Raman spectroscopy of seized drugs and density functional theory interpretation

The demand for a hand-held Raman spectrometer in the fast and accurate detection and identification of seized drugs is much higher than before, especially when facing unknown suspicious drugs. However, Raman spectra for the different drugs are less reported due to the inaccessibility of them. Here, we reported the experimental Raman spectra in detail of four typical drugs (such as methamphetamine, ketamine, caffeine, and magu). The Raman vibrational frequencies were also calculated by the method of density functional theory (DFT) at Becke-3-Lee-Yang-Parr (B3LYP) level with the 6-31 G and 6-31G(d,p) basis set. The results show that the experimental Raman spectra of these typical drugs are consistent with the theoretical Raman spectra. Using the potential energy distribution (PED) calculation with the GAR2PED program, the assignments of the observed Raman bands to the vibrational modes were presented. Further, methamphetamine and its camouflage N-benzylisopropylamine were analyzed by Raman spectroscopy and DFT calculations, and the result showed that the obvious differences of the Raman characteristic bands for these two samples could be found so that Raman technique could be used to identify the authenticity of methamphetamine. All the above results confirm the potential of the approach involving Raman spectroscopy combined with DFT calculations in the characterization of drugs. Based on this, the experimental spectra of seized drugs measured directly through a plastic package were studied. Raman spectroscopy has the advantage of being performed through packaging without disturbing the samples. Polypropylene transparent packaging does not alter the spectra of the drugs but will mask the corresponding bands if the Raman spectrum has a strong autofluorescence interference.