Elsevier

Talanta

Volume 167, 15 May 2017, Pages 75-85
Talanta

Rapid identification and desorption mechanisms of nitrogen-based explosives by ambient micro-fabricated glow discharge plasma desorption/ionization (MFGDP) mass spectrometry

https://doi.org/10.1016/j.talanta.2017.02.011Get rights and content

Highlights

  • The method proposed in this study can achieve in situ, on-line and fast detection (<5 s) of nitrogen-based explosives.

  • The method has a good performance on the selectivity and reliability to identify nitrogen-based explosives.

  • The limits of detection are at the level of pg mm−2 and even at fg mm−2. The recoveries of them are from 97.5% to 103.2%.

  • The detection of nitrogen-based explosives are free from the influence of the effect of the matrix by this method.

  • The article provides a guideline to explore the mechanisms of desorption.

Abstract

A novel technique of micro-fabricated glow discharge plasma desorption/ionization mass spectrometry was investigated for the first time in negative ion mode in this study. Negative ion micro-fabricated glow discharge plasma desorption/ionization mass spectrometry (NI-MFGDP-MS) was successfully applied to identify trace explosives in open air. Six explosives and explosives-related compounds were directly analyzed in seconds with this ion source. The ions of [M-H]- were predominant for 2-methyl-1,3,5-trinitrobenzene (trinitrotoluene, TNT) and 2,4,6-trinitrophenol (picric acid), and [M+NO3]- were dominant ions for 1,3,5-trinitro-perhydro-1,3,5-triazine (cyclonite, RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (octogen, HMX), 1,2,3-trinitroxypropane (nitroglycerin, NG), and pentaerythritol tetranitrate (PETN). The limits of detection (LOD) were from 87.5 pg mm−2 to 0.4 fg mm−2 and the relative standard deviation (RSD) ranged between 5.8% and 16.8% for the explosives involved in this study. The reliability of NI-MFGDP-MS was characterized by the analysis of a picric acid-RDX-PETN mixture and a mixture of RDX-pond water. NI-MFGDP-MS and ESI-MS were compared with these explosives and along with collision induced dissociation (CID) experiments. The results showed that electron capture, proton abstraction reaction, nucleophilic attack, ion–molecule attachment, decomposition and anion attachment took place during the NI-MFGDP-MS measurement. These findings provide a guideline and a supplement to the chemical libraries for rapid and accurate detection of explosives. The method shows great potential for fast, in situ, on-line and high throughput detection of explosives in the field of antiterrorism.

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.

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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.

References (56)

  • C.Y. Tian et al.

    Preliminary study of microfabricated glow discharge plasma for mass spectrometry imaging

    Chin. J. Anal. Chem.

    (2016)
  • J.T. Shelley et al.

    Characterization of direct-current atmospheric-pressure discharges useful for ambient desorption/ionization mass spectrometry

    J. Am. Soc. Mass Spectrom.

    (2009)
  • J. Kratzer et al.

    Comparison of dielectric barrier discharge, atmospheric pressure radiofrequency-driven glow discharge and direct analysis in real time sources for ambient mass spectrometry of acetaminophen

    Spectrochim. Acta B At. Spectrosc.

    (2011)
  • R.G. Ewing et al.

    A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds

    Talanta

    (2001)
  • N. Na et al.

    Direct detection of explosives on solid surfaces by mass spectrometry with an ambient ion source based on dielectric barrier discharge

    J. Mass Spectrom.

    (2007)
  • A.L. Juhasz et al.

    Explosives: fate, dynamics, and ecological impact in terrestrial and marine environments

    Rev. Environ. Contam. Toxicol.

    (2007)
  • M. Mäkinen et al.

    Ion spectrometric detection technologies for ultra-traces of explosives: a review

    Mass Spectrom. Rev.

    (2011)
  • G.P. Anderson et al.

    TNT detection using multiplexed liquid array displacement immunoassays

    Anal. Chem.

    (2006)
  • M. Sabo et al.

    Laser desorption with corona discharge ion mobility spectrometry for direct surface detection of explosives

    Analyst

    (2014)
  • H.E. Cullum et al.

    A second survey of high explosives traces in public places

    J. Forensic Sci.

    (2004)
  • J. Yinon et al.

    Electrospray ionization tandem mass spectrometry collision-induced dissociation study of explosives in an ion trap mass spectrometer

    Rapid Commun. Mass Spectrom.

    (1997)
  • M.Y.-M. Wong et al.

    Negative electrospray ionization on porous supporting tips for mass spectrometric analysis: electrostatic charging effect on detection sensitivity and its application to explosive detection

    Analyst

    (2014)
  • A.-C. Schmidt et al.

    Investigation of the ionisation and fragmentation behaviour of different nitroaromatic compounds occurring as polar metabolites of explosives using electrospray ionisation tandem mass spectrometry

    Rapid Commun. Mass Spectrom.

    (2006)
  • C.S. Evans et al.

    A rapid and efficient mass spectrometric method for the analysis of explosives

    Rapid Commun. Mass Spectrom.

    (2002)
  • G. Greg et al.

    Characterization of high explosive particles using cluster secondary ion mass spectrometry

    Rapid Commun. Mass Spectrom. Rcm

    (2006)
  • I. Cotte-Rodríguez et al.

    desorption electrospray ionization of explosives on surfaces: sensitivity and selectivity enhancement by reactive desorption electrospray ionization

    Anal. Chem.

    (2005)
  • X.L. Ding et al.

    Plasma-based ambient mass spectrometry techniques: the current status and future prospective

    Mass Spectrom. Rev.

    (2015)
  • T.J. Kauppila et al.

    Analysis of nitrogen-based explosives with desorption atmospheric pressure photoionization mass spectrometry

    Rapid Commun. Mass Spectrom.

    (2016)
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