Infrared spectroscopic identification of mineral drugs in herbal preparations with thermogravimetry-guided thermal separation–A case study of alum in a herbal powder for oral ulcer

Abstract Identification is the first step to ensure the quality and safety of mineral drugs in herbal preparations. Fourier transform infrared (FTIR) spectroscopy is a simple, rapid, cheap and green technique to identify mineral drugs that are solid materials mainly composed of inorganic ingredients. However, the spectral peaks of mineral drugs are often covered by the absorption bands of other ingredients in the FTIR spectra of herbal preparations. According to the different thermal stabilities of the compositions of herbal preparations, thermal separation procedures can be designed by thermogravimetry (TG) and FTIR analysis to eliminate the spectral masks of other ingredients on mineral drugs. As a result, FTIR spectra of the heated samples can provide more intuitive and reliable evidences for the identification of mineral drugs in preparations. In this research, an FTIR method with TG-guided thermal separation was developed to identify the alum in Kouqiang Kuiyang San (KKS) that is a traditional Chinese medicinal preparation for oral ulcer. TG and FTIR analysis of KKS simulation mixtures and related herbal materials suggested that heating at 250 °C for 1 h can remove the absorption peaks of water and organic molecules but remain the characteristic peaks of NH4+ and 2CaSO4⋅H2O above 3000 cm−1. Adulterated and deteriorated commercial KKS preparations were found by the FTIR method with thermal separation. This research illustrates the feasibility of FTIR spectroscopy and TG-guided thermal separation for the rapid and accurate identification of mineral drugs in herbal preparations.