Optimization of the single-step synthesis of hybrid C8 silica monoliths dedicated to nano-liquid chromatography and capillary electrochromatography

Abstract

Hybrid silica monoliths functionalized with octyl groups and dedicated to chromatographic separations in the reversed-phase mode were directly synthesized within capillaries according to the protocol described by Yan et al. [L.J. Yan, Q.H. Zhang, Y.Q. Feng, W.B. Zhang, T. Li, L.H. Zhang, Y.K. Zhang, J. Chromatogr. A 1121 (2006) 92]. Although these monoliths allowed reaching high efficiencies in capillary electrochromatography (CEC), serious limitations prohibited their application in nano-liquid chromatography (nano-LC). Such limitations observed as poor performances in the nano-LC mode and the lack of reproducibility of the synthesis were related to the longitudinal morphological inhomogeneities of the hybrid material along the capillary. Thus, several modifications were conducted in the synthesis protocol in order to improve the resulting morphology of the monolith making it suitable for nano-LC separations. The influence of several critical parameters (such as the addition temperature of the basic catalyst and the hydrolysis duration) on the textural and chromatographic properties had been extensively studied. It was found that a decrease (i.e. 0°C) of the temperature addition of the basic catalyst associated with a shorter hydrolysis duration (1h instead of 6h) allowed (i) delaying the gelation time and consequently facilitating the capillary filling step, (ii) increasing the structural homogeneity of the hybrid monoliths, i.e. their chromatographic performances in nano-liquid chromatography also (iii) greatly improving the reproducibility of the synthesis within the capillary without impairing the material's carbon load, i.e. the incorporation of the less hydrolysable C8 precursor. The resulting hybrid monoliths afforded retention factors comparable to that previously obtained for C18 grafted silica monoliths and efficiencies that are the best ever recorded in nano-LC with hybrid monoliths and that are close to the ones achieved with grafted silica monoliths. In fact, this modified protocol allowed a significant improvement of the performances in nano-LC which could be observed by the decrease of the mean value of Hmin going from 123μm (Yan's protocol) to 24μm (modified protocol) for a same length of capillary (l=8.5cm). In addition, the reproducibility of the synthesis was greatly improved through a factor six of reduction on the calculated standard deviation of these efficiencies. The high permeability and longitudinal homogeneity of the synthesized monolith allowed increasing the capillary length (for example, a 75-cm capillary was conveniently filled with hybrid silica monolith) and the column could be eluted at a very low backpressure leading to chromatographic performances up to 40,000 plates. Finally, the good efficiencies in the nano-LC mode combined with the excellent performances already present in the CEC mode led to fast (less than 1min) and high efficient separations in the pressurized capillary electrochromatography (p-CEC) mode.