Therapeutic drug assays with gas-liquid chromatography and optical detection

Clofibric acid (p-chlorophenoxyisobutyric acid), the major metabolite of Clofibrate, a drug used in the treatment of hyperlipemia, was assayed in blood serum using an ultraviolet absorbance monitor as a gas-liquid chromatographic detector. As in other gas-liquid chromatographic assays for this compound, an internal standard, p-chlorophenoxyacetic acid, was added, and the serum was acidified and extracted with organic solvent. The solvent was then evaporated and the acids converted into their methyl esters for analysis. The organic compounds in the effluent were scrubbed into a stream of 2-propanol, at a flow-rate of 0.5 ml/min. This was then "debubbled" and a portion drawn through the 20-microliters UV detector flow cell. With small-volume scubber and associated components, peak-broadening was minimal. Because of their moderately high extinction coefficients at 280 nm, the Clofibrate and the internal standard were detected in the submicrogram range without interference from long-chain fatty acid esters, which have similar retention times on the column used.