Solid-phase extraction and HPLC assay of nicotine and cotinine in plasma and brain

The aim of this study was to develop a simple and reliable assay for nicotine (NIC) and its major metabolite, cotinine (COT), in plasma and brain. A method was developed that uses an extraction method compatible with reverse-phase high-performance liquid chromatography (HPLC) separation and ultraviolet (UV) detection. Sequential solid-phase extraction on silica columns followed by extraction using octadecyl (C18) columns resulted in mean percent recovery (n = 5) of 51 +/- 5, 64 +/- 10, and 52 +/- 10% for NIC, COT, and phenylimidazole (PI), respectively, in spiked 1-mL serum samples. Recovery (mean +/- SEM) of the internal standard (PI) from spiked samples of nicotine-injected rats averaged 64.1 +/- 1.5% (n = 138) from plasma, and 20.7+/-0.8% (n = 128) from brain. The limits of detection of NIC in plasma samples were approximately 8 ng per mL, and of COT, 13.6 ng per mL. Further optimization of our extraction method, using slower flow rates and solid-phase extraction on silica columns, followed by C18 column extraction, yielded somewhat better recoveries (38 +/-3%) for 1-mL brain homogenates. Interassay precision (coefficient of variation) was determined on the basis of daily calibrations for 2 months and was found to be 7%, 9%, and 9% for NIC, COT, and PI, respectively, whereas intra-assay variability was 3.9% for both NIC and COT. Limited studies were performed on analytical columns for comparison of retention, resolution, asymmetry, and column capacity. We concluded that a simple two-step solid-phase extraction method, coupled with HPLC separation and UV detection, can be used routinely to measure NIC and COT in biological fluids and tissues.