Silica monoliths

Table 2. Absorption Peaks of Gel-Silica Monolith Pore Water and Surface Hydroxyl Groups ...

This section provides an overview of properties of polymer monolith columns related to 2D-HPLC. Monolithic organic polymer columns, having longer history than silica monoliths, have been reviewed in detail recently by S vec and by Eeltink including their preparation methods and performance (Eeltink et al., 2004 Svec, 2004a). Polymer monolith columns commercially available include polyfstyrene-co-di vinyl benzene) (PSDVB) columns and poly(alkyl methacrylate) columns. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

It is of much interest to compare polymer monoliths with monolithic silica columns for practical purposes of column selection. Methacrylate-based polymer monoliths have been evaluated extensively in comparison with silica monoliths (Moravcova et al., 2004). The methacrylate-based capillary columns were prepared from butyl methacrylate, ethylene dimethacrylate, in a porogenic mixture of water, 1-propanol, and 1,4-butanediol, and compared with commercial silica particulate and monolithic columns (Chromolith Performance). 

Table 7.1 shows the pore properties of several polymer monolithic columns prepared from styrene/DVB, methacrylates, and acrylamides along with the feed porosity and column efficiency, summarized from several recent publications. Some important points seem to be clearly shown in Table 7.1, especially by the comparison of the properties between methacrylate-based polymer monoliths and silica monoliths. 

A rather limited range of mesopores in terms of size and volume were observed in the skeletons of polymer monoliths. The porosity of the polymer monolith seems to be lower than that of silica monolith. The total porosity of these monoliths is in the range of 0.61-0.73, whereas interstitial (through-pore) porosity and mesopore porosity are 0.28-0.70 and 0.03-0.24, respectively. In the case of poly(butyl methacrylate-co-ethylene dimethacrylate), the observed porosity is around 0.61-0.71, resulting in permeability 0.15-8.43 x 10 14 m2, whereas the observed porosity of silica monoliths prepared in a capillary is 0.86-0.96 and the permeability is 7-120 x 10 14 m2. Higher permeability will be advantageous for 2D applications, as mentioned later. 

Hu, L., Chen, X., Kong, L., Su, X., Ye, M., Zou, H. (2005). Improved performance of comprehensive two-dimensional HPLC separation of traditional Chinese medicines by using a silica monolithic column and normalization of peak heights. J. Chromatogr. A 1092, 191 198. 

Miyabe, K., Cavazzini, A., Gritti, F., Kele, M., Guiochon, G. (2003). Moment analysis of mass-transfer kinetics in Ci8-silica monolithic columns. Anal. Chem. 75, 6975-6986. 

Rieux, L., Niederlander, H., Verpoorte, E., Bischoff, R. (2005). Silica monolithic columns Synthesis, characterisation and applications to the analysis of biological molecules. J. Sep. Sci. 28, 1628-1641. 

Brook, M.A., Chen, Y., Guo, K., Zhang, Z. and Brennan, J.D. (2004) Sugar-modified silanes precursors for silica monoliths. Journal of Materials Science, 14, 1469-1479. 

Brandhuber, D., Torma, V., Raab, C., Peterlik, H., Kulak, A. and Husing, N. (2005) Glycol-modified silanes in the synthesis of mesoscopically organized silica monoliths with hierarchical porosity. Chemistry of Materials, 17, 4262 1271. 

Another group working on silica monoliths is the one of A. and M. Kuehn [41]. Their Continuous-Bed-Silica (CB-Silica) is a highly porous monolith having meso- and micropores. The structure of the CB-Silica is very porous and con-... 

Another recent trend focused on supports in the shape of monolithic columns having the goal to benefit from the high permeability and the improved mass transfer characteristics of such structures. With this goal in mind, Lubda and Lindner [75] prepared enantioselective silica monolith columns with tert-butylcarbamoylquinine surface modification. A commercial sol-gel-derived Chromolith Performance Si (100 X 4.6 mm ID) monolith (1.9 tim macropore diameter, 12.5 nm mesopore... 

As with other types of solutes, chromatographers have attempted to improve the speed and efficiency of analysis of bases by fhe use of smaller particle (e.g., sub-2 p,m) or monolithic columns. Small particle columns have not yet been fully evaluated for the analysis of bases, to determine whether they give equivalent selectivity, and reduction in plate height commensurate with the reduction in particle size, as has been demonstrated for neutral compounds. Commercial silica monolith columns give reasonable performance for the analysis of bases at low pH, but show evidence of... 

Organo-Silica Monoliths for Affinity Ligand Encapsulation... 

In a tetralkoxysilane/alkyltrialkoxysilane system, the hydrolysis and condensation reactions proceed not only between similar precursor molecules but also between the two different precursor species. Figure 8.3 illustrates a simplified scheme of hydrolysis and condensafion reactions in a hybrid system containing TMOS and an alkyltrimethoxysilane. Notice the difference from the scheme in Figure 8.1 the alkyl group, represented by R, does not participate in the hydrolysis and condensation reactions that lead to the silica monolith. The sol-gel process kinetics in a hybrid sol-gel... 

In principle, many organosilanes can be used as a second precursor to form the organo-silica monolith. Various coprecursors reported for the fabrication of hybrid organo-silica monolithic columns for EC or CEC are listed in Table 8.1. The structure of the organo-silica material is affected by the ratio of the tetralkoxysilane to the alkyltrialkoxysilane in the sol-gel reactions, the type of precursor used, and the processing parameters. This can be seen, for example, in the phase separation behavior of... 

FIGURE 8.7 Separation of aldehydes and ketones by CEC in the organo-silica monolith shown in Figure 8.6. Column 50 cm x 50 xm ID (46 cm effective length), separation at —25 kV mobile phase 70% acetonitrile-30% 5 mM Tris-HCL (pH = 2.3) analytes (1) ben-zaldehyde, (2) o-tolualdehyde, (3) butyrophenone, (4) valerophenone, (5) hexaphenone, and (6) heptaphenone. (Reprinted from J. D. Hayes, A. Malik, Anal. Chem., 72 4090 (2000). With permission. Copyright American Chemical Society 2000.)... 

FIGURE 8.8 (a) SEM image of organo-silica monolith prepared with the APTES and TEOS ... 

TEOS/Cb-TEOS Volume Organo-Silica Monoliths ... 

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