LCxSFC Valve Technologies: Guidelines toward a Successful Online Modulation

Online two-dimensional liquid chromatography can be used under various separation modes but is sometimes limited in terms of orthogonality, especially for the analysis of neutral compounds. The use of SFC in the second dimension offers a wide choice of mobile and stationary phases, suggesting retention properties complementary to those of the first dimension. Initial works on online LCxSFC coupling published in the literature highlighted the first difficulties of solvent compatibility or potential problems with bubbles created by CO₂ in loops. The present work highlights the impact of the interface between the LC and SFC dimensions on the performance of the 2D separation. Six different configurations have been evaluated, differing by the addition of a makeup flow in the first dimension, a division, or a separation of flow paths in the second dimension. Their performances with empty loops eliminated the need for complex trapping columns, regardless of the LC mobile phase nature. Injection in partial fill mode was possible without any problems of CO₂ bubble formation, solvent miscibility, pressure, or modulation repeatability in each of the studied configurations. Selected configurations even allowed for the use of a mobile phase split upstream of the valve, so that the online LCxSFC configuration could be as flexible as LCxLC instrumentation in terms of the first dimension flow rate or second dimension injected volume. The obtained results allowed for building guidelines presenting the best interfaces to set up depending on the nature of the mobile phases in both dimensions. Two applications on monomers of lignin and industrial polymers confirmed preliminary results on configuration testing and illustrated that online LCxSFC could now be easily implemented. Precision tests on retention time were conclusive and are promising for the future use of this technique in industrial routines.