Environmental context. Although amoxicillin is a widely used antibiotic, it is yet to be detected in the aquatic environment. This study traces the production of the amoxicillin-S-oxide degradation product, and shows that it is consistently obtained only under sunlight irradiation. This is the first study to demonstrate the formation, under controlled environmental conditions, of this chemically stable product of amoxicillin. Abstract. Amoxicillin (AMX) is a widely used penicillin-type

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... c, and its presence in the environment has been widely investigated. The formation and structure of an oxidised degradation product (DP) of AMX are described in the present work. The experiments were carried out in buffer solution (pH 7.5) containing AMX at a concentration of 100 mg mL À1 , with and without acid and in field secondary effluent. The DP, AMX-S-oxide (sulfoxide), was consistently obtained only under sunlight irradiation and was significantly augmented by the addition of humic acid (5 mg L À1 ) and mainly in field secondary effluent, which acts as a natural photo-sensitiser. The structure of the AMX-S-oxide DP was determined by an LC-MS technique using a mobile phase of deuterated and non-deuterated solvents. A 1 H NMR spectrum was obtained for the pure compound isolated by preparative HPLC. Further confirmation of the AMX-S-oxide structure was achieved by comparison of its UV spectrum with those of the two oxidation products, AMX-S-oxide and hydroxylated AMX, obtained by the ozonolysis of AMX.