Chromatographic Properties of Minimal Aspect Ratio Monolithic Silica Columns

We report on a study wherein we synthesized TMOS-based silica monolithic skeletons in capillaries with an i.d. of 5 and 10 μm to produce skeleton structures with very low capillary-to-domain size aspect-ratios. These structures include the absolute minimal aspect-ratio case of a monolithic structure whose cross-section only contains a single node point. With domain-sized based reduced plate heights running as low as h_min = 1.3-1.5 for retained coumarin dyes providing a retention factor of k = 0.6-1.0, the study confirms the classic observation that ultralow aspect ratio columns generate a markedly lower dispersion than columns with a larger aspect ratio made in the past by Knox, Jorgenson, and Kennedy for the packed bed of spheres, but now for silica monoliths. The course of the reduced van Deemter curves, and more specifically the ratio of A-term versus C-term band broadening, could be interpreted in terms of the width and persistence length of the velocity bias zones in the columns. Considering the overall kinetic performance, it is found that the two best performing structures are also the structures with the lowest number of domains or node points, that is, with the lowest capillary-to-domain size aspect-ratio and, hence, resembling closest to the open-tubular format, which remains confirmed as the column format with the best kinetic performance. This is quantified by the fact that the minimal impedance values (order of E_min = 100) of the best performing ultralow aspect ratio monolithic columns are still significantly larger than the E_min values for the reference open-tubular columns (order of E_min = 15-20).