L-hydroxyproline has been derivatized with N-acetylsulfanilyl chloride and 5-chlorovaleric acid during the synthesis of the haptens HP1 and HP2.N-Acetylsulfanilyl chloride is used in the synthesis of N1,N1-dimethylsulfanilamide as the internal standard in the reaction. The reaction is used to determine tramadol and its O-desmethylated metabolite in blood plasma. L-hydroxyproline has been derivatized with N-acetylsulfanilyl chloride and 5-chlorovaleric acid during the synthesis of the haptens HP1 and HP2.