Investigations into fentanyl precursors method classification by handheld Fourier transform infrared and Raman spectroscopy combined with multivariate statistical analysis

Abstract

A goal of narcotic chemical attribution is to exploit a substance of interest for chemical forensic information and evidence. This could include determining the synthetic method used to make it and if that substance is forensically linked to any others. In most cases narcotic chemical attribution is profiling the impurities that are present, which give the substance a chemical fingerprint. One narcotic of significant concern to law enforcement and emergency responders is fentanyl due to the health risk it poses from very small amounts and its increasing prevalence in illicit drug markets. Portable handheld spectroscopic instruments have scope to become useful tools for substance identification in the field to ensure safety and to uncover possible investigative leads; however, they have limitations with interpretation of substances that have very small proportions of other components including impurities. In this study, two spectroscopic techniques, Fourier transform infrared (FTIR) and Raman spectroscopy, were applied to determine the synthetic method employed for the fentanyl precursors N-phenethyl-4-piperidone (NPP) and 4-anilino-N-phenethylpiperidine (ANPP). Principal Component Analysis (PCA) with k-nearest neighbours (k-NN) and Orthogonal Partial Least Squares - Discriminant Analysis (OPLS-DA) were used to establish classification models. The use of low-level and mid-level data fusion strategies were employed to integrate the data from the different handheld spectroscopic instruments, which allowed suitable classification of validation samples by their respective synthetic method. These results demonstrate the versatility of handheld spectroscopic instruments, with results easily being exported for data fusion analysis and the possibility for future instrument library enhancements.