Porous layer open tube column

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. 

PLOT Porous layer open tubular column used in capillary GC, consists of a thin layer (1-5 pm) of solid stationary phase material, e.g. modified alumina, coated onto the inner wall of silica capillary tubing see capillary GC. 

L of air is passed over 400 and 200 mg of coconut charcoal in separate tubes at a flow rate of 10-500 mL/min. The analyte is desorbed with methylene chloride and injected into a GC equipped with an FID. A fused-silica porous-layer open tubular column coated with AI2O3/KCI with or without a back-flushable precolumn may be used as a GC column. Alternatively, a 10% FFAP on a 80/100-mesh Chromosorb W column may be used. 

The other types of capillary columns are shown in Figure 6.3, the SCOT or support-coated open tubular column, on the left, and the PLOT or porous layer open tubular column, on the right. SCOT columns contain an adsorbed layer of very small solid support (such as Celite ) coated with a liquid phase. SCOT columns can hold more liquid phase, and have a higher sample capacity than the thin films common to the early WCOT columns. However, with the introduction of cross-linking techniques, stable thick films are possible for WCOT columns, and the need for SCOT columns has disappeared. A few SCOT columns are still commercially available but only in stainless steel tubing. 

Porous-layer- open tubular (PLOT) and support-coated open tubular (SCOT) columns are prepared by extending the inner surface area of the capillary tube. A layer of particles can be deposited on the surface or the column wall can be chemically treated to create a porous adsorbent layer. Obviously some of the wall-modified open tubular columns discussed in section 2.3.3 could be... 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency. Fused silica capillary tubes are used which have internal diameters ranging from 0.1 mm (small bore) to 0.53 mm (large bore) with typical lengths in excess of 20 m. The wall-coated open tubular (WCOT) columns have the internal surface of the tube coated with the liquid (stationary) phase and no particulate supporting medium is required. An alternative form of column is the porous-layer open tubular (PLOT) column, which has an internal coating of an adsorbent such as alumina (aluminium oxide) and various coatings. Microlitre sample volumes are used with these capillary columns and the injection port usually incorporates a stream splitter. 

For a period of time, OT columns that had characteristics intermediate between those of WCOT and packed columns were popular. There were two types, but they were similar. Support coated open tubular (SCOT) columns had a thin layer of solid support coated on the inside wall of a capillary tube of larger diameter than that used for WCOT columns. This layer was coated with stationary liquid similar to packed columns. Porous layer open tubular (PLOT) columns were similar but made differently for example, the solid support was added while the capillary tube was being drawn. With a few exceptions, SCOT and PLOT columns are no longer popular because wide diameter WCOT fused silica columns are as good, more stable (no layer to flake off), and easier to use. Subsequent discussion will be restricted to WCOT columns. 

Open tubular columns are simply capillary tubes in which the inside of the column wall is used as the support for the liquid phase. These wall-coated open tubular columns (WCOT) have the stationary phase distributed in the form of a thin film on the inside surface of the open capillary tube, the walls thus serving as the support. In order to reduce the thickness of the liquid phase film, a porous layer may be formed on the inside wall of the capillary tubing and then coated with the liquid phase to produce a support-coated open tubular column (SCOT). Porous-layer open tubular colunms (PLOT) are similar to SCOT colunms, the difference being that in the former, the stationary phase is deposited on fine crystalline particles or glass powder which is adsorbed onto the walls of the tube. In both cases, the available surface area of the wall is increased, and allows an increased amount of liquid phase to be accommodated in the same length and diameter of tubing. The whisker-walled (WW) colunm consists of whiskers chemically etched on the surface of the wall, which also result in a significant increase in the available surface area. Wall-coated, porous-layer, and support-coated capillary columns are all available as whisker-walled, i.e., WWCOT, WWPLOT, and WWSCOT, respectively. 

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. 

Several gas-solid adsorption capillary columns are available. They are commonly called porous-layer open tubular or PLOT columns. These columns contain a layer of adsorbent particles coated on the irmer wall of the fused silica tubing. Phases of aluminum oxide (alumina), molecular sieves, and porous polymers (Poraplot-like) are commercially available. Gas-solid adsorption rather than a gas-liquid partition is the separation mechanism involved. PLOT columns are well suited for the analysis of light hydrocarbons, sulfur gases, permanent gases, or other very volatile solutes at or... 

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. 

GSC is carried out using a different type of open tubular column. The stationary phase in such columns has the form of a layer of porous material coated on the walls of the tube. Such columns are... 

Patents [102, 103] describe the preparation of open tubular columns with an inside thickness-fixed porous layer by etching. First, the authors prepared a capillary column from a two-layer workpiece composed of two concentric tubes, one of them (external) being made of sodium borosilicate glass. To obtain an adsorption layer of defined thickness, the inner layer of the two-layer capillary was entirely leached. The method relies on porous glasses as the adsorbents. Porous glasses have been successfully used in gas capillary (see, for example, [104]). 

Open tubular or capillary columns have open unrestricted path for the gas within the column. These columns are about 15-30 meters in length with an inside diameter of about 0.25 mm. The inner wall of these columns is coated with the liquid stationary phase to about 1 m in thickness. The open tubular columns are of two kinds. One is known as the wall coated open tubular column [WCOT) in which the liquid phase is coated on the column wall. These columns have limited sample capacity and are unsuitable for large-scale separations. The second type is known as support coated open tubular columns (SCOT). In these columns a porous layer is formed on the inside wall of the tubing. The porous layer can either be formed by chemical treatment of the inner wall or is deposited on the inner wall. The support is coated in such a way that the inherent property of the capillary columns, i.e., the unrestricted gas flow is retained. The inert porous layer is then impregnated with the liquid stationary phase. These columns have a higher sample capacity. 

A year later silica-based fully porous 35-60-qm diameter beads were slurry packed in a tube and used for separation. This was the same material that had been used for open column or thin-layer chromatography. The only gain over these earlier techniques was in developmental time. Almost immediately, research was begun to optimize the packing in order to improve the separation. 

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