Modelling of analyte profiles and band broadening generated by interface loops used in multi-dimensional liquid chromatography

Currently, the shape and variance of the analyte band entering the second dimension column when injected from an open loop interface in two-dimensional liquid chromatography is not fully understood. This is however important as it is connected to several other variables encountered when developing 2DLC methods, including the first dimension flow rate, the sampling (modulation) time and the loop volume. Both numerical simulation methods and experimental measurements were used to understand and quantify the dispersion occurring in open tubular interface loops. Variables included are the analyte diffusion coefficient (D mol), loop filling and emptying rates (F-fill & F-empty), loop inner diameter or radius (R loop) and loop volume (V-loop). For a straight loop capillary, we find that the concentration profile (as measured at the loop outlet) depends only on a single dimensionless parameter t* (empty) = V (loop) F (empty) center dot D (mol) /R-loop(2) and theratio of the filling and emptying flow rates F-empty/F-fill. A model depending only on these two parameters was developed to predict of the peak variance resulting from the filling and emptying of a straight capillary operated in the first-in-last-out (FILO) modulation mode. Comparison of the concentration profiles and the corresponding variances obtained by either numerical simulation or experiments with straight capillaries shows the results generally agree very well. When the straight capillary is replaced by a tightly coiled loop, significantly smaller (20-40%) peak variances are observed compared to straight capillaries. The magnitude of these decreases is not predicted as well by simulations, however the simulation results are still useful in this case, because they represent an upper boundary (i.e., worst-case scenario) on the predicted variance. (C) 2021 Elsevier B.V. Allrightsreserved.

Automated summary

The study aimed to understand and quantify the dispersion occurring in open tubular interface loops using numerical simulation methods and experimental measurements. It was found that for a straight loop capillary, the peak variance depends on a dimensionless parameter and the ratio of filling and emptying flow rates significantly impacts the result. Additionally, when a tightly coiled loop is used instead of a straight capillary, smaller peak variances are observed.