High temperature separations on hybrid stationary phases

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.