5 Conclusion
In this work, the mass transfer mechanisms of small molecules along a 180 µm i.d. × 20 m long × 0.2 µm film thickness OTC using pre-turbulent and turbulent mobile phases (carbon dioxide/methanol mixtures) were determined and compared under retained conditions (0 < k < 1). Under preturbulent flow regime, the dispersion coefficient of the analytes is around 3 × 10−4 cm2 /s, which is six times as large as their bulk diffusion coefficient. This is explained by the presence of unstable and decaying pre-turbulent puffs generated by the imperfection of the SFC system (injection event, flow delivery of the mobile phase mixture, ABPR ripple). The mass transfer mechanism is still controlled by the slow molecular transport in the entire volume of the mobile phase despite the presence of vanishing turbulent puffs. Under sustained turbulent flow regime, the dispersion coefficient of the analyte is about four to five orders of magnitude larger than the bulk diffusion coefficient. Unlike the prediction of the general Golay HETP equation, which anticipates negligible mass transfer resistance in the mobile phase, experiments revealed that the analyte bandspreading is still controlled by the slow mass transfer of the analyte across the thin viscous layer and the film of stationary phase. This is directly explained by the presence of the viscous and buffer layers in the wall region region of the OTC. In these layers, which occupy about 30% of the capillary volume at a Reynolds number of 5000, the viscous forces are still dominant over the inertial forces and the molecular transport remains slow.
