Determination of hypericin in herbal medicine products by differential pulse polarography

Abstract

A differential pulse polarographic method is presented for the determination of total hypericin in phytotherapeutic preparations (drops, tablets and capsules). The polarographic behaviours of hypericin and of pseudohypericin were examined in various buffer systems over the pH range 3.5–10.0. In Britton Robinson buffer:methanol solution (at pH 6.0) the differential pulse polarograms exhibited reproducible peaks at Ep−1.02 V vs silver/silver chloride for hypericin, and at −1.00 V for pseudohypericin. Under these conditions, a plot of peak height against concentration of hypericin was found to be linear over the range 0.5–9.0 µg/mL (r = 0.9994) and 9.0–16.0 µg/mL (r = 0.9987). The polarographic method was applied to the determination of the content of total hypericin (hypericin, pseudohypericin, protohypericin and protopseudohypericin) in herbal medicinal products containing Hypericum perforatum. The precursors protohypericin and protopseudohypericin were converted into hypericin and pseudohypericin, respectively, by subjecting them to artificial light or daylight prior to analysis. Under these conditions, no separation step was required for the polarographic analysis. In order to evaluate the total concentration of hypericin, the standard addition method with hypericin as standard was applied. The relative standard deviation involved in analysing various herbal medicinal products ranged from ±1.9 to 2.9%. Copyright © 2000 John Wiley & Sons, Ltd.