Ibuprofen, naproxen, ketoprofen, 3-(4-chlorophenyl) propionic acid,
pyridine, 2-butanol,methyl chloroformate, ethyl chloroformate, isobutyl
chloroformate, sodium acetate, boric acid, potassium chloride and sodium
carbonate were purchased from Sigma-Aldrich (Germany) with
purity of at least p.a. or 97%. Methanol (Chromasolv) and di-sodium
hydrogen phosphate dihydrate (p. a.) was purchased from Fluka
(Germany). Acetone, ethanol and acetic acid (all p. a.) were purchased
from Penta (Czech Republic). Working mixed solutions of analytes in
methanol were prepared at two concentrations of 0.8 mg mL-1 and
0.04 mg mL-1. A standard solution of 3-(4-chlorophenyl) propionic