Silica monolithic porous columns

Monolith Column—Porous silica column prepared in situ to completely fill the column tube with a fully porous silica foam skelton. After the organic polymer support is heated off, the silica surface is silylated in place to product bonded-phase surface. Column is high resolution and can be used at high flow rates with relatively low back-pressure (see Chapter 16). 

Fields reported that continuous silica xerogels prepared from potassium silicate solutions could be used as highly permeable support media, and exhibit reasonable chromatographic efficiency in HPLC [23]. Minakuchi et al. reported the preparation and evaluation of continuous porous-silica columns that provide a much higher column efficiency in HPLC than do conventional columns packed with particles [13-16,18], The monolithic columns prepared in a capillary can also be used in CEC. 

In addition to utilization of monoliths as a column material, two reports describing respectively silicate and synthetic organic polymer based monolithic frits were published recently [85,86], The conventional method of frit fabrication for a particle packed column usually involves thermal sintering of a section of the packing material, such as bare or octadecyl silica, using a heating device. This approach has several weaknesses such as the lack of control of the temperature and porous properties of the frit that decreases reproducibly of the fabrication process. 

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... 

Recently a new type of column has appeared. Named monolithic It Is packed with a porous silica gel which unites to form a single entity (Figure 3.8). This material, more permeable to solvents than traditional bead type stationary phases, conserves a high efficiency even for rapid flow rates of the mobile phase. These columns, now fully reproducible, have equivalent performances to packed columns. 

Monolithic columns are another approach to provide lower pressure drops and higher rates of mass transfer. These are continuous solid columns of porous silica stationary phase instead of packed particles. Like perfusion packings, they have a bimodal pore structure (Figure 21.7). Macropores, which act as flowthrough pores, are about 2 fim in diameter. The silica skeleton contains mesopores with diameters of about 13 nm (130 A). It can be surface modified with stationary phases like Cig. The rod is shrink-wrapped in a polyetheretherketone (PEEK) plastic holder to prevent waU effects of solution flowing along the walls. The surface area of the mesopores is about 300 mVg, and the total porosity is 80%, compared with 65% for packed particles. The colunm exhibits a van Deemter curve approximating... 

Kobayashi, H. Tanaka, N. Ishizuka, N. Minakuchi, H. Nakanishi, K. The characterization of porous silica monolithic columns in HPLC and CEC. Chromatography 2000, 21, 404-405. 

Of the numerous methods developed to pack stationary porous medium/column in a microcapillary, two widely used approaches involve the packed beads column and the in situ polymerized monolithic column [5]. The preparation of the packed beads column includes the fabrication of retaining frits within a microcapillary as well as subsequent packing of micro-sized silica particles into the capdlary. To avoid the problems associated with frit... 

Monolithic CEC columns can be formed from organic porous polymeric monoliths or porous silica sol-gel monoliths. Other monolithic CEC column types, such as immobilized particles, have also been demonstrated but have not shown the potential of the first two types. The monolithic stationary phases are created in situ by polymerization reactions under controlled conditions, such that a porous bed is created. 

In recent years, the interest in using porous silica and polymer-based monolithic stationary-phase media for ion chromatographic separations of inorganic and organic ions has increased.As compared to particle bed columns, monolithic columns represent a single piece of porous cross-linked polymer or porous silica. Monoliths are made in different formats as porous rods, generated in thin capillaries or made as thin membrane or disks. 

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