Column support material, particle size

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

The purpose and role of the solid support is the accommodation of a uniform deposition of stationary phase on the surface of the support. The most commonly used support materials are primarily diatomite supports and graphitized carbon (which is also an adsorbent for GSC), to a lesser extent. Teflon, inorganic salts and glass beads. There is no perfect support material because each has limitations. Pertinent physical properties of a support for packed-column GC are particle size, porosity, surface area, and packing density. Particle size impacts column efficiency via the A term or eddy diffusion contribution in the van Deemter expression (Equation 2.44). The surface area of a support is governed by its porosity, the more porous supports requiring greater amounts of stationary phase... 

Studies on solid supports have shown that tlie pore size of the silica-based sorbents is of little importance in MSPD, but it should be considered as could vary with the sample. Particle size is of greater importance since particles as small as 3-20 m can lead to extended solvent elution times and plugged MSPD columns. However, 40 m particles with 60 Angstrom pores have been used extensively and successfully. Sorbents that have a blended range of particle size such as 40-100 m work equally well and can be used in most applications (101, 103). These materials also tend to be less expensive. Depending on the application, analysts can also use non-end-capped materials and materials with a range of carbon loading. Different applications may benefit, suffer, or be unaffected by these parameters, but workers should consider them to obtain the best extraction efficiency and the cleanest sample. 

A ternary liquid-liquid system for partition chromatography is prepared from a mixture of 2,2,4-trimethylpentane, ethanol and water (34 5 1). The less polar upper layer is used as the stationary phase. A diatomaceous material, Hyflow Super Cel (particle size, 7-11 jam), is used as the solid support. The columns (40 cm X 4 mm I.D.) are packed by the slurry technique, and the support material is coated in situ with the liquid stationary phase as described earlier [54]. A pre-column is inserted in order to maintain equilibrium between... 

GPC is a further special form of liquid chromatography. The separation column is packed with porous, polymer gels (e.g. polystyrene gel) as stationary phase. The particle size of the packing material and the size distribution of the pores are well defined and uniform. In GPC molecules are separated according to their effective size in solution, i.e., their hydrodynamic volume, and not according to their affinity for the support material. 

The behaviour of the column is largely dependent on the stationary phase but the support material can also play a very important role, particularly when a low concentration of liquid phase is used. A good support material should have a very large surface area and a uniform particle size, free from fines and sufficiently robust so that it does not break up into powder with normal handling. It should be chemically inert, without adsorptive effects, and, when coated with stationary phase, should pack easily and uniformly into the column. 

Column packing consists of two essential ingredients—the inert supporting phase and the stationary liquid phase. The solid supporting phase is usually an insert material of uniform particle size. Diatomaceous earth is a frequently used solid support, although celite, firebrick, and glass beads are also used. The particle size is important in achieving the maximum separation or efficiency. However, smaller particles inhibit the flow rate of gas due to the increased resistance of the denser medium. 

Particle size and size distribution define the quality of the support material and are the key determinants of efficiency and back-pressure of the column.1-3 The effect of dp on H is discussed in Chapter 2. For a well-packed column, Hmin is approximated to 2-2.5 dp. Also, since the van Deemter equation C term is proportional to dp2, columns packed with small particles have much less efficiency loss at high flow rates.14 However, since column back-pressure is inversely proportional to dp2, columns packed with sub-3-pm particles can easily exceed the pressure limit of most HPLC instruments at 6,000 psi. Note that decreasing particle size while keep the L constant can increase column efficiency and peak resolution (Figure 3.5A) and also increase peak height and sensitivity (Figure 3.5B). 

The solid support for the liquid phase should be chemically inert and stable at the operating temperatures, and should have a large surface area. The importance of uniform particle-size to facilitate even packing has already been emphasized (see Section 11,1, p. 96). Materials in most common use as inert supports are diatomaceous earth and glass beads, both of which are often pretreated chemically to minimize adsorption effects. Preparations of diatomaceous earth are available commercially, in narrow ranges of particle size, pretreated with acid, alkali, or dichlorodimethyl-silane. A coating of silver has been used for diminishing adsorption on the inert support it probably enhances heat transfer in the columns, as well. 

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