2. Materials and methods
2.1. Reagents and solutions
Reagents used were purchased from Sigma–Aldrich Chemical Corp. USA (purity: ACS). Stock solutions of Ade and Gua (0.01 M and 1.0 mg/L) were prepared by dissolving their hydrochloride salts in NaOH (0.05 M or 5.0 mM). All solutions were stored in the dark at 4 °C. Working standard solutions were prepared by stepwise dilution of the respective stock solution with a supporting electrolyte. These solutions should be used within 12 h. Different supporting electrolytes such as acetate buffer solutions (ABSs), Britton–Robinson buffer solutions (BRBSs) and phosphate buffer solutions (PBSs) containing 0.1 M KCl were tested. PBSs of various pH values were prepared from 0.1 M NaH2PO4 and 0.1 M Na2HPO4 and adjusting pH with 0.1 M H3PO4 or 5.0 mM NaOH. All of the other compounds were of analytical grade and used without further purification. Doubly distilled water with resistivity of 3–5 MΩ cm−1 at 25 °C was used throughout all the experiments. The solutions were deoxygenated by passing nitrogen gas.