Uso combinado de análisis directo en tiempo real/ espectrometría de masa Orbitrap y espectroscopía micro-Raman para la completa caracterización de muestras reales de explosivos

This work aims to develop a method that uses micro-Raman spectroscopy in combination with a DART mass spectral approach for the analysis of swab and wipe samples collected for the screening of explosives. The investigated samples are listed in Table 1. Our workflow of sample preparation intentionally excluded the time-consuming steps that would normally be required for either lixiviation of the analytes from the wipes and swabs or solid-phase extraction to separate the compounds from the matrix components prior to LC–MS analysis. Rather, we chose to use a vacuum impactor (Fig. 1) to collect micrometric explosive particles from the surface of the swab and wipe samples on an impaction plate made of carbon [10]. The transfer of particles from a wipe or swab sample to the impaction plate lasts only about 10 min. After sample collection, the carbon impaction plate was directly analyzed by micro-Raman spectrometry and then exposed directly to the metastable helium atoms (heated to 350 °C) of the DART ionization source coupled to an LTQ Orbitrap mass spectrometer. This study builds upon a small number of previous investigations that have demonstrated the potential of DART-MS for the analysis of reference explosive compounds spiked in various matrices [2, 3, 11]. Moreover, we recently showed that DART hyphenated to an Orbitrap mass analyzer permitted the direct and rapid characterization of synthetic and natural homopolymers whose molecular weights fall between 200 and 4000 Da [12]. The approach provided useful information concerning polymer compositions such as molecular weight distributions, the nature of terminal functionalities, and oligomeric sequences.

Raman spectroscopy provides a unique molecular spectral fingerprint, with each spectrum containing key signature bands that can be used for identification. All Raman spectra were acquired in the range 100–3200 cm⁻¹ to enable observation of most Raman bands for common explosives [5–7]. Raman microscopy was applied to focus the laser beam on micrometric targets and collect the Raman scattering from the explosive crystals collected by vacuum impactor directly onto a carbon disk. The NIR laser at 785 nm gave good-quality spectra of explosives, with limited background fluorescence.

Figure 2a displays the Raman microscopy image of a cluster of energetic crystals collected from the surface of sample 3 (wipe of a submunition). Crystal sizes are typically between 10 and 50 μm. The wavenumbers and relative intensities of peaks observed in the acquired Raman spectrum, represented in red, match well with those of the reference Raman spectrum of trinitrotoluene (TNT) from the Renishaw library (overlaid in blue). The observed peaks can be attributed to the vibrational modes of TNT [4, 5, 7, 13, 14], including characteristic symmetric NO₂ stretching vibrations at approximately 1360 cm⁻¹ and the asymmetric NO₂ stretching vibrations at 1534 and 1618 cm⁻¹. The shoulder at 1380 cm⁻¹ may be assigned to the –CH₃ symmetric bend, and its presence sets TNT apart from most of the explosives belonging to the nitro aromatic group. The other peaks can be assigned to the NO₂ scissoring modes (790, 820 cm⁻¹), to the –CH(ring) bending modes (800–1100 cm⁻¹), and to the ring-breathing mode (1210 cm⁻¹).

Figure 2b displays in red the Raman microscopy image of a crystal collected from the surface of sample 4 (wipe of a submunition). The experimental Raman spectrum obtained from the particle is dominated by a band at 884 cm⁻¹ which can be attributed to the symmetric C–N–C ring breathing mode, as well as the band at 944 cm⁻¹ which can be assigned to the ring stretching mode and to the N–O deformation; the band at 1278 cm⁻¹ can be attributed to CH₂ scissoring and the characteristic N–N stretch vibration [15]; the band at 1310 cm⁻¹ results from –CH₂ wagging; the band at 1388 is likely the υNO2 symmetric stretching vibration and βCH₂ scissoring; finally, the band at 1594 cm⁻¹ is attributed to the υNO2 asymmetric stretch in nitro amines [15, 16]. The reference Raman spectrum of RDX, obtained from the Renishaw library, is overlaid in blue (Fig. 2b). It is worth noting that the broad peaks observed in the experimental spectrum at 1310 and 1594 cm⁻¹ are due to the contribution of the vibrations of the graphite (carbon) support.

Figure 2c displays the Raman microscopy image of a crystal collected from the surface of sample 10 (wipe of a grenade). The experimental Raman spectrum (red trace) is characterized by Raman lines matching well with literature data [7, 17–19] and can be attributed to the vibrational modes of PETN. Indeed, the band observed at 622 cm⁻¹ is due to the rocking vibration mode of –ONO₂ groups; the band at 871 cm⁻¹ results from the O–N stretching mode, the band at 1042 cm⁻¹ from the CH₂ torsion and the bending of the C–C bond, and finally the bands at 1292 and 1469 cm⁻¹ result from the symmetric and asymmetric stretching of NO₂, respectively.

This investigation seeks to develop and apply negative ion mode DART-HRMS methods for the direct analysis of swabs and/or wipes holding explosives. To enable direct DART-HRMS analyses of explosive materials aspirated from swabs or wipes, the carbon impaction plate containing the transferred explosive particles was placed between the DART ion source outlet and the entrance of the LTQ-Orbitrap mass spectrometer. The DART parameters were optimized for each of the nitro-based explosives. We found that the optimal gas stream temperature was 350 °C for most explosives. A priori, we anticipated that the explosives bearing sufficiently acidic protons would yield abundant deprotonated molecules [M–H]⁻, whereas those products bearing very weakly acidic protons would be capable of forming anionic adducts, especially if chloride anions were made available, i.e., [M + Cl]⁻ [20]. To promote the formation of chloride adducts, chloroform vapor was added directly to the ionizing region, using a simple experimental setup where helium was gently bubbled into chloroform (ESM Fig. S2).

Figure 3 shows representative negative ion mass spectra of samples 5 (submunition, Fig. 3a), 9 (rocket, Fig. 3b), 10 (grenade, Fig. 3c), and 15 (bounding mine, Fig. 3d). In Fig. 3a and b, the [M–H]⁻ peak is readily observed at m/z 226.0106 for TNT, whereas in Fig. 3a and d, the chloride adduct anions of hexogen [RDX + ³⁵Cl]⁻ and octogen [HMX + ³⁵Cl]⁻ are observed at m/z 257.0043 and m/z 331.0159 along with their respective nitrate adduct anions [M + NO₃]⁻ at m/z 284.0233 and m/z 358.0350. All peak identifications are consistent with reference mass spectra obtained from pure standards. Deprotonated molecules were not detected for HMX, RDX, or PETN. Neither the nitramines nor the nitrate ester can readily transfer a proton to the strong reagent O₂ ^·− present in the DART plasma which has been postulated to become protonated to form · O₂H, a species having a low gas-phase acidity [21]. However, adding chloroform as dopant to the heated helium gas produced abundant Cl⁻ ions that readily formed anionic adducts with HMX, RDX, and PETN. The ability to observe highly abundant chloride adducts has been linked to an exceptional stability of these species in the gas phase [22].

Further DART-MS screening revealed that the three investigated submunition weapons (samples 3–5) did not contain the same explosives. For example, only TNT was detected in sample 3, whereas sample 4 revealed only RDX (Table 2). Other samples, however, contained mixtures of energetic nitro-based charges. For example, analysis of sample 5 by DART-MS revealed a mixture of HMX, RDX, and TNT, as deduced from the detection of [TNT–H]⁻ at m/z 226 and the RDX adducts [RDX + ³⁵Cl]⁻ and [RDX + NO₃]⁻ anions at m/z 257 and 284, respectively. In the case of the rocket weapons, only sample 6 contained RDX, whereas TNT was detected in samples 7–9. No differences were found in mortar weapons (samples 11–14) where only TNT was detected, and its structure is confirmed by the presence of the fragment ion m/z 196 formed under CID by loss of CH₂O [23]. On the other hand, sample 2 (dispersed explosive) contained a mixture of RDX and TNT, which can be attributed to the presence of “Composition B” [24], a mixture that is commonly used as explosive filling in mines. By comparison, the analysis of the bounding mine sample (no. 15) also revealed a mixture containing RDX, but this time the other main component was HMX. The analysis of the grenade swab (sample 10) revealed uniquely pentaerythritol tetranitrate (PETN), detected in three different forms: m/z 315.0064 [PETN–H]⁻, m/z 351.9830 [PETN + ³⁵Cl]⁻, and m/z 378.0019 [PETN + NO₃]⁻. All samples containing TNT displayed an abundant ion at m/z 197, whose signal intensity varied among the samples. The interpretation of this unassigned ion is discussed in the ESM.

All weapon samples were swiped several times, and each swipe was analyzed at least three times (three replicates) in different areas of the swipes. Although the relative intensity of the analytes detected in each replicate varied within a range of ±30 %, each replicate analysis revealed the same composition.

The analysis of explosives in the positive ion mode provides additional and valuable information for the characterization of explosives that complements that obtained in the negative ion mode. The latter is often used to ionize nitroaromatic-based explosives; however, analysis in positive ion mode can reveal the binder polymers used in manufacturing. These polymers are added to “phlegmatize” the main charge, i.e., they stabilize or desensitize the explosive. Other informative compounds can also be detected in positive ion mode, such as nonexplosive organic compounds, degradation products, plasticizers, and tagging agents.

Various polymers are commonly used as explosive fillers as well as plasticizers and binders in energetic materials. The use of these compounds in explosive formulations is mainly for desensitization purposes; however, with the development of more insensitive explosive fillers, the function of the polymer binder is shifting from that of a desensitizer to that of providing structural integrity [25]. Intact synthetic polymers are widely analyzed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) [26, 27], but this method is limited by the sensitivity when trace analysis is required. ESI-MS is another useful technique for the analysis of polymers, and it has the advantage that it can be easily interfaced with liquid separation methods (e.g., HPLC, CE). Some disadvantages of ESI are the tendency for signal suppression of less surface-active species when mixtures or co-eluting species are present, and the strict solvent requirements [28]. Alternatively, direct analysis of polymers using ambient ionization techniques has become useful [29], providing analysis methods which are fast and without special sample preparation requirements. In this way, DESI-MS was used for polymer analysis and showed good performance for the analysis of industrial synthetic polymers [30–32]; however, like ESI, DESI provides multiply charged species whose presence may complicate the mass spectral profile. Another direct sampling technique, DART-MS, provides only singly charged species, and when coupled to high-resolution instruments, represents a powerful tool for the analysis of polymers [12]. Thus, DART-MS has been demonstrated to be fast and nondestructive [33], and has utility for the analysis of synthetic and natural polymers [12].

In the current study, the analysis of explosive samples collected on an impaction plate using DART-MS produced only singly charged species. For most samples, the analysis of swabs showed the presence of polymers. Specifically, samples 3–9 and 11–14 all showed a low molecular weight polymer distribution, which was separated by a 44-Da repeat unit which we assign as ethylene oxide (−OCH₂CH₂–) characteristic of polyethylene glycol (PEG) polymer, as exemplified in Fig. 4a. This mass spectrum showed singly protonated [M + H]⁺ molecules and singly charged ammonium adducts [M + NH₄]⁺. Ammonium adducts likely arise from ammonium salts present in the samples, or present in residual amounts on the surface of the ionization source, because no ammonium compounds were added to the heated helium stream. Figure 4b displays the mass spectrum of sample 10 (grenade) showing singly charged species [M + NH₄]⁺ and [M + H]⁺. The spectrum shows a series of ions separated by 58 m/z units, which we assign to the propylene oxide (PO) repeat unit (−OCH₂CH₂CH₂–) corresponding to polypropylene (or isopropylene) glycols (PPG). Unlike the other samples analyzed, sample 15 shows a very complex mass spectrum displayed in Fig. 4c. Although DART-MS produces only singly charged species, the polymeric sample contains variable repeat units, and the number of possible combinations leads to a complex interpretation of the mass spectrum. Furthermore, the small quantity of sample did not allow us to perform additional experiments (e.g., MS/MS, FTIR, etc.), which can provide complementary information for structural elucidation and identification.

On the other hand, sample 1 (mine) did not show any polymer constituents. However, a plasticizer was detected at m/z 427 corresponding to protonated [M + H]⁺ diisooctyl sebacate (DOS) and its ammonium adduct [M + NH₄]⁺ was observed at m/z 444 (Fig. 5a). Other plasticizers were observed, such as tributyl phosphate (at m/z 267, corresponding to the protonated molecule and m/z 533, resulting from the dimerization of TBP, Fig. 5b); dibutyl phthalate ([M + H]⁺ observed at m/z 279, Fig. 5c); and bis(2-ethylhexyl)phthalate ([M + H]⁺ at 391, Fig. 5d). Plasticizers are critical components in explosive formulations. These compounds are used in explosive formulations to alter the mechanical characteristics of the polymer binders by increasing the flexibility of the polymer chain. In addition to improving the mechanical properties, plasticizers also tend to reduce the mix viscosity to improve processing, modify the oxygen balance, and modify the burn rate characteristics of the energetic material composite [26].