Evaluation of vibrational spectroscopic methods to identify and quantify multiple adulterants in herbal medicines

To counter the growth of herbal medicines adulterated with pharmaceuticals crossing borders, rapid, inexpensive and non-destructive analytical techniques, that can handle complex matrices, are required. Since mid-infrared (MIR), near infrared (NIR) and Raman spectroscopic techniques meet these criteria, their performance in identifying adulterants in seized weightloss herbal medicines is definitively determined. Initially a validated high pressure liquid chromatography methodology was used for reference identification and quantification of the adulterants sibutramine H2O·HCl, fenfluramine HCl and phenolphthalein. Of 38 products, only sibutramine and phenolphthalein were detected by HPLC. The spectroscopic measurements showed Raman was ill-suited due to sample burning and emission while NIR lacked adulterant selectivity. Conversely, MIR demonstrated apt identification performance, which manifested as spectrally meaningful separation based on the presence and type of adulterant during principal component analysis (test set validated). Partial least squares regression models were constructed from the MIR training sets for sibutramine and phenolphthalein - both models fitted the training set data well. Average test set prediction errors were 0.8% for sibutramine and 2.2% for phenolphthalein over the respective concentration ranges of 1.7-11.7% and 0.9-34.4%. MIR is apposite for the screening of anorectic and laxative adulterants and is the most viable technique for wider adulterant screening in herbal medicines.