Abstract
In the last decade, silica monolithic capillaries have focused more and more attention on miniaturized sepn. techniques like capillary electrochromatog. (CEC), nano-liq. chromatog. (nano-LC) and chip electrochromatog. owing to their unique chromatog. properties and their simplified prepn. compared with packed columns. They were synthesized according to a sol-gel multi-step process that includes, after a gelation step at 40° giving the macropores network and the silica skeleton, a post-gelation step (hydrothermal treatment at 120° in basic medium) that allows to tailor the mesopores and finally a calcination or a washing step to remove remaining polymers. To reduce the synthesis time, the no. of synthesis steps and above all the temp. synthesis, to adapt the synthesis of such silica monoliths in polymeric microsystem devices, the authors extensively studied the influence of the hydrothermal treatment and its duration on textural (pore size distribution) and chromatog. properties (retention, efficiency) of in situ-synthesized capillary monoliths in nano-LC and CEC. This study was performed on pure silica and octyl chains grafted silica monoliths. Untreated monoliths show small pores (<6 nm), whereas hydrothermally treated monoliths exhibit medium and large mesopores (8-17 nm). The hydrothermal treatment at 120° was not necessary for pure silica monolithic capillaries dedicated to normal phase liq. chromatog. or hydrophilic interaction liq. chromatog. (HILIC) and electrochromatog.: the suppression of the hydrothermal treatment did not impair efficiencies in CEC and in nano-LC but contributed to increase in retention factors. Minimal plate heights of .apprx.5 mm in CEC and 6 mm in nano-LC were obtained with or without hydrothermal treatment with bare silica. In the same way, the hydrothermal treatment was not necessary for grafted silica monoliths only dedicated to CEC. However, the hydrothermal treatment becomes essential before grafting to preserve the efficiency of the monolithic silica capillaries dedicated to nano-LC: in this particular case, the suppression of the hydrothermal treatment leads approx. to a loss of a factor two in efficiency.