Linear velocity optimum, supercritical fluids

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

Capillary supercritical fluid chromatography (SFC) columns are 0.1-0.025 mm ID and 3-20 m in length. Good reviews of the technique of SFC have been recently published [52-55]. It was reported that the optimum inner diameter for capillary SFC based on plate height, linear velocity, analysis time, and column length was around 0.050 mm. 

The purpose of Figure Id is to emphasize the wide range optimum linear velocities possible with supercritical fluids as the mobile phase. It Is important to bear in mind that the optimum linear velocity Is a function of k and D12. Furthermore, k Is a function of the mobile phase solvent power (itself a function of density and chemical composition (x ) with supercritical fluids) and the temperature, and the binary diffusivlty is a function of density and temperature ... 

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