Determination of isoflavones in commercial soy products by HPLC and coulometric electrode array detection

The concentrations of the isoflavones daidzein, genistein and glycitein were determined in a large number of commercially-available soy products by high-performance liquid chromatography with coulometric electrode array detection using estriol as internal standard. During extraction, the naturally occurring glycosides were converted into their respective aglycones by acid hydrolysis. The analytes were separated on a C-18 reversed phase column, eluted with methanol/acetonitrile/50 mM sodium acetate pH 4.8 (40/5/55, v/v/v), and detected by a coulometric electrode array detector using twelve electrodes set to potentials between +250 and +800 mV (in increments of 50 mV) against palladium reference electrodes. The isoflavones could be determined over a wide concentration range (0.8-1135.0 mg/kg for daidzein, 1.9-1442.5 mg/kg for genistein and 0.5-154.6 mg/kg for glycitein). The recovery of the isoflavones in the different matrices was determined by the standard addition method and varied between 40.9-94.4%. The detection limits (S/N=3) depend on the matrix of the soy product and were found for daidzein between 0.3-1.6 mg/kg, for genistein between 0.3-1.7 mg/kg and for glycitein between 0.5-2.3 mg/kg.