The influence of viscosity induced flow instability in liquid chromatography

Abstract

The flow instability process of viscous fingering has been widely researched for many years but its direct impact on liquid chromatography has not been extensively investigated. More specifically the morphology and flow patterns that the solute plugs experience as they pass through the column at various viscosities. The process of viscous fingering involves a fluid of a lower viscosity penetrating a fluid with a high viscosity. When the viscosity between the two liquids is significantly different the lower viscosity fluid enters the high viscosity fluid in a complex way such that the interface of the two fluids is augmented to resemble a series of fingers. The chromatographic solute plug as two liquid-liquid interfaces and depending on which is more viscous the plug (rear interface - fingers appear to trail plug) or the mobile phase (leading interface - fingers appear to proceed the plug) it will determine at which interface the instability will occur. High viscosity differences between the solute plug and the mobile phase has been linked to poor separations, but small viscosity differences have not been fully evaluated as the chromatographic peaks can maintain a normal distribution. This thesis investigates and describes the phenomena of viscous fingering as it applies to reversed phase high performance liquid chromatography (RP-HPLC) and size exclusion chromatography. Clearly visualising the solute plug as it passes through the liquid chromatography column and the way in which a change of viscosity influences the plug can be monitored using an optical visualisation technique. This technique involves the use of matching the refractive index of the fluid passing through the column to that of the stationary phase (silica based) within a glass column. The mobile phase used was a mixture so that the viscosity could be altered yet the refractive index could be maintained. This optical technique rendered the opaque stationary phase transparent and a coloured solute injected onto the column could be photographed and visual changes due to viscous fingering can be documented. Photographs were used to record the plugs movement along the column, and conventional post-column detection responses (chromatograms) were collected for comparison.

Date of Award	2009
Original language	English