The solid adsorbent was Silica gel 40 (0.063–0.200 mm) for
chromatographic columns from Merck (Darmstadt, Germany). Sil-
ica gel was activated at 600 ?C for 4 h, after cooling, the silica gel
was impregnated with sulphuric acid (100:30, w/v) and the mix-
ture was agitated with a mechanical vibrator.