Quantification for saponin from a soapberry (Sapindus mukorossi Gaertn) in cleaning products by a chromatographic and two colorimetric assays

The extract from the pericap of the Chinese Soapberry (Sapindus mukorossi Gaertn.) is increasingly being added into a variety of cleaning products. To establish a standardized quantification method, the accuracy of the reverse phase high performance liquid chromatography (HPLC), vanillin-sulfuric acid and antimony pentachloride colorimetric assays were compared. The chemical structural features of saponin of S. mukorossi for the major peak in the HPLC chromatograms were identified by LC/MS/MS, ¹H and ¹³C NMR spectroscopy. Organically extracted crude soapnut saponin was employed as the standard. Linearity for the calibration graphs was found for the range of organic extracts of crude saponin down to 0.2 g/L aqueous solutions, for all three assays studied. The area of the main peak in the HPLC chromatograms and the absorbance values of two colorimetric assays of the commercial glycerin clear soap base with predetermined amount of soapnut extract were recorded and compared with calibration graphs constructed with an aqueous saponin solution. It was found that HPLC analysis provided more precise values then the two colorimetric assays. Discrepancies were found between the saponin contents for 12 commercial cleaning products in Taiwan as analyzed by HPLC and the values estimated by antimony pentachloride and vanillin-sulfuric colorimetric assays.