Evaluation of chemical attribution signatures within gasoline over time using direct analysis in real time mass spectrometry and principal component analysis
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Abstract
Fire debris analysis requires fast, reliable, and reproducible instrumental methods to identify ignitable liquids, such as gasoline from fire debris evidence. Brand determination and source origin attribution should be utilized in forensic chemistry and fire debris analysis as determining the source of gasoline could be a vital investigative tool. Source attribution and brand determination of gasoline requires the analysis of unique components within the product to differentiate between brands and stations. Chemical attribution signatures (CAS) are components of a substance that are not a part of the substance itself, such as additives within gasoline. Direct analysis in real time mass spectrometry (DART-MS) has become an attractive analytical technique in forensic analysis because of its minimal sample preparation, rapid sampling, and its ability to be coupled to a variety of commercially available mass spectrometers. Principal Component Analysis (PCA) is a technique for reducing the dimensionality of large datasets in order to increase interpretability without losing any information about the spectral data.
Based on the results of past studies within our research group, differentiation between gasoline brands and between different gasoline stations of the same brand is possible using DART-MS analysis and PCA. It is theorized that as stations receive new shipments, the residual product within the underground tanks mixes with the new product and results in a different additive composition. The new mixture is thought to be different than a sample collected from that station before the delivery occurred. To test this hypothesis, several samples were collected from the same station over time and then DART-MS and PCA was completed. Comparison groups were made between samples based on the time that passed between them or what day of the week they were collected on. The PCA was able to evaluate the differences and similarities between the samples and group them based on those criteria. Additionally, to further apply this research to casework samples, DART-MS analysis of gasoline extracts collected from heated passive headspace concentration was completed to evaluate its efficacy at extracting higher molecular weight additives.
During initial experiments, the heated passive headspace concentration procedure was not efficient at extracting and concentrating gasoline additives. The sample that was used to complete the extraction was too dilute which would have contributed to the result of a spectrum not consistent with gasoline. Therefore, a dilution series was completed to evaluate the limit of detection of the DART-MS to detect additives in a gasoline sample. It was concluded that a 1:4 dilution of gasoline in pentane was the limit of detection for generation of a recognizable gasoline pattern .
A time course study was completed where gasoline samples were collected from the station over time to evaluate the change in product composition. The time course study concluded that additive composition of gasoline changes over time within the same station as a result of gasoline mixing. The gasoline mixing results in each sample developing a new set of CAS that are unique to that sample. This research aims to determine the difference between gasoline samples collected from the same station over time and how that could be applied to source determination in forensic casework.
Description
2024