Capillary GC Determination of Fentanyl and Midazolam in Human Plasma

A capillary GC method is described for simultaneously measuring fentanyl and midazolam in human plasma. Plasma was extracted at basic pH with cyclohexane–isopentanol (197:3) with Ro21-2212 added as internal standard and fentanyl derivative as carrier. The drug was back-extracted in H 2 SO 4 , and then the extract was made basic and reextracted with hexane. The organic phase was evaporated at 40°C under N 2 and the residue was dissolved in ethanol. A portion was analyzed on a wide-bore capillary column (10 m × 0.53 mm) of HP-1 (2.65 μm), operated at 255°C with N 2 as carrier gas and N–P detector. The calibration graph for fentanyl and midazolam was linear within the respective ranges of 0.5–100 and 5–1000 ng/ml with a correlation coefficient of 0.9998. The recovery of fentanyl and midazolam was 99.5 ± 4.5 and 99.9 ± 3.2%. The coefficient of variation within a run and between runs was less than 7%. The detection limit for fentanyl was 0.2 ng/ml plasma, and for midazolam was 0.5 ng/ml plasma. The method was applied to therapeutic drug monitoring and pharmacokinetic studies.