Analysis of smokeless powders is of interest from forensics and security perspectives. This article reports the detection of smokeless powder organic additives (in their pre-detonation condition), namely the stabiliser diphenylamine and its derivatives 2-nitrodiphenylamine and 4-nitrodiphenylamine, and the additives (used both as stabilisers and plasticisers) methyl centralite and ethyl centralite, by means of swab sampling followed by thermal desorption and direct injection soft chemical ionisation-mass spectrometry. Investigations on the product ions resulting from the reactions of the reagent ions H3O+ and O2+ with additives as a function of reduced electric field are reported. The method was comprehensively evaluated in terms of linearity, sensitivity and precision. For H3O+, the limits of detection (LoD) are in the range of 41–88 pg of additive, for which the accuracy varied between 1.5 and 3.2%, precision varied between 3.7 and 7.3% and linearity showed R2 ≥ 0.9991. For O2+, LoD are in the range of 72 to 1.4 ng, with an accuracy of between 2.8 and 4.9% and a precision between 4.5 and 8.6% and R2 ≥ 0.9914. The validated methodology was applied to the analysis of commercial pre-blast gun powders from different manufacturers.
Smokeless powders are a large and complex family of products used as propellants in ammunition cartridges , categorized as low explosives (they burn rapidly instead of detonating) . They are commonly employed in forensic analyses as their residues can be used as evidence for firearms discharge [3, 4]. They are also relevant from a Homeland Security perspective, as they are readily available and can be employed in the manufacturing of improvised explosive devices (IEDs) . They exhibit a complex composition, consisting of an explosive material (nitrocellulose, nitroglycerin, nitroguanidine or different mixtures of them) , heavy metals [1, 6] and a large number of different classes of organic compounds [1, 7, 8]. The latter became of great interest after the introduction of heavy metal–free ammunition in the market . Within the organic additive category, we can include plasticizers, stabilisers, opacifiers, flash suppressants, coolants, surface lubricants and dyes. [2, 9,10,11,12,13] The aim of these additives is to increase the shelf-life and modify the burning characteristics of the powder [5, 14]. Different concentrations and/or different additives are characteristic of a given manufacturer, producing therefore a chemical fingerprint for each powder . It is thus also important to determine their content throughout the manufacturing quality control process. Among all the possible additives, there are a number of key chemicals usually present and regarded as characteristic of smokeless powders [1, 12, 16]. The most common are the stabiliser diphenylamine (DPA) and its derivatives 2-nitrodiphenylamine (2-NO2-DPA) and 4-nitrodiphenylamine (4-NO2-DPA), and the additives (used both as stabilisers and plasticisers) methyl centralite (MC) and ethyl centralite (EC), which are the subject of this current paper—for structural information, see Table 1.
Several analytical techniques have been used for the qualitative and/or quantitative detection of smokeless powders, either in their pre- and/or post-blast forms [9, 10], including high-performance liquid chromatography (HPLC) [17,18,19], liquid chromatography-mass spectrometry (LC-MS) [8, 20,21,22], Fourier transform infrared spectroscopy , gas chromatography (GC) [12, 14, 24], capillary electrophoresis (CE) [25, 26], ion mobility spectrometry (IMS) , solid-phase microextraction-ion mobility spectrometry (SPME)-IMS [12, 28], (nano)electrospray ionization (nESI)-tandem mass spectrometry [29,30,31], laser electrospray-mass spectrometry (LEMS) [15, 32], desorption electrospray ionization-mass spectrometry (DESI) [33, 34], direct analysis in real time-mass spectrometry (DART-MS) , time-of-flight secondary ion-mass spectrometry (ToF-MS)  and Raman spectroscopy [23, 37]. Most of the abovementioned techniques require time-consuming sample preparation step(s)—exception of DESI and DART—or if not, they require complicated set-ups, such as the use of lasers as the means for sample vaporization (LEMS) or heated purified gases (DART). Here is where direct injection (DI) soft chemical ionisation-mass spectrometry (SCIMS) can compete (and/or be complementary) with these techniques for rapid, selective and sensitive detection of chemical compounds in complex environments. DI-SCIMS is an analytical technique for mass spectrometric gas analysis based on the ionization of neutrals by ion/molecule reactions with a reagent ion (such as H3O+, O2+ or NO+). This occurs within the controlled environment of a drift tube (DT) under the effect of an electric field E. The resulting ionised analyte molecules are then mass analysed by mass spectrometer. It is a direct injection technique as samples are injected directly into the drift tube of the instrument.
There are several analytical techniques that belong to the DI-SCIMS category [38, 39], with proton transfer reaction-mass spectrometry (PTR-MS) arguably the most widespread. PTR-MS was purposely designed for the monitoring of volatile organic compounds (VOCs) , but has developed further to analyse liquid and solid compounds , being successfully applied to the detection of explosives and explosive-related compounds in positive-ion mode [41,42,43,44,45,46,47,48,49,50]. Technically speaking, PTR-MS only refers to the use of hydronium as the reagent ion. Given that in this study we investigated reactions involving O2+ and H3O+, the term SCIMS is a more accurate description of the instrument for this work.
In this paper, we report the first DI-SCIMS studies of the additives to smokeless powders, namely DPA, 2-NO2-DPA, 4-NO2-DPA, MC and EC, using H3O+ and O2+ as the reagent ions. We can expect efficient reactions with H3O+ because the proton affinities for amine and amide-based compounds are higher than that of water. Certain studies involving ESI-MS  and IMS  show that these neutrals can be detected with a high sensitivity. Based on the identified ions, analytical figures of merit (limits of detection, linear dynamic range, repeatability and reproducibility) are established. This information should help in the development of a highly selective analytical technique for smokeless powder organic additive detection using DI-SCIMS.
Proton Transfer Reaction Mass Spectrometry
A Kore Technology Ltd. Series I PTR-ToF-MS instrument was used. Details of using PTR-MS are given in detail elsewhere [38, 47], and therefore, only pertinent issues will be briefly mentioned here. Recently, this instrument was equipped with a radiofrequency ion funnel drift tube and fast reaction region reduced electric field, E/N, switching capabilities . However, for these studies, the RF operation was not used.
Fast Reduced Electric Field Switching
Details of the fast switching have been given elsewhere . In brief, this new hardware development feature allows the rapid switching of the reduced electric field with transition times less than 140 ms (0.1–5 Hz) within the reaction region. This alters the reagent ion composition and ion molecule collisional energies, leading to differences in product ions between the two operational E/N values. This new hardware development allows for the manipulation of the ion chemistry, modifying the product ion distribution to provide more information to aid in assignment of the neutral responsible for the observed product ion(s).
Water vapour is introduced into a hollow cathode glow discharge where, after ionisation via electron impact and subsequent ion molecule processes, the terminal reagent ion is H3O+. These ions are transferred from the ion source into the drift tube by an applied voltage gradient where they react with the analyte M by donating their protons at the collisional rate; providing M has a proton affinity greater than that of water (PA(H2O) = 691 kJ mol−1). This process can be either non-dissociative (resulting in the protonated molecule MH+) and/or dissociative. Dissociative proton transfer results in product ions, which depend on their m/z values, may be useful for the identification of a compound. Fragmentation may be spontaneous upon proton transfer or may require additional energy which is supplied through collisions with the buffer gas resulting during the migration of ions under the influence of the electric field, E. Ions are separated using a time-of-flight mass analyser and detected by means of a multichannel plate. O2+ is also formed as an impurity due to air back flow from the reactor into the ion source region ; however, the instrument was operated in a manner that this was below 2% of the H3O+ signal intensity.
O2 + Production
For the production of O2+, water vapour in the discharge is replaced by pure oxygen (99.998% purity, BOC Gases, Manchester, UK). This leads to the formation of mainly O2+ reagent ions (> 95%) . Once injected into the DT, O2+ reacts with the analyte M via charge transfer, provided that the ionisation potential of M is less than that of O2 . Similarly to 2.1.2, this reaction may be non-dissociative and/or dissociative, and fragmentation may be spontaneous upon charge transfer or require additional energy. H3O+ is also observed due to residual water vapour in the system, with signal intensity below around 2.5% of the O2+ signal for the experimental conditions used throughout.
It is worth to highlight that when using O2+ as the reagent ion, it is possible to start measurements at lower E/N values than when using H3O+. This is a consequence of the lack of water clustering for O2+. This reagent ion signal had to be inferred from its corresponding isotopologue 16O18O at m/z 33.99, owing to detection saturation at m/z 31.99.
A thermal desorption unit (TDU) connected to the inlet of the drift tube through passivated (Silconert®) stainless steel (10-cm length) was used to introduce the samples. Details of the TDU have been given elsewhere . The TDU, connecting lines and drift tube were operated at a temperature of 150 °C (maximum possible temperature). PTFE swabs (ThermoFisher Scientific, Cheshire, UK) onto which known quantities of additives were deposited were placed into the TDU. For this study, laboratory air was used as the carrier gas. Prior to making contact with the swab, the carrier gas was passed through an oxygen/moisture trap (Agilent OT3-4)—not used for O2+ mode—and hydrocarbon trap (Agilent HT200-4). Upon closure of the TDU, a seal is created, and the carrier gas is heated to the temperature of the TDU before it flows through a series of apertures in the heated metal plate. This heated air then passes through the swab and into the inlet system driving any desorbed material through to the drift tube creating a temporal concentration “pulse” of typically between 10 and 20 s of an analyte in the drift tube . For the product ion distribution and branching ratio studies, each swab provided one measurement, which was replicated three times, and then, the results were averaged, and any background signals were subtracted.
The drift tube was maintained at a pressure of 1.1 mbar, and the glow discharge (for both water vapour and oxygen) was set at 1.3 mbar (which is a combination of the reagent neutral pressure and air back flowing from the drift tube).
For the fast switching experiments, the acquisition time per point was set to 40 ms and ion signals were averaged for each individual cycle.
In the following, only product ions with a product ion distribution (PID) greater than 1% are reported and the m/z of the lightest isotopologue will be given. However, when calculating the product ion distributions, all of the isotopologues are taken into account.
Chemical Standards and Smokeless Powder Samples
Table 1 gives details of the molar mass and structure of the five compounds investigated in this study.
These chemicals were individually purchased from AccuStandard Inc. (New Haven, CT, USA) and used without additional treatment. DPA is dissolved in MeOH; MC and EC are prepared in a mixture of MeOH/AcN 1:1 (v/v) and 2- and 4-NO2-DPA in AcN. Concentrations in all cases were 100 μg mL−1. Further dilutions of this mother solutions in the appropriate solvent(s) (HPLC grade) were prepared when needed. Typically, 1 μL of a solution of the required concentration was spotted onto the swab and left to evaporate the solvents for 1 min prior to insertion into the TDU.
Smokeless powders (either used for guns or rifles) were acquired in a local ammunition wholesaler. Rifle powders are typically single based (the only energetic material is nitrocellulose), and gun powders are double based (nitrocellulose together with nitroglycerine). When needed, 1 g of powder was dissolved in 10 mL of dichloromethane (HPLC grade) for 10 min at room temperature with the help of an ultrasonic bath. Once the solvent evaporated at room temperature, the residue was dissolved in 100 mL of a mixture of MeOH/acetonitrile 1:1 (v/v). Again, 1 μL was spotted onto the swab and left solvents to evaporate for 1 min prior to insertion into the TDU.
Results and Discussion
Analysis of Standard Additives, Fragmentation Patterns and Branching Ratio Studies in H3O+ and O2 + Modes
In H3O+ mode (data not shown), the protonated parent [DPA.H]+ at a m/z of 170.10 dominates across the E/N range studied (80–200 Td). One other product ion is observed at high E/N values (180 Td and above) at m/z 92.05. This is assigned to [C6H5NH]+, resulting from the loss of benzene from the protonated parent, increasing its intensity from negligible at low E/N to a maximum of 5% at 200 Td.
In O2+ mode (data not shown), only DPA+ at m/z 169.09, resulting from non-dissociative charge transfer, is observed for all the E/N values (60–200 Td).
2-Nitrodiphenylamine (2-NO2-DPA) and 4-Nitrodiphenylamine (4-NO2-DPA)
Figure 1 shows the PID plots for (a) 2-NO2-DPA and (b) 4-NO2-DPA for their reaction with H3O+ as a function of E/N (for the range from 80 to 200 Td). For both chemicals, the fragmentation pattern is very similar, and only differences ascribe to the ortho effect (amine and nitro groups in adjacent positions for 2-NO2-DPA) are observed. For both isomers, the protonated parent, m/z 215.08, is the dominant ion, with the exception of the 2-isomer at E/N values above 190 Td, where a product ion at m/z 197.07 (loss of H2O) takes over. For the 4-isomer, a loss of a hydroxyl group, giving a product ion at m/z 198.08, is also observed. This is consistent with the ortho observed behaviour, where [M-OH2]H+ replaces the [M-OH]H+ fragment ion [47, 54]. For the 2-isomer, a subsequent loss of an hydroxyl group (only observed at E/N > 160 Td) leads to the product ion at m/z 180.06, the intensity of which increases as the E/N increases. Finally, another product ion at m/z 169.07 corresponding to the nitro group loss from the protonated parent is observed in both isomers with different intensities (maximum values of ca. 8.5% for 2-isomer and ca. 14% for the 4-isomer, at 200 Td), and becoming relevant only above 150 Td in both cases.
In O2+ mode (PID plot not shown) and for both 2- and 4-NO2-DPA, the parent ion at m/z 214.07, the result of non-dissociative reaction channel, dominates. Its intensity decreases as the reduced electric field increases, dropping down to 25% at 180 Td of the initial intensity at 80 Td. Other fragment ion, at m/z 163.22 (unassigned in this paper), is observed in both cases, the intensity of which slightly decreases as the reduced electric field increases (from ~ 2% at 60 Td to 3.5% at 200 Td for 2-NO2-DPA and from ~ 3.5 to 7% for 4-NO2-DPA). This unidentified ion is consistent with the observations reported by Perez et al. 
In H3O+ mode, the protonated parent at m/z 241.13, [MC.H]+, is observed as the dominant ion up to around 190 Td (Fig. 2a). Other observed product ions are m/z 134.06, assigned to [PhNCH3CO]+ (resulting from the loss of N-methylaniline from the protonated parent), and m/z 106.07 (a subsequent loss of a CO molecule leaving a [PhNCH3]+ ion), which only yields a significant intensity above 140 Td and becomes dominant above 190 Td.
In O2+ mode (PID shown in Fig. 2b), non-dissociative charge transfer results in an ion at m/z 240.12, [MC]+, that dominates. Only at very high E/N values, > ca. 200 Td, the ion at m/z 106.10 becomes dominant. Additional observed product ions are m/z 225.10 (loss of a CH3 group) and m/z 183.01 (not assigned in this paper and being relevant only after 170 Td).
EC has a very similar structure to that of MC, so a similar fragmentation pattern is to be expected. For water chemistry, the protonated parent, [EC.H]+, at m/z 269.17, is dominant across all the E/N range (Fig. 3a). Observed fragment product ions are m/z 148.08 (via loss of N-ethylaniline from the protonated parent) and at m/z 120.08 (loss of CO), which only becomes relevant above 120 Td. Two more product ions are observed at m/z 93.06, assigned to be charged aniline [PhNH2]+, after the additional loss of a CH2CH molecule, and m/z 92.05, [PhNH]+, only becoming relevant at E/N > 140 Td. That these two ions are only and simultaneously observed for EC is consistent with results shown by Gilbert-López et al. in a LC/ESI-ToF-MS. 
For oxygen chemistry, PID shown in Fig. 3b, the ion resulting from charge transfer at m/z 268.16, dominates, and only at reduced electric field values above 200 Td loses its dominance. Identified product ions are to the same as those for water chemistry, namely m/z 148.08, 120.08, 93.06 and 92.05, but in addition, another product ion at m/z 164.00 is also observed, the intensity of which remains almost constant for the range 60 to 140 Td, after which its intensity decreases to ca. 5% at 220 Td.
Method Validation and Analytical Figures of Merit
Following the establishment of the product ions, the performance of the method was evaluated in terms of limits of detection (LoD), linear dynamic range and precision for both H3O+ and O2+ reagent ions (see Table 2). Serial calibration solutions of different concentrations for each standard additive were prepared. Calibration curves, using peak areas normalised to 1E6 reagent ions, as function of concentration using least square linear regression analysis were plotted. Instrumental LoDs were evaluated based on the minimum analyte concentration yielding to a signal-to-noise ratio equal to 3. Noise was defined as the average of 10 blank samples for a given mass. Although the conjunction of protonated parent and fragment ions is useful for selectivity purposes, to determine the sensitivity of the method, only the dominant ion resulting in the best LoD was used, i.e. the most intense ion signal (in terms of ncps) at a given E/N value was used to determine the LoD. Precision of the method was determined in terms of repeatability (measurements of five replicates within short intervals of time (typically 1–5 min) by the same operator under the same experimental conditions) and reproducibility (five replicates over five different days by the same operator under the same experimental conditions), with each replicate being the mean of three measurements. Linearity was studied covering a concentration range from 0.1 to 1500 ng of each compound at ten concentration values with three replicates at each concentration. No carryover effects were observed, and under the experimental conditions after ca. 10 s, the baseline was recovered for all the compounds of interest. Thirty-second integration time was used throughout in order to record a stable background prior and after a desorption event.
In H3O+ mode, the coefficient of determination R2 was higher than 0.9991 for all compounds. Instrumental limits of detection varied from 41 to 88 pg. Precision, expressed in terms of relative standard deviation (RSD), was found in all cases to be below around 3% for intra-day (repeatability) and below 7% for inter-day (reproducibility) studies.
In O2+ mode, the coefficient of determination R2 was higher than 0.9914 for all compounds. Instrumental limits of detection varied from 72 to 1.4 ng. Special mention should be noted to the cases of DPA, where the existence of an endogenous high background signal at the m/z of interest led to a LoD much higher than that of the rest of compounds, but still in the low nanogram region. Precision was found in all cases to be below around 5% for intra-day (repeatability) and below 8.6% for inter-day (reproducibility) studies.
Application to Commercial Samples
Six commercial smokeless powder samples from three different manufacturers were analysed. The concentration of additives was calculated using the standard calibration curves obtained for “Method Validation and Analytical Figures of Merit” section. Results, see Table 3, show the identified additives and its content in the smokeless gun powder (expressed as percentage) for the different samples for H3O+ and O2+ reagent ions at 140 Td (a compromise E/N value between high signal intensity and low fragmentation). These results are in good agreement with those found in the smokeless powder database . Figure 4 shows two mass spectra exemplifying two of the samples for water chemistry—similar plots (not shown) were found for the rest of the samples and for oxygen chemistry.
Based on our previous water chemistry work [41, 47, 50], and besides the detection of the additives studied for this work, dinitrotoluene was also clearly observed showing two intense product ion peaks at m/z 183.04 and 201.05, assigned to the protonated parent, DNT.H+, and its first water cluster, DNTH+.H2O. This was observed for three of the samples. It is possible to assign dinitrotoluene to be the 2,4-isomer. As reported recently by González-Méndez et al, [47, 50] monitoring product ions at m/z 183.04 and 201.05 allow assignment to 2,4-DNT, but that the presence of m/z 136.04 (elimination of HONO from the protonated parent) and m/z 91.06 (elimination of two nitro groups) observed at the high E/N setting (200 Td and above) indicates the presence of 2,6-DNT. These two latter peaks were not observed. No detailed product ion distribution studies for DNT and O2+ exist (to the best of our knowledge), but in O2+ mode, the charge transfer reaction channel leading to a peak at m/z 182.03 (assigned to [DNT]+) was observed.
The other three samples showed an intense peak at m/z 228.03. Fast switching experiments and previous studies dealing with 2,4,6-trinitrotoluene (TNT) and nitroglycerine (NG) confirmed this to be NG [41, 49, 50]. NG produces a characteristic signal at m/z 46.01 (NO2+) at high E/N values, whilst TNT does not; thus, a quick change in the E/N from low (80 Td) to high (180 Td) allows to assign this peak to NG.
Both Alliant powders show evidence of 2,4-DNT and also peaks at m/z 170.10, 215.08 and 269.17, assigned to [DPA.H]+, [2-,4-NO2-DPA.H]+ and [EC.H]+, respectively. Fast E/N switching experiments confirm the identity of these species based on the presence or absence of fragment ions at different reduced electric fields. Fast switching experiments based on Fig. 1a, b, for both Alliant Red Dot and Unique powder, as shown in Fig. 5, confirmed the identity of m/z 215.08 to be the 2-nitrodiphenylamine isomer. The presence or absence of m/z 197.07 and 198.08 would rule out one or another. Also, the presence of m/z 180.06 would confirm the existence of 2-NO2-DPA.
For the Hodgdon samples, only H322 did show evidences of 2,4-DNT, but BL- C showed a signal at m/z 228.03, assigned again to NG based on fast switching experiments. Both Hodgdon samples showed clear signals at m/z 170.10 and 215.08. Fast switching experiments confirmed m/z 215.08 to be 2-nitrodiphenylamine for Hodgdon-BL- C. However, in O2+ mode, no evidence for 2-nitrodiphenylamine was observed.
Both IMR samples showed a clear and intense peak for 2,4-DNT, and peaks at m/z 170.10, 215.08, 241.13 and 269.17 were also observed. Fast switching experiments confirmed the nitrodiphenylamine to be a mixture of the 2- and 4-isomers.
As stated in the introduction, owing to the complex composition of smokeless powders, the presence of unidentified peaks was expected, the majority coming from the plasticizers used in the manufacturing process. This was confirmed by additional unidentified peaks for all the powders at m/z 149.02 (reported by Scherperel et al. as a dibutyl phthalate fragment) , 205.09 and 279.16 (the latter is assigned to be protonated dibutyl phthalate [DBP.H]+ by Reese et al.  and Perez et al. ). It is evident that a more detailed study dealing with these is needed if a complete chemical analysis is required.
We have shown that direct injection soft chemical ionisation-mass spectrometry, using both water and oxygen reagent gases, can analyse smokeless powder organic additives. This has been applied to their identification for commercial powders in their pre-detonation condition.
This method makes use of commercially available swabs and thermal desorption, allowing complete analysis of samples within ~ 10 s. For a series of the most common organic additives for smokeless powders, fragmentation patterns have been established and analytical figures of merit have been reported. Achieving the best LoDs for oxygen chemistry requires using lower reduced electric field values than those used for water chemistry. Oxygen chemistry has been tested to be less sensitive than water chemistry for the compounds of interest. Fragmentation has been shown to be very similar in terms of the observed ions and their identity for both reagent ions. For H3O+ and O2+, the most intense ions are usually coming from the non-dissociative channels.
Fast switching experiments aided in the identification and distinguish between isomers, based on the presence or absence of fragment ions at different reduced electric fields.
When applied to commercial samples, results have shown that the content of the organic additives investigated in this study changed between the samples, helping to differentiate among samples and manufacturers.
Future work will include extending the number of additives and plasticisers and commercial samples. Moreover, and also importantly, analysis of post-blast samples to ensure organic gunshot residues can be detected in a rapid, sensitive and selective way using this DI-SCIMS technology. Similar to recent studies , the potential use of a radiofrequency ion funnel drift tube to check for improvements in both sensitivity and selectivity is worthwhile to mention.
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RGM is an early-stage researcher who acknowledges the support of the PIMMS Initial Training Network which in turn is supported by the European Commission’s Seventh Framework Programme under Grant Agreement Number 287382.
• Use of direct injection soft chemical ionisation-mass spectrometry for smokeless powder organic additive analysis
• Study of the underlying water and oxygen chemistry in positive-ion mode
• Comparison of fragmentation patterns for H3O+ and O2+ reagent ions
• Performance evaluation for the method in terms of sensitivity, linear dynamic range and precision
• Application to commercial pre-blast samples
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González-Méndez, R., Mayhew, C.A. Applications of Direct Injection Soft Chemical Ionisation-Mass Spectrometry for the Detection of Pre-blast Smokeless Powder Organic Additives. J. Am. Soc. Mass Spectrom. 30, 615–624 (2019). https://doi.org/10.1007/s13361-018-02130-1
- Soft chemical ionisation-mass spectrometry
- Proton transfer reaction mass spectrometry
- Smokeless powders
- Smokeless powder additives