2.3.1. Preparation of SPE columns and solid phase
For SPE, a Visiprep vacuum manifold for simultaneous processing of 12 samples was used. For the preparation of SPE, 0.1_0.01 g polyvinylpyrrolidone for every sample was weighed and 2 mL distilled water was added. After 24 h standing in a cool place the prepared polyvinylpyrrolidone samples were inserted into SPE columns (prepared from the manifold and 3 mL syringe, where about 1/3 of the volume was filled with packed cotton-wool).