Rapid identification and desorption mechanisms of nitrogen-based explosives by ambient micro-fabricated glow discharge plasma desorption/ionization (MFGDP) mass spectrometry
Graphical abstract
In consider of great advantages for in situ, on-line, high throughput detection and fast identification of explosives, micro-fabricated glow discharge plasma desorption/ionization mass spectrometry in negative ion mode (NI-MFGDP-MS) was used to identify explosives in open air. The capability and the reliability of fast identification of explosives at atmospheric pressure have been successfully demonstrated with NI-MFGDP-MS and the method constructed in this paper has a good performance on quantitative analysis. The mechanisms of desorption of explosives in this system were also explored to identify fast and accurately. The results provide a guideline and a supplement to chemical libraries for the rapid and accurate identification of explosives in the field of antiterrorism and environmental conservation.
Introduction
The efficient detection and accurate identification of explosives have become increasingly important due to the urgent needs of forensic investigations, security services and environmental monitoring [1], [2], [3], [4]. Meanwhile, the detection techniques to identify explosives rapidly and accurately from complex compounds even from real samples attract more attentions.
A number of analysis techniques including luminescence nano-sensors [5], near-field optical microscopy [6], raman spectroscopy [7] and ion mobility spectrometry [8], [9], [10] had already been developed and applied to detect explosives. However, the techniques mentioned above often have the deficiency of being costly, strong background noise and complicated procedure. Herein, a series of alternative methods for explosive analysis were proposed. Mass spectrometry (MS) has been of growing interest in recent years because of its inherent advantages of high sensitivity, good reproducibility and on-line detection of the analytes. Traditionally, gas chromatography/mass spectrometry (GC/MS) or liquid chromatography/mass spectrometry (LC/MS) had been used to detect explosives [3], [11]. However, both of them are time consuming due to complicated pretreatment and separation processes. Besides, the application of GC/MS is limited to thermostable explosives, whereas LC/MS is suitable for low vapor pressures or thermally labile compounds. Electrospray ionization (ESI) [12], [13], [14] and atmospheric pressure chemical ionization (APCI) [15], [16] mass spectrometry were representative techniques with the separation process for the detection of explosives during the period from the late of 20th century to the early of 21st century, and they have been employed by several research groups in the analysis of explosives. Some explosives, such as 2-methyl-1,3,5-trinitrobenzene (trinitrotoluene, TNT), 1,3,5-trinitro-perhydro-1,3,5-triazine (cyclonite, RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (octogen, HMX), pentaerythritol tetranitrate (PETN), glycerol trinitrate (nitroglycerin), ethylene glycol dinitrate (EGDN), and 2,4,6,N-Tetranitro-N-methylaniline (tetryl), were detected [12]. In spite of a wide range of mass spectra information of explosives have been obtained, limited sample size, complex sample preparation and the sample status make it challenging for direct detection by these mass spectrometric methods [17], [18]. Therefore, it is in urgent need to develop reliable, useable, fast, on-line and in situ detection techniques without complicated sample preparation to satisfy the increasing need of efficient and rapid detection of explosives.
Ambient mass spectrometry (AMS) is recognized as an extremely effective technique to detect explosives because of its fast response, high-throughput, high tolerance of impurities, high specificity, high sensitivity and no complicated or time-consuming sample preparation steps [3], [19], [20], [21], [22], [23], [24]. In ambient ionization techniques, the ionization/desorption sources are placed inline or incline with the MS inlet without any linked unit and the sample are ionized (and even desorbed) in ambient environment without any enclosure [19], [25]. With the in situ analysis from the contaminated surface, no separation process was needed for analyses enrichment. More importantly, the analysis time was shortened as no or little prior treatment was involved [22]. Since the introduction of desorption electrospray ionization (DESI) [26], AMS has become a hot spot in mass spectrometry. A series of newly developed ionization techniques, such as dielectric barrier discharge ionization (DBDI) [1], desorption atmospheric pressure chemical ionization (DAPCI) [27], low temperature plasma (LTP) [28], desorption atmospheric pressure photoionization (DAPPI) [20], microwave-induced plasma desorption/ionization source (MIPDI) [29], micro-fabricated glow discharge plasma desorption/ionization source (MFGDP) [30], [31] were successfully developed. Some of them have attracted more attentions for the detection of explosives and their ionization mechanisms are increasingly explicit. With DART [32], [33], in the negative ion mode, thermal electrons are produced by the collision between electrons generated by glow discharge and gas molecules in open air. And then, the thermal electrons are transmitted to the atmospheric oxygen generating [O2]-. These negative ions further react with analytes to yield sample ions.
Micro-fabricated glow discharge plasma desorption/ionization (MFGDP) mass spectrometry has been proposed by our previous work [30]. MFGDP can generate stable plasma at ambient conditions using either argon or helium as discharge gas by DC micro glow discharge. It has been proven to be efficient to analyze gas, liquid, solid, and creamy samples with molecular weight up to 1.5 kDa. Pharmaceuticals, amino acids, cholesterol, urea, agrochemicals, the extracts of fruits and vegetables, pesticide residues in fruits and vegetables and many other small molecular weight compounds were detected with good performance while surface shapes of the sample were ignored, and strong adduct ions and fragment ions were observed in some samples [30], [34]. Mass spectrometry imaging was also performed using MFGDP [35]. There is no heating damage to the real sample surface during analysis because of the low temperature of plasma flame. Similar to DART [36],corona-to-glow atmospheric discharge ion sources [37] and DBDI [38], protonated water clusters [(H2O)nH]+(n=2–5) were found when Ar or He acts as discharge gas. The hydronium ions play an important role in the protonation of sample molecules in positive ion mode. Extensive details on the composition of MFGDP, ionization mechanism, pathways and the assessment of analysis ability can be found in the literature [30], [34], [35].
Although various applications and ionization mechanism of MFGDP in positive ion mode have been studied, the performance of MFGDP in negative ion mode (NI-MFGDP) is still uncertain. Therefore, in this study, the nitro-based explosives were analyzed to investigate the performance of MFGDP in negative ion mode. Meanwhile, a mixture of three explosives and a mixture of one explosive with pond water were detected to evaluate the reliability and the potential of this system. The detection of six explosives was also performed by ESI-MS in negative ion mode in present work to provide a useful reference to analyze the data of NI-MFGDP-MS and to judge the formation mechanism of the major ions.
Section snippets
Chemicals and samples
2,4,6-Trinitrophenol (picric acid) and methanol (HPLC-grade) were purchased from Sigma-Aldrich(Steinheim, Germany). 2-Methyl-1,3,5-trinitrobenzene (trinitrotoluene, TNT), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(octogen, HMX) ,1,3,5-trinitroperhydro-1,3,5-triazine (cyclonite, RDX), 1,2,3-trinitroxypropane (nitroglycerin, NG) and pentaerythritol tetranitrate (PETN) were all purchased from AccuStandard Inc.(New Haven, CT). All samples were diluted with methanol before analysis. All other
Optimization of experimental parameters
TNT, picric acid, RDX, HMX, NG and PETN were used as target explosive samples (Fig. 2). The dominant ions of studied explosives were regarded as target ions during the processes of optimization. However, different ion transfer capillary temperatures were found to have different abundant ions intensities (Fig. 3). The phenomenon is believed to be relevant to the discrepant melting point, detonation point and vapor pressure.
The flow rate of discharge gas is also an important factor for the MFGDP.
Conclusions
Extensive MS and MS/MS analyses of six common explosives, a picric acid-RDX-PETN mixture and a mixture of RDX-pond water were performed using MFGDP with ion trap mass spectrometer in negative ion mode. The desorption mechanisms of several nitro-compounds were explored including aromatic ring nitro-compounds, atrazine ring nitro-compounds and linear nitro-compounds. The results show that the nitro compounds with high gas-phase acidities ionized via electron capture and proton abstraction
Safety hazard note
The MFGDP device calls for DC voltage up to several hundred volts to generate the plasma, thus care should be taken in order to avoid electric shot.
Acknowledgments
The authors are grateful to the Research Center of Analytical Instrumentation of Sichuan University for providing support for all of the devices and materials required for this work. They also thank Tian Yonghui, Niu Guanghui, Dai Jianxiong and Song Hao for their valuable advices.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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