All acidic analytes were quantified using an Agilent 1290 Infinity II UHPLC system (Agilent Technologies, Santa Clara, CA, USA), consisting of a binary pump, an autosampler, and a heated column compartment (40°C). The column was an Eclipse Plus C18 column (2.1 mm × 50 mm, 1.8 μm, Agilent Technologies) equipped with a guard column of the same type (5 mm). The mobile phase consisted of 0.5% v/v formic acid in each of A) 95:5 and B) 5:95 v/v water:acetonitrile, and elution was performed according to the following gradient. Mobile phase B was kept at 0% from 0.00 to 1.00 min, ramped to 53% from 1.00 to 6.00 min, ramped to 100% from 6.00 to 7.00 min and kept at 100% until 7.50 min when it was returned to 0% for a final 1.50 min re-equilibration. The flow of mobile phases was 0.4 mL min -1 during 0.00-7.00 min, 0.7 mL min -1 during 7.00-8.50 min, and 0.4 mL min -1 during 8.50-9.00 min. The injection volume was 5 µL.