Scheme 1 shows the extraction of optically pure ketoprofen from the microemulsion as well as formation of the diastereomer for GC analysis. Sample (0.1 g) was added to 1 ml of internal standard (5 μg hydrocinnamic acid /ml acetonitrile) and then evaporated to dryness in a vacuum evaporator for 10 min at 60 °C. Next, 5 ml of ethyl acetate were added to the dried samples, which were subsequently sonicated for 5 min and then centrifuged for 10 min at 3500 rpm to pipette the resulting supernatant. This procedure was repeated twice. Five ml of the supernatant was taken and evaporated to dryness in a vacuum evaporator for 10 min at 60 °C. Next, 5 ml each of acetonitrile and n-hexane were added to the dried sample three times to extract the acetonitrile layer. The extracts were subsequently evaporated to dryness in a vacuum evaporator for 15 min at 60 °C. Further enantiomeric separation of the optically pure ketoprofen was then conducted following the method described by Paik et al. (2004). Briefly, the residue was dissolved in dichloromethane (200 μl) and then sonicated (1 min) after the addition of 0.06 μmol of triethyl amine (50 mM, 1.2 μl) and 2.4 μmol of ethyl chloroformate (60 mM, 4 μl). Subsequently, 2 μl of (R)-(+)-1-phenyl ethyl amine (0.5 M) were added and the sample was then sonicated again (2 min). After acidification (pH ⩽ 2) with 0.1 M hydrochloric acid (200 μl), the amide derivatives were extracted with diethyl ether (600 μl) and ethyl acetate (600 μl) in sequence. The combined extracts were then evaporated to dryness (under gentle nitrogen stream) and the residue was reconstituted in a mixture (20 μl) of toluene and ethyl acetate (1:1). Finally, 1 μl of the sample was injected for direct analysis by gas chromatography (GC).