Detailed characterization of the kinetic performance of first and second generation silica monolithic columns for reversed-phase chromatography separations

The kinetic performance of commercially available first generation and prototype second generation silica monoliths has been investigated for 2.0 mm and 3.0–3.2 mm inner diameter columns. It is demonstrated that the altered sol–gel process employed for the production of second generation monoliths results in structures with a smaller characteristic size leading to an improved peak shape and higher efficiencies. The permeability of the columns however, decreases significantly due to the smaller throughpore and skeleton sizes. Scanning electron microscopy pictures suggest the first generation monoliths have cylindrical skeleton branches, whereas the second generation monoliths rather have skeleton branches that resemble a single chain of spherical globules. Using recently established correlations for the flow resistance of cylindrical and globule chain type monolithic structures, it is demonstrated that the higher flow resistance of the second generation monoliths can be entirely attributed to their smaller skeleton sizes, which is also evident from the external porosity that is largely the same for both monolith generations (ɛe ∼ 0.65). The recorded van Deemter plots show a clear improvement in efficiency for the second generation monoliths (minimal plate heights of 13.6–14.1 μm for the first and 6.5–8.2 μm for the second generation, when assessing the plate count using the Foley–Dorsey method). The corresponding kinetic plots, however, indicate that the much reduced permeability of the second generation monoliths results in kinetic performances (time needed to achieve a given efficiency) which are only better than those of the first generation for plate counts up to N ∼ 45,000. For more complex samples (N ≥ 50,000), the first generation monoliths can intrinsically still provide faster analysis due to their high permeability. It is also demonstrated that – despite the improved efficiency of the second generation monoliths in the practical range of separations (N = 10,000–50,000) – these columns can still not compete with state-of-the-art core–shell particle columns when all columns are evaluated at their own maximum operating pressure (200 bar for the monolithic columns, 600 bar for core–shell columns). It is suggested that monolithic columns will only become competitive with these high efficiency particle columns when further improvements to their production process are made and their pressure resistance is raised.