The anticonvulsant drug gabapentin and its heptaneacetic acid analog—used here as an internal standard—are isolated from serum (pH 9) with an octyldecyl (C-18) solid-phase sorbent column. To enhance analytical detection, trinitrobenzene derivatives of these extracted compounds are prepared quickly within 10 min. To furthur improve chromatographic selectivity, the derivatives are concentrated on a thin C-18 solid-phase membrane and interferences are washed away. The retained purified derivatives are eluted from the membrane with a small volume of solvent and the eluate is directly injected onto an Ultrasphere C-18 high-performance liquid chromatography column with quantification at 340 nm. No evaporation-concentration steps are necessary. Recoveries (extraction) of gabapentin and the internal standard are 94.2 ± 2.9% and 98 ± 2.0%, respectively. Analytical responses are linear from lower limit of sensitivity of 0.05 mg/L up to at least 10 mg/L. Between-run coefficients of variation (CV) range from 2.3 to 2.9% through the concentration range 0.5–4.0 mg/L. To illustrate the rationale for selection of test parameters for a robust method, we present optimization graphs for these processes. Moreover, we discuss the advantage of the packed cartridge and membrane sorbents as companion extraction devices.
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Lensmeyer et al. (1995) studied this question.