Four hundred microlitres of plasma spiked with ciprofloxacin were transferred to 1.5 ml Eppendorf tubes. Thirty microlitres of internal standard (IS) working solution in water (100 g/ml) were added to each tube. One drop of 10 M phosphate buffer (pH 2.7)was added and the tubes were vortex mixed for 3 min. Five hun-dred microlitres of ice cold acetonitrile were added using a glasssyringe. The tubes were vortex mixed for a further 5 min. The sam-ples were centrifuged at 3500 × g for 5 min. The supernatant waspoured into 8 ml silanised glass tubes. The silanised tubes wereplaced in a Turbovap Concentrator®with the water bath set at 50◦Cfor 20 min. The dried residue was reconstituted with 100 l mobilephase, vortex mixed for 3 min and re-centrifuged at 15,000 rpm for3 min. Fifty microlitres of the clear supernatant were injected intothe HPLC unit.