WCOT column

Gas-Liquid Chromatography. In gas-liquid chromatography (GLC) the stationary phase is a liquid. GLC capillary columns are coated internally with a liquid (WCOT columns) stationary phase. As discussed above, in GC the interaction of the sample molecules with the mobile phase is very weak. Therefore, the primary means of creating differential adsorption is through the choice of the particular liquid stationary phase to be used. The basic principle is that analytes selectively interact with stationary phases of similar chemical nature. For example, a mixture of nonpolar components of the same chemical type, such as hydrocarbons in most petroleum fractions, often separates well on a column with a nonpolar stationary phase, while samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases. Reference 7 is a metabolomics example of capillary GC-MS. 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency.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

The enantiomeric excess was determined to be >99% by capillary GLC analysis (30 ra x 32 tim WCOT column coated with Carbowax 20 M, hydrogen carrier gas, linear velocity ca. 94 cra/s, oven temperature 225°C) of the imide derived from the Mosher acid chloride. ... 

In an attempt to preserve the unique dual-nature selectivity of ILs while producing a stationary phase that is resilient to flowing at high temperatures, a method of immobilizing ILs as thin films in WCOT columns has been developed [42]. Figure 4.3 illustrates the steps used to form the immobilized IL stationary phase. 

For wall-coated open tubular (WCOT) columns, the stationary phase covers the inside surface of the column. The film thickness of the stationary phase can vary from 0.05 to 5 pm. It can be simply deposited on the surface, can originate from the reticulation of a polymer on the silica surface or can be bound to the silica through covalent bonds. The surface of the silica is treated before the stationary phase is deposited to avoid problems of wetability, desorption and stability over time. This treatment can involve attack by HC1 at 350 °C or the deposition of a fine coat of alumina particles. Afterward, the stationary phase is either deposited or prepared in situ by polymerisation at the inner surface of the column. Covalent bonding via Si-O—Si-C allows organic compounds to be bound to the silica surface. In the latter case, the columns are particularly stable and can be rinsed periodically allowing them to recover their initial performance. The efficiency of these columns can reach 150000 theoretical plates. 

WCOT Columns. Wall-coated open tubular columns wherein the internal wall is coated with a thin layer of liquid (liquid substrate). ... 

Since open tubular columns have low sample capacity this necessitates smaller sample sizes which in turn requires the use of sensitive detectors. The PLOT or SCOT columns, however, have a larger sample capacity than the WCOT columns, thus permitting larger sample sizes. These columns (PLOT, SCOT) can be operated without splitters and therefore, less sensitive detectors. One potential use for the SCOT columns is in the area of trace analysis. 

The column is the most critical part of a GC system and is chosen based on the nature of the analysis. There are two types of columns available for GC analysis packed columns and capillary (e.g., wall coated open tubular WCOT) columns. A packed column is not the primary choice for today s routine analysis of fatty acid composition due to its low resolution and requirement for large amounts of sample. Compared to a packed column, a capillary column needs far less sample and is able to achieve superior resolution. 

In particular, we can choose between open (capillary) columns and packed columns. A wall coated open tubular (WCOT) column has a much smaller phase ratio than a packed column, due to the small surface area of the wall. 

Finally, the sum of the first five McReynolds values has been used to compare the polarities of silicone polymers on OT columns. It has already been stated that the polarity of the silicone polymers can be increased by increasing the percentage of phenyl groups in the polymer. Figure 8.17 shows a plot for five polymers on bonded fused silica WCOT columns (except for SP-2250, which is from packed column data). The increasing McReynolds values clearly show the validity of this method of specifying stationary phase polarity. 

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. 

To increase the capacity of WCOT columns, wider diameters and heav-r loadings are used. These columns have largely replaced the SCOT... 

TABLE 4 Comparison of Fused Silica WCOT Columns... 

This type of column has become the most widely used because of its superior resolving power for complex mixtures compared to that of a packed column. Capillary columns are typically 5 to 50 metres long and between 0.1 mm and 0.60 mm internal diameter. A thin him (0.1 to 5 /zm thick) of the liquid stationary phase is coated or bonded onto the inner wall of the tube (hence the alternative description of wallopen tubular or WCOT columns)... 

It is usually necessary to modify the inside surface to assist wetting, e.g. by etching. If the glass surface itself is coated the column is referred to as a wall-coated open tubular (WCOT) column, whereas support-coated open tubular (SCOT) columns have a support bonded on to the glass walls, e.g. microcrystals of sodium or barium chloride. 

A range of open tubular GC columns were used. A 150 ft X 0.01 in. i.d. stainless steel wall-coated open tubular (WCOT) column (Perkin Elmer Corp., Norwalk, CT), a 50 ft X 0.02 in. i.d. support coated open tubular (SCOT) column (Perkin Elmer), and 33 ft and 100 ft X 0.03 in. i.d. porous layer open tubular (PLOT) columns. The latter were pre-... 

Advances in the manufacture of flexible fused silica WCOT columns will almost certainly extend the applications of GC by virtue of their high-resolution capability, while the advent of sophisticated, computerized detectors forecast the improvement in sensitivity and specificity. The use of chiral stationary phases for the resolution of enantiomers is becoming an increasingly important topic in pharmaceutical analyses. The range and availability of various liquid phases for chiral analysis by GC is bound to make this technique extremely valuable for the assay of pharmaceutical raw materials as well as for use in biological fluids. 

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