Simultaneous liquid chromatographic analysis for carbamazepine and carbamazepine 10,11-epoxide in plasma and saliva by use of double internal standardization.

High-performance liquid chromatography (HPLC) was used for simultaneous quantitation of carbamazepine (CBZ) and carbamazepine 10,11-epoxide (CBZ-EP) in plasma and saliva. Because concentrations of CBZ can greatly exceed those of CBZ-EP after single doses, two internal standards, lorazepam and N-desmethyldiazepam were added to all samples. Following extraction with chloroform, the components are separated on a muBondapak CN column with a mobile phase composed of 30% acetonitrile in water. Total chromatography time in 10 min. Concentrations of CBZ and CBZ-EP as low as 18 and 56 ng/ml, respectively, can be detected using 0.5 ml of plasma or saliva. The maximum within-day and day-to-day coefficients of variation for both compounds are 6.3 and 7.0%, respectively. Specificity of the method was supported by a significant correlation (r = 0.99) between assay results of the present method and those of a previously published HPLC assay. Application of the method to protein binding and salivary measurements in a single-dose CBZ disposition study is demonstrated.