Gas-solid chromatography adsorbents for

Limited number of commercially available adsorbents for gas-solid chromatography... 

TABLE 2.7 Porous Polymeric Adsorbents for Gas-Solid Chromatography... 

There are two basic disadvantages to the coated capillary column. First, the limited solute retention that results from the small quantity of stationary phase in the column. Second, if a thick film is coated on the column to compensate for this low retention, the film becomes unstable resulting in rapid column deterioration. Initially, attempts were made to increase the stationary-phase loading by increasing the internal surface area of the column. Attempts were first made to etch the internal column surface, which produced very little increase in surface area and very scant improvement. Attempts were then made to coat the internal surface with di-atomaceous earth, to form a hybrid between a packed column and coated capillary. None of the techniques were particularly successful and the work was suddenly eclipsed by the production of immobilize films firmly attached to the tube walls. This solved both the problem of loading, because thick films could be immobilized on the tube surface, and that of phase stability. As a consequence, porous-layer open-tubular (PLOT) columns are not extensively used. The PLOT column, however, has been found to be an attractive alternative to the packed column for gas-solid chromatography (GSC) and effective methods for depositing adsorbents on the tube surface have been developed. 

Porous layer open tube (PLOT) column A capillary column for gas-solid chromatography in which a thin layer of the stationary phase is adsorbed on the walls of the column. 

On the inside wall of (empty) capillary columns a thin (1—100 jxm) continuous layer of adsorbent is deposited for gas-solid chromatography and a continuous liquid film for gas-liquid chromatography. 

Methods for the prepetration of open capillary columns for gas-solid chromatography should meet very strict requirements 1) a rly uniform distribution of the adsorbent layer over the capillary surface 2) rigid attachment of the adsorbent par-... 

Some common adsorbents used for gas-solid chromatography are given below. [Some of these materials are commercially available as porous-layer open tubular columns (PLOT). PLOT columns yield higher sensitivity than do the traditional packed columns because of their higher column permeability and lower pressure drop (than packed columns).]... 

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 in gas-solid chromatography), and to a lesser extent, Teflon, inorganic salts, and glass beads. There is no perfect support material each has limitations. Pertinent physical properties of a solid support for packed column GC are particle size, porosity, surface area, and packing density. Particle size affects column efficiency by means of an eddy diffusion contribution in the van Deemter... 

In gas-solid chromatography (GSC) the stationary phase is a solid adsorbent, such as silica or alumina. The associated virtues associated therewith, namely, cheapness and longevity, are insufficiently appreciated. The disadvantages, surface heterogeneity and irreproducibility, may be overcome by surface modification or coating with small amounts of liquid to reduce heterogeneity and improve reproducibility 4,15). Porous polymers, for example polystyrene and divinyl benzene, are also available. Molecular sieves, discussed in Chapter 17, are used mainly to separate permanent gases. 

The difference between IGC and conventional analytical gas-solid chromatography is the adsorption of a known adsorptive mobile phase (vapour) on an unknown adsorbent stationary phase (solid state sample). Depending on experiment setup, IGC can be used at finite or infinite dilution concentrations of the adsorptive mobile phase. The latter method is excellent for the determination of surface energetics and heat of sorption of particulate materials [3]. With IGC at finite dilution, it is possible to measure sorption isotherms for the determination of surface area and porosity [4], The benefits of using dynamic techniques are faster equilibrium times at ambient temperatures. 

Zeolites. Porous structure and good adsorption properties make them usable in gas-solid chromatography. They are a specific type adsorbent, with cavities allowing sieving action for molecules able to enter "holes" (windows). 

Van Hook and Phillips (34) and Van Hook (37) have discussed the application of Equation 1 to gas-liquid and gas-solid chromatography respectively. First consider the corrective terms. The third term in Equation 1 corrects for the isotope effect on the partial molal volumes of the condensed phase. In the case of the two dimensional adsorbed film the term should be rewritten in terms of the surface tension and the molar coverage. In either event the correction is expected to be of the same order of magnitude as that for the pure liquids (where it reduces simply to the isotope effect on molar volumes). These corrections are negligibly small. They amount to only about 0.1% of the total isotope effect per D atom for representative hydrocarbons (35). Similarly, the fourth term which corrects for the isotope effect on the nonideality of the gas phase is readily shown to be negligibly small (31, 35) under normal chromatographic conditions. 

Gas-solid chromatography is based on the fact that molecules migrating down the column experience numerous adsorption-desorption events on the walls of the tube and on the surface of an eventual packing. Let us assume that the surfaces are smooth and homogeneous. The simplest theory [6] suggests that at each collision with surface the molecule is adsorbed for a mean time interval ra obeying... 

Gas-solid chromatography is performed with both packed and open tubular columns. For the latter, a thin layer of the adsorbent is affixed to the inner walls of the capillary. Such columns are sometimes called porous-layer open tubular columns, or PLOT columns. Figure 31-16 shows a typical application of a PLOT column. 

Gas chromatography is often divided into categories based on the type of stationary phase used. Gas-liquid chromatography (GLC) implements a porous, inert solid support that is coated with a viscous, nonvolatile liquid phase. On the other hand, gas-solid chromatography (GSC) uses a solid adsorbent as the stationary phase. Klee offers these general rules-of-thumb for selection of stationary phase materials use solid adsorbents to separate room-temperature gases, liquid stationary phases to separate room-temperature liquid and solid mixtures, polar phases for polar solutes, and nonpolar phases for nonpolar solutes. Table 1 lists common liquid- and solid-stationary phase materials available for use in capillary columns. Barry cross-refers numerous column materials from nine different manufacturers. ... 

A technique akin to adsorbent deactivation is thb modification of the total surface by covering the adsorbent with, sev al. monolayers of some compound or by reacting all the surface sites. In eitfier case a new surface results, of generally higher linear capacity and totally different adsorption characteristics. This general technique has so far beeit jeserved for adsorbents used in gas-solid chromatography (see Section 9-2B). 

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