Collected influent wastewater samples were transported on ice to the lab (<1 hr). On arrival, samples were portioned into 50 mL and spiked with 50 ng of each internal standard and shaken. Samples were then filtered through GF/F filters (Whatman, UK). Solid phase extraction (SPE) was used to extract target AAs from wastewater, Oasis HLB cartridges were preconditioned using 2 mL of MeOH followed by 2 mL of MilLi-Q H2O at a flowrate of 1 mL min -1 . Wastewater filtrates were then loaded onto the preconditioned cartridges at a rate of 5 mL min -1 . Cartridges were dried for at least 30 minutes under vacuum. For the elution step, 4 mL of MeOH at a rate of 1 mL min -1 with the eluate collected in silanised glass vials. Eluates were then dried at 40 °C under N2 via a TurboVap evaporator. Dried residues were reconstituted with 500 μL of 80:20 H2O: MeOH and transferred into polypropylene vials and kept at -18 °C until analysis.