Columns packings,supports,stationary phases,tubing

An open-tubular column is a capillary bonded with a wall-supported stationary phase that can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The inner diameters of open-tubular CEC columns should be less than 25 pm that is less than the inner diameters of packed columns. The surface area of fused silica tubing is much less than that of porous packing materials. As a result, the phase ratio and, hence, the sample capacity for open-tubular columns are much less than those for packed columns. The small sample capacity makes it difficult to detect trace analytes. 

In column chromatography, the stationary phase is coated onto or chemically bonded to support particles that are then packed into a tube, or the stationary phase is coated onto the inner surface of the tube, GC and LC are subclassifica-tions of column chromatography (see Figure 6-1). 

A gaseous mobile phase flows under pressure through a heated tube either coated with a liquid stationary phase or packed with liquid stationary phase coated onto a solid support. The analyte is loaded onto the head of the column via a heated injection port where it evaporates. It then condenses at the head of the column, which is at a lower temperature. The oven temperature is then either held constant or programmed to rise gradually. Once on the column separation of a mixture occurs according to the relative lengths of time spent by its components in the stationary phase. Monitoring of the column effluent can be carried out with a variety of detectors. 

Two types of columns are used. A packed column is one filled with inert, solid particles coated with a liquid stationary phase. Standard tubing is about 0.5 cm in diameter, with lengths ranging from 1 m to 20 m however, columns for large-scale preparative work may be up to 5 cm in diameter and several meters long. Commonly used solid supports are diatomaceous earth, Teflon powder, and glass beads. The stationary liquid must be chosen on the basis of the compounds to be analyzed. A more recently developed and more widely used type of column is the open-tubular or capillary column. This is prepared by coating the inner wall of the column with the stationary liquid phase. The inside diameter of a typical capillary tube is 0.25 mm, and... 

The stationary phase can be a liquid or a solid. If it is a liquid, it can be coated directly on the inside walls of a capillary tube (column), or it can be coated on an inert solid support and be handled like a solid. In effect, then, there are three stationary phase configurations in the first type, a solid (with or without stationary liquid) is packed into a column in the second type, a solid is coated on the surface of a flat, plane material like glass (TLC), and in the third type, a liquid is coated on the inside wall of an open tube (OT). 

The stationary liquid phase is coated on a solid support and packed into a column (packed column GC), or it is coated on the wall of an open tube (WCOT), as has been discussed. The higher efficiency of OT columns has reduced the necessity for many selective liquids, and the number of OT columns necessary to analyze for all types of analytes is smaller than for packed columns. 

The stationary phase in HPLC is the solid support contained in within a specified column over which the mobile phase flows effecting the separation of the individual components. The HPLC column is normally fabricated using 100- to 300-mm long stainless steel tubes with an internal diameter of 2-5 mm. They are packed with porous, microporous, spherical, or irregularly shaped particles, or particles with specific coatings with the following characteristics. 

The pioneering gas-liquid chromatographic studies in the early 1950s were carried out on packed columns in which the stationary phase was a thin film of liquid retained by adsorption on the surface of a finely divided, inert solid support. From theoretical studies made during this early period, it became apparent that unpacked columns having inside diameters of a few tenths of a millimeter could provide separations that were superior to those of packed columns in both speed and column efficiency. In such capillary columns, the stationary phase was a film of liquid a few tenths of a micrometer thick that uniformly coated the interior of capillary tubing. In the late 1950s, such open tubular columns were constructed the predicted... 

In open tubular columns, the stationary phase is held on the inner surface of a capillary, whereas in packed columns, the stationary phase is supported on particles that are contained in a glass or metal tube. Open tubular columns contain an enormous number of plates that permit rapid separations of closely related species. They suffer from small sample capacities. 

Historically, GC was first performed using packed columns. Such columns were usually made of coiled metal or glass tubes, with length of 1 to 5 m and internal diameter of a few millimeters. These were packed with small particles of sohd support coated with nonvolatile liquid stationary phases. The solid particles themselves played the role of the stationary phase in GSC. Packed columns were characterized by poor efficiencies related to multiple flow paths among the packing particles (the A term in the van Deemter equation), as well as uneven distribution of the hquid phase within the particles and at the contact points between the particles. The number of theoretical plates in packed columns was several thousand at the most, therefore improvements in resolution were usually achieved by the use of more selective stationary phases. Consequently, hundreds of different stationary phases were available at the peak of packed column development. 

Open tubular, or capillary, columns are of two basic typos wall-coated open tubular (W( OT) and support-coated open tubular (SCOT) columns. Wall-coated columns are simply capillary tubes coated with a thin layer of the stationary phase. In support-coated open tubular columns, the inner surface of the capillary is lined with a thin film (—.30 pm) of a support material, such as diatomaceous earth. This type of column holds several times as much stationary phase as docs a wall-coated column and thus has a greater sample capacity. Generally, the efficiency of a SCOT column is less than that of a WCOT column but significantly greater than that of a packed column. 

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