A reliable and simple differential pulse polarographic method is described for the determination of thymoquinone in black seed oil. The polarographic behaviour of thymoquinone was examined in various buffer systems over the pH range 5.0-10.0. Thymoquinone is reduced in a single, reversible peak at the dropping mercury electrode. The differential pulse polarogram showed a distinct peak in Sörensen buffer:methanol (3:7, v/v; pH 8.5) at a peak potential of -0.095 V (vs. silver/silver chloride electrode), and a plot of peak height against concentration was found to be linear over the range 0.2-15.0 microg/mL (R = 0.9998). The limit of detection was calculated to be 0.054 microg/mL. The polarographic method has been applied to determine thymoquinone in two black seed oil preparations available on the Austrian pharmaceutical market.

A differential pulse voltammetric method is presented for the determination of isopropylmethylphenols (carvacrol and/or thymol) in phytotherapeutic black seed oils. The voltammetric behaviours of these phenols were examined in various buffer systems over the pH range 3.5-10.0. In Sörensen buffered methanol solution (3:7; v:v; pH 8.5), the differential pulse voltammograms exhibited reproducible peaks at Ep + 0.49 V vs. silver-silver chloride-potassium chloride 3 M for both carvacrol and thymol. Under these conditions, a plot of peak height against concentration of the isopropylmethylphenols was found to be linear over the range 0.25-2.5 microg/mL (r = 0.999). The detection limit was 0.04 microg/mL. The described voltammetric method was tested on two black seed oils available on the Austrian market.

A simple and accurate differential pulse polarographic method has been developed for the determination of oosporein in the culture broth of the fungus Beauveria brongniartii. This hydroxybenzoquinone derivative is the only major secondary metabolite secreted by this entomopathogenic fungus, which is used as biological pest control agent (BCA) against Melolontha melolontha larvae. It can be found in the host organism as well as in the formulated product. The polarographic behavior of oosporein was examined in various buffer systems over the pH range 3-10. In Britton-Robinson buffer/methanol solution (3:7 v/v, pH 5.5) the differential pulse polarograms exhibited reproducible peaks at E(p) = -0.18 V vs silver/silver chloride/potassium chloride (3 M). Under these conditions, a plot of peak height vs concentration of oosporein was found to be linear over the range 5.9 x 10(-)(7) to 2.5 x 10(-)(5) M (0.18-7.74 microg mL(-)(1); r = 0.9998). The detection limit was calculated to be 54 ng mL(-)(1). To evaluate the concentration of oosporein, the standard addition method was applied. The analysis of oosporein in the culture broth led to a mean value of 524.9 microg mL(-)(1) broth with a relative standard deviation (S(rel)) of +/-2.6%. The proposed polarographic method is accurate, not time-consuming, and it is of low cost because no separation steps are necessary.

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