Identifying Insects with Spectroscopy

Entomology, that is the study of insects, can provide vital information during a forensic investigation. After an individual dies their body begins to undergo a complex decomposition process almost immediately, attracting a variety of insects along the way who wish to colonise, feed on the temptingly putrefying remains and reproduce.

Specialists have been taking advantage of this fact for hundreds of years, allowing us to discover that the types of insects present on a cadaver and the age of these insects can prove invaluable in estimating how much time has passed since the victim died (known as the post-mortem interval). Simply put, certain species prefer the decomposing corpse at different stages in the decay process, and with the right information, investigators can study the insects and their ages and begin to develop a kind of timeline.

Currently, accurately identifying species and establishing the development stage of an insect can be time-consuming and requires the expertise of an entomologist and potentially DNA analysis. This is obviously not ideal – your average police force does not have an entomologist on hand, nor do they have oodles of times to dedicate to insect identification. Even with the assistance of an entomologist, accurately determining the age of maggots can be problematic. Although larvae may be of a certain age, their length and weight can be affected by a variety of factors that may not be accounted for, such as starvation (Singh and Bala, 2009).

As you might expect, researchers are searching for ways to resolve this issue, and analytical chemistry might just be the answer.

As analytical chemistry progresses and increasingly advanced analytical techniques are developed, we are seeing more and more fascinating applications of these instruments to established areas of study. In a recent study published in Forensic Science International, researchers took a well-established technique and applied it to forensic entomology. In this case, they used a form of infrared spectroscopy.

Infrared spectroscopy is an analytical technique which determines the amount of radiation absorbed by a molecule. Infrared light is directed towards to sample and, depending on the molecule, a certain amount of radiation will pass through the sample and some will be absorbed.  When a molecule absorbs radiation, the bonds within it begin to vibrate. Different bonds will vibrate and be influenced by surrounding atoms to a different extent, thus allowing for a unique ‘spectrum’ to be produced. This spectrum is essentially a graph displaying how much radiation was absorbed by the sample at what wavelength. Scrutinising this spectrum can allow the analyst to determine what kind of molecules are present. Although this is not sufficient to specifically identify compounds, the spectrum produced can at least be used to distinguish between different samples, which will produce different spectra. The spectra essentially act as ‘fingerprints’ for different substances.

FTIRSpectrum

Typical IR spectroscopy spectra.

If you want to know a bit more about this technique, Compound Chemistry has a great little page on IR spectroscopy.

So back to how this analytical technique can be useful in forensic entomology. The proof of principle study to which I’m specifically referring aimed to both identify the species of larvae and the life cycle stage using vibrational spectroscopy, in this case Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy. A slightly long-winded name, but in short this is simply a form of IR spectroscopy that allows in situ analysis of solid or liquid samples without the need for sample preparation. Anyone who has spent many painful hours preparing samples for analysis will appreciate the benefit of this.

Three species commonly encountered at incident scenes were used in the study; C. vomitoria, L. sericata, and M. domestica (that is the bluebottle fly, the green bottle fly and the common housefly respectively). One of these species (the C. vomitoria) was also selected for a study focussing on the life cycle, in which spectra were collected for each time point in the insect’s life cycle. Scans were based on a crushed mixture of epidermis and internal matter (not possible for a ‘no maggots were harmed during the making of’ notice then). The results were promising, indicating FTIR spectroscopy could be a great tool in forensic entomology.

But surely there is a whole range of analytical instruments out there (yes, there sure is), so why would this one be any more suitable for forensic entomology? One of the major benefits of FTIR is the possibility of handheld IR instrumentation, which basically means it can be used in situ at the scene of a crime or other incident. Granted the investigator would need the appropriate equipment, but it beats shipping samples back to the lab and waiting for analysis. IR spectroscopy is a non-destructive technique (okay, the insects were somewhat mutilated in this study, but nevertheless the samples themselves remained after analysis). The ability to perform analyses without destroying the sample has a huge benefit, particularly if the available sample is limited, allowing for alternative tests and future analysis to be conducted if necessary. This of course is an advantage in forensic science. Also of great benefit to a legal investigation, IR instrumentation is fast, with spectra being collected in a matter of minutes.

There is however the glaring problem of the cost of analytical instrumentation. As I previously stated, your average police force may not have a forensic entomologist on hand… they equally may not have the funds to purchase analytical instrumentation such as IR spectrometers.

Bearing in mind this was merely a pilot study, using a very limited sample size, the research shows some promising results – that it is possible to classify species and life cycle stage using IR spectroscopy. Were this to be expanded upon, you could theoretically develop a database of IR spectra collected from different species of insects at different stages of development, allowing future spectra obtained from unknowns to be compared and, hopefully, identified.

References

Pickering, C. L. Hands, J. R. Fullwod, L. M. Smith, J. A. Baker, M. J. Rapid discrimination of maggots utilising ATR-FTIR spectroscopy. Forensic Sci Int. 249 (2015), pp 189-196.

Singh, D. Bala, M. The effect of starvation on the larval behaviour of two forensically important species of blow flies (Diptera: Calliphoridae). Forensic Sci Int. 193 (2009), pp. 118-121.

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