Tuesday, August 20, 2013

CSI: Smell Squad

Via Drew Friedman.

Faithful readers of I Smell Dead People (FN’s most wildly popular ongoing feature) know that people unlucky enough to catch the scent of decomposing human remains often remark on how unlike it is to any other smell. At the same time, neighbors sometimes fail to report the telltale odor because they misattribute it to a dead rat or other animal.

So how similar, in fact, are the odors of decomposing human and animal remains? It’s more than an academic question: police “cadaver dogs” in some jurisdictions are trained on decomposing pig carcasses. Well, thanks to researchers in Reno, Nevada, we have an answer.

The research team analyzed volatile organic compounds in the headspace of decomposing cow, pig, chicken, and human remains. They separately analyzed VOCs from bone, fat, muscle and skin tissue samples, both fresh and decomposed, using solid-phase microextraction and GC-MS.

Altogether, they found almost four dozen different volatile molecules. The distribution of VOCs across species and specimens was complex. Here’s a flavor:
Aldehydes were relatively numerous from fresh and decomposed tissues across the species. Nonanal was one of only a few compounds to be found in all decomposed samples, and all bone, fat and muscle, but absent from the fresh skin. Decanal was present only in decomposed samples, and in all of the cow and pig tissues, but only the chicken muscle. Although few in total count, aromatic hydrocarbons were prevalent in the different tissue types, both fresh and decomposed, and were generally prevalent across species. Toluene was the only compound found in every sample, both fresh and decomposed. It is a very chemically stable molecule and likely arises from the decomposition of the aromatic amino acids.
Despite the complexity, the researchers draw a couple of broad conclusions:
Pig VOC signatures were not found to be a subset of human; in addition to sharing only seven of thirty human-specific compounds, an additional nine unique VOCs were recorded from pig samples which were not present in human samples. The VOC signatures from chicken and human samples were most similar sharing the most compounds of the animals studied.
It is worth pointing out that this sort of chemical checklist approach (molecule X present or absent) is not an olfactory evaluation of the samples, which might result in somewhat different conclusions. Still, these results remind us that we must constantly question our assumptions. Pig physiology is sufficiently similar to human that they are useful in pharmacological and other types of research. As cadaver samples, maybe not so much.

A final point for the tech-heads out there: this is typical of the real-world complexity a forensic e-nose will have to sort out.

The study discussed here is “Characterization of the volatile organic compounds present in the headspace of decomposing animal remains, and compared with human remains,” by Mary E. Cablk, Erin E. Szelagowski, & John C. Sagebiel, published in Forensic Science International 220:118-125, 2012.

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