In this study a new
method for evaluating the pressure effect on
separations of oligonucleotides and proteins on an anion exchange
column was developed. The pressure rise of up to 500 bar was attained
by coupling restriction capillaries to the column outlet to minimize
differences in pressure over the column. Using pH transient measurements
it was demonstrated that no shift in ion exchange equilibria occurs
due to a pressure increase. Results from isocratic and gradient separations
of oligonucleotides (model compounds) were evaluated by stoichiometric
displacement and linear gradient elution model, respectively. Both
elution modes demonstrated that for smaller oligonucleotides the number
of binding sites remained unchanged with pressure rise while an increase
for large oligonucleotides was observed, indicating their alignment
over the stationary phase. From the obtained model parameters and
their pressure dependencies, a thermodynamic description was made
and compared between the elution modes. A complementary pattern of
a linear increase of partial molar volume change with a pressure rise
was established. Furthermore, estimation of the pressure effect was
performed for bovine serum albumin and thyroglobulin that required
gradient separations. Again, a raise in binding site number was found
with pressure increase. The partial molar volume changes of BSA and
Tg at the maximal investigated pressure and minimal salt concentration
were −31.6 and −34.4 cm3/mol, respectively,
indicating a higher rigidity of Tg. The proposed approach provides
an insight into the molecule deformation over a surface at high pressures
under nondenaturing conditions. The information enables a more comprehensive
UHPLC method development.