Formic acid was purchased from Riedel-de Häen (Seelze, Germany) and L-ascorbic acid was from Sigma–Aldrich (Steinheim,
Germany). Water used for all solutions was obtained from a Milli-Q water purification system (Millipore; Bedford, MA). Stock solutions of L-ascorbic acid were prepared daily in acidified Milli-Q water
(0.1% v/v formic acid) and stored in amber flasks at 4 C prior to chromatographic analysis. Working standard solutions were prepared by dilution.