Determination of free concentrations of ropivacaine and bupivacaine in plasma from neonates using small-scale equilibrium–dialysis followed by liquid chromatography-mass spectrometry

Attempts to determine a safe plasma concentration of ropivacaine and bupivacaine in neonates have not been consistent. This might be due to an underestimation of free drug in small plasma samples by currently used techniques, e.g., ultrafiltration. We describe a simple microscale equilibrium–dialysis technique for the separation of free and bound ropivacaine and bupivacaine. The free drug in the dialysate was determined using solid-phase extraction and liquid chromatography with mass spectrometry. Pentycaine was used as an internal standard and added to the dialysates prior to extraction. The method is very selective and sensitive, as no compounds other than the analyte and internal standard were observed in the resulting chromatograms at low ng/ml levels. The limit of quantitation was 2.5 ng/ml. The calibration curve was linear in the range of 2 to 1000 ng/ml. The precision of the whole procedure was 8.1% (n=10) and 6.5% (n=7) for ropivacaine and bupivacaine, respectively. The method was tested in the analysis of plasma samples taken from neonates who had received epidural injections.