Capillary column wall coated, efficiency

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. 

These columns (i.d. < 200 pm) are characterized by a high specific efficiency (number of theoretical plates per meter and per second). Cartoni and co-workers [44] described the technique for preparing glass and fused-silica capillary columns (100 pm i d ), which were precoated with a very thin layer of graphitized carbon black and then coated with polar liquid phases. The layer of carbon black increased the wettability of the capillary columns walls and a very uniform coating was obtained. Columns coated with Carbowax 20 M, 40 M and 600 M were prepared. Polar liquid phases were strongly retained on carbon black, and these column showed higher temperature stability. 

Capillary column A narrow bore tube (0.25-1 mm ID) typically 30-100 m long (usually of deactivated fused silica), whose walls are coated with a liquid stationary phase to produce high-efficiency separations (N > 100,000). 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency.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. 

Suzuki et al. [819] studied the determination of chlorinated insecticides in river and surface waters using high resolution electron capture gas chromatography with glass capillary columns. They compared resolution efficiencies of organochlorine insecticides and their related compounds with wall-coated open tubular (WCOT) and support-coated open tubular (SCOT) glass capillary columns with those of conventional... 

The development of modem capillary columns has led to improved resolution and has also simplified the process of running gcs considerably. The columns are normally made from fused silica capillary with an inside diameter of between 0.2 and 0.5mm, and are polymer coated. They have no packing, but instead the liquid stationary phase is bonded to the inside wall of the capillary, and this allows gas to flow very easily. Because of this the columns can be made much longer than packed columns (between 12 and 100m) and they are typically ten times as efficient. Capillary columns give extremely high sensitivity and only a very small quantity of material is required. For this reason the injector normally incorporates a splitter, so that only a small portion of the sample injected actually enters the column. 

Golayf ° and Giddings, respectively, described a modification of the rate theory for capillary columns (hollow tube with inner wall coated with liquid phase) and the random walk, non-equilibrium theory. The former derived an equation to describe the efficiency of an open tubular column, while the random walk theory describes chromatographic separation in terms of statistical moments. The non-equilibrium theory involves a rigorous mathematical treatment to account for incomplete equilibrium between the two phases. ... 

Two types of columns are used in SFC packed columns containing solid particles of small inner diameter or wall-coated open-tubular columns (WCOT), usually called just capillary columns. Packed columns have been preferred when capacity is the most relevant issue capillary columns are selected when efficiency is the goal. 

Capillary Columns. Capillary columns are long, open tubes of small diameter. They have high efficiencies, low sample capacity, and low pressure drop. Commercially available capillary columns range from 0.1 to 0.53 nun in internal diameter and from 5 to 50 m in length. The inside wall of the tubing is coated with a film ranging from 0.1 im to thick films of 3.0 im. 

A comparison of efficiency of HPLC and GC columns was made by Wldmer et al (20). For example, 3 ym packed 10 cm column gives 15,000 plates or 150,000 plates/meter. This compares with 11,880 plates obtained with 0.1 mm l.d. wall coated capillary GC columns. These data suggest that greater efficiency In terms of number of plates/meter Is possible with HFLC. 

As is the case for both gas and liquid chromatography, the analytical column is the heart of the SFC technique. The quality of the separation can be no better than the quality of the column. This is normally measured in terms of efficiency, deactivation, and stationary phase film stability. Column technology for capillary SFC more closely resembles that for capillary GC than for HPLC the column wall should be deactivated and coated with a thin film of stationary phase, after which the stationary phase is rendered nonextractable by free-radical crosslinking. 

For packed columns the efficiency is given by the quality and particle size of the column packing. For wall coated capillaries the efficiency is a matter of the coating film properties. Furthermore, the efficiency depends on the injection mode, solvent effects, flow rate and column dimensions. 

Table III. Efficiency of Wall Coated Capillary Columns...

While the developments in capillary GC were slow in coming in the late 1960 s, many researchers then considered the support-coated open tubular and micropacked columns to be viable alternatives to the conventional capillaries. Although some interesting results were reported about 10 years ago [94,95] on the performance of such columns, they were largely overshadowed by the rapid advances in technology of wall-coated columns. The limited column permeability of micropacked columns and an excessive surface activity of support-coated open tubular columns are the major drawbacks of these column types. However, they may still offer a suitable compromise between sample capacity and column efficiency in certain special instances. 

Capillary columns GC columns, e.g. WCOT, PLOT, having a bore of 0.1 -0.7 mm with a 0.1 -5 pm film of stationary phase coated onto the inner wall of a pure silica glass column 10-50 m long, high column efficiency, N ff, and excellent resolution are achieved. Capacity of the stationary phase is very low so a sample injection splitter is used to introduce approximately 0.01 pi of sample onto the column. 

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