Wall-coated open tubular

Figure 2.21 shows the on-line extraction gas chromatogram of 2.25 ml of water spiked at 5 ppb levels with 14 different organic pollutants (40). In this case, the authors concluded that wall-coated open tubular traps (thick-film polysiloxane phases) can be used for the on-line extraction of organic compounds from water. However, when using swelling agents such as pentane, non-polar analytes can be trapped quantitatively, while for more polar compounds chloroform is the most suitable solvent. 

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... 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency.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

Vindevogel, J., and Sandra, P. (1994). On the possibility of performing chiral wall-coated open-tubular electrochromatography in 50 pm internal diameter capillaries. Electrophoresis 15, 842-847. 

Surface tension modifications for producing wall-coated open tubular columns... 

The term A is related to the flow profile of the mobile phase as it traverses the stationary phase. The size of the stationary phase particles, their dimensional distribution, and the uniformity of the packing are responsible for a preferential path and add mainly to the improper exchange of solute between the two phases. This phenomenon is the result of Eddy diffusion or turbulent diffusion, considered to be non-important in liquid chromatography or absent by definition in capillary columns, and WCOT (wall coated open tubular) in gas phase chromatography (Golay s equation without term A, cf. 2.5). 

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. 

The chromatographic column used was a wall-coated, open tubular column (WCOT) (J W Scientific) with a DB-1 Durabond chemically bonded stationary phase that had a nominal film thickness of 0.25 pm. The column was 60 m long X 0.32 mm i.d. The DB-1 stationary phase has chromatographic properties similar to SE-30. 

Standards with a poor UV chromophore (4-methyl-2-pentanone, 1-chlorododecane, and stearic acid) were analyzed on a Perkin-Elmer 3920 gas chromatograph with a flame ionization detector and a 30-m SE-54 wall-coated open tubular (WCOT) fused-silica capillary column (J W Scientific). The injector temperature was 200 °C the detector interface temperature was 280 °C. The carrier gas was He at 16.5 lb/in.2, and the makeup gas was nitrogen at a flow of 40 cm3/min. Splitless l-/zL injections of the 25,000 1 concentrates were made by starting with an oven temperature of 45 °C and the oven door open after 2.75 min, the oven door was closed and the temperature was programmed at 4 °C/min to 280 °C, which was held for 8 min. The syringe was kept in the injection port for 15 s after injection. 

Table 23-3 Comparison of packed and wall-coated open tubular column performance0...

Wall-coated open tubular column (WCOT) liquid stationary phase on inside wall of column... 

Figure 24-12 Separation of two polyaromatic hydrocarbons on a wall-coated open tubular column with different carrier gases. Resolution. R. increases and analysis time decreases as we change from N2 to He to H2 carrier gas. [Courtesy J W Scientific, Folsom, CA.]...

WCOT. Wall Coated Open Tubular column. 

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

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. 

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