3. Results and discussion
3.1. Development and finalisation of HPLC method
Many of the published methods for examining the metabolites
found in pomegranate have focused on determining ellagic and
gallic acids, or the punicalagins A and B, but not all four of the
metabolites in a single HPLC analysis. This divided approach has
largely resulted from the differences in the chromatographic properties
of these four compounds when applied to C-18 stationary
phases. Gallic and ellagic acids represent the extremes in terms
of retention and are relatively simple to separate if the oligomeric
hydrolyzable polyphenols that elute in between them are ignored.
In contrast, punicalagins A and B are part of the oligomeric hydrolyzable
polyphenols that elute in the between the extremes and
require elongated chromatographic runs focused on that region
to resolve. We selected a 2.6 lm Kinetex C-18 column with the
hope that the additional resolution afforded by a smaller particle
size would facilitate resolution of the target compounds, while
minimising sample run time.
We initially employed the aggressive parameters recommend
by the column manufacturer for the HPLC–UV analysis of phenolics
in green tea to the analysis of a pomegranate marc extract, but
quickly learned that the suggested chromatographic conditions
did not adequately resolve the targeted compounds and resulted
in gallic acid eluting in the column void volume. Since our objective
was to minimise or eliminate any pretreatment steps we
elected to modify the chromatographic conditions, including the
overall run time, slope of the gradient, and phosphoric acid composition
of the mobile phase. In the course of our method