A sample of 0.5 g of cream or gel was weighed into a 50 ml
conical centrifuge tube using a Mettler balance model PB-303S
(Mettler-Toledo, Switzerland). The sample was extracted by
addition of 30 ml of acetonitrile followed by vortex mixing
for 3 min. An aliquot of 1.5 ml was transferred to a microcentrifuge
tube and subjected to centrifugation at 10,000 rpm for
5 min using an Eppendorf microcentrifuge (Brinkmann Inst.
Inc., NY, USA). An aliquot of 50 l of the supernatant was
injected directly into the HPLC.