All samples were analyzed using an auto-sampler HPLC on a Shimadzu (Kyoto, Japan) series LC-10A and a SPD-10AVP dual absorbance detector set at 260 nm and a SPD-M10AVP diode array detector at 200-370 nm. A Phenomenex (Torrance, CA, USA) C-18 reverse-phase Acqua end-capping 125 Å, 5 µm column (150×4.6 mm) with a reverse-phase Acqua 5 µm guard column (15×3.2 mm) was used. The mobile phase consisted of phosphate buffer (20 mM) containing 0.2% of triethylamine: acetonitrile (8:2). The final pH was adjusted to 6.9 with phosphoric acid. The flow rate was 1 mL min-1. The injection volume was 50 µL, the run time was 7 min, and the retention time was approximately 4 minutes. All samples were filtered using a 0.45-µm syringe filter composed of modified hydrophilic PTFE into silanized HPLC vials prior to injection.