Large-Volume Sample Staking of Rice Polyphenols Prior to Their Determination by Non-aqueous Capillary Electrophoresis

Abstract

A rapid non-aqueous capillary electrophoretic (NACE) method for the separation of (−)-epicatechin (EPI), (+)-catechin (−CAT), kaempferol (KAE), quercetin (QUR), naringanin (NAR), ferulic acid (FA), and p-coumaric acid (p-CA) has been developed and applied to the determination of these compounds in different rice varieties. All seven compounds were separated on capillary of 50 μm × 68 cm (60-cm effective length) using 20 mmol L−1 borate buffer of pH 9.0 and 5 % acetonitrile in methanol. Large-volume sample stacking (LVSS) technique was optimized and used to preconcentrate non-detectable polyphenols of white polished rice. Rice extracts were concentrated on-line by LVSS prior to separation by non-aqueous capillary electrophoresis. An improvement of 10–55 times in detectability was achieved with injection at 50 mbar for 30 s followed by voltage inversion (−20 kV) for 5 s. Linear calibration range of 1–300 μg L−1 and 0.01–60 μg L−1 was observed for NACE and NACE-LVSS method respectively, with the detection limit of 0.33–2.0 and 0.006–0.19 μg L−1. Good reproducibility with standard deviations of less than 5 % was achieved. Polyphenol contents of different rice varieties were determined using developed method.