The high performance liquid chromatography (HPLC) apparatus consists of a Waters (Milford, MA, USA) Model 600E pump associated with a Waters Model 600 gradient controller and a Rheodyne (Cotati, CA, USA) Model 7725i. HPLC can be coupled with a fluorescence detector (FL) Jasco 821-FP Inteligent spectrofluorometer (Japan spctreoscopic Co, LTD, Japan) or with a Conductivity detector (CD) Waters 432 (Millipore, Milford, MA, USA). The system is operated with Millenium software.