Direct injection analysis of oxypurinol and metformin in wastewater by hydrophilic interaction liquid chromatography coupled to tandem mass spectrometry

The increasing global prevalence of gout and diabetes has led to a rise in the use of their respective medications, allopurinol and metformin. These are excreted via urine as oxypurinol and metformin and are discharged into wastewater and the environment. Current environmental monitoring of those two polar chemicals requires labour intensive and potentially inefficient sample pre-treatments, such as using solid-phase extraction or freeze-drying. This study validated a sensitive and simple method using direct-injection LC–MS/MS for the simultaneous measurement of oxypurinol and metformin in wastewater. The final method utilised a hydrophilic interaction liquid chromatography together with simple filtration through 0.2 μm regenerated cellulose filter followed by dilution in acetonitrile with a dilution factor of 10. The developed method was validated with the limit of quantifications (LOQ) of 0.11 and 0.34 μg/L for metformin and oxypurinol, respectively. The new method was applied to 42 influent wastewater samples and 6 effluent samples collected from 6 Australian wastewater treatment plants. Both compounds were detected well above the LOQ at concentrations 29–214 μg/L in influent and 2–53 μg/L in effluent for metformin, and 24–248 μg/L in influent and 4–81 μg/L in effluent for oxypurinol, demonstrating its high applicability.