Solid phase extractions of the acid wash solutions were performed as follows. Oasis HLB solid-phase extraction cartridges (Waters Corp., Milford, MA, USA) were sequentially conditioned with 1ml of hexane, 1ml of ethyl acetate, 1ml of methanol, and 1ml of HPLC-grade water. Following conditioning, five ml of an acid wash sample were loaded onto the cartridge.