Quantitative analysis of fentanyl in rat plasma by gas chromatography with nitrogen–phosphorus detection

A sensitive assay method was developed to determine fentanyl, an opiate agonist, in rat plasma by gas chromatography with nitrogen–phosphorus detection. For the pretreatment of plasma samples, sodium hydroxide was added to denature protein and n-butyl chloride was used to extract fentanyl. The calibration curve was linear within the concentration range 0.5 to 50 ng/ml (r=0.9997). The limit of detection was 0.1 ng/ml, and 0.5 ng/ml could be quantified with acceptable precision. Furthermore, fentanyl could be determined in only 200 μl of rat plasma. The method has been successfully applied to an intramuscular pharmacokinetic study at a dose of 10 μg/kg. Therefore, the current method is a valuable analytical tool for investigating the pharmacokinetics of fentanyl at low clinical doses.