Determination of dexamethasone in bovine tissues by coupled-column normal-phase high-performance liquid chromatography and capillary gas chromatography-mass spectrometry.

A new method for the determination of dexamethasone in bovine liver and muscle tissues has been developed. Crude tissue extracts were obtained by means of a three-phase liquid-liquid extraction scheme. The resulting residue was subjected to coupled-column normal-phase high-performance liquid chromatography which served to isolate the drug for the purpose of screening and quantification. Sample was injected onto the first column of the system, a phenyl column, from which a heart-cut was diverted to a short silica column which retained dexamethasone. The contents of this column were backflushed onto a cyanopropyl column which isolated dexamethasone. Mobile phases consisted of hexane modified with 2-propanol, acetic acid, and water. Analysis of each sample was completed in 15 min. Quantitation was performed by external standard calibration of ultraviolet response at 239 nm. Limits of detection were estimated to be 4 and 6 ppb in muscle and liver, respectively. In addition to screening and quantitation, the coupled-column system purified tissue extracts for gas chromatographic-mass spectrometric analysis which, in the selected-ion monitoring mode, confirmed the identity of the trimethylsilyl-enol-trimethylsilyl derivative of dexamethasone.