Abstract：In this study, the infrared spectra (IR) of 301 new psychoactive substances (NPS) reference substances, including 100 synthetic cannabinoids, 81 synthetic cathinone, 42 phenethylamines, 9 tryptamines, 5 piperazines, 7 phencyclidine-type substances, 2 aminoindanes, 55 other types, were analyzed. The discriminant ability of IR for NPS, especially for NPS structural analogues, was investigated. The result showed that IR could well distinguish all kinds of NPS, even for some analogues with high spectrum similarity. IR has a high discrimination ability for 97% NPS, including alkyl chain-substituted regioisomers, ring-substituted regioisomers, different benzene ring disubstituted analogues, etc. The matching coefficient method is commonly used for spectral library search. The calculated matching coefficient value depends on several factors such as the quality of spectrum, software, algorithm, etc., so it is difficult to determine a universal threshold for positive detection. Therefore, the matching coefficient method can be only used for preliminary screening. Compared with the matching coefficient method, the characteristic peak method is more accurate and reliable for qualitative identification. Generally, eight absorption peaks with relatively high intensity in the range of 2 500～650 cm-1 are selected as characteristic peaks, but for some NPS structural analogues with similar spectra, it is necessary to select some low intensity but distinguishable peaks as characteristic absorption peaks. Only when all the characteristic absorption peaks are detected can the positive detection be concluded. According to this principle, characteristic absorption peaks of 168 regulated NPS were determined, and formed the IR method part of the industry standard “Forensic sciences—Examination methods for 168 new psychoactive substances including 2-FA in suspected drugs—GC-MS, IR and LC”. The rapid qualitative analysis of NPS by infrared spectroscopy based on a characteristic peak method will greatly improve the efficiency and reduce the cost of NPS qualitative identification analysis.
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