Evaluation of the phototransformation of the antiviral zanamivir in surface waters through identification of transformation products

• Sunlight photodegradation of zanamivir in surface waters was investigated.
• Four new TPs were tentatively identified of which one was persistent.
• A new methodology using HILIC-LTQ Orbitrap-MS for identification of the TPs was established.
• Phototransformation pathways of zanamivir were mechanistically explained.

The antiviral zanamivir has been recently reported to occur in surface waters where its presence may lead to the selection of resistant strains of virus in aquatic fauna. In order to evaluate the fate of zanamivir in surface waters, its susceptibility to phototransformation was evaluated using simulated and natural sunlight. Upon exposure of aqueous solutions (20 μg L−1) to simulated sunlight, zanamivir in surface water degraded at t1/23.6 h. Under natural sunlight in surface water about 30% of the initial concentration of the antiviral disappeared within 18 days. The experiments with surface water showed similar effect as humic acid addition with expected decreasing effect on degradation while nitrate addition showed increasing effect. In the experiments with artificial sunlight at high concentrations of zanamivir, four photoproducts were tentatively identified by hydrophilic interaction chromatography-LTQ-Orbitrap-MS, showing [M+H]+ ions at m/z 112 (TP111), m/z 275 (TP274), m/z 323 (TP322), and m/z 333 (TP332). However at 20 μg L−1 only the formation of the recalcitrant TP111 was observed. The proposed structures were rationalized by photolysis mechanisms. Photoproduct TP111 was confirmed with a commercially available standard (isocytosine). In summary, the findings suggest that the photodegradation of zanamivir in surface waters proceeds with slow kinetics.