Determination of free concentration of ropivacaine and bupivacaine in blood plasma by ultrafiltration and coupled-column liquid chromatography.

A coupled-column liquid chromatographic method for determining the free concentration of ropivacaine and bupivacaine in blood plasma was developed. Following adjustment of the temperature and pH, the plasma samples were ultrafiltrated. Ropivacaine or bupivacaine in the ultrafiltrate was determined by direct injection into a coupled-column liquid chromatographic system, consisting of one reversed-phase and one ion-exchange column. The system was highly selective. Ropivacaine and bupivacaine were detected by UV at 210 nm. The limit of determination was 10 nM and the inter-assay precision at a concentration level of about 100 nM was 6% (R.S.D., n = 30) for ropivacaine and 7% (R.S.D., n = 30) for bupivacaine.