Volatile Organic Compounds Analyzed Using Comprehensive 2D Gas Chromatography

In a recent study led by scientists from Chaminade University of Honolulu in Hawaii, United States, the chromatographic profiles of postmortem bacterial species and volatile organic compounds (VOCs) were analyzed. Their findings were published in the Journal of Chromatography A (1).

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Volatile organic compounds (VOCs) are gases emitted into the air from products or processes, some of which can be harmful (2). These gases typically have high vapor pressure at room temperature, allowing it to evaporate or sublimate and enter the surrounding environment. This class of compounds is believed to have vast probative potential in forensic science, as it is associated with several types of physical evidence, such as accelerants, drugs, currency, and decomposing remains. Specifically, VOCs associated with decomposing remains are of interest because these gases are thought to have potential in aiding medicolegal death investigations. Decomposition odor has been exploited as a forensic trace in the past, with it having potential to demonstrate the prior location of decomposing remains; additionally, the core VOCs in decomposition odor have also been shown to change predictably over time, thus presenting the possibility to further develop this forensic trace as a biochemical marker to estimate postmortem interval.

However, prior research has not thoroughly investigated which VOCs can be directly linked to individual bacterial species on decomposing remains. For this study, the goal was to profile VOCs produced over time by individual bacteria species using comprehensive two-dimensional gas chromatography (GC×GC) to expand our foundational knowledge of what each bacterial species contributes to decomposition odor. Five different species of bacteria (Bacillus subtilis, Ignatzschineria indica, Ignatzschineria ureiclastica, Curtobacterium luteum, and Vagococcus lutrae) were individually cultured on standard nutrient agar and monitored daily using solid phase microextraction arrow (SPME Arrow) and GC×GC in combination with quadrupole mass spectrometry (qMS) and flame ionization detection (FID). With the GC×GC-qMS/FID approach, rich VOC profiles were generated to represent the bacterial species’ metabolic VOC production longitudinally.

The data obtained from the chromatographic output was used to be compared to a prior study using one-dimensional GC-qMS and between the five species to allow for investigating the extent of overlap between species. However, no single VOC could be found in all five bacterial species investigated, with little overlap being in the profile between species. To further visualize these differences, chromatographic peak data was investigated using two different ordination strategies, principal component analysis (PCA) and principal coordinate analysis (PCoA). Afterwards, the two ordination strategies were compared with each other using a Procrustes analysis, which is a form of statistical shape analysis which focusses on comparing the geometry of two objects or data sets (3). This was done to understand differences in ordination strategies between the separation science community and chemical ecological community.

The ordination strategies were found to produce similar results, as evidenced by the correlation of PCA and PCoA in the Procrustes analysis. For the analysis strategies, the scientists yielded distinct VOC profiles for each species. Though there is more research to be done, the scientists are encouraged by this prospect. “Further study of additional species will support understanding of the holistic view of decomposition odor from a chemical ecology perspective, and further support our understanding of the production of decomposition odor that culminates from such a complex environment,” they wrote (1).

References

(1) Furuta, K.; Byrne, J.; Luat, K.; Cheung, C.; Carter, D. O.; Tipton, L.; Perrault Uptmor, K. A. Volatile Organic Compounds Produced During Postmortem Processes Can Be Linked via Chromatographic Profiles to Individual Postmortem Bacterial Species. J. Chromatogr. A 2024, 1728, 465017. DOI: 10.1016/j.chroma.2024.465017

(2) Volatile Organic Compounds. American Lung Association 2024. https://www.lung.org/clean-air/indoor-air/indoor-air-pollutants/volatile-organic-compounds (accessed 2024-6-11)

(3) Procrustes alignment. Elizabeth DuPre 2021. https://neurodatascience.github.io/fmralign-tutorials/1-2-procrustes.html (accessed 2024-6-11)