Silica rods, monolithic

Wenk, M., Droll, A., and Krahenbuhl, S. 2006. Fast and reliable determination of the antifungal drug voriconazole in plasma using monolithic silica rod liquid chromatography. J Chromatogr B. 832 313. 

Monolithic column — The trend to use shorter columns in liquid chromatography means that the resultant lower separation efficiency is of concern. One way to improve HPLC separation efficiency on a shorter column is to reduce the size of the packing material, but at the cost of increased backpressure. Another approach to improve performance is increasing permeability with a monolithic column. Such a column consists of one solid piece with interconnected skeletons and flow paths. The single silica rod has abimodal pore structure with macropores for through-pore flow and mesopores for nanopores within a silica rod8182 (Figure 12.1). 

FIGURE 13.7 Scanning electron micrograph showing flow-through channels in silica-based monolithic rods. 

The basic studies dealing with the preparation of continuous porous silica materials date back to 1991 [76-78]. Two years later, Nakanishi and Saga applied for a patent describing the fabrication of monolithic silica rods for chromatographic application [79-81], whereas a second protocol for the preparation of continuous silica rods was independently filed by Merck KGaA in Germany [82]. 

First comprehensive investigations with respect to the properties of continuous porous silica rods were, however, carried out by Tanaka and Fields in 1996 [33,34,83], who reported on two different methods for the preparation of silica monoliths. 

The morphology of a typical inorganic monolith is fundamentally different from that of organic polymers (Figure 1.4a). The sttucture is rather sponge- than brush-like and is consttucted by interconnected silica rods in the low micrometer size. This composition leads to a discrete distribution of flow channels, which can be deduced by comparison of macropore disttibution of a typical organic... 

FIGURE 1.4 Morphology and porosity of a typical monolithic rod, prepared by copolymerization of silane precursors, (a) SEM micrograph of the fractured surface of a monolithic silica gel rod. (b) Pore size distribution of a representative monolithic silica rod. (Reprinted from Guiochon, G., J. Chromatogr. A, 1168, 101, 2007. Copyright 2007, with permission from Elsevier.)... 

Many contributions regarding silica monolithic columns were published by the group of Tanaka [93,189,196]. In their early work, they reported on the successful separation of alkyl benzenes, which are representative for the separation of many low-molecular-weight compounds, containing aromatic groups. Tanaka et al. also combined a conventional column in the first dimension with a silica rod column for the fractionation of aliphatic and aromatic hydrocarbons [197]. The successful separation of the 16 EPA priority pollutants PAHs was carried out by Nunez et al. [93] and is shown in Eigure 1.15. 

Fig. 5.3. Plots of the skeleton size against the through-pore size of the continuous monolithic silica prepared in a capillary (O), and the larger-sized silica rod columns (7 mm x 83 mm) having constant through-pore size/skeleton size ratio ( ) [15]. Also plotted are the particle size (vertical axis) against the size of interstitial voids (25-40% of dp as indicated by the bars) found with a conventional particle-packed column.

Improvements in the selectivity of the separation of microcystins and nodnlarin have been achieved by selecting the most efficient stationary phase, with this aim (Spoof 2002) compared a monolithic C-bonded silica rod colnmn (Merck Chromolith) to particle-based C and antide C 18 18 16 sorbents in the HPLC separation of eight microcystins and nodularin-R. Two gradient mobile phases of aqneons trillnoroacetic acid modified with acetonitrile or methanol, different flow-rates, and different gradient lengths were tested. The performance of the Chromolith colunrn measured the resolution of some microcystin pairs. The selectivity, efficiency (peak width), and peak asymmetry equalled, or exceeded, the performance of traditional particle-based columns. The Chromolith 21 colnmn allowed a shorteiting of the total analysis time to 4.3 minutes with a flow rate of 4 ml/minute. 

The preparation of polymeric monoliths is relatively simple compared with those of the silica rods. A polymerization mixture consisting of monomer, cross-linker, initiator, and porogenic solvent is introduced into a sealed tube. The reaction can be temperature or redox initiated and in the case of transparent molds UV light can also be used to trigger the polymerization. At the end of the reaction the seals are removed and the tubes are attached to a pump, which flushes solvent through the monolith to remove the porogens and the unreacted components. The obtained monolith can be radial or axial compressed to prevent the formation of voids and further functionalized for different chromatographic modes. The majority of current monolithic supports... 

Figure 1.2 LC-MS chromatograms of a drag and its metabolites on conventional packed column and a monolithic silica rod at various flow-rates. Reprinted from [16] with permission, and adapted. 2002, John Wiley and Sons Ltd.

Instead of packed columns, monolithic (continuous bed), analytical, or capillary columns in the form of a rod with flow-through pores offer high porosity and improved permeability. Silica-based monolithic columns are generally prepared by gelation of a silica sol to a continuous sol-gel network, onto which a Cjg or another stationary phase is subsequently chemically bonded. Such columns provide comparable efficiency and sample capacity as conventional columns packed with 5-pm particle materials, but have three to five times lower flow resistance, thereby allowing higher flow rates and fast HPLC analyses. Rigid polyacrylamide, polyacrylate, polymethacrylate, or polystyrene monolithic columns are prepared by in sim polymerization. 

Despite the fact that the first silica monolithic supports have been already reported in the late 1970s, the first uniform porous silica rods used for reversed-phase chromatography... 

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