Porosity monolith silica columns

Chromatographic use of monolithic silica columns has been attracting considerable attention because they can potentially provide higher overall performance than particle-packed columns based on the variable external porosity and through-pore size/skeleton size ratios. These subjects have been recently reviewed with particular interests in fundamental properties, applications, or chemical modifications (Tanaka et al., 2001 Siouffi, 2003 Cabrera, 2004 Eeltink et al., 2004 Rieux et al., 2005). Commercially available monolithic silica columns at this time include conventional size columns (4.6 mm i.d., 1-10 cm), capillary columns (50-200 pm i.d., 15-30 cm), and preparative scale columns (25 mm i.d., 10 cm). 

The porosities in parentheses were obtained with a C g-bonded phase b Develosil-Cis particles packed in a column, 4.6 mm diameter, 10 cm in length c Monolithic silica column prepared in a mould and inserted into a PTFE tube d Monolithic silica column prepared in a mould and PEEK-resin clad e Monolithic silica column prepared in a 100 pm capillary... 

When monolithic sihca columns are prepared in a fused sihca capillary, the silica network structure can be bonded to the tube wall. They can be used as a column directly after preparation, or as a reversed-phase adsorbent after alkyl or some other type of modihcation. The porosity of monolithic silica columns is much greater than that of a particle-packed column. A major difference is seen in interstitial porosities 65-70% for monolithic sihca prepared in a mold, and higher than 80% for those prepared in a capillary, compared to 40% for a particle-packed column. A comparison of the separations of cytochrome triptic digest on packed and monolithic colums is shown in Figure 3-23. The separations are nearly identical except that on monolithic column it is ten times faster. Figure 3-24 shows the dependence of the backpressure generated on the system as a function of the flow rate for packed column and a set of different monolithic columns. The slope on all monohthic columns is the same, and it is approximately live times lower than that on a packed column. Additional information on fast FIPLC on monolithic columns is given in Chapter 17. 

Taguchi et al. [97] and Liang et al. [98,99] reported on the preparation of monolithic carbon columns, which exhibit a hierarchical, fully interconnected porosity. Silica particles (10 pm) have been suspended in an aqueous solution, containing ethanol, FeClj, resorcinol, and formaldehyde. After polymerization, the solid rod was dried, cured, and carbonized by raising temperature to 800°C and finally up to 1250°C. Finally, concentrated FIF was used to remove silica and iron chloride. Even if carbon have been shown to possess a high specihc surface area (up to lllSmVg), their chromatographic efficiency is moderate (FIETP of 72 pm). 

The introduction of monolithic columns in the 1990s was another and more successful attempt to increase column permeability while decreasing the gap in column dual porosity. Macropores in the monolith are between 4000 and 6000 A in diameter, and they occupy almost 80% of the column volume. Compared to the conventional packed column with 5- or even 3-pm particles, the silica skeleton in monolith is only approximately 1 pm thick, which facilitates accessibility of the adsorbent surface inside the mesopores of the skeleton (pores between 20 and 500 A in diameter are usually called mesopores). Comparison of the spherical packing material and monolithic silica is shown in Figure 3-1. 

This paper presents a study on selected particulate and monolithic silicas with a high porosity designed for preparative liquid phase separation processes. The aim is to elucidate the decisive pore structural properties of the bulk materials and to correlate these data with those obtained at column operation, i.e. column pressure drop and column performance. 

The Kf values for the particulate silica columns indicate a decrease of the permeability with increasing average mesopore diameter, increasing total porosity and increasing pore connectivity at constant average particle diameter of 10 pm. The monolithic column show a slight increase of the permeability with increasing macropore diameter at constant total porosity which is to be expected. 

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. 

Another approach is the use of monolithic columns consisting of silica based rods of bimodal pore structure. They contain macropores (-1-2 pm) and smaller mesopores ( 10-20nm) [38]. The macropores allow for low backpressure at high flow rates. The mesopores provide the needed surface area for interactions between the solute and stationary phase. The macropores result in higher total porosity as compared to porous silica particles. Flow rates of 5 mL/min can be tolerated on a 10-cm column without an appreciable loss in... 

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