Simultaneous determination of saponins and isoflavones in soybean (Glycine max L.) by reversed-phase liquid chromatography with evaporative light-scattering and ultraviolet detection.

The first validated analytical method permitting the simultaneous qualitative and quantitative determination of isoflavones and saponins in soy has been developed. It combines liquid chromatography with an ultraviolet and evaporative light-scattering detector (ELSD). Within less than 30 min, 6 isoflavones (detected at 254 nm) and 4 triterpene saponins (monitored with the ELSD) were baseline separated, using a reversed-phase C18 column and a mobile phase consisting of water and acetonitrile, both containing 0.025% triflouroacetic acid. The method was validated for limit of detection (LOD), linearity, repeatability, precision, and accuracy. LOD was 3.2-6.0 ng/mL for isoflavones and 10.4-14.2 microg/mL for saponins, and linearity was indicated by R2-values of 0.997 and higher. Intra- and interday precisions of the assay were below 7.0% for all of the compounds except for one, which was only present in trace amounts in the samples. Repeatability was indicated by very stable retention times and relative standard deviations well below 4.0% for multiple injections (n = 3). Accuracy was confirmed by recovery rates between 96.8 and 101.0%, respectively. Different sample matrixes were successfully analyzed, proving the wide range of applicability of this method, including soybeans, capsules, liquids, and instant soy drinks.