Column chromatography reservoirs

Use of steel columns and high pressures showed that column chromatography could be "fast" and "reproducible" as well as flexible. A basic instrument, shown in diagrammatic form in Figure 19-1, p. 184, consists of a solvent reservoir, a pump, a gradient chamber, an injection port, a column, a detector, a iraction collector, and a recorder. Depending upon the quality of the individual components and the number of components actually used, the cost of such a combination can vary from 4,000 to 35,000. 

Semimicroscale Columns. An alternative apparatus for small-scale column chromatography is a commercial column, such as the one shown in Figure 19.8. This type of column is made of glass and has a solvent-resistant plastic stopcock at the bottom. The stopcock assembly contains a filter disc to support the adsorbent column. An optional upper fitting, also made of solvent-resistant plastic, serves as a solvent reservoir. The colunm shown in Figure 19.8 is equipped with the solvent reservoir. This type of column is available in a variety of lengths, ranging from 100 to 300 mm. Because the column has a built-in filter disc, it is not necessary to prepare a support base before the adsorbent is added. 

Another development arising from FAB has been its transformation from a static to a dynamic technique, with a continuous flow of a solution traveling from a reservoir through a capillary to the probe tip. Samples are injected either directly or through a liquid chromatography (LC) column. The technique is known as dynamic or continuous flow FAB/LSIMS and provides a convenient direct LC/MS coupling for the on-line analysis of mixtures (Figure 40.2). 

Figure 4-1. Components of a simple liquid chromatography apparatus. R Reservoir of mobile phase liquid, delivered either by gravity or using a pump. C Glass or plastic column containing stationary phase. F Fraction collector for collecting portions, called fractions, of the eluant liquid in separate test tubes.

Figure 2, Block diagram of a liquid chromatograph. A, solvent reservoir B, filter C, pump D, pulse dampener (optional) E, pre-column (used only in liquid-liquid chromatography) F, pressure gauge G, infector H, column I, detector J, fraction collector K, recorder or oth readout device.

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Sulfur compounds in the gas oil fractions from two bitumens (Athabasca oil sand and Cold Lake deposit)> a heavy oil (Lloydminster) from Cretaceous reservoirs along the western Canada sedimentary basin, and a Cretaceous oil from a deep reservoir that may be mature (Medicine River) are investigated. The gas oil distillates were separated to concentrates of different hydrocarbon types on a liquid adsorption chromatographic column. The aromatic hydrocarbon types with their associated sulfur compounds were resolved by gas chromatographic simulated distillation and then by gas solid chromatography. Some sulfur compounds were further characterized by mass spectrometry. The predominant sulfur compounds in these fractions are alkyl-substituted benzo- and dibenzothiophenes with short side chains which have few dominant isomers. 

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