A local straight-line screening (LSLS) algorithm was recently designed as a method to detect synthetic drug(s) in adulterated herbal medicines based on infrared spectroscopy. Some modifications are made in this paper to improve the existing LSLS algorithm, including interpolation, second derivation, and change of calculation regions from 3 to 7 data points. These modifications have decreased the effect of unpredicted noises and baseline shift on infrared spectroscopy, resulting in outstanding detailed spectral characteristics of the suspected synthetic drugs. The algorithm has been tested using five kinds of synthetic drugs (sibutramine, fenfluramine, lovastatin, sildenafil, and methyldopa) in 40 herbal medicine samples. The concentration of the synthetic drug(s) predicted by the modified LSLS algorithm is closer to those determined by high-performance liquid chromatography. Consequently, the correct results rise from 30 obtained using the original LSLS to 36 obtained using the modified LSLS in 40 samples, the false negative responses drop from 5 to 1, and the false positive responses drop from 5 to 3. The results obtained using the M-LSLS algorithm based on the sibutramine spectrum collected at different times and on different instruments also vary within acceptable ranges. These allow the method to be more appropriate for the preliminary screening of herbal medicines suspected of adulteration with synthetic drugs, with high rapidity, accuracy, and cost effectiveness.

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