Mobile phase buffers included formic acid (HCOOH, >95 %), ammonium acetate (NH4OAc), acetic acid (CH3COOH) and ammonium fluoride (NH4F), purchased from either Merck or Fischer Scientific. All glassware was deactivated by rinsing once with 5 % dimethylchlorosilane (DMDCS), twice with toluene and three times with MeOH. This was to avoid any losses of basic compounds on the -OH sites present on the glass surface. Oasis HLB SPE cartridges (60 mg, 3 mL) were purchased from Waters. The filters (Whatman GF/F 0.7-μm and GF/D) and polyproplene LC vials were also purchased from Waters.