Determination of 5-hydroxynaphthoquinones in phytotherapeutic Drosera preparations by differential pulse polarography

A differential pulse polarographic method is described for the determination of three pharmacologically active 5-hydroxynaphthoquinones in Drosera-containing phytomedicines available on the Austrian market. The polarographic behaviours of plumbagin, 7-methyljuglone and droserone were examined in various buffer systems in the pH-range from 3.5 to 10.0. The compounds were reduced in a single reversible peak at the dropping mercury electrode. The differential pulse polarograms of the naphthoquinones showed distinct peaks in sodium acetate/acetic acid buffer:methanol solution, and a plot of peak height against concentration was found to be linear over the range 0.16–30.0 µg/ml for droserone, 0.06–8.0 µg/ml for plumbagin and 0.16–25.0 µg/ml for 7-methyljuglone. The polarographic method was applied to determine the amounts of these naphthoquinones in three Drosera-containing phytomedicines. In one preparation the droserone concentrations, depending on the sample taken, were found to be 6.81 and 5.78 µg/ml, whilst in a second preparation the droserone concentrations were 1.87, 3.67 and 0.51 µg/ml. The content of plumbagin and 7-methyljuglone was lower than the polarographic detection limit (0.4 µg/ml of each). In a third preparation neither droserone, plumbagin, nor 7-methyljuglone were polarographically detectable (<0.2 µg/ml). Copyright © 1999 John Wiley & Sons, Ltd.