/m3 , but it kept very low humidity level of the air as the carrier gas. To retain very high sensitivity to organophosphate substances (from a few ␮g/m3 ) the only solution is membraneless system and the necessity of making correction on account of humidity. Parameters of the control unit for the DMS spectrometer: • High-speed, high-voltage (HSV) generator frequency, 3 MHz. • Selective voltage (peak-to-peak) 100–1200 V, asymmetry factor, 0.27. • Compensation voltage −30 V to +8 V (−5.2 to 1.4 Td), 10 Hz. • Maximum electric field intensity, 50 kV/cm. • Maximum normalized intensity (E/N) of electric field, 160 Td. • Length of DMS electrodes, 25 mm. • Distance between DMS electrodes, 0.25 mm. • Gas flow rate through detector, 3 l/min.

Identification of the type of substance detected is made based on the compensation voltage (CV). CV as a function of HSV for the four different organophosphate nerve agents is shown in Fig. 4. For immediate identification (within a few seconds), several HSV values are selected and checked against peaks within a certain CV. For each HSV value, the data collection time is 0.4 s. Measurement of several consecutive HSV values (600, 700, 800, 900, 1000, 1100 and 1200 V) to produce a full cycle of data collection requires approximately 2.8 s. Thus, in such an analysis, it may be concluded that identification of trace chemicals is immediate. Table 1 summarizes data on the identification of positive ion CWAs. Based on the curves shown in Fig. 4, it was found that all the organophosphate substances were distinguishable. Identification of the substance was based solely on the CV value for monomers. Thus, for each HSV value (UHSV ), a CV value (UCV ) can be assigned that is characteristic for a given compound, and can be used for identification. For sarin and tabun, both monomers and dimers were detected across the range of UHSV values. In the case of soman, monomer 2 was evident at UHSV > 800 V and a concentration of >50 ␮g/m3 of soman. Distinction between sarin and monomer 1 of soman was only possible for low UHSV values. At >800 V UHSV , sarin and soman were only distinguishable from one another by detecting monomer 2 of soman. For VX and UHSV values ≥800 V, the peak of the monomer was observed at a positive UCV value. This constitutes an additional criterion for identifying VX because for the other CWAs, peaks corresponding to monomers occurred only at negative UCV values. Of note, PRS-1W responses to sarin and soman were only possible for positive ions. In addition to positive ions, tabun and VX reacted with negative ions (at concentrations >150 ␮g/m3 ), which is another criterion for distinguishing them from other organophosphate CWAs. 3.2. Identification of compounds based on points (temperature) of defragmentation One advantage of DMS is a significant increase in the energy of ions, which often leads to defragmentation of the lighter ion fragments. This may result in ion fragments being disconnected from the cluster neutral particles or a portion of the molecule chain

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Table 1 Values of the compensation voltage UCV for positive ion CWAs organophosphate nerve agents. UHSV [V]

600 700 800 900 1000 1100 1200








Monomer 1

−1.65 −2.02 −2.24 −2.64 −2.69 −2.69 −2.32

−0.54 −0.16 0.28 0.95 1.69 2.66 3.92

−2.09 −2.46 −4.99 −6.55 −8.33 −10.11 −11.74

−0.54 −0.24 0.21 0.80 1.62 2.66 3.84

−2.76 −3.73 −4.99 −6.55 −8.18 −10.04 −11.74

VX Monomer 2 – – −1.05 −1.05 −0.91 −0.61 –

Dimer −0.76 −0.31 0.20 0.80 1.69 2.66 3.77

−0.61 −0.39 0.06 0.58 1.32 2.29 3.47

Fig. 5. Chemical structures of the most common nerve agents.

may be detached. Identification of chemical compounds can also be achieved based on the defragmentation points of gases and vapors, and discontinuity peaks on 3D spectrograms [30]. Organophosphate nerve agents include an asymmetrical P-atom and consist of at least two stereoisomers. GA, GB and VX contain a chiral phosphorus center and thus each of these nerve agents has

two stereoisomers, while soman has four stereoisomers because of an additional chiral center within the pinacolyl substituent (Fig. 5). On the spectrograms for soman, sarin, tabun and VX, clear peaks from monomers and dimers were evident. In the case of soman and tabun, defragmentation of dimers to monomers was observed, and, as a consequence, disintegration of the monomer into two different

Fig. 6. Differential mobility spectra of (a) positive ions of soman, (b) negative ions of soman, (c) positive ions of soman (blue line) and negative ions of soman (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 7. Differential mobility spectra of (a) positive ions of tabun, (b) negative ions of tabun, (c) positive ions of tabun (blue line) and negative ions of tabun (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Differential mobility spectrum of (a) positive ions of sarin, (b) negative ions of sarin, (c) positive ions of sarin (blue line) and negative ions of sarin (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 9. Differential mobility spectra of (a) positive ions of VX, (b) negative ions of VX, (c) positive ions of VX (blue line) and negative ions of VX (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

compounds (Figs. 6 and 7). Defragmentation of the dimer proceeds according to Eq. (4). M2 H+ → MH+ + M


At a specific ion energy, peaks appear from defragmentation of the monomer, which are characteristic for the compound. The characteristic branching that is shown in the graph is the basis for the identification of compounds. MH+ → M1 + M2 monomer

M1 , M2 − decomposition products of the (5)

The peaks of monomers 1 and 2 may represent molecule fragmentation or isomers derived from soman. In the case of sarin, the spectrogram reveals discontinuity peaks (Fig. 8). These may reflect fragmentation of monomers into molecules with lower proton affinities, or another isomer. Sarin and soman gave rise to the phosphorous-containing ion [CH5 FO2 P]+ . This ion results from loss of an alkenyl radical from the ionized molecule, and it is characteristic of sarin and soman. Fragmentation of tabun showed distinct characteristics from those observed with soman and sarin. The mass spectrum of tabun was dominated by the dimethylamino group, with production of the ion [C2 H6 N2 ]+ .

In the case of VX, both the monomer and dimer are shown for negative ions (Fig. 9), but only the monomer was detectable for positive ions. Confirmation of the identity of compounds that are the result of discontinuities or degradation of the molecule is possible by using a mass spectrometry detector connected to the DMS spectrometer. 4. Conclusions The PRS-1W detector can detect CWAs at very low concentrations with a very short detection time and a low number of scans (UHSV values of 600, 700, 800, 900, 1000, 1100 and 1200 V). PRS-1W is able to detect not only CWAs, but can also detect toxic industrial agents, volatile organic compounds, and other gases and vapors. The unique feature of the PRS-1W device is its ability to defragment ions, and to determine the type of substance according to the fragments produced. Thus, DMS technology, with the ability to defragment ions as well as the possibility of 3D spectra, allows the identification of chemical substances with a higher level of credibility compared with classical IMS technology. Acknowledgment This research was supported by the National Centre for Research and Development. The funder had no role in study design, data


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Biographies Mirosław Maziejuk since 1995 he is researcher in Military Institute of Chemistry and Radiometry and since 2010 he is in charge of Radiometry and Camouflage Department. He also was working in PIMCO Sp. z o.o. and ProVision (Warsaw, Poland). In 1995 he obtained his Ph.D. in the Polish Military University of Technology and his Ph.D. thesis was concerned on construction detection elements, which can be used to determine nuclear explosion parameters. Since 1996 he is working under developing detection techniques used in toxic substances determination. He is a specialist in ion mobility spectrometry and differential ion mobility spectrometry. Michał Ceremuga is a Head of Department of CBRN Reconnaissance and Decontamination of the Military Institute of Chemistry and Radiometry. He obtained degree of MSc. from the Military University of Technology in 1999. In 2014 he obtained degree of Ph.D. from Warsaw University of Technology. Since 1999 he has worked in the Military Institute of Chemistry and Radiometry. He has been involved in many projects in area of defence research/development. He is co-author of many patents. He was giving a speech and participation in numerous International and National Conferences. He is a member of working group NATO/NAAG/JCGCBRN/RNDSG/EDA/JIP CBRN/ISIMS. ´ she obtained degree of MSc. from the University of Warsaw Monika Szyposzynska in the Faculty of Chemistry in 1995. In 2000 she obtained degree of Ph.D. from the University of Warsaw in the Faculty of Chemistry. Since 2005 she has worked in the Department of CBRN Reconnaissance and Decontamination in the Military Institute of Chemistry and Radiometry. She worked as research and development on projects connected with detection and identification CWA. She is on position of adjunct. She is deputy head of the Analytical Laboratory for CWC Verification. Tomasz Sikora is a specialist in the field of Analytical Chemistry. His knowledge on pesticides detection was explored during the Masters study at the Department of Chemistry, University of Warsaw. He defended his doctoral thesis on “Synthesis and design of electroactive polymers for applications in sensors and biosensors, and use in the detection of organophosphate pesticides” at the University of Perpignan. Since 2011 he is a specialist at the Military Institute of Chemistry and Radiometry, Warsaw, Poland in the field of trace analysis. Anna Zalewska is a specialist in the field of explosives, she has a well-established expertise in the detection of explosives vapors gained during doctoral studies, carried out in the Department of Energetic Materials at the Faculty of Chemistry, Warsaw University of Technology. She defended doctoral thesis on “New possibilities of the screening mobile devices in the detection of the explosives” in 2014. Between 2007 and 2012 she was a research worker in Warsaw University of Technology. From 2013 she is a specialist at the Military Institute of Chemistry and Radiometry in the field of trace analysis, detection and identification traces of explosives and chemical warfare agents. She was giving a speech and participation in numerous International and National Conferences.