Cross linked stationary phase

To meet the high demands of organic trace analysis,21 GC columns have been subject to continuous refinement. This refers not only to the reduction in diameter of the nowadays almost exclusively used capillary columns (separation efficiency increases with decreasing capillary diameter), but also reflects the development in stationary phase technology In order to reduce column bleed (which is essential for mass spectrometric detection), highly cross-linked stationary phases are used to... 

Critical temperature The temperature above which a substance can no longer exist in the liquid state, regardless of pressure. Cross-linked stationary phase A polymer stationary phase in a chromatographic column in which covalent bonds link different strands of the polymer, thus creating a more stable phase. Crystalline membrane electrode Electrode in which the sensing element is a crystalline solid that responds selectively to the activity of an ionic analyte. 

Commercial columns are advertised as having bonded or cross-linked stationary phases. The purpose of bonding and cross-linking is to provide a longer-lasting stationary phase that is not disrupted at elevated temperatures or during temperature programming. With use, untreated colutnns slowly lose their stationary... 

Fused silica introduced by Hewlett-Packard 1981-1984 Deactivation procedures and cross-linked stationary phases 1981-1988 Interfacing capillary columns with spectroscopic detectors (MS, FTIR, AED) 1983 Megabore column introduced as an alternative to the packed column... 

Chemically bonded stationary phases are covalently bonded to the fused silica surface, are highly temperature stable, and can be solvent rinsed for cleaning from matrix contaminations. In contrast, cross-linked stationary phases are not bonded to the surface, only use chemical links between the polymer chains. 

Column Conditioning—Capillary columns with bonded (or cross-linked) stationary phases do not normally need to be conditioned however, it is good chromatographic practice to briefly condition a new column as described below. 

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

An example of a size-exclusion chromatogram is given in Figure 7 for both a bench-scale (23.5 mL column) separation and a large-scale (86,000 mL column) mn. The stationary phase is Sepharose CL-6B, a cross-linked agarose with a nominal molecular weight range of 5000-2 x 10 (see Fig. 6) (31). 

The materials originally used as stationary phases for GPC were the xerogels of the polyacrylamide (Bio-Gel) and cross-linked dextran (Sephadex) type. However, these semi-rigid gels are unable to withstand the high pressures used in HPLC, and modern stationary phases consist of microparticles of styrene-divinylbenzene copolymers (Ultrastyragel, manufactured by Waters Associates), silica, or porous glass. 

One solution is to replace the column, but a less expensive approach is to attempt to clean the column. Baking is one approach that removes some forms of contamination, but also shortens the column life because it removes some of the stationary phase. A solvent rinse is the most effective means of cleaning a bonded or cross-linked phase column. Solvent rinse kits are available with instructions from most column manufacturers. The procedure involves forcing solvents through the GC column, usually in the following order—water, methanol, methylene chloride, and hexane—using 10-15 psi back pressure. 

In relation to separation of nucleotides, Hoffman61 found that adenine nucleotides interacted most strongly with cycloheptaamylose, presumably by inclusion of the base within the cavity of cyclodextrin. When epichlorohydrin-cross-linked cycloheptaamylose gel was used as a stationary phase for nucleic acid chromatography, adenine-containing compounds were retarded most strongly. 

There are two types of stationary phases commonly used in exclusion chromatography silica gel and micro-reticulated cross-linked polystyrene gels. A third type of exclusion media is comprised of the Dextran gels. Dextran gels are produced by the action of certain bacteria on a sucrose substrate. They consist of framework of glucose units that can form a gel in aqueous solvents that have size exclusion properties. Unfortunately the gels are mechanically weak and thus, cannot tolerate the high pressures necessary for HPLC and, as a consequence, are of very limited use to the analyst. 

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