Testing Method for On-Site Measurement of Explosive Materials Contaminated on Travel Luggage Bag and Backpack Using Ion Mobility Spectrometry

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1 DOI: /bkcs S.-S. Choi and C. E. Son Contaminated on Travel Luggage Bag and Backpack Using Ion Mobility Spectrometry Sung-Seen Choi* and Chae Eun Son Department of Chemistry, Sejong University, Seoul 05006, Korea. * Received September 26, 2017, Accepted November 2, 2017, Published online December 1, 2017 Testing method for the on-site detection of explosive materials contaminated on the travel luggage bag (TLB) and backpack (BP) surfaces was established using ion mobility spectrometry (IMS) and smear matrix. Trinitrotoluene and 1,3,5-trinitro-1,3,5-triazacyclohexane were used as model explosive materials. Two sampling methods of rolling (with a metal roller) and handrubbing were used, and stainless steel mesh was used as the smear matrix for collection of explosive material. Testing parts of the TLB and BP were selected in consideration of contaminant accumulation, physical contact, and sample collection. Explosive materials deposited on polytetrafluoroethylene (PTFE) sheet were transferred to the testing part, the explosive materials existing on the testing part were collected using the smear matrix, and then the collected explosive materials were analyzed using IMS. High signal-to-noise ratio over 10 was applied for the determination of the minimum initial explosive concentration (C min_ptfe ) to detect in IMS. The handrubbing method was much more efficient than the rolling method. The C min_ptfe values were different according to the testing parts. The C min_ptfe values of the TLB were higher than those of the BP. The experimental results were explained by difference in surface morphology of the testing areas. The testing method can be helpful to select the sampling parts and to collect the explosive materials for on-site security checks. Keywords: Explosive, On-site measurement, Ion mobility spectrometry, Smear matrix, Sampling, Baggage Introduction By increasing terror threat, security programs have been reinforced and detection of explosives is critical for security applications. Ion mobility spectrometry has been widely used for detection of explosives due to the low detection limits, fast response, and simplicity. 1 7 Ion mobility spectrometer (IMS) is one of portable and hand-held explosive detectors, and it is used by security personnel to detect the presence of explosives in vehicles, packages, and other items. It is very important to select a sampling method for the explosive detection on-site using IMS. A sampling method for the on-site detection of explosive materials requires simplicity, fast collection, and no unpleasant feeling. Possible sampling methods for the on-site explosive detection are air sampling and smearing methods. Air sampling method can be working only when explosive vapor exists sufficiently in the air. However, vapor pressures of explosive materials at ambient temperature are not sufficiently high. 8 For IMS measurement in security checks, smearing method is a proper method to collect explosive materials contaminated on the target surface due to its simplicity and convenience. It was reported that smearing method was used as a trace explosive preparation methodology for surface analysis such as morphology observation and infrared analysis, 9,10 and it was also good for collection of solid-state explosive materials. 11 In the present work, testing method for the on-site detection of explosive materials contaminated on the bag surfaces using IMS and smear matrix was established. Commercial travel luggage bag (TLB) and backpack (BP) were used as model bags. Polytetrafluoroethylene (PTFE) sheet was used as the mother matrix for the transfer of solid-state explosive materials to the bag surface because it is chemically inert and is not swollen by organic solvent. The initial solid-state explosive material was prepared by dropping the explosive solution on a PTFE sheet and drying. 2,4,6-Trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) were employed as model explosives. The testing method is composed of two principal sampling steps of transfer of solid-state explosive materials from the PTFE sheet to a target surface of luggage and baggage and collection of the explosive materials from the target part to the smear matrix for IMS detection. And the same sampling method was applied both for the transfer and collection of solid-state explosive materials for comparison of total efficiency of the transfer and collection. In this study, rolling method with a stainless steel roller and hand-rubbing method were employed as the sampling Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley Online Library 45

2 ISSN (Print) (Online) methods. In our previous work, 11 transfer efficiency of solid-state explosive materials from a PTFE sheet to a smear matrix using a small stainless steel roller was examined. Solid-state explosive materials deposited on the PTFE sheet were transferred to the smear matrix by rolling with the small stainless steel roller without extra force. Stainless steel mesh made of fine stainless steel wires was used as the smear matrix and it is an adequate smear matrix for IMS detection. 11,12 The rolling method is relatively objective but it is hard to apply for narrow or curved region of bags. The hand-rubbing method is a simple and appropriate method in the field. Though sampling wands were reported, 13,14 use of a sample wand was not considered because of requirement of extra steps. As a sample wand is not directly inserted into IMS and it is not easy for solidstate explosive materials to transfer to the bag surface using a sample wand, extra sampling steps must be required, which leads to decrease in transfer efficiency. Testing parts of the TLB and BP were selected in consideration of capability of contaminant accumulation, frequency of physical contact, and easiness of sample collection. Differences in the transfer and collection efficiencies of explosive materials according to the sampling methods and parts were investigated. The sampling methods were consisted of (1) loading explosive chemical on the detection area and (2) collecting it from the detected spot. Appropriate selection of the sampling parts is also important for efficiency of security checks. We believed that the testing method is applicable to the on-site security checks for the detection of the presence of explosive materials in vehicles, packages, and other items. Experimental TNT and RDX were supplied from Hanwha Co. (Jeollanamdo, Korea). Photographs of the TLB and BP used in this study were shown in Figures 1 and 2, respectively. Acetone was purchased from Aldrich Chemical Co. (WI, USA) Figure 1. Photographs of the travel luggage bag used in this study. The testing parts are (a) smooth surface (TLB_SS), (b) fixed handle (TLB_FH), and (c) adjustable handle (TLB_AH). Figure 2. Photographs of the backpack used in this study. The testing parts are (a) outer surface (BP_OS), (b) bag strap (BP_BS), and (c) buckle strap (BP_BKS). PTFE sheet (thickness of 90 μm) was purchased from Sungjin Co. (Seoul, Korea), and it was used as a mother matrix to deposit and transfer the explosive materials. Stainless steel mesh (thickness 170 μm and size 5 8cm 2 ) made of fine stainless steel wires was supplied by IMS Technology (Seoul, Korea), and it was used as a smear matrix for collection of the explosive materials. A stainless steel roller (weight 494 g, diameter 40 mm, and length 40 mm) was used for transferring explosive materials. The explosive solutions were prepared by dissolving each explosive in acetone. Concentrations of the explosive solutions were controlled by diluting the initial solution of 1000 μg/ml. The sample solutions of 5, 10, 20, 50, 100, 200, 500, and 1000 μg/ml were prepared and 1 μl was used for initial deposition of the explosive materials on the PTFE sheet (corresponding to 5, 10, 20, 50, 100, 200, 500, and 1000 ng, respectively). The experiments were carried out from the lowest concentration. If 1000 ng sample was not detected in IMS, higher concentration explosives of 2, 5, and 10 μg were used. Testing parts of the TLB were the smooth surface (TLB_SS), fixed handle (TLB_FH), and adjustable handle (TLB_AH) as marked in Figure 1. Testing parts of the BP were the outer surface (BP_OS), bag strap (BP_BS), and buckle strap (BP_BKS) as marked in Figure 2. For the rolling method, transfer procedure of the explosive materials from the explosive-deposited PTFE sheet to the testing part and collection procedure of the explosive materials from the testing part to the smear matrix using the stainless steel roller were described in Figure 3. First, the explosive solution of 1 μl was dropped on the PTFE sheet and the solvent was evaporated. Second, the explosivedeposited side of the PTFE sheet was placed on the bag surface, and the stainless steel roller was placed on the PTFE sheet and was moved back-and-forth three times. Third, the smear matrix was placed on the explosive-transferred bag surface and the stainless steel roller was also placed on the smear matrix and was moved back-and-forth three times. Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 46

3 sheet was softly rubbed with hand three times. Third, the smear matrix was also placed on the explosive-transferred part and the smear matrix was softly rubbed with hand three times. And then, the explosive-collected smear matrix was inserted in the sampling region of IMS. Surfaces of the PTFE sheets, TLB, BP, and smear matrix were observed with an image analyzer (EG Tech video microscope IT Plus 4.0, Gyeonggi-do, Korea). IMS equipped with corona discharge ionization source, Ionab IMS of IMS Technology (Korea), was used. The analysis conditions of IMS were as follows: temperature of the sampling region was 150 C, temperature of the drift tube was 100 C, the electric field was 154 V/cm, and the drift distance of ion was 11.2 cm. The IMS analysis was performed in the negative mode. Minimum explosive concentration to detect in IMS was the initial explosive concentration deposited on the PTFE sheet and it was determined considering sufficient signal-to-noise ratio (S/N) over 10. Figure 3. Experimental processes (rolling method) of explosive transfer from the PTFE sheet to the bag surface and explosive collection from the bag surface to the smear matrix. Rolling with a stainless steel roller was used as the sampling method. And then, the explosive-collected smear matrix was inserted in the sampling region of IMS. The transfer and collection procedures by handrubbing were described in Figure 4 (hand-rubbing method). First, the explosive-deposited PTFE sheet was prepared as described above. Second, the explosive-deposited side of the PTFE sheet was placed on the testing part and the PTFE Figure 4. Experimental processes (hand-rubbing method) of explosive transfer from the PTFE sheet to the testing part and explosive collection from the testing part to the smear matrix. Handrubbing was used as the sampling method. Results and Discussion Explosive materials were transferred from the mother matrix of PTFE sheet to the testing parts and were collected using the smear matrix as shown in Figures 3 and 4 for the rolling and hand-rubbing methods, respectively. The rolling method is more reasonable than the hand-rubbing method, but applicable parts of the rolling method are restricted due to use of a rigid metal roller. It is hard that the stainless steel roller applies to the parts of narrow area and soft materials. We tried to apply the rolling method to all the testing parts of TLB_SS, TLB_FH, TLB_AH, BP_OS, BP_BS, and BP_BKS. Of the six testing parts, only the rolling method was successfully applied to the TLB_SS part, which has wide and hard surface area. Hence, the rolling method is not adequate to apply to the small, soft, and porous surfaces because the stainless steel roller has rigid and wide surface. Widths of the handle parts of the TLB are small related to the metal roller, and most parts of the BP are composed of soft and porous layers. As marked in Figures 1 and 2, excluding the TLB_SS part, the other testing parts are narrow, curved, or soft. In order to compare the transfer/collection efficiencies of explosive materials for the two sampling methods, first of all magnified images of the sample surfaces (PTFE sheet, testing parts, and smear matrix) before and after the transfer and collection processes were observed. The magnified images of the TLB_SS part were observed because the TLB_SS part has wider and smoother surface than the other testing parts. As the crystal formation of RDX is much more favorable than that of TNT and synthesized state of RDX is a crystalline solid, RDX was used for observation of the explosive morphology on the PTFE sheet, TLB_SS part, and smear matrix. Several tens of RDXdeposited PTFE sheets were prepared, and the PTFE sheets having the similar RDX deposition shapes were selected to minimize the experimental errors. Figures 5 and 6 show Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 47

4 Figure 5. Magnified images ( 200) of RDX remained on the PTFE sheet before and after RDX transfer to the TLB_SS part, those of RDX remained on the TLB_SS part before and after RDX collection to the smear matrix, and that of RDX collected on the smear matrix. The initial amount of RDX deposited on the PTFE sheet was 1.0 μg. Rolling with a stainless steel roller was used as the sampling method. magnified images of RDX shapes on the PTFE sheets, on the TLB_SS parts, and on the smear matrices for the rolling and hand-rubbing methods, respectively. Changes of RDX shapes on the PTFE sheets before and after RDX transfer to the TLB_SS part were examined, and those on the bag surface before and after RDX collection using the smear matrix were also observed. And the relative amounts and shapes of RDX collected on the smear matrices from the bag surfaces by the rolling and hand-rubbing methods were investigated. Initial shapes of RDX deposited on the PTFE sheets were nearly the same. After transferring RDX from the PTFE sheet to the bag surface, lots of RDX were observed on the TLB_SS part whereas only some spots and small particles were observed on the PFTE sheets irrespective of the sampling methods. This means that most of RDX deposited on the PTFE sheet successfully transferred to the bag surface. This is due to very low interactions of RDX with PTFE. The magnified images of the TLB_SS part and smear matrix after collection of RDX from the TLB_SS part to the smear matrix show very different states depending on the sampling methods. For the rolling method, most of RDX still remained on the TLB_SS part after the RDX collection process and it was hard to find RDX on the smear matrix. This means that most of RDX deposited on the bag surface were not collected to the smear matrix. For Figure 6. Magnified images ( 200) of RDX remained on the PTFE sheet before and after RDX transfer to the TLB_SS part, those of RDX remained on the TLB_SS part before and after RDX collection to the smear matrix, and that of RDX collected on the smear matrix. The initial amount of RDX deposited on the PTFE sheet was 1.0 μg. Handrubbing was used as the sampling method. the hand-rubbing method, only small spots and particles were observed on the bag surface after the RDX collection and lots of RDX was observed on the smear matrix. This indicates that most of RDX deposited on the bag surface were collected by handrubbing with the smear matrix. The minimum initial concentrations (C min_ptfe s) of TNT and RDX deposited on the PTFE sheets to detect in IMS were measured, and high signal-to-noise ratio over 10 was applied for determination of the C min_ptfe. When the rolling method was applied, any ion peak related to the explosive materials was not observed until the initial TNT and RDX concentrations of 1000 ng. The C min_ptfe values of TNT and RDX for the TLB_SS part using the rolling and hand-rubbing methods were compared. For the rolling method, the C min_ptfe values of TNT and RDX for the TLB_SS part were 5 and 10 μg, respectively. The C min_ptfe values of TNT and RDX for the TLB_SS part using the hand-rubbing method were the same of 1000 ng. The C min_ptfe values were much larger than the limit of detection (LOD) ones and were also higher than the minimum amounts of the explosives deposited on the PTFE sheet to detect with IMS. 11,12 This is because amounts of the solidstate explosive materials are remarkably reduced through the transfer and collection processes. The experimental results indicate that the rolling method has lower transfer/ collection efficiencies of explosive materials compared to Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 48

5 Table 1. The minimum initial explosive concentrations (C min_ptfe ) deposited on the PTFE sheets to detect in IMS for the testing parts of travel luggage bag (ng). Explosive Testing part of travel luggage bag TNT RDX Smooth surface (TLB_SS) Fixed handle (TLB_FH) Adjustable handle (TLB_AH) The hand-rubbing method was used. Intensity [TNT - H] - [RDX + Cl] - Table 2. The minimum initial explosive concentrations (C min_ptfe ) deposited on the PTFE sheets to detect in IMS for the testing parts of backpack (ng). Explosive Testing part of backpack TNT RDX Outer surface (BP_OS) Bag strap (BP_BS) Buckle strap (BP_BKS) The hand-rubbing method was used. the hand-rubbing method. As discussed above, collection efficiency of the rolling method was not good though the testing part surface is smooth and wide. The C min_ptfe values for the hand-rubbing method were much lower than those for the rolling method. Therefore, the hand-rubbing method is more efficient to collect explosive materials deposited on the bag surface than the rolling method. Experimental results (C min_ptfe s) using the hand-rubbing method were summarized in Tables 1 and 2 for the TLB and BP, respectively. Figure 7 shows IMS spectra of TNT and RDX at the C min_ptfe of TLB_AH part, while Figure 8 shows those of BP_BKS part. The typical ions of TNT and RDX detected in IMS were [TNT H] and [RDX + Cl], respectively. 11,12,18,19 Drift times of the TNT and RDX ions Intensity [TNT - H] - [RDX + Cl] Drift time (ms) Figure 7. IMS spectra of TNT (500 ng) and RDX (500 ng) at the minimum initial concentrations of the TLB_AH part Drift time (ms) Figure 8. IMS spectra of TNT (50 ng) and RDX (20 ng) at the minimum initial concentrations of the BP_BKS part. were 35.9 and 37.7 ms, respectively. The C min_ptfe values of TNT and RDX for the TLB (TLB_* parts) were larger than those for the BP (BP_* parts). Principal difference in the testing parts of the TLB and BP is fiber bundles as shown in Figures 9 and 10. The testing parts of BP are composed of fabric with fine fiber bundles, whereas those of the TLB are composed of polymer sheets and accessories. The testing parts of the TLB are not made of fabric and do not have any fiber bundles. The testing parts composed of fiber bundles have larger surface areas compared to the testing parts of smooth or embossing surface of the TLB. Hence, efficiencies for the explosive transfer to the testing parts of BP from the PTFE sheets could be greater than those of the TLB. For the TLB, the C min_ptfe values of TNT and RDX were similar, and the order according to the testing part was TLB_SS > TLB_FH ~ TLB_AH. For the BP, the C min_ptfe values of TNT and RDX were different depending on the testing parts as listed in Table 2. The C min_ptfe values of TNT for the BP_OS and BP_BS parts were similar and were larger than that for the BP_BKS one. The C min_ptfe values of RDX for the BP_BS and BP_BKS parts were similar and were smaller than that for the BP_OS one. Combining the two results of TNT and RDX, the order of C min_ptfe according to the testing part was BP_OS > BP_BS > BP_BKS. From the experimental results, it can lead to a conclusion that the hand-rubbing method is more adequate sampling method than the rolling one and the C min_ptfe values for the BP are lower than those for the TLB. It is very important to select sampling parts of travelers luggage and baggage for accurate and fast inspection of the presence of explosive materials in the on-site security checks. Sampling from the parts composed of fabric (made of fine fiber bundle) is more efficient than that composed of polymer article, sheet, and film. Sampling from the rough surface is also more efficient than sampling from the smooth one. This Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 49

6 Article Figure 9. Magnified images ( 200) of the testing parts of travel luggage bag. (a) Smooth surface (TLB_SS), (b) fixed handle (TLB_FH), and (c) adjustable handle (TLB_AH). Figure 10. Magnified images ( 200) of the testing parts of backpack. (a) Outer surface (BP_OS), (b) bag strap (BP_BS), and (c) buckle strap (BP_BKS). Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 50

7 testing method will be applicable for the other explosives and things. Conclusions Testing method for the on-site detection of explosive materials contaminated on the TLB and BP surfaces was developed using IMS and smear matrix. The hand-rubbing method was found to be more efficient than the rolling one. Initial solid-state TNT and RDX samples were prepared by depositing the explosive materials on PTFE sheet using the solutions, and the explosives deposited on PTFE sheet were transferred to the testing parts. Explosive materials transferred on the testing parts were collected with the smear matrix by handrubbing, and the collected explosive materials were directly measured using IMS by inserting the smear matrix in the IMS sampling region. The C min_ptfe values for the TLB were larger than those for the BP due to the fiber bundles of BP. For the TLB, the C min_ptfe values of TNT and RDX were similar, and the order according to the testing part was TLB_SS > TLB_FH ~ TLB_AH. For the BP, the order of C min_ptfe according to the testing part was BP_OS > BP_BS > BP_BKS. For efficient IMS detection of explosive materials in the on-site security checks, it is recommendable that sampling is carried out from the fabric parts using handrubbing. Acknowledgments. This study was supported by Korea Testing Laboratory and IMS Technology (Korea). References 1. J. Lee, S. Park, S. G. Cho, E. M. Goh, S. Lee, S.-S. Koh, J. Kim, Talanta 2014, 120, A. Zalewska, W. Pawłowski, W. Tomaszewski, Forensic Sci. Int. 2013, 226, M. Najarro, M. E. D. Morris, M. E. Staymates, R. Fletcher, G. Gillen, Analyst 2012, 137, K. M. Roscioli, E. Davis, W. F. Siems, A. Mariano, W. Su, S. K. Guharay, H. H. Hill Jr., Anal. Chem. 2011, 83, S.-S. Choi, O.-B. Kim, Y.-K. Kim, S. G. An, M.-W. Shin, S.-J. Maeng, G. S. Choi, Bull. Korean Chem. Soc. 2011, 32, S. Armenta, M. Alcala, M. Blanco, Anal. Chim. Acta 2011, 703, T. Khayamian, M. Tabrizchi, M. T. Jafari, Talanta 2003, 59, H. Ostmark, S. Wallin, H. G. Ang, Propellants Explos. Pyrotech. 2012, 37, M. Wrable-Rose, O. M. Primera-Pedrozo, L. C. Pacheco-Londono, S. P. Hernandez-Rivera, Sens. Imaging 2010, 11, O. M. Primera-Pedrozo, Y. M. Soto-Feliciano, L. C. Pacheco-Londono, S. P. Hernandez-Rivera, Sens. Imaging 2009, 10, S.-S. Choi, C. E. Son, Anal. Methods 2017, 9, S.-S. Choi, C. E. Son, M.-W. Shin, G. S. Choi, Bull. Kor. Chem. Soc. 2016, 37, M. E. Staymates, J. Grandner, J. R. Verkouteren, IEEE Sensors J. 2013, 13, J. R. Verkouteren, N. W. M. Ritchie, G. Gillen, Environ. Sci.: Process. Impacts 2013, 15, A. Strachan, A. C. T. van Duin, D. Chakraborty, S. Dasgupta, W. A. Goddard III., Phys. Rev. Lett. 2003, 91, A. E. D. M. van der Heijden, R. H. B. Bouma, Cryst. Growth Des. 2004, 4, N. I. Golovina, A. N. Titkov, A. V. Raevskii, L. O. Atovmyan, J. Solid State Chem. 1994, 113, M. Tam, H. H. Hill Jr., Anal. Chem. 2004, 76, S. D. Harvey, R. G. Ewing, M. J. Waltman, Int. J. Ion Mobil. Spec. 2009, 12, 115. Bull. Korean Chem. Soc. 2018, Vol. 39, Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 51