Liquid chromatography preparative

Liquid chromatography (Hostettman et al., 1986) in its many forms is a separation technique based on the polarity of the analytes and their partition between the mobile and stationary phases, and is therefore complementary to fractional distillation, which separates materials by their boiling point. The usual sequence for fractionating an essential oil or extract is to distil it first and then apply liquid chromatography to the distillation fractions as a further fractionation procedure, rather than as an analytical tool. The selectivity of the technique is achieved by choosing a stationary phase, usually from the various activities of silica gel, and varying the polarity of the mobile phase, the solvent, by mixing a non-polar component (such as hexane or pentane) with different amounts of a more polar component (such as diethyl ether, ethyl acetate or chloroform). 

In the simplest form of liquid chromatography, the solvent is applied to the top of the column by gravity from a reservoir, a slow but reproducible process. Greater speed and resolution can be achieved, 

Analytical HPLC necessarily includes a detector on the outlet from the column, which responds to the presence of analytes in the solvent stream. Narrow-bore columns and fine particle sizes are used to achieve the best possible resolution. Although analytical HPLC is used in the fragrance industry to investigate the non-volatile fractions of essential oils and product bases, for example, it is much more widely used in other industries (such as the pharmaceutical industry, for which it is the main research tool). 

Liquid chromatography (Hostettman et al., 1986) in its many forms is a separation technique based on the polarity of the analytes and their 

Thin-layer chromatography (TLC) [5] is routinely used in dendrimer synthesis for purity checks and identification of individual constituents of a dendrimer sample and is suitable for monitoring column chromatographic separations. Since chemistry students are introduced to this analytical technique during their first semesters, we shall not go into further details here. 

In addition to the analytical-scale assay of trace impurities modern LC can be employed in the preparative mode (PLC) for the isolation of appreciable amounts of pure component [97,98], Where only a few milligrams ( 5mg) of a compound is required then a few repetitive injections on an analytical scale column ( 4.6 mm i.d.) may provide the requisite amount. The analytical scale equipment required has been described in detail earlier in this chapter. 

However, there are many other instances when larger quantities (50 mg-50 g) of pure material may be required. Separations of this scale are normally accomplished using larger bore columns which are designed for use with proprietary LC analytical-scale equipment The principal features of the methods are as follows. 

The second mode is referred to as the sample overload technique and sacrifices column efficiency and resolution in favour of sample throughput. The use of wide diameter columns ( 10mm) packed with large microporous particles ( 50pm) allows isolation of gram quantities of material in relatively short elution times as the column can be operated at high flow rates without seriously degrading resolution. By its nature this technique 

Regardless of the procedure employed preparative HPLC has most commonly been used in the adsorption mode with silica packings due to their sample versatility and loading capacity however large bore preparative columns are now commercially available, packed with any of the proprietary reverse-phase and ion exchange media. 

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B. A. Bidlingmeyer, Preparative Liquid Chromatography, 2nd Reprint, Elsevier, Amsterdam, 1991. ISBN 0444428321. 

W. H. Pirkle and B. C. Hamper, The direct preparative resolution of enantiomers by liquid chromatography on chiral stationary phases in Preparative Liquid Chromatography, B. A. Bidling-meyer (Ed.), Journal Chromatography Library Vol. 38, 3 Edition, Elsevier Science Publishers B. V, Amsterdam (1991) Chapter 7. 

Racemic mixtures of sulfoxides have often been separated completely or partially into the enantiomers. Various resolution techniques have been used, but the most important method has been via diastereomeric salt formation. Recently, resolution via complex formation between sulfoxides and homochiral compounds has been demonstrated and will likely prove of increasing importance as a method of separating enantiomers. Preparative liquid chromatography on chiral columns may also prove increasingly important it already is very useful on an analytical scale for the determination of enantiomeric purity. 

Although fractional crystallization has always been the most common method for the separation of diastereomers. When it can be used, binary-phase diagrams for the diastereomeric salts have been used to calculate the efficiency of optical resolution. However, its tediousness and the fact that it is limited to solids prompted a search for other methods. Fractional distillation has given only limited separation, but gas chromatography and preparative liquid chromatography have proved more useful and, in many cases, have supplanted fraetional crystallization, especially where the quantities to be resolved are small. 

Sherma, J. and Fried, B., Preparative thin layer chromatography, in Preparative Liquid Chromatography, Journal of Chromatography Library, Bidlingmeyer, B.A., Ed., Vol. 38, Elsevier, Amsterdam, 105-127, 1987. 

Colin, H., Large-scale high-performance preparative liquid chromatography, in Preparative and Production Scale Chromatography, Ganetsos, G. and Barker, P. E. Eds., Marcel Dekker, New York, 1988, 11. 

Golshan-Shirazi, S. and Guiochon, G., Theory of optimization of the experimental conditions of preparative liquid chromatography optimization of column efficiency, Anal. Chem., 61, 1368, 1989. 

Siitfeld, R., Preparative liquid chromatography with analytical separation quality interval injection/displacement reversed-phase high-performance liquid chromatography, ]. Chromatogr., 464, 103, 1989. 

Colin, H., Hilaireau, P., and Martin, M., Flip-flop elution concept in preparative liquid chromatography, /. Chromatogr., 557, 137, 1991. 

Felinger, A. and Guiochon, G., Comparing the optimum performance of the different modes of preparative liquid chromatography, /. Chromatogr. A, 796, 59, 1998. 

Gradient Elution Conditions in Multicomponent Preparative Liquid Chromatography. 

S. Dontas, S. Lodiakis and G. Parissakis, Isolation of palm oil carotenoids using preparative liquid chromatography. Tiv. Ital. EPPOS 26 (1998) 29-39. 

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