Analysis of household ignitable liquids and their post-combustion weathered residues using compound-specific gas chromatography-combustion-isotope ratio mass spectrometry

Determination of stable nitrogen isotopic ratios of nitrate ions in ammonium nitrate

Ammonium nitrate (AN) is one of the most commonly used explosives in criminal cases. The comparison and source-tracing of AN is important for investigation of attribution and fingerprinting of an explosive used at different events. The stable isotope signature of AN is an important index for comparison and tracing. However, the characteristics of the stable nitrogen isotopic ratios of AN (δ¹⁵ N_NH4NO3 ) alone are not sufficient to achieve a fine comparison between different AN samples. To increase the comparison index and further improve the discriminability between stable nitrogen isotopic ratios of different ANs, a method of isolation and analysis of nitrate ions in AN was established using stable-isotope-ratio mass spectrometry (IRMS). The method was based on the principle that strong alkali react with AN to produce ammonia and nitrate. After the isolation, stable nitrogen isotopes of nitrate ions (δ¹⁵ N_NO3 ) were obtained using IRMS, and then the stable nitrogen isotopes of ammonium ions from AN (δ¹⁵ N_NH4 ) was calculated according to the principle of mass balance. The results show that the method is effective for the isolation of nitrate ions without notable isotope fractionation. The developed method was applied to analyze and discriminate AN samples from eight different cities in China. Three samples out of the initial eight AN samples with similar δ¹⁵ N_NH4NO3  values were further distinguished by their δ¹⁵ N_NH4 and δ¹⁵ N_NO3  values. The isolation and stable-nitrogen isotopic analysis method developed for nitrate ions in AN is simple and effective, thereby increasing the discriminability of the stable isotope ratios in AN.

Purification of ammonium nitrate via recrystallization for isotopic profiling using isotope ratio mass spectrometry

Stable-isotope analysis of ammonium nitrate (AN) plays an important role for comparing and tracing the sources of AN samples. Numerous studies have reported the application of isotope ratio mass spectrometry (IRMS) for the stable-isotope analysis of AN. However, in real cases, AN is often mixed with liquid hydrocarbons, organic explosives, wood flour, and sodium chloride. Therefore, the purification of AN samples prior to IRMS analysis is essential. Based on the different solubilities of AN at different temperatures, AN samples were purified by recrystallization. The results show that recrystallization can effectively purify AN, and the isotope fractionation effect caused by recrystallization increases the stable-isotope ratio (δ¹⁵N increased by 0.07-0.14‰, δ¹⁸O increased by 0.15-0.33‰, and δ²H increased by 4.61-18.16‰). The change of δ¹⁵N and δ¹⁸O were close to the standard deviation values, and did not affect the differentiation of AN. δ²H was deemed unsuitable for AN differentiation, owing to isotopic exchange during recrystallization. Eight AN samples from different cities and four AN samples produced in different years and batches from the same manufacturer in China were analyzed after recrystallization. Both groups of AN samples could be distinguished by combining the nitrogen and oxygen isotope ratios. Therefore, recrystallization can be used as a means of AN purification for the accurate determination of AN in ammonium nitrate fuel oil and explosive residues.

Detection and Characterization of Ignitable Liquid Residues in Forensic Fire Debris Samples by Comprehensive Two-Dimensional Gas Chromatography

This study covers an extensive experimental design that was developed for creating simulated fire debris samples under controlled conditions for the detection and identification of ignitable liquids (IL) residues. This design included 19 different substrates, 45 substrate combinations with and without ignitable liquids, and 45 different ILs from three classes (i.e., white spirit, gasoline, and lamp oil). Chemical analysis was performed with comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS) for improved separation and compound identification. The enhanced peak capacity offered by GC×GC-TOFMS allowed the use of a target compound list in combination with a simple binary decision model to arrive at quite acceptable results with respect to IL detection (89% true positive and 7% false positive rate) and classification (100% correct white spirit, 79% correct gasoline, and 77% correct lamp oil assignment). Although these results were obtained in a limited set of laboratory controlled fire experiments including only three IL classes, this study confirms the conclusions of other studies that GC×GC-TOFMS can be a powerful tool in the challenging task of forensic fire debris analysis.

Study of the Weathering Process of Gasoline by eNose

In a fire investigation the rapid detection of the presence of ignitable liquids like gasoline is of great importance as it allows appropriate treatment of the remains, the identification of prevention methods and detects the possible presence of an arsonist. In some cases, analysts cannot access the fire scene in the first few hours due to the dangers involved in the situation and, as a consequence, phenomena such as weathering start. Ignitable liquid weathering is an evaporation process that results in an increase in the abundance of non-volatile compounds relative to volatile compounds, and this process changes the chemical composition. In the present work, the weathering of samples of gasoline at different times (from 0 h to a month) has been studied using an electronic nose (eNose). The influence of the volume used (40 µL and 80 µL) and the type of support (cork, wood, paper and cotton sheet) has been studied. Chemometric tools have been used with the aim of ascertaining the weathering time for which the developed method is capable of detecting the presence of gasoline. The eNose was able to discriminate samples of weathered gasoline. The support used for the samples did not seem to have an influence on the detection and the system.

Forensic Mass Spectrometry

Developments in forensic mass spectrometry tend to follow, rather than lead, the developments in other disciplines. Examples of techniques having forensic potential born independently of forensic applications include ambient ionization, imaging mass spectrometry, isotope ratio mass spectrometry, portable mass spectrometers, and hyphenated chromatography-mass spectrometry instruments, to name a few. Forensic science has the potential to benefit enormously from developments that are funded by other means, if only the infrastructure and personnel existed to adopt, validate, and implement the new technologies into casework. Perhaps one unique area in which forensic science is at the cutting edge is in the area of chemometrics and the determination of likelihood ratios for the evaluation of the weight of evidence. Such statistical techniques have been developed most extensively for ignitable-liquid residue analyses and isotope ratio analysis. This review attempts to capture the trends, motivating forces, and likely impact of developing areas of forensic mass spectrometry, with the caveat that none of this research is likely to have any real impact in the forensic community unless: (a) The instruments developed are turned into robust black boxes with red and green lights for positives and negatives, respectively, or (b) there are PhD graduates in the workforce who can help adopt these sophisticated techniques.

Multidimensional gas chromatography methods for bioanalytical research

Multidimensional gas chromatography (MDGC) methods are high-resolution volatile chemical separation techniques, and comprise classical heart-cutting MDGC and its more recent incarnation, comprehensive 2D GC. Although available for a long period, MDGC approaches are still not widely practiced in the field of bioanalysis, possibly reflecting the general preference for regular GC versus MDGC approaches. With the recent introduction of ‘-omic’ techniques that emphasize global nontargeted profiling of metabolites within living systems, it is evident that MDGC is gaining momentum as a separation tool, since it offers very high resolution. By untangling metabolites within highly complex biological matrices, and expanding the metabolic coverage, MDGC plays a frontline role in ‘-omics’ based studies. This review highlights state-of-the-art MDGC approaches, and summarizes the recent developments in bioanalytics.