The derivatisation with OPA was carried out based on the modified
method described by Thippeswamy, Gouda, Rao, Martin, and
Gowda (2006). The derivatisation solution was freshly prepared
every day as follows: 5 mg of OPA were dissolved in 0.05 mL of
methanol, and mixed with 0.45 mL of 0.4 M boric acid/borate buffer
(pH = 9.5) and 25 lL of b-mercaptoethanol (Wang et al., 2010).
The 70 lL of sample infusion (or standard amino acid) were mixed
with 10 lL OPA solution, incubated at 25 C for 2 min, and used
immediately for HPLC analysis.