Abstract: AIMTo analyse the impurities of gatifloxacin. METHODSThe impurity of gatifloxacin were analysized and determinated by RP-HPLC/electrospray ionization mass spectrometry with a Zorbax SB-C₁₈ (4.6 mm×150 mm ID, 5 μm) column. The mobile phase was 3% acetic acid/acetonitrile-3% acetic acid/water (15∶85). The two compounds were synthesized: 1-cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-7-(1-piperazinyl)-4-oxo-3-quinolinecarboxylic acid (DMP) and 1-cyclopropyl-6-fluoro-1,4-dihydro-8-hydro-7-(3-methy-1-piperazinyl)-4-oxo-3-quinolinecarboxylic acid (DMO). Their liquid chromatogram, UV, MS were compared with those of the impurity of gatifloxacin. RESULTSThe mass of the impurity was 14 less than that of gatifloxacin. It means the impurity was CH₂ less than gatifloxacin. The tR(HPLC), UV and MS of DMP were the same as those of the impurity of gatifloxacin. CONCLUSIONBased on the t_R(HPLC), UV and MS, the impurity of gatifloxacin is confirmed as DMP.