**2.1. Materials and reagents**

Benzophenone (Reagent plus, 99%), sulisobenzone (5-benzoyl-4-hydroxy-2-methoxy-benzenesulfonic acid) (HPLC, ≥97.0%) and acetonitrile (for HPLC Plus) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ethyl acetate was purchased from Merck (Merck, Darmstadt, Germany), ethanol and methanol were purchased from Associated Chemical Enterprises (Johannesburg, South Africa). Stock solutions of benzophenone and 5-benzoyl-4-hydroxy-2-methoxy-benzenesulfonic acid (10 mg L−1) were prepared in ultra-pure water (Direct-Q® 3UV-R purifier system, Millipore, Merck). Working standards of 100 μg L−1 were prepared daily by diluting appropriate volumes of the stock solution in ultra-pure water. The ion exchange resins used in this study as packing materials were Dowex 1x8 (Chloride form) (Sigma Aldrich).
