A novel pretreatment method, which was performed using a two-dimensional high-performance liquid chromatography (2D-HPLC) system, was proposed for the direct analysis of drugs in human serum. A temperature-responsive column was used as a pretreatment column. The stationary phase of the temperature-responsive column exhibits temperature-regulated hydrophilic/hydrophobic characteristics. Controlling the ionic strength of the eluent enables human serum albumin (HSA) to pass through the column without retention. When serum samples containing barbiturates or benzodiazepines were injected into the temperature-responsive column using 10 mM of ammonium acetate (pH 6.5) as the mobile phase and in the temperature range of 10–40 °C, HSA was eluted from the column near the dead time, followed by the individual drugs. When the column temperature was changed, the retention times of the drugs were altered owing to surface property changes within the pretreatment column. These closely eluted compounds were subsequently introduced into the analytical column using a column-switching valve, with a minimal gap time to avoid foreign substance contamination. This new 2D-HPLC method afforded high-quality chromatograms of multiple drugs without unwanted peaks from foreign substances. The present technique could be an attractive choice in selecting the analytical method for drug analysis.
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