As analytical chemistry continues to push the limits of resolution and complexity, even subtle physical effects can determine the quality of results. A recent study by Rick van den Hurk, Tijmen Bos, Dwight Stoll, and Bob Pirok, published in the Journal of Chromatography A (2025), examines one such effect: radial dispersion, the way liquids mix across the width of a flow channel.
Their findings reveal that inadequate mixing of solvent streams, often overlooked in two-dimensional liquid chromatography (2D-LC), can cause distorted peaks and unpredictable separations. The solution, however, turns out to be elegantly simple: coiling the connecting tubing.
The Subtle Challenge of Mixing in Chromatography
In 2D-LC, effluent from a first separation is transferred to a second column using a modulation interface. This often requires combining two solvent streams, typically an organic-rich and an aqueous flow, using a T-shaped connector. Under the low-flow, laminar conditions typical of LC, the two liquids can flow side by side without mixing, forming radially segregated layers.
The team visualized this phenomenon using coloured dyes and confirmed it through computational fluid dynamics (CFD) simulations. Both approaches revealed the persistence of segregated streams after the junction.
These unmixed layers reach the column inlet, causing uneven solvent composition across the capillary radius. The result is poor analyte retention, peak splitting, and fronting, all of which compromise data quality and quantitative accuracy.
To address this, they tested whether changing the geometry of the connecting tubing could promote mixing. When the tubing was coiled, the curvature generated Dean vortices, small secondary flows that enhance radial dispersion without turbulence. The effect was clear: analyte peaks became narrower, more symmetrical, and more reproducible.
Even a modest number of coils significantly improved chromatographic performance. Beyond roughly 20 coils, additional curvature offered diminishing returns, suggesting an optimal balance between mixing efficiency and minimal dead volume.
Practical Implications
This work highlights how physical design can be as crucial as chemical optimization in modern chromatography. Coiled tubing offers a low-cost, low-complexity way to improve solvent homogeneity at modulation interfaces, outperforming conventional mixers that primarily promote axial, not radial, dispersion.
The findings are relevant not only for active modulation strategies such as Active Solvent Modulation (ASM) or At-Column Dilution (ACD), but also for feed injection, post-column derivatization, and make-up flow configurations. In all cases, inducing radial dispersion enhances stability, reproducibility, and analytical precision.
van den Hurk, R.S., Bos, T.S., Stoll, D.R., & Pirok, B.W.J. (2025). Significance of radial dispersion to effective modulation in two-dimensional liquid chromatography. Journal of Chromatography A, 1763, 466456. https://doi.org/10.1016/j.chroma.2025.466456
