Solvent was removed from the polar fraction using a rotary
evaporator (Büchi Labortechnik AG, Flawil, Switzerland). Shortly
before termination of distillation, nitrogen was introduced into
the system from a rubber bulb. The residues were dried to constant
weight at 105 C using a convection oven (Weiss–Gallenkamp,
Loughborough, UK). The mass of polar compounds was determined
from the residue obtained from the polar fraction.