Chromatograms showing the change in day-to-day repeatability over three days for the standard test mixture before (A) and after (B) treatment using 10 mM ascorbic acid. Column was 4.6 mm × 100 mm packed with 3 μm Hypercarb. Mobile phase conditions were 5/95 (v/v) water/MeOH + 0.1% FA at a flow rate of 1 mL/min and 30 °C. For (A), measurements were taken over the course of 2 weeks at day 1 (red), day 4 (black) and day 14 (blue). For (B), ascorbic acid treatment was performed before use each day by flowing 10 mM ascorbic acid in mobile phase overnight. Before measurements, ascorbic acid was washed off column by flushing with mobile phase. Measurements were taken over a similar time span as in (A). Elution order: mandelic acid, 2-MHA, hippuric acid, 3-MHA, and 4-MHA. UV Detection: 240 nm.