Piperacillin Impurity B(EP):(2R,4S)-2-[(1R)-carboxy[2-(4-ethyl-2,3-dioxopiperazine-1-carboxamido)-2-phenylacetamido]methyl)-5,5-dimethylthiazolidine-4-carboxylic acid(Piperacillin penicilloic acid).