This project has focused on the study of hybrid phases at low percentage of
organic modifier and pure water using high temperature in liquid
chromatography (HTLC). It examined the effect of temperature on the
retention of a range of test solutes and phenol homologues. The hybrid
columns all showed linear van't Hoff relationship at low percentages
of methanol but at higher temperature non linear van't Hoff curves were
observed. Non·linear van't Hoff curves were observed with pure water on
hybrid phases. This was thought to be because the retention mechanism
changed at a discontinuity point above 100 ·C due to change in the properties
of the mobile phase as well as changes in the entropy. The methylene
selectivity decreased with temperature and increase with increasing pressure.
XTerra phenyl and XBridge phenyl columns were stable up to 200 ·C and also
in different flow rates without distortion in peaks. The efficiency of both
columns was significantly decreased at low linear velocities due the
domination of the B-term but at higher linear velocities the C-term dominated
the separation. XBridge phenyl showed a flattened van Deemter curve
indicates that high flow rate enabled a better separation.
The application of high temperature on hybrid phases was studied for the
separation of the selected steroids on XTerra MS C18 at low percentages
methanol and pure water. A high flow rate was used to decrease the retention
on XTerra MS C18 column at low percentage of methanol.
In this project, variable back-pressure was used as an external parameter at
constant flow rate and temperature to study the effect of pressure on retention
in liquid chromatography. Hydrophobicity and shape selectivity, which depend
on retention factor, increased due to increases in the pressure.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.