During the process development of ceftazidime, a new impurity which exceeded the limit of 0.1% was detected by a simple HPLC method. The molecular weight of the target impurity was determined by LC/MS. This suspected impurity was synthesized and purified for characterization. When co-injected with ceftazidime in HPLC, the retention time of the impurity was the same as the ceftazidime sample containing the impurity. The structural determination of the suspected impurity was conducted by IR, MS, 1H-NMR and 13C-NMR spectroscopic techniques. This new impurity was the methyl ester of ceftazidime, and its structure was determined as (6R,7R)-7-[[(Z)-2-(2-aminothiazol-4-yl)-2-[(1-methoxycarbonyl-1-methylethoxy) imino]acetyl]amino]-8-oxo-3-[(1-pyridinio)methyl]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate, with molecular formula
of C23H24N6O7S2 and molecular weight of 560 Da.