With the increased interest in multi-dimensional LC separations in recent years, several sophisticated commercial 2D-LC systems have been introduced to the market. However, there is still a lack of a complete theoretical foundation upon which sound method development and instrumental design decision can be made. One parameter that is not fully understood is 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. This is however important as it is connected to several other variables encountered when developing 2D-LC methods, including the first-dimension flow rate, the sampling (modulation) time and the loop volume.
In our recent work, 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 and the ratio 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. In the first-in-first-out (FIFO) modulation mode a model with the same function was also developed however in this case the filling fraction of the loop (f) was also included as it plays a role in determining the peak shape and variance in this modulation mode. It was found that the FILO mode yields lower variances than the FIFO mode because the emptying of the loop in the opposite direction of its filling balances out the dispersion created by the parabolic flow profile during the filling phase.
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. These results are important for the overall quality of the 2D-LC separation, due to the sensitivity of small second dimension columns to peak dispersion due to the injection plug.

Ken Broeckhoven

Professor, Vrije Universiteit Brussel

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