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Determination of triacetonetriperoxide in ambient air
Analytica Chimica Acta 482 (2003) 183–188
Determination of triacetonetriperoxide in ambient air Rasmus Schulte-Ladbeck, Uwe Karst∗ Department of Chemical Analysis, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 2 December 2002; received in revised form 3 February 2003; accepted 11 February 2003
Abstract A method for the analysis of the explosive triacetonetriperoxide (TATP) in ambient air is introduced. The high volatility of the peroxide leads to significant concentrations in the air surrounding even minute quantities of TATP, thus enabling the analyst to avoid direct contact with the sensitive explosive. Air sampling is performed using gas-washing bottles filled with acetonitrile and air sampling pumps at a flow-rate of 0.6 l min−1 . A sampling and a back-up gas-washing bottle are connected in series to allow monitoring of possible breakthroughs in the sampling gas-washing bottle. After sampling, two different analytical methods were used: first, reversed-phase high-performance liquid chromatography (HPLC) with subsequent post-column UV irradiation and electrochemical detection; and second, photochemical degradation of TATP with enzyme-catalyzed photometric detection. The limits of detection for 20 min of sampling time (12 l sample volume) were 190 ng l−1 air for the photometric method and 550 ng l−1 air for LC with electrochemical detection. The recovery was at least 75%. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Explosives; Air analysis; HPLC; Electrochemical detection
1. Introduction Detection and identification of explosives such as triacetonetriperoxide (TATP) is a very difficult task, especially for explosives which are very sensitive. TATP can easily be synthesized, and the starting chemicals are readily available. The number of incidents involving TATP has been increasing in the last few years. TATP appears in drug crimes [1] and amateur chemist accidents [2]. It is also used by terrorists in many areas of the world [3,4]. Furthermore, as descriptions of the synthesis are becoming more and more available to the wider public due to new media, the number of cases is likely to increase in the forthcoming years.
∗ Corresponding author. Tel.: +31-53-489-2983; fax: +31-53-489-4645. E-mail address: [email protected] (U. Karst).
The synthesis of TATP was first carried out by Wolffenstein in the 19th century [5]. His proposed structure of TATP which was later confirmed by Groth [6] is presented in Fig. 1. TATP is one of the most sensitive explosives known. It is sensitive to shock, friction, static electricity and temperature changes. Therefore, safe handling of TATP is extremely problematic. Danger significantly increases if TATP is allowed to dry. To date, there are no known industrial or military applications of TATP. One major reason for this is the tendency of TATP to sublime rapidly [7]. The most frequently used methods for the identification of TATP are infrared (IR) spectroscopy or chemical ionization–mass spectrometry (CI–MS) [1,2,8]. Two methods for trace level analysis of solid and/or liquid TATP samples have recently been published, which comprise reversed-phase high-performance liquid chromatography (HPLC) with post-column UV irradiation and fluorescence or electrochemical
0003-2670/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0003-2670(03)00212-5
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Fig. 1. Structure of triacetonetriperoxide (TATP).
detection, respectively [9,10]. A rapid test scheme for the qualitative analysis applies UV irradiation for the decomposition of TATP to hydrogen peroxide. The latter is subsequently detected by the peroxidase (POD)-catalyzed formation of the green radical cation of 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) [11]. Recently, LC/MS methods have been described as well for the determination of TATP and the related hexamethylenetriperoxide diamine (HMTD) [12,13]. Applications of these methods comprise on the one hand the identification of unknown white powders, which may contain TATP or related compounds. Due to the sensitivity of the explosive, sampling of the dry materials poses a severe risk of unwantedly causing an explosion to the analyst. On the other hand, post-explosion identification of TATP residues, which are spread in a large amount of debris requires laborious leaching of the debris with organic solvents to obtain peroxide solutions. In both cases, the analysis of air samples taken either over the solid sample or at the site of an earlier explosion would be helpful. In the first case, sampling could be performed without coming close to or even touching the dangerous sample. In the second case, peroxide identification could be performed by legal authorities taking only one air sample at the detonation site. Within this work, the high volatility of TATP has therefore been used as basis for the development of the first known air analysis method of a peroxide-based explosive.
2. Experimental 2.1. Safety note TATP and its homologues are very dangerous materials, which may lead to severe and spontaneous
explosions under impact, friction, static electricity and temperature changes. The synthesis of these substances may only be carried out by highly qualified and experienced personnel, under the use of appropriate safety precautions (reinforced goggles and gloves, splinter-proof vessels, protective shield etc.), and only in small quantities. For this work, TATP was synthesized according to literature procedures (see later) in quantities not exceeding 100 mg. Working with larger amounts of the substance strongly increases the danger associated with spontaneous explosions. 2.2. Reagents All chemicals were purchased from Aldrich (Steinheim, Germany), Merck (Darmstadt, Germany), Sigma (Deisenhofen, Germany) and Fluka (Neu-Ulm, Germany) in the highest quality available. Acetonitrile for HPLC was Merck gradient grade. Horseradish peroxidase was purchased from Sigma (Deisenhofen, Germany) (EC 1.11.1.7). 2.3. Instrumentation 2.3.1. UV-Vis A handheld photometer SQ118 from Merck (Darmstadt, Germany) was used for field measurements. For the recording of the UV-Vis spectra, a diode array photometer model HP 8453 with the software UV-Vis Chemstation 845x from Hewlett-Packard (Waldbronn, Germany) was used. For UV irradiation, a low-pressure mercury lamp (6 W) was used. 2.4. HPLC–electrochemical instrumentation and analysis A liquid chromatographic system consisting of the following components (all from Shimadzu, Duisburg, Germany) was used: two LC-10AS pumps, degasser GT-154, SPD-M10Avp diode-array detector, SIL-10A autosampler, software Class LC-10 (Version 1.6) and CBM-10A controller unit. A Coulochem II multi-electrode detector (ESA, Chelmsford, MA, USA) was used. The electrochemical cell was a Standard Analytical Cell Model 5010 also from ESA. The injection volume was 10 l. The column material was Merck LiChroSpher RP18; particle size 5 m; pore size 300 Å; column dimensions 250 mm × 3 mm. For
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UV irradiation, a low-pressure mercury lamp (6 W) was used. For HPLC separations, a solvent mixture consisting of 65% of acetonitrile and 35% of a 4 mmol l−1 phosphate buffer, pH 8, at a flow-rate of 0.5 ml min−1 was used. For buffer preparation, 7.7 mg NaH2 PO4 ·H2 O and 61.3 mg Na2 HPO4 ·H2 O were dissolved in 100 ml of deionized water. The voltage in the detector cell was 900 mV [10].
polymer materials for the sampling tubes. Sampling time was 20 min and the flow-rate was adjusted to 0.6 l min−1 . One sampling gas-washing bottle and a second gas-washing bottle were connected in series to detect possible breakthroughs of the analyte, which would then be trapped in the second gas-washing bottle. The contents of the sampling and the subsidiary gas-washing bottle were both investigated by the different detection methods.
2.5. Synthesis
2.6. Experimental set-up
2.6.2. Post-column irradiation with HPLC–electrochemical detection The post-column irradiation set-up has been described previously for the analysis of liquid peroxide samples [10]. Immediately after the separation of the analytes, the latter were irradiated with UV light of 254 nm. A 25 m knitted Teflon tubing, with a diameter of 0.3 mm, which was wrapped around the UV lamp was used as a reaction loop. After the decomposition of the analytes to hydrogen peroxide, the latter was detected in the coulometric cell [10].
2.6.1. Air sampling set-up The gas-washing bottle-based air sampling set-up is schematically presented in Fig. 2, for the case of measurements of TATP in the gas phase over an unknown white powder. Two gas-washing bottles were filled with 20 ml of acetonitrile each. The air flow was pumped into the sampling solution through a glass pipe, topped with a porous frit. The whole set-up comprised a minimum amount of Teflon tubing to thoroughly connect the different glass pipes, as the loss of sample material was observed when using other
2.6.3. Rapid UV-Vis-based test for the identification of TATP Initially, the sample solution was irradiated with UV light of 254 nm for 30 min. The reagent solution (5 mg POD and 27.2 mg 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate in 50 ml of a 0.01 mol l−1 acetate buffer, pH 5.5) was mixed with the sample in a ratio of 1:1. After an incubation period of 5 min, detection was performed with a handheld photometer at a wavelength of 415 nm as described for the analysis of liquid samples in [11].
The synthesis of TATP and HMTD was performed as described in the literature [5,14]. For this work, the synthesis was carried out to obtain 100 mg of the explosives in case of quantitative yield of the product. For safety precautions, refer to the safety note given earlier. Excess amounts of the explosives can be destroyed according to [7].
Fig. 2. Air sampling set-up.
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3. Results and discussion The method presented is making use of the high volatility of TATP even at room temperature. An air sample containing gaseous TATP is thus lead through an gas-washing bottle filled with acetonitrile, where the analyte is trapped and enriched. For air sampling efficiency, the selection of the solvent is of high importance. On the one hand, it has to serve for sufficient, ideally quantitative trapping of the analytes. On the other hand, it should be compatible with both methods used for subsequent analysis of the peroxide. This is particularly important when if the analytical technique is foreseen to be used under field conditions, because solvent changes will not be possible in this case. Acetone and methanol turned out to be inadequate for the water insoluble TATP because of their low boiling points and the unsatisfactory solubility of TATP in these solvents. Acetonitrile, on the other hand, fulfilled the requirements of being not too volatile, of being compatible with direct injection into a reversed-phase HPLC system and of trapping TATP appropriately. An amount of 20 ml of acetonitrile were used for 20 min of sampling applying an air flow-rate of 0.6 l min−1 . For the preliminary experiments, the connections between the glass flasks as well as the sampling tube were mostly out of PET tubing. The recovery of the analytes with this set-up was in the range of 60% and lower. Owing to possible adsorption of the analytes on the tubing, these were replaced by Teflon material and glass pipes. After the replacement, the recovery of the analytes was significantly increased to 77% at a concentration of 9.25 g l−1 . At a concentration of 1.85 g l−1 the recovery was 75%. It should be noted that the term “recovery” mentioned here refers to the recovery of the complete process of vaporization, trapping and analysis and not only to trapping and analysis alone, as done usually. The reason for this is the fact that only unpurified TATP could be used for recovery determination for safety reasons. A stock solution of TATP in acetonitrile was vaporized by pumping a constant air stream over it and transferring this into the gas-washing bottle set-up. Another quantity of the TATP solution was immediately analyzed by HPLC. The recovery was then quantified by calculating the ratio of TATP amounts determined in the sampling gas-washing bot-
tle and in the TATP stock solution. At the flow-rate of 0.6 l min−1 , which was applied here, no TATP was found in the back-up gas-washing bottle. At higher flow-rates, however, a breakthrough occurred. Due to the lack of breakthroughs at the low flow-rate, it is reasonable to assume that the observed loss of TATP may be mainly due to a decomposition of the analyte during evaporation, and only to a lesser extent due to non-quantitative trapping. It is well possible that parts of the loss can still be traced back on an adsorption of the TATP even to the remaining low surface area of polymer materials in the collection system. The provided recovery percentage can therefore be considered as the minimum recovery for trapping and analysis. An indoor sampling situation for the analysis of an unknown solid sample was simulated. For this purpose, the inlet of the sampling device was fixed in different heights (2–20 cm) above an amount of TATP (∼100 mg). TATP was detected up to a distance of 10 cm between the tubing inlet and the sample amount. For sampling, an air flow-rate of 0.6 l min−1 was applied for 20 min. Analysis was performed according to [11] applying the rapid test with photometric detection. While the established method for TATP determination comprised the dilution of the organic sample with water, to present deactivation of the enzyme [11] this could be avoided within this work. However, the amount of peroxidase used for the analysis was doubled to guarantee sufficient enzymatic activity. The sensitivity could be significantly improved by this variation of the method. The irradiation time of the original method was 15 min. Fig. 3 shows the signal intensity against the irradiation time. The curve reaches its maximum at 30 min of irradiation time. For the fast test [11], a short analysis time was a prime objective. Fifteen minutes of irradiation were therefore applied for the fast test. However, for the air analysis, sensitivity was much more important. The lower concentrations commonly appearing during air analysis demand a longer irradiation time, and 30 min of photochemical treatment were therefore applied in this method. In this application, the pretreatment with catalase as described in [11] was not necessary, as hydrogen peroxide is not enriched in the absorbance solution, which consists of pure acetonitrile. This is due to the fact that hydrogen peroxide is much more polar and therefore better soluble in polar solvents, especially in
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Fig. 3. Signal intensity vs. irradiation time. Here, the ABTS method was used for detection. The sample investigated contained 9.25 g l−1 air.
Fig. 4. Formation of the green radical cation of 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS).
water. After sampling, TATP is irradiated by UV light at a wavelength of 254 nm and is thus decomposed to hydrogen peroxide. Afterwards, hydrogen peroxide is determined on the base of the POD-catalyzed reaction of hydrogen peroxide with ABTS (see Fig. 4). Detection is performed with a portable photometer. Limits of detection in the gas phase for this method are 190 ng l−1 air. The R.S.D. (n = 4) for the concentration of 1.85 g l−1 air was 10%, while at a concentration of 9.25 g l−1 air the R.S.D. (n = 4) was 7%. As the ABTS method does not distinguish between TATP and further analytes with similar characteristics, a second detection method to enhance selectivity was introduced. Therefore, reversed-phase high-performance liquid chromatography in combi-
nation with post-column UV irradiation and electrochemical detection was used for the analysis of TATP. As described in the literature [10], this method separates HMTD from TATP and is a sensitive way to quantify TATP. For the analysis, the set-up presented in [10] was used. TATP and HMTD (for the structure of HMTD, see Fig. 5), as a reference substance were dissolved in acetonitrile. Subsequently, the solution was vaporized, and afterwards, an air sample
Fig. 5. Structure of hexamethylenetriperoxide diamine (HMTD).
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Fig. 6. Chromatogram of an acetonitrile solution spiked with TATP (c = 1 × 10−4 mol l−1 ) and HMTD (c = 1 × 10−4 mol l−1 ), before vaporization of the spiked solution (dotted line) and a chromatogram of the air sampling solution (solid line).
was taken. Finally, the air sample was analyzed by means of HPLC with photochemical decomposition and electrochemical detection (see Fig. 6). It was found that the less volatile HMTD was obviously not at all vaporized within this approach. The limits of detection for the gas phase analysis of TATP were 550 ng l−1 air for this method. The R.S.D. (n = 4) for this method was 3% at a concentration in the gas phase of 1.85 g l−1 air and 1.4% at a concentration of 9.25 g l−1 air.
4. Conclusions In this paper, the first air sampling method for a peroxide-based explosive was introduced. TATP was collected by means of gas-washing bottle sampling and the analyte was determined subsequently using different selective approaches. The method presented is characterized by a good recovery of the analytes. It is now possible to examine large amounts of unknown substances, which are supposed to contain TATP from a position of relative safety. Furthermore, this air sampling method is an option to check post-explosion sites for TATP, even when no immediately visible residues of the respective explosive are found.
Acknowledgements Financial support by the German Minister of the Interior (Berlin, Germany) is gratefully acknowledged. References [1] G.M. White, J. Forensic Sci. 37 (1992) 652–656. [2] H.K. Evans, F.A.J. Tulleners, B.L. Sanchez, C.A. Rasmussen, J. Forensic Sci. 31 (1986) 1119–1125. [3] The Independent, London, vol. 8, 1996, p. 7. [4] R.T. Cooper, Los Angeles Times, vol. 29, 2001, p. A12. [5] R. Wolffenstein, Chem. Ber. 28 (1895) 2265–2269. [6] P. Groth, Acta Chem. Scand. 23 (1969) 1311–1329. [7] A.J. Bellamy, J. Forensic Sci. 44 (1999) 603–608. [8] S. Zitrin, S. Kraus, B. Glattstein, in: Proceedings of the International Symposium on the Analysis and Detection of Explosives, US Government Printing Office, Washington, DC, 1984, pp. 137–141. [9] R. Schulte-Ladbeck, P. Kolla, U. Karst, Anal. Chem. 75 (2003) 731–735. [10] R. Schulte-Ladbeck, U. Karst, Chromatographia, in press. [11] R. Schulte-Ladbeck, P. Kolla, U. Karst, Analyst 127 (2002) 1152–1154. [12] L. Widmer, S. Watson, K. Schlatter, A. Crowson, Analyst 127 (2002) 1627–1632. [13] A. Crowson, M.S. Beardah, Analyst 126 (2001) 1689–1693. [14] W.P. Schaefer, J.T. Fourkas, B.G. Tiemann, J. Am. Chem. Soc. 107 (1985) 2461–2463.
Determination of triacetonetriperoxide in ambient air Rasmus Schulte-Ladbeck, Uwe Karst∗ Department of Chemical Analysis, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 2 December 2002; received in revised form 3 February 2003; accepted 11 February 2003
Abstract A method for the analysis of the explosive triacetonetriperoxide (TATP) in ambient air is introduced. The high volatility of the peroxide leads to significant concentrations in the air surrounding even minute quantities of TATP, thus enabling the analyst to avoid direct contact with the sensitive explosive. Air sampling is performed using gas-washing bottles filled with acetonitrile and air sampling pumps at a flow-rate of 0.6 l min−1 . A sampling and a back-up gas-washing bottle are connected in series to allow monitoring of possible breakthroughs in the sampling gas-washing bottle. After sampling, two different analytical methods were used: first, reversed-phase high-performance liquid chromatography (HPLC) with subsequent post-column UV irradiation and electrochemical detection; and second, photochemical degradation of TATP with enzyme-catalyzed photometric detection. The limits of detection for 20 min of sampling time (12 l sample volume) were 190 ng l−1 air for the photometric method and 550 ng l−1 air for LC with electrochemical detection. The recovery was at least 75%. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Explosives; Air analysis; HPLC; Electrochemical detection
1. Introduction Detection and identification of explosives such as triacetonetriperoxide (TATP) is a very difficult task, especially for explosives which are very sensitive. TATP can easily be synthesized, and the starting chemicals are readily available. The number of incidents involving TATP has been increasing in the last few years. TATP appears in drug crimes [1] and amateur chemist accidents [2]. It is also used by terrorists in many areas of the world [3,4]. Furthermore, as descriptions of the synthesis are becoming more and more available to the wider public due to new media, the number of cases is likely to increase in the forthcoming years.
∗ Corresponding author. Tel.: +31-53-489-2983; fax: +31-53-489-4645. E-mail address: [email protected] (U. Karst).
The synthesis of TATP was first carried out by Wolffenstein in the 19th century [5]. His proposed structure of TATP which was later confirmed by Groth [6] is presented in Fig. 1. TATP is one of the most sensitive explosives known. It is sensitive to shock, friction, static electricity and temperature changes. Therefore, safe handling of TATP is extremely problematic. Danger significantly increases if TATP is allowed to dry. To date, there are no known industrial or military applications of TATP. One major reason for this is the tendency of TATP to sublime rapidly [7]. The most frequently used methods for the identification of TATP are infrared (IR) spectroscopy or chemical ionization–mass spectrometry (CI–MS) [1,2,8]. Two methods for trace level analysis of solid and/or liquid TATP samples have recently been published, which comprise reversed-phase high-performance liquid chromatography (HPLC) with post-column UV irradiation and fluorescence or electrochemical
0003-2670/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0003-2670(03)00212-5
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Fig. 1. Structure of triacetonetriperoxide (TATP).
detection, respectively [9,10]. A rapid test scheme for the qualitative analysis applies UV irradiation for the decomposition of TATP to hydrogen peroxide. The latter is subsequently detected by the peroxidase (POD)-catalyzed formation of the green radical cation of 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) [11]. Recently, LC/MS methods have been described as well for the determination of TATP and the related hexamethylenetriperoxide diamine (HMTD) [12,13]. Applications of these methods comprise on the one hand the identification of unknown white powders, which may contain TATP or related compounds. Due to the sensitivity of the explosive, sampling of the dry materials poses a severe risk of unwantedly causing an explosion to the analyst. On the other hand, post-explosion identification of TATP residues, which are spread in a large amount of debris requires laborious leaching of the debris with organic solvents to obtain peroxide solutions. In both cases, the analysis of air samples taken either over the solid sample or at the site of an earlier explosion would be helpful. In the first case, sampling could be performed without coming close to or even touching the dangerous sample. In the second case, peroxide identification could be performed by legal authorities taking only one air sample at the detonation site. Within this work, the high volatility of TATP has therefore been used as basis for the development of the first known air analysis method of a peroxide-based explosive.
2. Experimental 2.1. Safety note TATP and its homologues are very dangerous materials, which may lead to severe and spontaneous
explosions under impact, friction, static electricity and temperature changes. The synthesis of these substances may only be carried out by highly qualified and experienced personnel, under the use of appropriate safety precautions (reinforced goggles and gloves, splinter-proof vessels, protective shield etc.), and only in small quantities. For this work, TATP was synthesized according to literature procedures (see later) in quantities not exceeding 100 mg. Working with larger amounts of the substance strongly increases the danger associated with spontaneous explosions. 2.2. Reagents All chemicals were purchased from Aldrich (Steinheim, Germany), Merck (Darmstadt, Germany), Sigma (Deisenhofen, Germany) and Fluka (Neu-Ulm, Germany) in the highest quality available. Acetonitrile for HPLC was Merck gradient grade. Horseradish peroxidase was purchased from Sigma (Deisenhofen, Germany) (EC 1.11.1.7). 2.3. Instrumentation 2.3.1. UV-Vis A handheld photometer SQ118 from Merck (Darmstadt, Germany) was used for field measurements. For the recording of the UV-Vis spectra, a diode array photometer model HP 8453 with the software UV-Vis Chemstation 845x from Hewlett-Packard (Waldbronn, Germany) was used. For UV irradiation, a low-pressure mercury lamp (6 W) was used. 2.4. HPLC–electrochemical instrumentation and analysis A liquid chromatographic system consisting of the following components (all from Shimadzu, Duisburg, Germany) was used: two LC-10AS pumps, degasser GT-154, SPD-M10Avp diode-array detector, SIL-10A autosampler, software Class LC-10 (Version 1.6) and CBM-10A controller unit. A Coulochem II multi-electrode detector (ESA, Chelmsford, MA, USA) was used. The electrochemical cell was a Standard Analytical Cell Model 5010 also from ESA. The injection volume was 10 l. The column material was Merck LiChroSpher RP18; particle size 5 m; pore size 300 Å; column dimensions 250 mm × 3 mm. For
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UV irradiation, a low-pressure mercury lamp (6 W) was used. For HPLC separations, a solvent mixture consisting of 65% of acetonitrile and 35% of a 4 mmol l−1 phosphate buffer, pH 8, at a flow-rate of 0.5 ml min−1 was used. For buffer preparation, 7.7 mg NaH2 PO4 ·H2 O and 61.3 mg Na2 HPO4 ·H2 O were dissolved in 100 ml of deionized water. The voltage in the detector cell was 900 mV [10].
polymer materials for the sampling tubes. Sampling time was 20 min and the flow-rate was adjusted to 0.6 l min−1 . One sampling gas-washing bottle and a second gas-washing bottle were connected in series to detect possible breakthroughs of the analyte, which would then be trapped in the second gas-washing bottle. The contents of the sampling and the subsidiary gas-washing bottle were both investigated by the different detection methods.
2.5. Synthesis
2.6. Experimental set-up
2.6.2. Post-column irradiation with HPLC–electrochemical detection The post-column irradiation set-up has been described previously for the analysis of liquid peroxide samples [10]. Immediately after the separation of the analytes, the latter were irradiated with UV light of 254 nm. A 25 m knitted Teflon tubing, with a diameter of 0.3 mm, which was wrapped around the UV lamp was used as a reaction loop. After the decomposition of the analytes to hydrogen peroxide, the latter was detected in the coulometric cell [10].
2.6.1. Air sampling set-up The gas-washing bottle-based air sampling set-up is schematically presented in Fig. 2, for the case of measurements of TATP in the gas phase over an unknown white powder. Two gas-washing bottles were filled with 20 ml of acetonitrile each. The air flow was pumped into the sampling solution through a glass pipe, topped with a porous frit. The whole set-up comprised a minimum amount of Teflon tubing to thoroughly connect the different glass pipes, as the loss of sample material was observed when using other
2.6.3. Rapid UV-Vis-based test for the identification of TATP Initially, the sample solution was irradiated with UV light of 254 nm for 30 min. The reagent solution (5 mg POD and 27.2 mg 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate in 50 ml of a 0.01 mol l−1 acetate buffer, pH 5.5) was mixed with the sample in a ratio of 1:1. After an incubation period of 5 min, detection was performed with a handheld photometer at a wavelength of 415 nm as described for the analysis of liquid samples in [11].
The synthesis of TATP and HMTD was performed as described in the literature [5,14]. For this work, the synthesis was carried out to obtain 100 mg of the explosives in case of quantitative yield of the product. For safety precautions, refer to the safety note given earlier. Excess amounts of the explosives can be destroyed according to [7].
Fig. 2. Air sampling set-up.
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3. Results and discussion The method presented is making use of the high volatility of TATP even at room temperature. An air sample containing gaseous TATP is thus lead through an gas-washing bottle filled with acetonitrile, where the analyte is trapped and enriched. For air sampling efficiency, the selection of the solvent is of high importance. On the one hand, it has to serve for sufficient, ideally quantitative trapping of the analytes. On the other hand, it should be compatible with both methods used for subsequent analysis of the peroxide. This is particularly important when if the analytical technique is foreseen to be used under field conditions, because solvent changes will not be possible in this case. Acetone and methanol turned out to be inadequate for the water insoluble TATP because of their low boiling points and the unsatisfactory solubility of TATP in these solvents. Acetonitrile, on the other hand, fulfilled the requirements of being not too volatile, of being compatible with direct injection into a reversed-phase HPLC system and of trapping TATP appropriately. An amount of 20 ml of acetonitrile were used for 20 min of sampling applying an air flow-rate of 0.6 l min−1 . For the preliminary experiments, the connections between the glass flasks as well as the sampling tube were mostly out of PET tubing. The recovery of the analytes with this set-up was in the range of 60% and lower. Owing to possible adsorption of the analytes on the tubing, these were replaced by Teflon material and glass pipes. After the replacement, the recovery of the analytes was significantly increased to 77% at a concentration of 9.25 g l−1 . At a concentration of 1.85 g l−1 the recovery was 75%. It should be noted that the term “recovery” mentioned here refers to the recovery of the complete process of vaporization, trapping and analysis and not only to trapping and analysis alone, as done usually. The reason for this is the fact that only unpurified TATP could be used for recovery determination for safety reasons. A stock solution of TATP in acetonitrile was vaporized by pumping a constant air stream over it and transferring this into the gas-washing bottle set-up. Another quantity of the TATP solution was immediately analyzed by HPLC. The recovery was then quantified by calculating the ratio of TATP amounts determined in the sampling gas-washing bot-
tle and in the TATP stock solution. At the flow-rate of 0.6 l min−1 , which was applied here, no TATP was found in the back-up gas-washing bottle. At higher flow-rates, however, a breakthrough occurred. Due to the lack of breakthroughs at the low flow-rate, it is reasonable to assume that the observed loss of TATP may be mainly due to a decomposition of the analyte during evaporation, and only to a lesser extent due to non-quantitative trapping. It is well possible that parts of the loss can still be traced back on an adsorption of the TATP even to the remaining low surface area of polymer materials in the collection system. The provided recovery percentage can therefore be considered as the minimum recovery for trapping and analysis. An indoor sampling situation for the analysis of an unknown solid sample was simulated. For this purpose, the inlet of the sampling device was fixed in different heights (2–20 cm) above an amount of TATP (∼100 mg). TATP was detected up to a distance of 10 cm between the tubing inlet and the sample amount. For sampling, an air flow-rate of 0.6 l min−1 was applied for 20 min. Analysis was performed according to [11] applying the rapid test with photometric detection. While the established method for TATP determination comprised the dilution of the organic sample with water, to present deactivation of the enzyme [11] this could be avoided within this work. However, the amount of peroxidase used for the analysis was doubled to guarantee sufficient enzymatic activity. The sensitivity could be significantly improved by this variation of the method. The irradiation time of the original method was 15 min. Fig. 3 shows the signal intensity against the irradiation time. The curve reaches its maximum at 30 min of irradiation time. For the fast test [11], a short analysis time was a prime objective. Fifteen minutes of irradiation were therefore applied for the fast test. However, for the air analysis, sensitivity was much more important. The lower concentrations commonly appearing during air analysis demand a longer irradiation time, and 30 min of photochemical treatment were therefore applied in this method. In this application, the pretreatment with catalase as described in [11] was not necessary, as hydrogen peroxide is not enriched in the absorbance solution, which consists of pure acetonitrile. This is due to the fact that hydrogen peroxide is much more polar and therefore better soluble in polar solvents, especially in
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Fig. 3. Signal intensity vs. irradiation time. Here, the ABTS method was used for detection. The sample investigated contained 9.25 g l−1 air.
Fig. 4. Formation of the green radical cation of 2,2 -azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS).
water. After sampling, TATP is irradiated by UV light at a wavelength of 254 nm and is thus decomposed to hydrogen peroxide. Afterwards, hydrogen peroxide is determined on the base of the POD-catalyzed reaction of hydrogen peroxide with ABTS (see Fig. 4). Detection is performed with a portable photometer. Limits of detection in the gas phase for this method are 190 ng l−1 air. The R.S.D. (n = 4) for the concentration of 1.85 g l−1 air was 10%, while at a concentration of 9.25 g l−1 air the R.S.D. (n = 4) was 7%. As the ABTS method does not distinguish between TATP and further analytes with similar characteristics, a second detection method to enhance selectivity was introduced. Therefore, reversed-phase high-performance liquid chromatography in combi-
nation with post-column UV irradiation and electrochemical detection was used for the analysis of TATP. As described in the literature [10], this method separates HMTD from TATP and is a sensitive way to quantify TATP. For the analysis, the set-up presented in [10] was used. TATP and HMTD (for the structure of HMTD, see Fig. 5), as a reference substance were dissolved in acetonitrile. Subsequently, the solution was vaporized, and afterwards, an air sample
Fig. 5. Structure of hexamethylenetriperoxide diamine (HMTD).
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Fig. 6. Chromatogram of an acetonitrile solution spiked with TATP (c = 1 × 10−4 mol l−1 ) and HMTD (c = 1 × 10−4 mol l−1 ), before vaporization of the spiked solution (dotted line) and a chromatogram of the air sampling solution (solid line).
was taken. Finally, the air sample was analyzed by means of HPLC with photochemical decomposition and electrochemical detection (see Fig. 6). It was found that the less volatile HMTD was obviously not at all vaporized within this approach. The limits of detection for the gas phase analysis of TATP were 550 ng l−1 air for this method. The R.S.D. (n = 4) for this method was 3% at a concentration in the gas phase of 1.85 g l−1 air and 1.4% at a concentration of 9.25 g l−1 air.
4. Conclusions In this paper, the first air sampling method for a peroxide-based explosive was introduced. TATP was collected by means of gas-washing bottle sampling and the analyte was determined subsequently using different selective approaches. The method presented is characterized by a good recovery of the analytes. It is now possible to examine large amounts of unknown substances, which are supposed to contain TATP from a position of relative safety. Furthermore, this air sampling method is an option to check post-explosion sites for TATP, even when no immediately visible residues of the respective explosive are found.
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