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A graphical and textual abstract for the Table of contents entry The sensitivity, specificity, and repeatability for explosives detection were improved by a dopant-assisted reactive low temperature plasma (DARLTP) probe.
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Dopant-assisted reactive low temperature plasma probe for sensitive and specific detection of explosives Wendong Chen, Keyong Hou, Lei Hua and Haiyang Li* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A dopant-assisted reactive low temperature plasma (DARLTP) probe was developed for sensitive and specific detection of explosives by a miniature rectilinear ion trap mass spectrometer. The DARLTP probe was fabricated by a T shaped quartz tube. The dopant gas was introduced into the plasma stream through a side-tube. Using CH2Cl2 doped wet air as the dopant gas, the detection sensitivities were improved to about 4-fold (RDX), 4-fold (PETN), and 3-fold (Tetryl), respectively, comparing with those obtained by the conventional LTP. Furthermore, the formation of [M + 35Cl]– and [M + 37Cl]– for these explosives enhanced the specificity for their identification. Additionally, fragment ions of Tetryl and adduct ions such as [RDX + NO2]– and [PETN + NO2]– were dramatically reduced, which simplified the mass spectra and avoided the overlap of mass peaks for different explosives. The sensitivity improvement might be attributed to the increased intensity of reactant ion [HNO3 + NO3]–, which was enhanced to 4fold after the introduction of dopant gas. The limits of detection (LODs) for RDX, Tetryl, and PETN were down to 3, 6, and 10 pg, respectively. Finally, an explosive mixture was successfully analyzed, demonstrating the potentials of DARLTP probe for qualitative and quantitative analysis of complicated explosives.
Introduction Sensitive analytical method for accurate identification of explosives is of great importance for security services1, 2 and forensic investigations.3 A variety of techniques have been developed for the sensitive detection of explosives, such as cyclic voltammetry,4 luminescence nanosensor,5 ion mobility 6-8 spectrometry, and mass spectrometry (MS).9, 10 Particularly, due to the attractive features of fast response, high sensitivity, as well as high specificity from the tandem MS analysis and ion/molecule reactions, MS has been widely used for the accurate measurement of trace explosives.11, 12 The ionization source plays an important role for the performance of MS in detecting explosives, such as sensitivity and specificity. By switching reagent ions, the proton transfer reaction and selected ion flow tube mass spectrometry have been applied to improve the selectivity and sensitivity for the explosive detection.13-16 Another kind of ionization source with similar functions is ambient ionization source. Up to date, more than thirty types of such sources have been developed, including atmospheric pressure chemical ionization (APCI),11 desorption electrospray ionization (DESI),17, 18 direct analysis in real time (DART),3, 19 flowing atmospheric-pressure afterglow (FAPA),20, 21 and low temperature plasma (LTP).22-24 Among these sources, the APCI source could enhance the specificity for 2,4,6trinitrotoluene (TNT) analysis with nitromethane as the reagent gas,11 while the reactive DESI could increase the selectivity for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and cyclo-1,3,5,7tetra-methylenetetranitrate (HMX) detection by adding HCl and This journal is © The Royal Society of Chemistry [year]
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trifluoroacetate into the spray solvent.25 In particular, the LTP probe based on dielectric barrier discharge was a versatile, gentle, and bipolar ionization source, which had been successfully applied to directly analyze the organic explosives such as TNT, RDX, and pentaerythritol tetranitrate (PETN) in the negative ion mode.22 Recently, the LTP probe was extended for the measurement of inorganic explosives such as black powder and firecracker.26 To improve the specificity for RDX and HMX detection, a “reactive” LTP mode was developed by Cooks and co-workers.27 It was accomplished via merging discharge gas with trifluoroacetic acid (TFA) vapour while additional adduct ion [M + TFA]– was formed to confirm the identifications. Nevertheless, this approach might cause the corrosion of electrodes which would shorten the lifetime of LTP probe because that the TFA vapour was directly flowed through the discharge electrodes. To avoid the corrosion of electrodes and retain the advantage of specificity enhancement, a reactive LTP array was newly designed by adding trifluoroacetic anhydride into the plasma plume through a separated stainless steel capillary.24 However, the detection sensitivity was not improved via above-mentioned two different reactive LTP probes. In a previous report by our group, a water-assisted low temperature plasma (WALTP) probe was presented, and its capability of enhancing the responses for RDX, PETN, and 2,4,6trinitrophenylmethylnitramine (Tetryl) has been demonstrated by introducing the wet air into the plasma stream.28 In this work, a novel dopant-assisted reactive low temperature plasma (DARLTP) probe was developed to simultaneously [journal], [year], [vol], 00–00 | 1
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Fig. 1 (a) Schematic diagram showing the configuration of dopant-assisted reactive low temperature plasma (DARLTP) probe for miniature rectilinear ion trap mass spectrometer (RIT-MS). The halogen lamp was used for the thermal desorption of explosive samples deposited on the polytetrafluoroethylene (PTFE) swab. (b) Photograph of the DARLTP probe.
improve the sensitivity and specificity for explosives detection with a miniature rectilinear ion trap mass spectrometer (RIT-MS). Two different dopant gases were tested for the DARLTP probe, and the probable mechanism for the enhancement was investigated. Finally, the repeatability and sensitivity of DARLTP probe was evaluated while an explosive mixture was detected to demonstrate the potentials of this probe for the analysis of real samples.
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Dopant-assisted reactive low temperature plasma probe The schematic diagram and photograph of DARLTP probe are illustrated in Fig. 1. Similar to the configuration of WALTP probe,28 the DARLTP was fabricated by a T shaped quartz tube (6 mm o.d. and 4 mm i.d.). A stainless steel cylinder set outside of the tube and a stainless steel rod centred axially (both above the side-tube) were employed as the high voltage (HV) electrode and the internal electrode, respectively. An alternating voltage (1.6 kVp-p, 25 kHz) was applied to the HV electrode. The discharge gas was helium, with an experimentally optimized flow rate of 150 mL min–1 for RDX and PETN detection, and the flow rate of 100 mL min–1 for Tetryl (This flow rate was chosen because the same dominant peaks of Tetryl ([Tetryl + NO3]–) can be obtained by conventional LTP and DARLTP probe at the flow rate of 100 mL min–1). The dopant gas such as dry/wet air doped with CH2Cl2 was introduced into the probe via a side-tube. The flow rate for the dopant gas was controlled at 25 mL min–1 unless otherwise specified. For comparison purpose, the side-tube of DARLTP probe was sealed to alternately obtain experimental results with a conventional LTP probe.
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Miniature rectilinear ion trap mass spectrometer The miniature RIT-MS used in the experiments had been described in detail elsewhere.26 As shown in Fig. 1a, the RIT-MS with mass range of m/z 70-600 was equipped with a discontinuous atmospheric pressure interface (DAPI),29, 30 which provided a pulse sampling method that dramatically reduced the gas intake. Analytical procedure 2 | Journal Name, [year], [vol], 00–00
A certain volume of sample solutions was dripped onto the surface of a polytetrafluoroethylene (PTFE) swab (a piece of fabric made of PTFE material) by a micropipettor. After the evaporation of solvent at room temperature, the PTFE swab was placed on the sample holder and heated for 3 s by a halogen lamp (LCB-50, Inflidge Industry Co., Ltd., Japan).26 Once the halogen lamp was turned off, the mass spectrum was recorded with single scan. Reagents, samples, and dopant gases
Experimental 15
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Methanol and dichloromethane were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China) with analytical grade. The purified water was purchased from Hangzhou Wahaha Group Co., Ltd. (Hangzhou, China). RDX, PETN, and Tetryl were purchased as 0.1 or 1 mg mL–1 solutions in methanol or methanol/acetonitrile (1:1) from AccuStandard, Inc. (New Haven, CT, USA). The explosive samples were prepared by diluting the stock solutions with pure methanol. The dopant gases used in this work were dry air doped with CH2Cl2 or wet air doped with CH2Cl2. The dry air was compressed air purified and filtrated by silica gel, activated carbon, and 13X molecular sieve traps. Its relative humidity was 0% measured by a dew point sensor (DP300, CS Instrument GMH). As shown in Fig. 1a, the wet air with relative humidity of 100% was prepared by bubbling method (namely, by flowing the dry air through purified water). The pure liquid CH2Cl2 was sealed in a vessel with a silicone cap. Three quartz capillaries were inserted into the silicone cap to allow the CH2Cl2 vapour diffusing out. The dry/wet air doped with CH2Cl2 was prepared by flowing the dry/wet air over the headspace of the CH2Cl2 vessel, with a valve to switch according to the need. The concentration of CH2Cl2 was calculated to be ca. 550 ppmv by daily consecutive weighting method.
Results and discussion
Enhancements of sensitivity and specificity by CH2Cl2 doped dry air
As dopant, chlorohydrocarbon has been employed to enhance the specificity for explosive analysis.11, 31 Herein, CH2Cl2 doped dry
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air was selected as the dopant gas for DARLTP probe, and its analytical performances were demonstrated by the measurement of three explosive samples, 0.5 ng RDX, 0.5 ng PETN, and 0.5 ng Tetryl. For comparison, these samples were also detected by the use of conventional LTP probe. As depicted in the lower part of Fig. 2, with conventional LTP, the characteristic ions of RDX and PETN are [M + NO2]– and [M + NO3]–, while the characteristic ion of Tetryl is [Tetryl + NO3]–. The middle part in Fig. 2 displays the mass spectra of three explosives acquired by the DARLTP with CH2Cl2 doped dry air as dopant gas. It is notable that the ions [M + Cl]– for RDX, PETN, and Tetryl were formed after the addition of CH2Cl2. Since the concentration of CH2Cl2 was relatively low (ca. 550 ppmv), the intensity of chloride adduct of explosives were lower than those of nitrate adduct, although the explosives such as RDX has a higher affinity for Cl– than that for NO3–. Whereas, as the relative abundances of isotopes 35Cl and 37Cl are always in a ratio of 3:1, [M + 35Cl]– and [M + 37Cl]– can be specifically characterized, which makes the qualitative analysis of explosives more accurate. What is more, the introduction of CH2Cl2 doped dry air brings an enhancement for [M + NO3]– intensity, about 3-fold (RDX), 3-fold (PETN), and 2-fold (Tetryl) respectively as high as those obtained by the conventional LTP. In addition, the intensities of [RDX + NO2]– and [PETN + NO2]– are almost decreased to zero; and the intensities of fragment ions for Tetryl are reduced to about threetwentieths (m/z 181), one-sixth (m/z 210), one-sixth (m/z 241), and one-tenth (m/z 257) of those acquired by the conventional LTP, respectively, simplifying its mass spectrum. Moreover, the
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reduction of Tetryl fragment ions by the DARLTP would avoid their serious overlaps with the deprotonated ion of 2,4View Article Online dinitrotoluene (m/z 181) and the fragmentDOI: ion 10.1039/C5AN00816F of TNT (m/z 210), which is beneficial for the qualitative analysis of mixed explosives.27, 28
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Enhancements of sensitivity and specificity by CH2Cl2 doped wet air
Since both of the WALTP probe and the DARLTP probe with CH2Cl2 doped dry air as dopant gas were able to improve the explosive response and decrease the intensities of Tetryl fragment ions,28 we predicted that further improvement of explosive response and reduction of Tetryl fragment ions could be acquired via the introduction of CH2Cl2 doped wet air. To verify this prediction, RDX, PETN, and Tetryl were analyzed by the DARLTP with CH2Cl2 doped wet air as dopant gas, and their mass spectra were shown in the top part of Fig. 2. As expected, comparing with the cases of WALTP probe28 and CH2Cl2 doped dry air as dopant gas, the explosive responses are further improved, approximately 4-fold (RDX), 4-fold (PETN), and 3fold (Tetryl) as high as those obtained by the conventional LTP, respectively. Almost no peaks are observed for the fragment ions of Tetryl as well as [RDX + NO2]– and [PETN + NO2]–, simplifying the mass spectra while avoiding the overlap of mass spectra peaks for different explosives. In addition, the formation of [M + Cl]– for these explosives is also capable of enhancing the specificity for their identification.
Fig. 2 Mass spectra of (a) 0.5 ng RDX, (b) 0.5 ng PETN, and (c) 0.5 ng Tetryl obtained by the DARLTP probe with CH2Cl2 doped wet air (top part) or CH2Cl2 doped dry air (middle part) as dopant gas and the conventional LTP probe (lower part), respectively.
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In order to understand the mechanism for the performance enhancement by the DARLTP probe, the mass spectra of reactant ions were recorded for the DARLTP with CH2Cl2 doped wet/dry air as dopant gases, as well as for the conventional LTP, respectively. As illustrated in Fig. 3, the intensity of dominant reactant ion [HNO3 + NO3]– is improved for the DARLTP with CH2Cl2 doped dry air as dopant gas, about 3 times as high as that for the conventional LTP. The improvement factor of [HNO3 + NO3]– intensity is 4-fold with CH2Cl2 doped wet air as dopant gas. Evidently, the enhancement factors of [HNO3 + NO3]– intensity are generally in agreement with those of [M + NO3]– intensity for the three explosives, which implies that the explosive product ions [M + NO3]– might be mainly generated by the reaction of their molecules M with [HNO3 + NO3]–, and the improvement of [M + NO3]– intensities was due to the enhanced intensity of [HNO3 + NO3]– in the DARLTP probe. Consistent with the atmospheric pressure corona discharge source, the reactant ions in the DARLTP probe included O2–, NO2–, and NO3–.32, 33 When CH2Cl2 doped wet air was used as dopant gas, more nitrogen oxides could be generated inside the quartz tube. The Penning ionization of H2O molecules could yield abundant OH˙ radicals,19 which would rapidly react with NO2 to form neutral HNO3.33-35 The produced HNO3 would increase [HNO3 + NO3]– via the association reaction with NO3–,35, 36 which would finally improve the [M + NO3]– intensities for the studied explosives. On the other hand, dissociative electron attachment ionization37, 38 and charge-transfer ionization39 of CH2Cl2 with O2– ions would conduce the formation of Cl– and consumption of thermal electrons and O2– respectively, which would lead to a prominent reduction of O2– while certainly attenuate NO2– as well.40 Hence, [M + Cl]– for three explosives could be formed through anion attachment with Cl–, while the intensities of [M + NO2]– adduct ions for RDX and PETN were dramatically decreased.
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RDX, PETN, and Tetryl rise rapidly with an increase of flow rate from 0 to 25 mL min–1, which are followed by the slowly View Article Online decreases with further increase of flow rate to 100 mL min–1. As DOI: 10.1039/C5AN00816F –1 the flow rate was increased from 0 to 25 mL min , the proportion of dopant gas in the plasma was increased. Hence, the amount of reactant ions and the intensities of [M + NO3]– were increased and then achieved the maximum values. After that, the intensities of [M + NO3]– decreased with further increase of flow rate due to the dilution of explosive concentration. In the flow rate of 25 mL min–1, higher enhancements for the intensities of [M + NO3]– were acquired. Therefore, the flow rate of dopant gas was selected to be 25 mL min–1 in the subsequent experiments.
Fig. 4 The relative intensities of [M + NO3]– for three explosives as a function of the flow rate of dopant gas (CH2Cl2 doped wet air).
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The intraday repeatability of DARLTP probe for explosive analysis was assessed by the measurement of 0.5 ng RDX, 0.5 ng PETN, and 0.5 ng Tetryl for 5 times each, and the relative standard deviation (RSD) of [M + NO3]– intensity was 11.0%, 11.3%, and 10.2%, respectively, which was comparable to that of WALTP (11.6%) and much better than that of conventional LTP (28.5%).28 The inter-day repeatability was also estimated by the analysis of 0.5 ng RDX for 5 days, and the RSD was 12.2%. The limits of detection (LODs) are defined as the minimum amount of explosives with responses that can be distinguished from the background (~ 20 mV), namely, with S/N ratio ≥ 3. The LODs for RDX, Tetryl, and PETN were 3, 6, and 10 pg, respectively, which were lower than those acquired using conventional LTP and WALTP probe.26, 28 Analysis of an explosives mixture
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Fig. 3 Mass spectra of the reactant ions for the DARLTP probe with CH2Cl2 doped wet air or CH2Cl2 doped dry air as dopant gas and the conventional LTP probe, respectively. 40
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Optimization of the flow rate of dopant gas In order to further improve the enhancements for explosive detection by CH2Cl2 doped wet air, its flow rate was optimized. As shown in Fig. 4, the relative intensities of [M + NO3]– for 4 | Journal Name, [year], [vol], 00–00
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As an application, the DARLTP probe was utilized to analyze a mixture consisting of 3 ng RDX, 6 ng Tetryl, and 10 ng PETN. From Fig. 5, it is clear that comparing with the case of conventional LTP, the intensities of [M + NO3]– for these explosives are all improved, while the fragment ions of Tetryl are dramatically reduced. Besides, the existence of [M + Cl]– for these explosives enhances the specificity for their identification, which makes the qualitative analysis of complicated explosive samples more reliable. On the other hand, it was observed that the signal intensity of each explosive in the mixture (Fig. 5) was lower than that of individual explosive (Fig. 2). The reason might be that during the
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ionization of a mixed sample, the competitive reactions among each explosive with reactant ions might lead to a lower ionization efficiency comparing to the case of single pure sample. Therefore, the explosive intensities in the mixture were decreased.
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Fig. 5 Mass spectra of a mixture of 3 ng RDX, 6 ng Tetryl, and 10 ng PETN obtained by the DARLTP probe with CH2Cl2 doped wet air as dopant gas and the conventional LTP probe, respectively.
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A novelly-constructed DARLTP probe has been developed and evaluated for sensitive and specific detection of explosives. When CH2Cl2 doped wet air was used as dopant gas, the DARLTP probe could improve the sensitivity, specificity, and repeatability for the analysis of explosives. In addition, the significant reductions of the fragment ions of Tetryl, as well as the adduct ions [RDX + NO2]– and [PETN + NO2]– could simplify the mass spectra and avoid the overlap of mass peaks for different explosives. The enhancement of explosive response was probably attributed to the increase of reactant ion [HNO3 + NO3]–. Finally, the successful determination of mixed explosives demonstrated that the DARLTP probe had potentials for the qualitative and quantitative analysis of complicated explosive samples.
Acknowledgements 25
This work is partially supported by NSF of China (Grants: 21375129) and the National Science & Technology Pillar Program (Grants 2013BAK14B04).
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Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. Fax: +86-411-84379517; Tel: +86-411-84379509; Email: [email protected] 1
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Analyst Accepted Manuscript
Analyst
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Analyst Accepted Manuscript
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A graphical and textual abstract for the Table of contents entry The sensitivity, specificity, and repeatability for explosives detection were improved by a dopant-assisted reactive low temperature plasma (DARLTP) probe.
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Dopant-assisted reactive low temperature plasma probe for sensitive and specific detection of explosives Wendong Chen, Keyong Hou, Lei Hua and Haiyang Li* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A dopant-assisted reactive low temperature plasma (DARLTP) probe was developed for sensitive and specific detection of explosives by a miniature rectilinear ion trap mass spectrometer. The DARLTP probe was fabricated by a T shaped quartz tube. The dopant gas was introduced into the plasma stream through a side-tube. Using CH2Cl2 doped wet air as the dopant gas, the detection sensitivities were improved to about 4-fold (RDX), 4-fold (PETN), and 3-fold (Tetryl), respectively, comparing with those obtained by the conventional LTP. Furthermore, the formation of [M + 35Cl]– and [M + 37Cl]– for these explosives enhanced the specificity for their identification. Additionally, fragment ions of Tetryl and adduct ions such as [RDX + NO2]– and [PETN + NO2]– were dramatically reduced, which simplified the mass spectra and avoided the overlap of mass peaks for different explosives. The sensitivity improvement might be attributed to the increased intensity of reactant ion [HNO3 + NO3]–, which was enhanced to 4fold after the introduction of dopant gas. The limits of detection (LODs) for RDX, Tetryl, and PETN were down to 3, 6, and 10 pg, respectively. Finally, an explosive mixture was successfully analyzed, demonstrating the potentials of DARLTP probe for qualitative and quantitative analysis of complicated explosives.
Introduction Sensitive analytical method for accurate identification of explosives is of great importance for security services1, 2 and forensic investigations.3 A variety of techniques have been developed for the sensitive detection of explosives, such as cyclic voltammetry,4 luminescence nanosensor,5 ion mobility 6-8 spectrometry, and mass spectrometry (MS).9, 10 Particularly, due to the attractive features of fast response, high sensitivity, as well as high specificity from the tandem MS analysis and ion/molecule reactions, MS has been widely used for the accurate measurement of trace explosives.11, 12 The ionization source plays an important role for the performance of MS in detecting explosives, such as sensitivity and specificity. By switching reagent ions, the proton transfer reaction and selected ion flow tube mass spectrometry have been applied to improve the selectivity and sensitivity for the explosive detection.13-16 Another kind of ionization source with similar functions is ambient ionization source. Up to date, more than thirty types of such sources have been developed, including atmospheric pressure chemical ionization (APCI),11 desorption electrospray ionization (DESI),17, 18 direct analysis in real time (DART),3, 19 flowing atmospheric-pressure afterglow (FAPA),20, 21 and low temperature plasma (LTP).22-24 Among these sources, the APCI source could enhance the specificity for 2,4,6trinitrotoluene (TNT) analysis with nitromethane as the reagent gas,11 while the reactive DESI could increase the selectivity for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and cyclo-1,3,5,7tetra-methylenetetranitrate (HMX) detection by adding HCl and This journal is © The Royal Society of Chemistry [year]
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trifluoroacetate into the spray solvent.25 In particular, the LTP probe based on dielectric barrier discharge was a versatile, gentle, and bipolar ionization source, which had been successfully applied to directly analyze the organic explosives such as TNT, RDX, and pentaerythritol tetranitrate (PETN) in the negative ion mode.22 Recently, the LTP probe was extended for the measurement of inorganic explosives such as black powder and firecracker.26 To improve the specificity for RDX and HMX detection, a “reactive” LTP mode was developed by Cooks and co-workers.27 It was accomplished via merging discharge gas with trifluoroacetic acid (TFA) vapour while additional adduct ion [M + TFA]– was formed to confirm the identifications. Nevertheless, this approach might cause the corrosion of electrodes which would shorten the lifetime of LTP probe because that the TFA vapour was directly flowed through the discharge electrodes. To avoid the corrosion of electrodes and retain the advantage of specificity enhancement, a reactive LTP array was newly designed by adding trifluoroacetic anhydride into the plasma plume through a separated stainless steel capillary.24 However, the detection sensitivity was not improved via above-mentioned two different reactive LTP probes. In a previous report by our group, a water-assisted low temperature plasma (WALTP) probe was presented, and its capability of enhancing the responses for RDX, PETN, and 2,4,6trinitrophenylmethylnitramine (Tetryl) has been demonstrated by introducing the wet air into the plasma stream.28 In this work, a novel dopant-assisted reactive low temperature plasma (DARLTP) probe was developed to simultaneously [journal], [year], [vol], 00–00 | 1
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Fig. 1 (a) Schematic diagram showing the configuration of dopant-assisted reactive low temperature plasma (DARLTP) probe for miniature rectilinear ion trap mass spectrometer (RIT-MS). The halogen lamp was used for the thermal desorption of explosive samples deposited on the polytetrafluoroethylene (PTFE) swab. (b) Photograph of the DARLTP probe.
improve the sensitivity and specificity for explosives detection with a miniature rectilinear ion trap mass spectrometer (RIT-MS). Two different dopant gases were tested for the DARLTP probe, and the probable mechanism for the enhancement was investigated. Finally, the repeatability and sensitivity of DARLTP probe was evaluated while an explosive mixture was detected to demonstrate the potentials of this probe for the analysis of real samples.
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Dopant-assisted reactive low temperature plasma probe The schematic diagram and photograph of DARLTP probe are illustrated in Fig. 1. Similar to the configuration of WALTP probe,28 the DARLTP was fabricated by a T shaped quartz tube (6 mm o.d. and 4 mm i.d.). A stainless steel cylinder set outside of the tube and a stainless steel rod centred axially (both above the side-tube) were employed as the high voltage (HV) electrode and the internal electrode, respectively. An alternating voltage (1.6 kVp-p, 25 kHz) was applied to the HV electrode. The discharge gas was helium, with an experimentally optimized flow rate of 150 mL min–1 for RDX and PETN detection, and the flow rate of 100 mL min–1 for Tetryl (This flow rate was chosen because the same dominant peaks of Tetryl ([Tetryl + NO3]–) can be obtained by conventional LTP and DARLTP probe at the flow rate of 100 mL min–1). The dopant gas such as dry/wet air doped with CH2Cl2 was introduced into the probe via a side-tube. The flow rate for the dopant gas was controlled at 25 mL min–1 unless otherwise specified. For comparison purpose, the side-tube of DARLTP probe was sealed to alternately obtain experimental results with a conventional LTP probe.
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Miniature rectilinear ion trap mass spectrometer The miniature RIT-MS used in the experiments had been described in detail elsewhere.26 As shown in Fig. 1a, the RIT-MS with mass range of m/z 70-600 was equipped with a discontinuous atmospheric pressure interface (DAPI),29, 30 which provided a pulse sampling method that dramatically reduced the gas intake. Analytical procedure 2 | Journal Name, [year], [vol], 00–00
A certain volume of sample solutions was dripped onto the surface of a polytetrafluoroethylene (PTFE) swab (a piece of fabric made of PTFE material) by a micropipettor. After the evaporation of solvent at room temperature, the PTFE swab was placed on the sample holder and heated for 3 s by a halogen lamp (LCB-50, Inflidge Industry Co., Ltd., Japan).26 Once the halogen lamp was turned off, the mass spectrum was recorded with single scan. Reagents, samples, and dopant gases
Experimental 15
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Methanol and dichloromethane were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China) with analytical grade. The purified water was purchased from Hangzhou Wahaha Group Co., Ltd. (Hangzhou, China). RDX, PETN, and Tetryl were purchased as 0.1 or 1 mg mL–1 solutions in methanol or methanol/acetonitrile (1:1) from AccuStandard, Inc. (New Haven, CT, USA). The explosive samples were prepared by diluting the stock solutions with pure methanol. The dopant gases used in this work were dry air doped with CH2Cl2 or wet air doped with CH2Cl2. The dry air was compressed air purified and filtrated by silica gel, activated carbon, and 13X molecular sieve traps. Its relative humidity was 0% measured by a dew point sensor (DP300, CS Instrument GMH). As shown in Fig. 1a, the wet air with relative humidity of 100% was prepared by bubbling method (namely, by flowing the dry air through purified water). The pure liquid CH2Cl2 was sealed in a vessel with a silicone cap. Three quartz capillaries were inserted into the silicone cap to allow the CH2Cl2 vapour diffusing out. The dry/wet air doped with CH2Cl2 was prepared by flowing the dry/wet air over the headspace of the CH2Cl2 vessel, with a valve to switch according to the need. The concentration of CH2Cl2 was calculated to be ca. 550 ppmv by daily consecutive weighting method.
Results and discussion
Enhancements of sensitivity and specificity by CH2Cl2 doped dry air
As dopant, chlorohydrocarbon has been employed to enhance the specificity for explosive analysis.11, 31 Herein, CH2Cl2 doped dry
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air was selected as the dopant gas for DARLTP probe, and its analytical performances were demonstrated by the measurement of three explosive samples, 0.5 ng RDX, 0.5 ng PETN, and 0.5 ng Tetryl. For comparison, these samples were also detected by the use of conventional LTP probe. As depicted in the lower part of Fig. 2, with conventional LTP, the characteristic ions of RDX and PETN are [M + NO2]– and [M + NO3]–, while the characteristic ion of Tetryl is [Tetryl + NO3]–. The middle part in Fig. 2 displays the mass spectra of three explosives acquired by the DARLTP with CH2Cl2 doped dry air as dopant gas. It is notable that the ions [M + Cl]– for RDX, PETN, and Tetryl were formed after the addition of CH2Cl2. Since the concentration of CH2Cl2 was relatively low (ca. 550 ppmv), the intensity of chloride adduct of explosives were lower than those of nitrate adduct, although the explosives such as RDX has a higher affinity for Cl– than that for NO3–. Whereas, as the relative abundances of isotopes 35Cl and 37Cl are always in a ratio of 3:1, [M + 35Cl]– and [M + 37Cl]– can be specifically characterized, which makes the qualitative analysis of explosives more accurate. What is more, the introduction of CH2Cl2 doped dry air brings an enhancement for [M + NO3]– intensity, about 3-fold (RDX), 3-fold (PETN), and 2-fold (Tetryl) respectively as high as those obtained by the conventional LTP. In addition, the intensities of [RDX + NO2]– and [PETN + NO2]– are almost decreased to zero; and the intensities of fragment ions for Tetryl are reduced to about threetwentieths (m/z 181), one-sixth (m/z 210), one-sixth (m/z 241), and one-tenth (m/z 257) of those acquired by the conventional LTP, respectively, simplifying its mass spectrum. Moreover, the
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reduction of Tetryl fragment ions by the DARLTP would avoid their serious overlaps with the deprotonated ion of 2,4View Article Online dinitrotoluene (m/z 181) and the fragmentDOI: ion 10.1039/C5AN00816F of TNT (m/z 210), which is beneficial for the qualitative analysis of mixed explosives.27, 28
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Enhancements of sensitivity and specificity by CH2Cl2 doped wet air
Since both of the WALTP probe and the DARLTP probe with CH2Cl2 doped dry air as dopant gas were able to improve the explosive response and decrease the intensities of Tetryl fragment ions,28 we predicted that further improvement of explosive response and reduction of Tetryl fragment ions could be acquired via the introduction of CH2Cl2 doped wet air. To verify this prediction, RDX, PETN, and Tetryl were analyzed by the DARLTP with CH2Cl2 doped wet air as dopant gas, and their mass spectra were shown in the top part of Fig. 2. As expected, comparing with the cases of WALTP probe28 and CH2Cl2 doped dry air as dopant gas, the explosive responses are further improved, approximately 4-fold (RDX), 4-fold (PETN), and 3fold (Tetryl) as high as those obtained by the conventional LTP, respectively. Almost no peaks are observed for the fragment ions of Tetryl as well as [RDX + NO2]– and [PETN + NO2]–, simplifying the mass spectra while avoiding the overlap of mass spectra peaks for different explosives. In addition, the formation of [M + Cl]– for these explosives is also capable of enhancing the specificity for their identification.
Fig. 2 Mass spectra of (a) 0.5 ng RDX, (b) 0.5 ng PETN, and (c) 0.5 ng Tetryl obtained by the DARLTP probe with CH2Cl2 doped wet air (top part) or CH2Cl2 doped dry air (middle part) as dopant gas and the conventional LTP probe (lower part), respectively.
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Proposed mechanism for the enhancements
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In order to understand the mechanism for the performance enhancement by the DARLTP probe, the mass spectra of reactant ions were recorded for the DARLTP with CH2Cl2 doped wet/dry air as dopant gases, as well as for the conventional LTP, respectively. As illustrated in Fig. 3, the intensity of dominant reactant ion [HNO3 + NO3]– is improved for the DARLTP with CH2Cl2 doped dry air as dopant gas, about 3 times as high as that for the conventional LTP. The improvement factor of [HNO3 + NO3]– intensity is 4-fold with CH2Cl2 doped wet air as dopant gas. Evidently, the enhancement factors of [HNO3 + NO3]– intensity are generally in agreement with those of [M + NO3]– intensity for the three explosives, which implies that the explosive product ions [M + NO3]– might be mainly generated by the reaction of their molecules M with [HNO3 + NO3]–, and the improvement of [M + NO3]– intensities was due to the enhanced intensity of [HNO3 + NO3]– in the DARLTP probe. Consistent with the atmospheric pressure corona discharge source, the reactant ions in the DARLTP probe included O2–, NO2–, and NO3–.32, 33 When CH2Cl2 doped wet air was used as dopant gas, more nitrogen oxides could be generated inside the quartz tube. The Penning ionization of H2O molecules could yield abundant OH˙ radicals,19 which would rapidly react with NO2 to form neutral HNO3.33-35 The produced HNO3 would increase [HNO3 + NO3]– via the association reaction with NO3–,35, 36 which would finally improve the [M + NO3]– intensities for the studied explosives. On the other hand, dissociative electron attachment ionization37, 38 and charge-transfer ionization39 of CH2Cl2 with O2– ions would conduce the formation of Cl– and consumption of thermal electrons and O2– respectively, which would lead to a prominent reduction of O2– while certainly attenuate NO2– as well.40 Hence, [M + Cl]– for three explosives could be formed through anion attachment with Cl–, while the intensities of [M + NO2]– adduct ions for RDX and PETN were dramatically decreased.
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RDX, PETN, and Tetryl rise rapidly with an increase of flow rate from 0 to 25 mL min–1, which are followed by the slowly View Article Online decreases with further increase of flow rate to 100 mL min–1. As DOI: 10.1039/C5AN00816F –1 the flow rate was increased from 0 to 25 mL min , the proportion of dopant gas in the plasma was increased. Hence, the amount of reactant ions and the intensities of [M + NO3]– were increased and then achieved the maximum values. After that, the intensities of [M + NO3]– decreased with further increase of flow rate due to the dilution of explosive concentration. In the flow rate of 25 mL min–1, higher enhancements for the intensities of [M + NO3]– were acquired. Therefore, the flow rate of dopant gas was selected to be 25 mL min–1 in the subsequent experiments.
Fig. 4 The relative intensities of [M + NO3]– for three explosives as a function of the flow rate of dopant gas (CH2Cl2 doped wet air).
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The intraday repeatability of DARLTP probe for explosive analysis was assessed by the measurement of 0.5 ng RDX, 0.5 ng PETN, and 0.5 ng Tetryl for 5 times each, and the relative standard deviation (RSD) of [M + NO3]– intensity was 11.0%, 11.3%, and 10.2%, respectively, which was comparable to that of WALTP (11.6%) and much better than that of conventional LTP (28.5%).28 The inter-day repeatability was also estimated by the analysis of 0.5 ng RDX for 5 days, and the RSD was 12.2%. The limits of detection (LODs) are defined as the minimum amount of explosives with responses that can be distinguished from the background (~ 20 mV), namely, with S/N ratio ≥ 3. The LODs for RDX, Tetryl, and PETN were 3, 6, and 10 pg, respectively, which were lower than those acquired using conventional LTP and WALTP probe.26, 28 Analysis of an explosives mixture
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Fig. 3 Mass spectra of the reactant ions for the DARLTP probe with CH2Cl2 doped wet air or CH2Cl2 doped dry air as dopant gas and the conventional LTP probe, respectively. 40
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Optimization of the flow rate of dopant gas In order to further improve the enhancements for explosive detection by CH2Cl2 doped wet air, its flow rate was optimized. As shown in Fig. 4, the relative intensities of [M + NO3]– for 4 | Journal Name, [year], [vol], 00–00
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As an application, the DARLTP probe was utilized to analyze a mixture consisting of 3 ng RDX, 6 ng Tetryl, and 10 ng PETN. From Fig. 5, it is clear that comparing with the case of conventional LTP, the intensities of [M + NO3]– for these explosives are all improved, while the fragment ions of Tetryl are dramatically reduced. Besides, the existence of [M + Cl]– for these explosives enhances the specificity for their identification, which makes the qualitative analysis of complicated explosive samples more reliable. On the other hand, it was observed that the signal intensity of each explosive in the mixture (Fig. 5) was lower than that of individual explosive (Fig. 2). The reason might be that during the
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ionization of a mixed sample, the competitive reactions among each explosive with reactant ions might lead to a lower ionization efficiency comparing to the case of single pure sample. Therefore, the explosive intensities in the mixture were decreased.
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Fig. 5 Mass spectra of a mixture of 3 ng RDX, 6 ng Tetryl, and 10 ng PETN obtained by the DARLTP probe with CH2Cl2 doped wet air as dopant gas and the conventional LTP probe, respectively.
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A novelly-constructed DARLTP probe has been developed and evaluated for sensitive and specific detection of explosives. When CH2Cl2 doped wet air was used as dopant gas, the DARLTP probe could improve the sensitivity, specificity, and repeatability for the analysis of explosives. In addition, the significant reductions of the fragment ions of Tetryl, as well as the adduct ions [RDX + NO2]– and [PETN + NO2]– could simplify the mass spectra and avoid the overlap of mass peaks for different explosives. The enhancement of explosive response was probably attributed to the increase of reactant ion [HNO3 + NO3]–. Finally, the successful determination of mixed explosives demonstrated that the DARLTP probe had potentials for the qualitative and quantitative analysis of complicated explosive samples.
Acknowledgements 25
This work is partially supported by NSF of China (Grants: 21375129) and the National Science & Technology Pillar Program (Grants 2013BAK14B04).
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Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. Fax: +86-411-84379517; Tel: +86-411-84379509; Email: [email protected] 1
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