Viscosity contrast effects in analytical scale chromatography - Evidence and impact

• Pre-viscous and viscous fingering in narrow chromatographic column was visualised.
• Both viscous processes are strongly dependent on the radius of the column.
• Viscous fingers were noted at high diluent-eluent viscosity ratios.
• Pre-viscous fingering elongates the sample zones at small viscosity ratios.

The phenomenon known as viscous fingering has been shown to be very detrimental to separation performance in preparative and size exclusion chromatography, and also in multidimensional HPLC. However, there are few reports of viscous fingering in analytical scale high performance liquid chromatography (HPLC), despite samples very often being introduced to the analytical-size HPLC columns in a solvent have a substantially different viscosity than the mobile phase. With this study we aim at first hand to investigate if viscous fingering or any similar and related effects to these in preparative levels also take place in analytical scale HPLC and if so, what impact this have on the separation performance. We could show that not only viscous finger does occur in analytical scale columns but also that peak distortions are apparent already at viscosity ratios between eluent and sample solution approaching unity. The latter indicates that a pre-viscous phenomenon is occurring that could be more important than the viscous fingering itself at the analytical format. As the viscosity contrast increases, the leading edge of the sample band distorts and the band volume increases, both leading to a decrease in performance.