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

•Thermal separation was proposed to help FTIR identify mineral drugs in herb products.•Thermal separation procedures can be designed according to FTIR and TG analysis.•Alum in a herbal powder for oral ulcer was detected by FTIR with thermal separation.•Adulterated and deteriorated products of the herbal powder preparation were found.