Simultaneous determination of fentanyl and alfentanil in rat tissues by capillary column gas chromatography

Fentanyl and alfentanil were determined in blood and in thirteen tissues from rats by gas chromatography, with splitless injections on a fused-silica capillary column and nitrogen-selective detection. Sufentanil was used as the internal standard for both drugs and the acetyl analogue of sufentanil was used as an external standard to quantitate extraction recoveries. The extraction solvent was isopentanol-pentane (1:49) and a back-extraction into 0.1 M hydrochloric acid was used for sample clean-up. The extraction recoveries from tissue homogenates averaged 88% for fentanyl, 72% for alfentanil and 87% for sufentanil. The standard curves were linear over a range of 0.5-800 ng per sample for both drugs and the coefficients of variation for eight determinations of the drugs in tissue homogenates were 2-9% at 0.8-32 ng per sample. Assays of tissues from rats treated with simultaneous intravenous infusions of fentanyl and alfentanil confirmed the high precision and sensitivity of the method, which should therefore prove feasible for studies of the tissue distribution of these opioids.