Generating highly specific spectra and identifying thermal decomposition products via Gas Chromatography / Vacuum Ultraviolet Spectroscopy (GC/VUV): Application to nitrate ester explosives

Limits of identification using VUV spectroscopy applied to C8H18 isomers isolated by GC×GC

The vacuum ultraviolet detector for gas chromatography can be used to identify structural differences between isomers with similar chromatographic elution times, which adds detail to characterization, valuable for prescreening of sustainable aviation fuel candidates. Although this capability has been introduced elsewhere, vacuum ultraviolet spectroscopy for saturated hydrocarbons has been examined minimally, as the similarities between their spectra are much less significant than their aromatic counterparts. The fidelity with which structural differences can be identified has been unclear. In this work, all possible structural isomers of C₈H₁₈ are measured and determined to have unambiguously unique vacuum ultraviolet spectra. Using a statistically based residual comparison approach, the concentration limits at which the spectral differences are interpretable are tested in both a controlled study and a real fuel application. The concentration limit at which the spectral differences between C₈H₁₈ isomers are unambiguous is below 0.40% by mass and less than 0.20% with human discretion in our experimental configuration.

Quantitative analysis of smokeless powder particles in post‐blast debris via gas chromatography/vacuum ultraviolet spectroscopy ( GC / VUV )

Forensic analysis of smokeless powder particles recovered from the debris of an improvised explosive device can provide information about the type of smokeless powder used and can aid investigation efforts. In this study, quantitative methods were used to yield information about the difference in the chemical composition of the particles pre‐ and post‐blast. The technique, gas chromatography/vacuum ultraviolet spectroscopy (GC/VUV), was able to quantify nitroglycerin, 2,4‐dinitrotoluene, diphenylamine, ethyl centralite, and di‐n‐butyl phthalate in pre‐ and post‐blast smokeless powder particles using heptadecane as an internal standard. Post‐blast debris was obtained via controlled explosions with assistance from the Indiana State Police Bomb Squad. Two galvanized steel and two polyvinyl chloride pipe bombs were assembled. Two devices contained single‐base smokeless powder and two contained double‐base smokeless powder. 2,4‐dinitrotoluene and diphenylamine were successfully quantified in the single‐base smokeless powder post‐blast debris while nitroglycerin, diphenylamine, and ethyl centralite were successfully quantified in the double‐base smokeless powder post‐blast debris. Compounds were detected at concentrations as low as 9 μg of 2,4‐dinitrotoluene per mg, <3 μg of diphenylamine per mg, 131 μg of nitroglycerin per mg, and <3 μg of ethyl centralite per mg. Concentration changes between pre‐ and post‐blast smokeless powder particles were determined as well as microscopic differences between pre‐ and post‐blast debris for both smokeless powders in all devices. To our knowledge, this is the first use of GC/VUV for the quantification of explosives.

A systematic study of the absorbance of the nitro functional group in the vacuum UV region

The nitro functional group (NO₂) features strongly in compounds such as explosives, pharmaceuticals, and fragrances. However, its gas phase absorbance characteristics in the vacuum UV region (120-200 nm) have not been systematically studied. Gas chromatography/vacuum UV spectroscopy (GC/VUV) was utilized to study the gas phase VUV spectra of various nitrated compounds (e.g., nitrate esters (-R-O-NO₂), nitramines (R-N-NO₂), nitroaromatics (Ar-NO₂), and nitroalkanes (R-NO₂)). The nitro absorption maximum appeared over a wide range (170-270 nm) and its wavelength and intensity were highly dependent upon the structure of the rest of the molecule. For example, the nitroalkanes exhibited a trend in that the ratio of the relative absorption intensity between these two absorption features between the alkyl group (<150 nm) and the nitro group (200 nm) increases as the molecular weight increases. It was observed that the addition of multiple nitro functional groups on benzene or toluene resulted in an increase in intensity and blue shift from approximately 240 nm-210 nm. Nitrate esters exhibited an absorption between 170 nm and 210 nm and absorbance increased with increasing nitrogen content. The relative diversity of the spectra obtained was analyzed by Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA). These calculations revealed that the spectra of all the compounds analyzed could be reliably differentiated without any misclassifications.

Preliminary stable isotope analyses for propellant discrimination in shotshells

RATIONALE

This study aimed to develop methods to determine the identity and trace the origin of propellants used in shotshells. Specifically, the use of organic component and stable isotope analysis techniques, such as bulk stable isotope analysis (BSIA) and compound-specific isotope analysis (CSIA) techniques, for the study of shotshell propellants was investigated.

METHODS

Nine samples of shotshell propellants from different manufacturing countries and brands were analyzed for explosive and additive components by gas chromatography/mass spectrometry and thin-layer chromatography. BSIA of the propellants was achieved using elemental analysis/isotope ratio mass spectrometry without a pretreatment process. For the CSIA of nitroglycerin, double-base powder propellants were extracted with ether, and the isotope ratios of carbon and nitrogen were measured by gas chromatography/isotope ratio mass spectrometry.

RESULTS

Nine samples drawn from seven brands in four countries were classified into five groups by organic component analysis, while eight classification groups were identified by BSIA. Thus, two samples could not be distinguished from each other by either BSIA or organic component analysis. Subsequently, with the use of results obtained with CSIA for nitroglycerin, all the samples could be classified into different groups. These findings suggest that the nine propellant samples were all composed of different ingredients or raw materials from different sources.

CONCLUSIONS

Stable isotope ratio analyses were performed for propellant discrimination. The combined BSIA, CSIA and organic component analysis techniques were able to successfully distinguish the nine shotshell propellants from seven brands sourced from four different countries, and the results suggested that the samples contained different ingredients or raw materials from different sources. We therefore can conclude that reliable results can be obtained using combined isotope analysis methods such as CSIA and BSIA for origin tracing and identity determination.

A critical comparison of vacuum UV (VUV) spectrometer and electron ionization single quadrupole mass spectrometer detectors for the analysis of alkylbenzenes in gasoline by gas chromatography: Experimental and statistical aspects

Recent advances in benchtop vacuum ultraviolet (VUV) spectrometers have yielded effective universal detectors for gas chromatography (GC). The ability of these detectors to acquire absorbance spectra from 125 nm to 430 nm poses an alternative to the gold standard of mass spectrometry (MS) as a sensitive and selective GC detector. The applications of GC/VUV extend into many areas. Featured here is the potential application of GC/VUV to the analysis of ignitable liquids, which may be found on debris from suspected arson fires. A particular compound class of interest is the alkylbenzenes, as they are a significant component in fuels such as gasoline, petroleum distillates, and aromatic solvents such as degreasers and cleaning solvents. To measure the sensitivity, selectivity and specificity of GC-VUV and GC-MS for alkylbenzenes we employed both library search methods and chemometric analysis using discriminant analysis. The GC-VUV detector was found to have superior specificity to the GC-MS detector in full scan mode. The GC-VUV detector was able to identify all alkylbenzenes correctly, including the correct identification of all structural isomers. LODs for both GC-VUV and GC-MS were found to be picograms on column.

Spotting isomer mixtures in forensic illicit drug casework with GC-VUV using automated coelution detection and spectral deconvolution

Analysis of isomeric mixtures is a significant analytical challenge. In the forensic field, for example, over 1000 new psychoactive substances (NPSs), comprising of many closely related and often isomeric varieties, entered the drugs-of-abuse market within the last decade. Unambiguous identification of the isomeric form requires advanced spectroscopic techniques, such as GC-Vacuum Ultraviolet Spectroscopy (GC-VUV). The continuous development of NPSs makes the appearance of a novel compound in case samples a realistic scenario. While several analytical solutions have been presented recently to confidently distinguish NPS isomers, the presence of multiple isomers in a single drug sample is typically not considered. Due to their structural similarities it is possible that a novel NPS coelutes with a known isomer and thus remains undetected. This study investigates the capabilities of VUV spectral deconvolution for peak detection and identification in incompletely resolved drug mixtures. To mimic worst case scenarios, severe coelution was deliberately induced at elevated GC temperatures. The deconvolution software was nevertheless able to correctly detect both substances, even in case of near-identical VUV spectra at almost full coelution. As a next step, spectra were subsequently removed from the reference library to simulate the scenario in which a novel substance was encountered for the first time in forensic case work. However, also in this situation the deconvolution software still detected the coelution. This work shows that a VUV library match score below 0.998 may serve as a warning that a novel substance may be present in a street sample.

Gas chromatography-vacuum ultraviolet spectroscopic analysis of organosilanes

Organosilanes are used in a broad range of industrial, cosmetic, and personal care products. They serve as bridges between inorganic or organic substrates and organic/polymeric matrices. They are also versatile intermediates and can be used for a variety of synthetic applications. They do not exist naturally and have to be synthesized. Evaluation of intermediates and products resulting from the synthesis processes of organosilanes can be challenging. In this study, gas chromatography with vacuum ultraviolet spectroscopic detection (VUV) was used to analyze Si-containing compounds that are commercially available or were synthetically prepared. VUV measures full scan absorption in the range of 120-240 nm, a region that provides unique absorption signatures for chemical compounds. VUV absorption spectra of organosilanes showed rich and featured characteristics in this wavelength range. Theoretical computations of VUV absorption spectra based on time-dependent density functional theory were also explored as a complementary tool for identification. In addition, the synthesis process of isomeric benzodioxasiline compounds (ortho-, meta-, and para-) was monitored by GC-VUV. It was demonstrated that GC-VUV can be used for easy and rapid differentiation of organosilanes, including structural isomers.

Thermal and spectroscopic analysis of nitrated compounds and their break-down products using gas chromatography/vacuum UV spectroscopy (GC/VUV)

Gas chromatography/vacuum UV spectroscopy (GC/VUV) was utilized to study various explosives and pharmaceuticals in the nitrate ester and nitramine structural classes. In addition to generating specific VUV spectra for each compound, VUV was used to indicate the onset of thermal decomposition based upon the appearance of break-down products such as nitric oxide, carbon monoxide, formaldehyde, water, and molecular oxygen. The effect of temperature on decomposition could be fit to a logistical function where the fraction of intact compound remaining decreased as the transfer line/flow cell temperature was increased from 200 °C to 300 °C. Utilizing this relationship, the decomposition temperatures for the nitrate ester and nitramine compounds were determined to range between 244 °C and 277 °C. It was also discovered that the decomposition temperature was dependent on the GC carrier gas flow rate and, therefore, the residence time of the compounds in the transfer line/flow cell. For example, the measured decomposition temperature of nitroglycerine ranged from 222 °C to 253 °C across four flow rates. Tracking the appearance/disappearance of decomposition products across this temperature range indicated that NO, CO, and H₂CO are final decomposition products while O₂ and H₂O are intermediate products. The decomposition temperatures for all explosives were highly correlated to similar decomposition measurements taken by differential scanning calorimetry (DSC) (r = 0.91) and thermal gravimetric analysis (TGA) (r = 0.90-0.98). In addition, the decomposition temperatures for all explosives were negatively correlated to the heat of explosion at constant volume (r = -0.68) and strongly positively correlated to the oxygen balance (r = 0.92).

Optimization of the qualitative and quantitative analysis of cocaine and other drugs of abuse via gas chromatography - Vacuum ultraviolet spectrophotometry (GC - VUV)

Gas Chromatography-Vacuum UV Spectroscopy (GC-VUV) has seen increased attention in many areas, however, a statistical optimization of VUV method parameters has not been published. This article presents the first statistical optimization of parameters influencing analytes such as cocaine in the VUV flow-cell. Flow-cell temperature, make-up gas pressure, and carrier gas flow rate from the GC were examined and optimized for the detection of controlled substances. The accuracy, precision, linearity, and optimized detection limits for drugs such as cocaine (98.5%, 1.2%, 0.9998, 1.5 ng), heroin (99.3%, 0.94%, 0.9998, 2.0 ng), and fentanyl (98.5%, 1.7%, 0.9752, 9.7 ng) are reported. In general, the limits of detection for cocaine, heroin, fentanyl, and methamphetamine after optimization were comparable to gas chromatography-mass spectrometry (GC-MS) in "scan mode", which had detection limits of 1.1-38 ng on column. The VUV absorption spectra of cocaine, PCP, lorazepam, and HU-210 are also reported. And three samples of "real world" cocaine are analyzed to demonstrate applicability to forensic drug analysis.

Identifying Thermal Decomposition Products of Nitrate Ester Explosives Using Gas Chromatography-Vacuum Ultraviolet Spectroscopy: An Experimental and Computational Study

Analysis of nitrate ester explosives (e.g., nitroglycerine) using gas chromatography-vacuum ultraviolet spectroscopy (GC-VUV) results in their thermal decomposition into nitric oxide, water, carbon monoxide, oxygen, and formaldehyde. These decomposition products exhibit highly structured spectra in the VUV that is not seen in larger molecules. Computational analysis using time-dependent density functional theory (TDDFT) was utilized to investigate the excited states and vibronic transitions of these decomposition products. The experimental and computational results are compared with those in previous literature using synchrotron spectroscopy, electron energy loss spectroscopy (EELS), photoabsorption spectroscopy, and other computational excited state methods. It was determined that a benchtop GC-VUV detector gives comparable results to those previously reported, and TDDFT could predict vibronic spacing and model molecular orbital diagrams.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Instrumental and chemometric analysis of opiates via gas chromatography-vacuum ultraviolet spectrophotometry (GC-VUV)

Since its introduction, gas chromatography (GC) coupled to vacuum ultraviolet spectrophotometry (VUV) has been shown to complement mass spectrometry (MS) for materials such as petrochemicals, explosives, pesticides, and drugs. In forensic chemistry, opioids are commonly encountered but rarely are the samples pure. This work focuses on GC-VUV analysis applied to naturally occurring (e.g., morphine), semi-synthetic (e.g., heroin), and synthetic (fentanyl) opioids as well as common adulterants and diluents (e.g., lidocaine and quinine). The specificity of the VUV spectra were examined visually as well as via descriptive statistical methods (e.g., correlation coefficients and sums of square residuals). Multivariate pattern recognition techniques (principal component analysis and discriminant analysis (DA)) were used to prove the opioid spectra can be reliably differentiated. The accuracy of the DA model was 100% for a test set of VUV spectra. Finally, three "street" heroin samples were analyzed to show "real-world" performance for forensic analyses. These samples contained adulterants such as caffeine, as well as by-products of heroin manufacture.

Simulation of Vacuum Ultraviolet Absorption Spectra: Paraffin, Isoparaffin, Olefin, Naphthene, and Aromatic Hydrocarbon Class Compounds

The advent of a new vacuum ultraviolet (VUV) spectroscopic absorption detector for gas chromatography has enabled applications in many areas. Theoretical simulations of VUV spectra using computational chemistry can aid the new technique in situations where experimental spectra are unavailable. In this study, VUV spectral simulations of paraffin, isoparaffin, olefin, naphthene, and aromatic (PIONA) compounds using time-dependent density functional theory (TDDFT) methods were investigated. Important factors for the simulations, such as functionals/basis sets and formalism of oscillator strength calculations, were examined and parameters for future PIONA compound simulations were obtained by fitting computational results to experimental spectra. The simulations produced satisfactory correlations between experimental observations and theoretical calculations, and enabled potential analysis applications for complex higher distillate fuels, such as diesel fuel. Further improvement of the methods was proposed.

Gas chromatography vacuum ultraviolet spectroscopy: A review

Accelerated technological progress and increased complexity of interrogated matrices imposes a demand for fast, powerful, and resolutive analysis techniques. Gas chromatography has been for a long time a 'go-to' technique for the analysis of mixtures of volatile and semi-volatile compounds. Coupling of the several dimensions of gas chromatography separation has allowed to access a realm of improved separations in the terms of increased separation power and detection sensitivity. Especially comprehensive separations offer an insight into detailed sample composition for complex samples. Combining these advanced separation techniques with an informative detection system such as vacuum ultraviolet spectroscopy is therefore of great interest. Almost all molecules absorb the vacuum ultraviolet radiation and have distinct spectral features with compound classes exhibiting spectral signature similarities. Spectral information can be 'filtered' to extract the response in the most informative spectral ranges. Developed algorithms allow spectral mixture estimation of coeluting species. Vacuum ultraviolet detector follows Beer-Lambert law, with the possibility of calibrationless quantitation. The purpose of this article is to provide an overview of the features and specificities of gas chromatography-vacuum ultraviolet spectroscopy coupling which has gained interest since the recent introduction of a commercial vacuum ultraviolet detector. Potentials and limitations, relevant theoretical considerations, recent advances and applications are explored.

Evaluation and application of gas chromatography - vacuum ultraviolet spectroscopy for drug- and explosive precursors and examination of non-negative matrix factorization for deconvolution

Since the introduction of a benchtop vacuum ultraviolet (VUV) absorption spectroscope with an increased wavelength range towards to the high energetic ultraviolet radiation, gas chromatography coupled to VUV has been proven a powerful tool in several fields of application such as petroleomics, permanent gas analytic, pesticide analytic and many more. In this study, the potential of GC-VUV for investigations was examined, focusing on drug- and explosive precursors as well as chemical warfare simulants. The ability of VUV absorption spectra to differentiate isomers is presented, among others for nitroaromatics. In addition, the limit of detection for target compounds was determined to 0.7 ng absolute on column. Furthermore, non-negative matrix factorization (NMF) was successfully implemented as alternative deconvolution approach and evaluated for the deconvolution of unknown substances. In comparison, the spectral library-based deconvolution was applied to a standard mixture and a simulated case study. The results reveal that the NMF is a useful additional tool for deconvolution because, unlike library-based deconvolution, it allows to investigate unknown substances as well.

Distinguishing drug isomers in the forensic laboratory: GC-VUV in addition to GC-MS for orthogonal selectivity and the use of library match scores as a new source of information

Currently, forensic drug experts are facing chemical identification challenges with the increasing number of new isomeric forms of psychoactive substances occurring in case samples. Very similar mass spectra for these substances could easily result in misidentification using the regular GC-MS screening methods in combination with colorimetric testing in forensic laboratories. Building on recent work from other groups, this study demonstrates that GC-VUV is a powerful technique for drug isomer differentiation, showing reproducible and discriminating spectra for aromatic ring-isomers. MS and VUV show complementary selectivity as VUV spectra are ring-position specific whereas MS spectra are characteristic for the amine moieties of the molecule. VUV spectra are very reproducible showing less than 0.1‰ deviation in library match scores and therefore small spectral differences suffice to confidently distinguish isomers. In comparison, MS match scores gave over 10‰ deviation and showed significant overlap in match score ranges for several isomers. This poses a risk for false positive identifications when assigning compounds based on retention time and GC-MS mass spectrum. A strategy was developed, based on Kernel Density Estimations of match scores, to construct Receiver Operating Characteristic (ROC) curves and estimate likelihood ratios (LR values) with respect to the chemical differentiation of drug related isomers. This approach, and the added value of GC-VUV is demonstrated with the chemical analysis of several samples from drug case work from the Amsterdam area involving both compounds listed in Dutch drug legislation (3,4-MDMA; 3,4-MDA; 4-MMC; 4-MEC and 4-FA) as well as their unlisted and thus uncontrolled isomers (2,3-MDMA; 2,3-MDA; 2- and 3-MMC; 2- and 3-MEC and 2- and 3-FA).