Synthesis of propyl-functionalized hybrid monolithic silica capillaries and evaluation of their performances in nano-LC and CEC.

During the last decade, silica monolithic capillaries have focused more and more attention on miniaturized separation techniques like CEC, nano-LC, and chip electrochromatography owing to their unique chromatographic properties and to their possible in situ synthesis. Nevertheless, the preparation of conventional silica-based individual monolithic columns is time consuming, owing to the individual steps involved, including the synthesis of the silica matrix and its subsequent on-column chemical grafting. The hybrid organic-inorganic monoliths, whose synthesis is based on the polycondensation of siloxane with organosiloxane precursors, seems to be an attractive alternative since their direct synthesis leads to silica monoliths with organic moieties covalently linked to the inorganic silica matrix through hydrolytically stable Si-C bonds. This study describes the synthesis of hybrid monoliths using propyltrimethoxysilane (C3-TriMOS) as a new kind of silica coprecursor to subsequently increase the hydrophobicity of the stationary phase. The influence of several experimental parameters (pH, gelation temperature, relative proportion of the precursors) on the textural (skeleton and macropore size) and chromatographic properties (efficiency, retention, and electroosmotic mobility) of the obtained monoliths are discussed. The results show that the optimal coprecursor incorporation is obtained after a postgelation step during which the condensation of the C3-TriMOS coprecursor is favored by an increase in the pH medium. Thermal hydrolysis of urea previously added to the polymerization mixture allows this in situ pH increase. These hybrid monoliths present hydrophobic properties and allow the separation of test mixtures in the RP mode without any further modification. Moreover, they present excellent efficiencies since reduced plate height as low as 5 and 15 microm are obtained in the electrodriven mode (CEC) and in the hydrodynamic one (nano-LC), respectively.

Electrochromatographic behavior of silica monolithic capillaries of different skeleton sizes synthesized with a simplified and shortened sol-gel procedure.

Silica monolithic capillaries (SMCs) were synthesized by a sol-gel process. First, a simplification of the synthesis was proposed by replacing the calcination and the drying steps which can have tremendous effects on chromatographic and physical properties, by a single water or methanol 2 h washing step. The efficiency of such a washing step was demonstrated and the comparison of the chromatographic and electrochromatographic properties between calcined and washed SMCs has shown that such a modification did not impair retention, efficiency, and stability of the monolith. This simplified procedure was carried out to synthesize SMCs with two different skeleton sizes. These capillaries were evaluated in electrochromatography and present high efficiencies (H = 5 microm) at least equal to the best ones reported in the literature. Furthermore, the influence of the skeleton size on the EOF of the second kind (EOF-2) was investigated with unmodified SMCs used under various experimental conditions including electrical field strength and buffer concentration. The ionic strength of the mobile phase and the applied electrical field that enable this EOF-2 were related to the size of the skeleton which was tuned by the synthesis conditions.

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography.

Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.

Effect of temperature on the retention of ionizable compounds in reversed-phase liquid chromatography: application to method development.

The analysis of pharmaceutical compounds is often a difficult challenge which requires mathematical tools to improve the quality of the separation method. This work is an attempt to rationalize the anomalous variation of the logarithm of the retention factor with temperature in case of ionizable compounds. The effect of temperature on ionizable compounds was studied within a large range of temperature, ranging from 30 to 130 degrees C. The determination of the so-called chromatographic pKa and the study of its variation with temperature allow to explain why the forms of the van't Hoff curves are so different depending on the type of solute, the type of buffer and the type of the mobile phase. A retention model along with a computation procedure is proposed to optimize both temperature and mobile phase composition and to provide good and robust conditions as shown by illustrative examples.

Synthesis of zirconia monoliths for chromatographic separations.

The aim of this work is to join the advantages of two different kinds of stationary phases: monolithic columns and zirconia-based supports. On the one hand, silica monolithic columns allow a higher efficiency with a lower back-pressure than traditional packed columns. On the other hand, chromatographic stationary phases based on zirconia have a higher thermal and chemical stability and specific surface properties. Combining these advantages, a zirconia monolith with a macroporous framework could be a real improvement in separation sciences. Two main strategies can be used in order to obtain a zirconia surface on a monolithic skeleton: coating or direct synthesis. The coverage by a zirconia layer of the surface of a silica-based monolith can be performed using the chemical properties of the silanol surface groups. We realized this coverage using zirconium alkoxide and we further grafted n-dodecyl groups using phosphate derivatives. Any loss of efficiency was observed and fast separations have been achieved. The main advance reported in this paper is related to the preparation of zirconia monoliths by a sol-gel process starting from zirconium alkoxide. The synthesis parameters (hydrolysis ratio, porogen type, precursor concentration, drying step, etc.) were defined in order to produce a macroporous zirconia monoliths usable in separation techniques. We produced various homogeneous structures: zirconia rod 2 cm long with a diameter of 2.3 mm, and zirconia monolith inside fused silica capillaries with a 75 microm I.D. These monoliths have a skeleton size of 2 microm and have an average through pore size of 6 microm. Several separations have been reported.