Fractions and acids were in contact for 72 h to dissolve as much
particles as possible. Digestion of samples continued at a heating
plate at, 100 C, in order to obtain slow evaporation and the heating
program stopped when solutes approached dryness. Then the
solutions were diluted with 20 ml of pure water. Subsequently,
samples were filtrated with filter paper and diluted again in
100 ml volumetric flasks with de-ionized water.