Determination of the free concentration of ropivacaine in plasma by packed capillary liquid chromatography: A comparison of ultrafiltration and microdialysis as sample preparation methods

In this study, ultrafiltration and microdialysis have been compared as sample preparation methods to separate the free fraction of ropivacaine from the protein bound in 150 μL plasma. A liquid chromatographic system with packed capillary columns (0.2 mm internal diameter) was used to enhance sensitivity when analyzing the small sample volumes obtained after the ultrafiltration and the microdialysis. The microdialysis was performed with probes of our own construction, and to save analysis time, the microdialysis sampling was coupled on‐line to the liquid chromatographic system. The reduction of back pressure in the microdialysis outlet tube and in the injector was found to be essential. The free fraction obtained with each method was equivalent: both gave a free fraction of 6%.