Sample analysis. Dried leaf material was ground with a pestle and mortar, and around 100 mg weighed accurately into a 4-cm3 glass vial (Fisher), to which was added 1 cm3 of acetonitrile and the vial capped prior to shaking at around 50 r.p.m. for 24 h in the dark. This technique has been found to consistently extract .90% of the artemisinin in a herbal sample. After this maceration 10 mL of liquid was removed and placed in a 2-cm3 crimp vial with 100 mL of santonin stock solution (0.1 mg cm23 in acetonitrile) and the sample was made up to 1 cm3 with HPLC-grade acetonitrile. Samples were analysed blind, in a programme that periodically included previously analysed samples to monitor consistency. Precision and accuracy of analyses were measured by the inclusion of samples of known artemisinin concentration.