Chromatographic separation was conducted on an Agilent 1290 HPLC system consisting of an auto-sampler and a binary pump system (Agilent Corporation, CA, USA) coupled with a UV-Vis detector. HPLC grade reagents, acetonitrile, and methanol were purchased from Burdick & Jackson (Morristown, NJ, USA). All other chemicals were of analytical grade unless otherwise noted. Double-distilled water was prepared using a Millipore system and filtered through a 0.22 μm Millipore aqueous phase filter before HPLC analysis.