Sample preparation methods for high-performance liquid chromatographic
analysis of birch leaf
phenolics were compared. The methods tested were: (1) air-drying
at ambient temperature, (2)
oven-drying at 40 °C, (3) oven-drying at 80 °C, (4) freeze-drying,
prefreezing with liquid N2, (5)
freeze-drying, prefreezing at −18 °C, (6) freeze-drying without
prefreezing, (7) storing frozen for 12
days without drying, and (8) immediate extraction of fresh samples.
Although there were significant
differences among methods in absolute concentrations of phenolics, none
of the methods altered
considerably the phenolic profile of birch leaves. For
quantitative analysis, the samples should be
analyzed immediately. Alternatively, the samples may be stored
fresh frozen, at least for a short
time. Among the drying methods, the highest concentrations of
phenolics were obtained by freeze-drying of slowly frozen leaves (method 5). Freeze-drying without
prefreezing resulted in significantly
lower concentrations of flavonoid glycosides than method 5.
Oven-drying did not prove to be a good
alternative to conventional freeze-drying, nor did it show any
advantage over simple air-drying.
Keywords: Betula pendula; sample preparation; drying methods; flavonoid
glycosides; phenolic
compounds