New qualitative analysis strategy for illicit drugs using Raman spectroscopy and characteristic peaks method

Performing fast qualitative identification of seized illegal drugs by Raman spectroscopy is challenging due to fluorescence interference as well as chemical complexity. Spectrometers with 785-nm excitation, 1,064-nm excitation, and sequentially shifted excitation (SSE) were compared for their effect on fluorescence reduction. The characteristic peaks method, which is independent of cutting agents, was tested as a new strategy to broaden the application of the Raman technique. The suitability of the characteristic peaks method was fully examined by analyzing a large amount of seized illegal drugs, including 72 methamphetamine hydrochloride (concentration range of 13.9%-99.4%), 68 ketamine hydrochloride (17.7%-99.8%), 176 heroin hydrochloride (5.2%-79.5%), 51 cocaine hydrochloride (21.1%-94.5%), and 33 cocaine base (30.9%-92.5%) samples. The results showed that seized methamphetamine, ketamine, and cocaine samples had no or little fluorescence. Hence, in regard to detection of these samples, the advantage of using 1,064-nm excitation and SSE compared with 785-nm excitation was quite limited. Regarding the heroin samples, a significant improvement of the "high" confident positive detected rate was evident for 1,064 nm excitation (60.8%) and SSE (61.4%), compared with 785-nm excitation (13.1%). However, it was also seen that even if 1,064-nm excitation and SSE were applied, the fluorescence of heroin samples was still unable to be fully overcome. By using the characteristic peaks method, low LOD results of 5%-20% were acquired for 40 types of drug mixtures, and lower LODs were obtained for the 60% of the drug mixtures compared with library searching method. Raman spectroscopy in conjunction with the characteristic peaks method was shown to be fast, simple, accurate, and sensitive in the qualitative analysis of seized drug samples.