Measurement of the molecular diffusion coefficient and the effective longitudinal diffusion under supercritical fluid chromatography conditions in packed bed columns.

• Adapted and improved set-up to measure molecular diffusion coefficient D mol in SFC. • New method to measure longitudinal effective diffusion Deff inside the column. • Unique data for D eff /D mol are presented as a function of the retention factor. • Estimation of the surface diffusion coefficient γ s D s /D mol vs. retention are made. • γ s D s /D mol is found to decrease strongly with increasing retention in SFC. The improvement of supercritical fluid chromatography (SFC) instrumentation enhanced its reliability and utility over the past decade. The further development of high speed and high resolution separations is however obstructed by the lack of accurate models for axial dispersion in SFC. This work is a first step to tackle this by developing more reliable methods to measure molecular (D mol) and longitudinal diffusion (D eff) in SFC, as these affect all aspects of separation efficiency. In the present contribution, we report on an improved method, to enable more flexible, reliable and accurate measurements of D mol in SFC using commercial instrumentation. A two-column variant of the stopped-flow experiment is proposed as an adapted set-up for measuring the effective longitudinal diffusion coefficient D eff in SFC-conditions. Using the set-ups for a number of test-compounds, it has been found that D eff , and the coefficients describing its constituent sub-processes (cf. particle diffusion D part and surface diffusion γ s D s), all vary in a linearly proportional way with the bulk diffusion coefficient D mol within a high degree of accuracy. It has also been found that D eff decreases much more sharply with increasing retention factor compared to LC. By applying the effective medium theory, it was found that the relative surface diffusion coefficient γ s D s /D mol decreases strongly with retention factor for the investigated solutes and column, in contrary to what is typically observed in reversed phase liquid chromatography. Results indicate that this might be related to a change in retention behavior of the analytes. Obviously, more analytes and conditions need to be explored to complete this picture and the extend range of applicability of these observations. [ABSTRACT FROM AUTHOR]