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

- Extra-column pressure drops can exceed the column pressure drop.
- Flow in SFC needs to be ≤2 mL-min−1 to avoid excessive extra-column ΔP.
- Accurate calculations showing significant turbulent flow under most conditions used in SFC.
- Turbulent flow is at least proportional the square of flow or worse.
- 4.6 mm ID columns with sub-2 μm particles should not be used with sub-2 μm particles.

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 × 100 mm, 1.8 μm R,R-Whelk-O1 column was used with low dispersion LD (120 μm) plumbing to generate sub-1 min chiral separations. However, the optimum, or near optimum, flow rate was 5 mL-min−1, producing a system pressure of 580 bar (with 40% methanol, outlet pressure 120 bar). 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 200 bar, 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 400 bar 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 120 μm ID tubing was calculated to be laminar below 1.0 mL-min−1, mostly transitional up to 2.5 mL-min−1 and virtually always turbulent at 3 mL-min−1 and higher. Flow in 170 μm 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, 4.6 mm columns are not very user friendly for use with sub-2 μm 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.