Characterizing pressure issues due to turbulent flow in tubing, in ultra-fast chiral supercritical fluid chromatography at up to 580 bar

Abstract

It has been widely suggested that the outlet pressure be changed to maintain constant density (“isopycnic” conditions) when comparing the kinetic performance of different columns in supercritical fluid chromatography (SFC). However, at high flow rates, flow in the tubing is turbulent, causing large extra-column pressure drops that limit options for changing outlet pressure. Some of these pressure drops occur before and some after the column, obscuring the actual column inlet and outlet pressures.In this work, a 4.6×100mm, 1.8μm R,R-Whelk-O1 column was used with low dispersion LD (120μm) plumbing to generate sub-1min chiral separations. However, the optimum, or near optimum, flow rate was 5mL-min−1, producing a system pressure of 580bar (with 40% methanol, outlet pressure 120bar). Both the flow rate and pump pressure required were near the limits of the instrument, and significantly exceeded the capability of many other SFC’s. Extra-column pressure drops (ΔPec) were as high as 200bar, caused mostly by turbulent flow in the tubing. The ΔPec increased by more than the square of the flow rate.Reynolds Numbers (Re) were calculated for tubing as a function of flow rate between 100 and 400bar and 5–20% methanol in CO2, and 40°–60°C. This represents the most extensive analysis of turbulence in tubing in the SFC literature. Flow in 120μm ID tubing was calculated to be laminar below 1.0mL-min−1, mostly transitional up to 2.5mL-min−1 and virtually always turbulent at 3mL-min−1 and higher. Flow in 170μm tubing is turbulent at lower flows but generates half the ΔPec due to the lower mobile phase linear velocity.The results suggest that, while sub-minute chromatograms are easily generated, 4.6mm columns are not very user friendly for use with sub-2μm packings. The high flow rates required just to reach optimum result in high ΔPec generated by the tubing, causing uncertainty in the true column inlet, outlet, and average column pressure/density. When comparing kinetic performance of columns with different dimensions, the pressure drops in the tubing must be considered.