One milliliter of filtrate was diluted with 2
mL of 4.5 M solution of sodium acetate at pH 6.2, 4 mL of 0.3
M solution of TCA, and 2 mL of 1,2-phenylenediamine (2.5 m
dissolved in 25 mL) and allowed to stand for 30 min at 37 °C
in the dark; 20 íL of this solution was injected onto the HPLC
system.