FTIR microspectroscopy coupled with variable selection methods for the identification of flunitrazepam in necrophagous flies

The detection and identification of a drug in a corpse through the analysis of fly larvae feeding on the body by spectroscopic techniques promises to be of great value, because of their sensitivity, promptness, low cost and simplicity. Therefore, the purpose of this study was to develop a method based on Fourier-transform infrared (FTIR) microscopy to identify and discriminate flunitrazepam in necrophagous flies (Chrysomya megacephala, Chrysomya albiceps and Cochliomyia macellaria) as a non-invasive and non-destructive technique. Thirty-two Wistar mice were divided into two groups of sixteen and supplemented in two categories: group 1 - ethanol; and group 2 - standard flunitrazepam at a dose of 2 mg kg(-1). Spectra from the larvae samples were analyzed by principal component analysis-linear discriminant analysis (PCA-LDA), and variable selection techniques such as successive projection algorithm (SPA-LDA) and genetic algorithm (GA-LDA) to determine if control versus flunitrazepam could be segregated. In addition, the multivariate classification accuracy results were tested based on sensitivity, specificity, positive (or precision) and negative predictive values, Youden index, and positive and negative likelihood ratios. For control vs. flunitrazepam category, the sensitivity and specificity levels, using 46 wavenumbers by SPA-LDA, gave relatively good accuracy (up to 82.3% control vs. flunitrazepam). The resulting GA-LDA model also successfully classified both classes with respect to the main biochemical alterations induced by flunitrazepam using only 40 wavenumbers (up to 88.2% control vs. flunitrazepam). Compared to classical methods, this new approach could represent an alternative and an innovative tool for faster and cheaper evaluation in entomotoxicology.