Sensitive and selective assay for fentanyl using gas chromatography with mass selective detection

A modified gas chromatographic assay, using mass-selective detection, has been developed for the quantitation of fentanyl in swine serum. Fentanyl and sufentanil, the internal standard, were extracted using a single-step liquid-liquid extraction with dichloromethane. Sensitivity and selectivity were improved by using electron-impact ionization (EI) in the selected-ion monitoring (SIM) mode, where fentanyl and sufentanil were monitored using the fragment ions at m/z 245 and 289, respectively. The limit of quantitation (LOQ) is 0.05 ng/ml, using 1 ml of sample, with a C.V. of 10.8% and a signal-to-noise ratio of 29. Standard curves were linear (r2 = 0.999) over the working range of 0.05-1/5 ng/ml, using 1/y2 as a weighting factor. Recoveries averaged 69.8 +/- 4.7%, 91.0 +/- 13.0% and 90.9 +/- 10.3% at serum concentrations of 1.5, 0.5 and 0.1 ng/ml, respectively. Intra- and inter-day variances, were < 12% at 0.1 ng/ml, and < 10% at concentrations of 0.5, 1 and 1.5 ng/ml. Bias was 6.2% at the LOQ and < or = 12.8% at every other standard curve concentration. Applicability of the assay is demonstrated for the pharmacokinetic study of transdermally administered fentanyl in a postoperative swine.