Standards of salicylic acid (1 ), cinnamic acid (2 ), syringic acid (3 ), rosmarinic acid (4 ), caffeic acid (5 ) and chlorogenic acid (6 ) were purchased from Sigma Aldrich (St. Louis, MO, USA); their purity was ≥98%. All solvents (acetonitrile, methanol) and chemicals (ammonium acetate and ammonia) required for CEC analysis had p.A. quality and were bought from Merck Eurolab (Vienna, Austria). Ultrapure water was prepared using a Sartorius arium purification system (Göttingen, Germany).
Chemicals for preparation of the monolith, i.e. 1,4 butanedione, 1-propanol, 2,2-diphenyl-1-picryl-hydrazyl, 3-(trimethoxysilyl) propyl methacrylate, dimethylformamide, allylimidazole, 1,3-propanesultone, dimethylformamide, 2,2-dimethoxy-2-phenylacetophenone and ethyleneglycol dimethacrylate were also obtained from Sigma Aldrich.
Chemicals for preparation of the monolith, i.e. 1,4 butanedione, 1-propanol, 2,2-diphenyl-1-picryl-hydrazyl, 3-(trimethoxysilyl) propyl methacrylate, dimethylformamide, allylimidazole, 1,3-propanesultone, dimethylformamide, 2,2-dimethoxy-2-phenylacetophenone and ethyleneglycol dimethacrylate were also obtained from Sigma Aldrich.