Fundamental investigation of the dispersion caused by a change in diameter in nano liquid chromatography capillary tubing

We report on a Computational Fluid Dynamics (CFD) study of the extra dispersion caused by the change in diameter when coupling two pieces of capillary tubing with different diameter. In this first investigation into the problem, the focus is on the typical flow rates (0.25≤F≤2μL/min) and diameters (d≤40μm) used in nano-LC, considering both the case of either a doubling or halving of the diameter. The CFD simulations allow to study the problem from a fundamental point of view, i.e., under otherwise perfect conditions (perfect alignment, zero dead-volume). Flow rates, capillary diameters, diffusion coefficients and liquid viscosities have been varied over a range relevant for nano-LC (Reynolds-numbers Re ≤ 1), with also an excursion made towards high-temperature nano-LC conditions (Re ≥ 10 and more). The extra dispersion caused by the change in diameter has been quantified via a volumetric variance σ 2 conn, defined in such a way that the overall dispersion across the entire capillary system can be easily reconstructed from the known analytical solutions in the individual segments. When the two capillaries are longer than their diffusion entry length, covering most of the practical cases, σ 2 conn converges to a limiting value σ 2 conn,∞ which varies to a close approximation with the square of flow rate. Under the investigated nano-LC conditions, the σ 2 conn,∞ -values are surprisingly small (e.g., on the order of 0.01 to 0.15 nL 2 in a 20 to 40μm connection) compared to the dispersion occurring in the remainder of the capillaries.