Temporal and qualitative analysis of injured decomposed skin tissues using ATR-FTIR combined with chemometrics

Injury timing estimation and the differentiation between antemortem and postmortem injuries are of considerable importance in forensic practice; however, the discriminatory performance of conventional methods declines markedly as decomposition progresses. In this study, we established a rabbit sharp-force injury model, collected full-thickness skin samples at 1-7 days postmortem, and used attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy combined with chemometric methods to analyze the biochemical alterations in superficial injured tissues under natural environmental conditions. The results showed that characteristic bands at 1740 cm^-1, 1650 cm^-1, 1540 cm^-1, and 1080-1030 cm^-1, corresponding primarily to lipid carbonyl, amide I, amide II, and phosphate/carbohydrate-related components, respectively, exhibited pronounced changes with increasing postmortem interval. Principal component analysis (PCA) of the spectral data revealed time-dependent variation trends in the biochemical composition of injured tissues among different groups. Among six machine-learning regression models, partial least squares regression (PLS-R) achieved effective prediction of the postmortem interval, with a highest R²(Pred) value of 0.932. Among three classification models, partial least squares discriminant analysis (PLS-DA) showed the best performance for distinguishing abdominal antemortem from postmortem injuries, with an accuracy of 0.976 in the independent prediction set. These findings indicate that ATR-FTIR spectroscopy combined with chemometric analysis can effectively capture dynamic biochemical changes in injured tissues and may provide a rapid and label-free spectroscopic approach for injury timing estimation and the discrimination between antemortem and postmortem injuries.