Column wall-coated

The wall-coated column in Figure 24-2c features a 0.1- to 5-p.m-thick film of stationary liquid phase on the inner wall of the column. A support-coated column has solid particles coated with stationary liquid phase and attached to the inner wall. In the porous-layer column... 

Figure 24-4 Effect of open tubular column inner diameter on resolution. Narrower columns provide higher resolution. Notice the increased resolution of peaks 1 and 2 in the narrow column. Conditions DB-1 stationary phase (0.25 xm thick) in 15-m wall-coated column operated at 95°C with He linear velocity of 34 cm/s. [Courtesy JSW Scientific. Folsom. CA.]...

Figure 24-6 Effect of stationary phase thickness on open tubular column performance. Increasing thickness increases retention time and Increases resolution of early-eluting peaks. Conditions DB-I stationary phase in 15-m-long x 0.32-mm-diameter wall-coated column operated at 40°C with He linear velocity of 38 cm/S. [Courtesy J6W Scientific. Folsom. CA]...

Figure 24-11 Von Deemter curves for gas chromatography of n-C,7H36 at 175"C using N2, He, or Hj in a 0.25-mm-diameter x 25-m-long wall-coated column with OV-101 stationary phase. [From R. R. Freeman, ed.. High Resolution Gas Chromatography (Palo Alto, CA Hewlett Packard Co.. 1981).]...

The capillary column used was a 50 mX 0,32 mm (i.d.) fused silica UCON 75H 90000 wall coated column. The column oven temperature was programmed from 30 to 180°C at a rate of 3"C/min with a 10-min post injection hold and a 10-min hold at a final limit. Column inlet pressure of Helium was 7 psl and the split 30 mL/min. 

Table 31-4 compares the performance characteristics of fused-silica capillary columns with other types of wall-coated columns as well as with support-coated and packed 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. 

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. 

Gas phase Liquid phase Wall-coated column Frequency Response Pressure... 

Compared to wall-coated columns, dynamically coated SCOT columns have higher capacity ratios. 

The third type of a GC column (Fig. 7) is a porous-layer, open tubular column. While such a column has an inner diameter and a length comparable to the wall-coated columns, its inner wall is modified through a chemical treatment or deposition of finely dispersed particles. The porous layer can be either an adsorbent or a thin layer of the solid support impregnated with a liquid stationary phase. Efficiencies of the porous-layer, open tubular columns are not as high as those of true capillary columns, but their greater sample capacity is an advantage to some separations. The enhanced sample capacity is obtained because... 

Capillary columns are of two principal types. Wall-coated open tuhular columns (WCOT) contain a thin layer of stationary phase, typically 0.25 pm thick, coated on the capillary s inner wall. In support-coated open tuhular columns (SCOT), a thin layer of a solid support, such as a diatomaceous earth, coated with a liquid stationary phase is attached to the capillary s inner wall. 

Before discussing column preparation procedures a few comments on nomenclature are in order. Open tubular columns are also widely known as capillary columns. The characteristic feature of these columns is their openness, which provides an unrestricted gas path through the column. Thus open tubular colximn rather than capillary column is a more apt description. However, both descriptions appear frequently in the literature and can be emsidered interchangeable. The type of columns discussed so far are also known as wall-coated open tubular columns (WCOT). Here the liquid phase is deposited directly onto the column wall without the inclusion of any additive that might be considered as... 

WCOT Wall-coated open-tubular column... 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

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