Thermal decomposition of the chloride and nitrate adducts of pentaerythritol tetranitrate in air at ambient pressure using a cross flow design tandem ion mobility spectrometry
Graphical abstract
Introduction
Chloride adducts of explosives in the gas phase form the basis for response in ion mobility spectrometry (IMS) used to screen hand-carried personal items at airports worldwide [[1], [2], [3]]. In common embodiments of IMS-based analyzers, explosive particulate obtained from residue on the surface of an object is vaporized into the reaction region of an IMS drift tube [[4], [5], [6]]. In the reaction region, explosives are ionized through association reactions with Cl¯, formed from a chlorocarbon precursor [7,8]. All association reactions are exothermic and mobility spectra or mass spectra should exhibit the Cl¯ adduct as the major ion, in the absence of other processes at suitably low temperatures [1]. Since adducts are temperature sensitive, mobility spectra routinely contain ions that are understood to arise from decomposition of the explosive during thermal desorption, or from decomposition of the Cl¯ adduct in the drift tube of the IMS analyzer [[9], [10], [11]]. The uncertainty of the temperature ranges over which Cl¯ adducts of nitrate and nitrite explosives are stable led to the initiation of a program in our laboratory to determine the energetics of their dissociation [[12], [13], [14]]. Ion decompositions at ambient pressure were studied using tandem ion mobility spectrometers to mobility select an ion in a first drift region and then to pass the selected ion into a final drift stage where thermal decomposition causes baseline distortions in the mobility spectra. While this method was developed for dissociation of proton bound dimers [15], the method has been used to explore reaction enthalpies for chloride adducts of explosives [[12], [13], [14],16,17]. In this method, rate constants are extracted from the baseline profile between an ion and the decomposition product ion. Spectra obtained at temperatures over relatively narrow ranges of 10 °C–15 °C provide the energy of activation, Ea, for thermal decomposition. Two main decomposition pathways are observed for chloride adducts, Cl¯ displacement of NO3¯ with nitrate explosives and loss of Cl¯ with nitro-containing explosives. Experimental values for Ea using the baseline methods differed on averaged by ∼6% from those obtained using density funcational theory (DFT) models [13]. The measurements were made under the same conditions as found in IMS-based explosive trace detectors (ETDs), making the findings directly applicable to instruments in-service world-wide at airport security check points. The extent of the study was limited by the fact that only eight of twenty explosives had the necessary vapor pressure, thermal stability, ion stability, or simplicity of decomposition pathway for Ea determinations. Consequently, the baseline method with tandem ion mobility spectrometer as developed was not directly useful to characterize chloride adducts of some explosives of high significance.
Pentaerythritol tetranitrate (PETN) is an exemplary nitrate explosive whose thermal decomposition is too extensive to permit the creation of sufficiently abundant Cl¯ adducts by the method described above. Under vacuum, solid PETN decomposition begins at ca. 75 °C with Ea = 192 ± 5 kJ mol−1, determined between 75 °C and 130 °C [18]. The distribution of mass-analyzed decomposition products was shown to be temperature dependent and findings suggested that PETN decomposed in the vapor phase. The primary process for decomposition was attributed to the rupture of a RO-NO2 bond and to a later elimination of formaldehyde. This cleavage results in multiple pathways for decomposition that are dependent on reaction conditions and are driven by autocatalytic reactions [19]. In practical studies, the response to PETN in an IMS-based commercial ETD, equipped with a heated anvil inlet at ambient pressure, exhibited a maximum (for intact PETN) at 100 °C, above which response was significantly lessened and attributed to thermal decomposition [6]. Vaporization and elution of PETN through a gas chromatographic column with low time of residence and high carrier gas flow rate showed a maximum electron capture detector response at 175 °C but mass spectrometry showed extensive ion fragmentation [19].
Gaseous ions from PETN can be formed directly from solution at ambient pressure using a selection of ion sources without need for vaporization of solid PETN. Principal among these is electrospray ionization [[20], [21], [22]]. PETN adducts may be formed using solutions containing suitable anions such as acetate and formate in addition to chloride [20,23]. Chloride adducts were produced through the addition of ammonium chloride to the solvent and PETN∙Cl¯ examined by MS/MS produced NO3¯ [21]. In a chemical ionization (CI) source with methane as the reagent gas, PETN alone produced NO3¯ and PETN∙NO3¯ and, on the addition of a chlorine-containing compound, a Cl¯ adduct was produced in amount comparable to the NO3¯ adduct [24]. In a corona discharge ion source PETN desorbed into a chlorocarbon vapor yielded a chloride adduct [25]. In contrast, Xu et al. found [PETN-H]¯ and PETN∙NO3¯ with APCI [23] while Kinghorn found m/z 316 (C6H10N3O12), which was proposed as a formate adduct with trinitropentaerythritol [26]. In the absense of additives, NO3¯ and nitrite adducts were formed. In summary, PETN∙Cl¯ has been formed in several soft ionization sources when a chlorine-containing reagent was introduced to the ionization volume. Significant amounts of NO3¯ or NO3¯ adduct are also observed, showing that the instability of PETN and/or PETN∙Cl¯ under experimental conditions in ESI, APCI, and CI sources is significant and probably unavoidable.
Electrospray ionization would be the method of choice for the formation of PETN∙Cl¯, however, a temperature of around 200 °C was required for stable signals with the IMS drift tube and precluded the observation of ion thermal decomposition at the required lower temperatures. The objective of this study was to investigate the thermal stability of PETN∙Cl ¯ and to determine the activation energy for its thermal decomposition. In addition, the thermal properties of the ubiquitous nitrate adduct, PETN∙NO3¯ were examined. A new ion mobility spectrometer design for PETN∙Cl¯ formation is described that has pre-formation of Cl¯ reactant ion that subsequently reacts with PETN introduced via evaporation from solution delivered from a nebulizer source.
Section snippets
Instrumentation
A schematic diagram of the tandem ion mobility spectrometer equipped with a 10 mCi 63Ni foil beta source is shown in Fig. 1(a). The two ion shutters, 4.9 cm apart allow, with the second shutter opening at an appropriate time after the first, the gating of chosen mobility separated ions into the final, 9.1 cm long, drift region. The drift tube structure is similar to that previously described [14] with a slight difference in the drift ring dimensions, 2.85 mm thick x 18.9 mm ID x 38.1 mm OD. A
Results
The Cl¯ reactant ion was produced by the dissociative capture of thermalized electrons by CCl4 in the source region. The mobility spectrum in Fig. 2(a) was obtained at 155 °C using ion shutter 2 with ion shutter 1 open and nebulizer input of PETN. The ion identities are assigned by their reduced mobility coefficients (Ko), measured with a reference Ko of TNT (= 1.54 cm2 V−1 s−1) nebulized separately. The Ko values (cm2 V−1 s−1) compared with those reported from IMS-MS experiments [6,7] in
Models
The structures of PETN∙Cl¯ and PETN∙NO3¯ were modeled using DFT and the B3LYP functional with the 6-311+ G(d,p) basis set, as were the intermediate structures and final products of their thermal dissociations. The energies, electronic plus zero-point energy, of the steps in the determined reaction paths are presented in Fig. 5 and given in Table 1, Table 2. Each structure, except those at the central energy barriers, was at a minimum on the potential energy surface with no imaginary vibrational
Discussion
The cross-flow design of the dual shutter mobility instrument physically separates reactant ion formation from analyte introduction. Analyte introduction using a nebulizer allows the operation of an atmospheric ion mobility spectrometer at lower temperatures than with an ESI source, often the preferred method of ion formation with low vapor pressure analytes. Whereas ESI requires a temperature of around 200 °C for the efficient desolvation of the ions produced at atmospheric pressure, the
Conclusions
A new design of an ion mobility spectrometer has analyte introduced downstream from the ion source and orthogonal to the reactant ion drift direction. The nebulizer sample introduction allows operation of the drift tube at temperatures lower than required for electrospray ion desolvation and is a method for the introduction of low vapor pressure explosives that are themselves thermally unstable as solids or produce thermally unstable adducts. PETN produces two ion-molecule adducts with chloride
Author statement
All three authors contributed to the concepts in this manuscript. BG performed the experiments and drafted the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
“This material is based upon work supported in part by the U.S. Department of Homeland Security, Science and Technology Directorate, Office of University Programs, under Grant Award 2013-ST-061-ED0001. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Department of Homeland Security.” The grant was administered first by Center for Excellence for
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