Improved gas chromatographic method of determining diclofenac in plasma

Diclofenac was converted into either its methyl or ethyl ester in methanol or ethanol containing 0.1% or 0.5% sulfuric acid, respectively. The ester was extracted and subjected to gas--liquid chromatography with electron-capture detection. The esterification resulted in an increased sensitivity of the gas chromatographic detection, three times better than that previously reported for the formation of indolone ring in trifluoroethanol containing 0.5% sulfuric acid.