Packed columns slurry preparation

Glass chromatography column 400 x 15-mm i.d. with a stopcock Column preparation A silica gel column is prepared by packing a slurry of silica gel (10 g) in n-hexane-acetone (9 1, v/v) into a glass chromatography column. About a 1-cm layer of anhydrous sodium sulfate is placed above and below the... 

Slurry packing techniques are required for the preparation of efficient columns with rigid particles of less than 20 micrometers in diameter. The same general packing apparatus. Figure 4.8, can be used to pack columns by the balanced-density slurry, liquid slurry, or the viscous slurry techniques. Down-fill slurry packing is the method of choice for small bore columns and packed capillary columns. 

Tang et al. used columns packed with a slurry of beads suspended in supercritical C02. This packed column was filled with a dilute sol solution prepared by hydrolysis and polycondensation of tetramethoxysilane and ethyltri-methoxysilane precursors. The column was dried using supercritical C02 and heated first to 120 °C for 5 h followed by another 5 h at a temperature of 250 °C [108-110]. Column efficiencies of 127,000 and 410,000 plates/m were reported... 

Retention times of molecules separated over mesoporous silica are much longer than those obtained by using commercially available silica, this is due to the increased surface area of mesoporous silica, which in turn increases molecular capacity factors. Differences between capacity factors are also enhanced Thus, molecules which elute with similar retention times on commercial HPLC columns, with overlapping peaks, can be successfully separated by using HPLC columns slurry packed with mesoporous silica. The long retention times are somewhat of a drawback in that large amounts of solvent must be used and the peak shapes of molecules with long retention times can be broad. Mesoporous silica may not be ideal for routine analytical separations but provides an excellent and cost-effective preparative separation medium. 

A carbon column was initially presented by Jensen and Sundstrom [137] for separation of non-planar and planar PCBs. It is often applied in the cleanup of PCDDs and PCDFs as presented by Smith et al. [138], Kuehl et al. [116] used a carbon-glass column to separate non-planar and planar compounds. They prepared the column by blending AMOCO PX-21 carbon (50 mg) with a shredded glass filter pad (600 mg) in dichloromethane and packed this slurry into a glass column. The sample was fractionated by eluting with dichloromethane (50 ml), dichloromethane benzene (1 1 50 ml), and finally in reverse with toluene (50 ml). Non-planar compounds, loosely bound planar compounds, and tightly bound planar compounds were isolated in these three fractions. PCDEs have been reported to elute in the same fraction as PCDDs and PCDFs on activated carbon [36]. 

The preferred method for packing silica gel and alumina columns is the slurry method, whereby a slurry of the adsorbent and the first eluting solvent is made and poured into the column. When nothing is known about the mixture being separated, the column is prepared in petroleum ether, the least polar of the eluting solvents. 

A packed column for LSC ordinarily consists of a glass tube packed with the adsorbent which is finely divided solid. Packed columns are usually prepared by pouring a slurry of the stationary phase in the mobile phase into a glass tube closed at the bottom by a cottonwool pad or perforated plate having openings too small to permit passage of the solid particles used. The column is then allowed to form by use of one or more of the following methods ... 

At present it seems that immobilization of silica-based particles within a packed capillary by hydrothermal treatment or sol-gel adhesion represent a simpler approach to the preparation of silica-based monoliths for capillary electrochromatography [302,332-334]. Particle fixation is achieved through adhesion by silica precipitated in the interparticle space released from the particles by hydrothermal treatment, or formed by hydrolysis and polycondensation of a solution of alkoxysilanes (sol-gel process). Since only relatively low temperatures are used in both processes, chemically bonded phases can be immobilized as easily as silica. The selectivity and separation efficiency of immobilized particle beds is generally similar to that of slurry packed columns prepared from the same stationary phases. 

A pump is necessary to pack, unpack, and provide buffer from the slurry preparation tank to the column. The ideal pump provides constant pressure and flow rate at low shear stress. Because of shear stress reasons rotating parts have to be avoided, therefore diaphragm pumps are preferred. To dampen the pulsing characteristic of a diaphragm pump, typically 2 or 3 pistons are used. 

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