Subsequently, the cartridges were cleaned up with
5 mL of water:MeOH (9:1, v/v) and completely dried under vacuum.
Analytes were eluted from the cartridge with 2.5 mL of MeOH
and 2.5 mL of MeOH:NH4OH (95:5, v/v) and then the eluates were
evaporated under a flow of nitrogen gas to dryness. The residues
were redissolved with water:MeOH (9:1, v/v) to 2 mL for surface
water and 5 mL for influent and effluent sewage.