Estimating the weathering time of the final instar exuviae of Dermestes frischii by ATR-FTIR spectroscopy and GC–MS analysis

Dermestidae is a group of beetles common in forensic and storage research. The weathering patterns of their exuviae may have potential value for the estimation of the postmortem interval (PMI) in forensic entomology, burial time in forensic taphonomy and invasion time in storage containers. However, no relevant research has been done. In this study, a large number of final instar Dermestid exuviae were kept at 16 °C, 25 °C, 34 °C and indoor variable temperatures, allowing them to naturally weather for 90 days. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and gas chromatography-mass spectrometry (GC–MS) were used to detect 13 time points during this period. The corresponding spectra and peak areas of n-alkanes were obtained to explore whether some functional groups or n-alkanes undergo temporal changes in weathering. The results showed that 90 days of weathering did not significantly change the chromatographic peak area or the corresponding spectral absorption peaks of n-alkanes in Dermestid exuviae, while the amides showed an attenuation trend. The accuracy of the training set and test set of the established partial least square-discriminant analysis (PLS-DA) models were 89.04 % and 75.54 %, respectively, and that of the random forest (RF) models were 100 % and 75.00 %, respectively, achieving good classification results. This indicates that the weathering patterns of Dermestid exuviae have value for estimating the PMI. This study is the first to investigate the weathering of necrophagous beetle exuviae with the aim of estimating the PMI. In the future, additional research should be performed under outdoor conditions or a longer weathering time.