Direct injection of large volumes of plasma in a column-switching system for the analysis of local anaesthetics. I. Optimization of semi-permeable surface precolumns in the system and characterization of some interference peaks.

Possibilities for accomplishing direct injection of large volumes (500 microliters) of plasma samples into a column-switching HPLC system were investigated. A new format of precolumn containing a semi-permeable surface (SPS) support (1 cm x 1 cm) was used for the sample clean-up and trace enrichment and was combined with a Kromasil C18 column for the final separation. A stable chromatographic system with respect to the separation selectivity and separation time was constructed and evaluated. The main parameters were the hydrophobicity of the SPS column, pH of the eluents, concentration of the organic modifier in the eluents and the detection wavelength. Two main interference peaks that were eluted in front of the ropivacaine peak were systematically characterized by varying the loading conditions for the SPS precolumn. The SPS column could tolerate large volumes (< or = 500 microliters) of plasma injections with a total volume of more than 50 ml. The developed system is stable, which permits the detection of 30 ng/ml ropivacaine in human plasma.