Classical Open-Column Chromatography

FIGURE 11.14 The classic open-column chromatography configuration with fraction collector. 

The length of the column is determined according to the degree to which the mixture components separate on the stationary phase chosen. Difficult separations would require more contact with the stationary phase and thus may require longer columns. Again, all four types (adsorption, partition, ion exchange, and size exclusion) can be used with this technique. 

---

Classical separations by open column chromatography with different stationary phases (silica gel, reversed-phase C-18 or C-8, polyamide, cellulose) and elution with appropriate solvent mixtures are also useful for flavonoid fractionation and purification. Different column systems can be used. The classical open column chromatography uses relatively large particle sizes (0.2-6 mm), with limited resolution, and solvent filtration through the column proceeds by the pressure of the solvent column placed on top of the stationary phase. In other cases, smaller... 

Figure 21.14. Comparison of columns used in liquid chromatography. A Classical open column chromatography with large porous particle packings B HFLC with pellicular packings C HPLC with microparticulate packings.

Principles and Characteristics Column liquid chromatography is the parent of all other types of chromatography. The technique used by Tswett is now called classical open-column liquid chromatography or simply LC. In column chromatography the stationary phase is contained in a column and the mobile phase flows... 

The content of aroma compounds is, in general, low, and compositions of these compounds are often complex. Therefore, at the dawn of analytical chemistry, aroma compounds were extracted from a huge mass of raw material. Fractionation was carried out by means of distillation, and various other classical procedures (e.g., crystallization, pH control in extraction, derivatization) were employed. Quite obviously, compounds revealed using these procedures were inevitably restricted to a set of major constituents, if any. Occasionally, before the 1950s, additional techniques like UV-IR spectroscopy and open-column chromatography were employed and were helpful to some extent. 

In conventional open column chromatography, solvent is gravity fed onto a column of large ( 150-250 pm) particles, and the components of the mixture are then carried through the packed column by the eluant, separation being achieved by differential distribution of the sample components between the stationary and mobile phases. However, open column classical liquid chromatography suffers from a number of disadvantages, for example ... 

OPEN-COLUMN CHROMATOGRAPHY. Liquid-column chromatography performed in the classical manner in a relatively large bore, usually glass column under gravity or low-pressure flow. 

Five types of columns are routinely used in gas chromatography classical packed columns with internal diameters greater than 2 mm containing particles in the range 100 to 250 micrometers micropacked columns having diameters less than 1 mm with a packing density similar to classical packed columns (dp/d less than 0.3, where dp is the particle diameter and d the column diameter) packed capillary lumns have a column diameter less than 0.5 mm and a packing density less than classical packed columns (dp/d 0.2-0.3) SCOT columns (support-coated open... 

Over the last two decades, HPLC has to a large extent superseded the classical modes of open column, thin-layer or paper chromatography previously used for natural product separation and has become an integral part of natural product analysis and preparative isolation. This can be attributed to various factors, including (1) availability of numerous chromatographic modes, robust high-resolution chromatographic materials and... 

Wall-coated open tubular columns (WCOT columns), or simply capillary columns, and classical packed columns dominate the practice of gas-liquid chromatography today. Porous layer open tubular columns (PLOT columns) and classical packed columns dominate the practice of gas-solid chromatography. WCOT columns are typically up to 100 m... 

LC uses mostly packed columns, as the use of open tubular columns in this method is not practical because of the extremely small column diameters required for good separation. In gas chromatography, both packed and open tubular columns can be used, but the latter are far more popular because of their vastly superior properties. The mobile phase is usually forced through the stationary phase at elevated pressure, although other approaches are also possible (e.g., electrically driven flow in electrochromatography (EC), gravity driven flow in classical LC or flow driven by capillary forces in TLC). 

In spite of very diverse successful practical applications, the mechanism of com-plexation and the relationship between structure and selectivity are still at best only partly solved and remain open for discussion. Thermodynamic studies could supply some valuable information facilitating an understanding of the physicochemical basis of the complexation processes. The GC modified with CyDs is one of a variety of experimental methods employed in the determination of thermodynamic quantities for the formation of CyD inclusion complexes (see Chapters 8-10). The thermodynamic parameters for separation of the enantiomers were determined for various derivatives of CyDs dissolved in various stationary phases [63-65] or as a Uquid derivatized form [66]. Interesting observations were made by Armstrong et al. [66]. The authors postulated two different retention mechanisms. The first involved classical formation of the inclusion complex with high thermodynamic values of AH, AAH, and AAS and a relatively low column capacity and the second loose, probably external, multiple association with the CyD characterized by lower AH, AAH, and AAS values. The thermodynamic parameters of complexation processes obtained from liquid and gas chromatography measurements are collected in Table 5.2. It is clear from those data that for all the compounds presented the complexation processes are enthalpy-driven since in all cases AH is more negative than TAS. 

No column is required for isolation purposes hence affinity chromatography is frequently carried out in open systems, e.g. suction filtering. Classical columns with a hydrostatic eluent feed offer a further possibility. This chapter, however, is confined to a description of separations with high-performance stationary phases (10 pm and below) with which rapid chromatography can be achieved. Very small columns may be used. 

---