The organic residues from each plant material were re-dissolved
in methanol (1 ml), filtered through a membrane filter (0.45 m
pore size) and analyzed by HPLC, as described previously by Palacio
et al. [29]. The phenolic compounds were detected at 265 nm
for coniferyl alcohol, 272 nm for cinnamic acid, 275 nm for sinapyl
alcohol, 281 nm for NDGA, 310 nm for p-coumaric acid, 325 nm for
ferulic acid and caffeic acid, 328 nm for sinapic acid, and 370 nm for
quercetin and kaemferol aglycones. The peak areas corresponding
to each detected phenolic compound from the samples, and having
the same retention time as authentic compounds, were integrated
for comparison with an external standard calibration curve.