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the technique of chromatography, originally invented by Tswett in the latter part of the nineteenth century, was not initially developed for analytical purposes, but for the isolation of some specific pigments from plant extracts. In fact, all the early applications of chromatograph were exclusively for preparative purposes and it was not until gas chromatography (GC) was introduced by Martin and Synge (1) was the technique used for analytical purposes. Even after the introduction of GC, liquid chromatography (then called column chromatography) was still used largely for preparative work. Liquid column chromatography evolved from a preparative procedure into an analytical technique during the late nineteen sixties, largely provoked by the development of high performance liquid chromatography (HPLC), which, in turn,  was largely sparked off by the successful development of GC. Initially, column loads were increased for preparative purposes by increasing the dimensions of the column both

Preparative Gas Chromatography Gas chromatography has not been used extensively for preparative work although its counterpart, liquid chromatography, has been broadly used in the pharmaceutical industry for the isolation and purification of physiologically active substances. There are a number of unique problems associated with preparative gas chromatography. Firstly, it is difficult to recycle the mobile phase and thus large volume of gas are necessary. Secondly, the sample must be

is quickly distributed across the plate surface and then onto the column through the slots. In this way the sample is injected across the top surface of the packing. Preparative Detectors Preparative chromatography detectors can have very limited specifications, compared with their analytical counterparts. They need not be particularly sensitive (in fact too great a sensitivity is a distinct disadvantage) as the sample size and the eluent solute concentrations are very large. Preparative chromatography detectors can have large sensor volumes and, as the detector is required only to monitor the separation, they need not have a linear response. They do need to tolerate high flow rates and thus, must have low flow impedance. Analytical detectors can be used for preparative purposes but a portion is usually split from the column eluent, diluted with more mobile phase and then passed through the detector. In practice this becomes a rather clumsy procedure. The most commonly used

Introduction X Chromatography, although primarily a separation technique, is mostly employed in chemical analysis. Nevertheless, to a limited extent, it is also used for preparative purposes, particularly for the isolation of relatively small amounts of materials that have comparatively high intrinsic value. Chromatography is probably the most powerful and versatile technique available to the modern analyst. In a single step process it can separate a mixture into its individual components and simultaneously provide an quantitative estimate of each constituent. Samples may be gaseous, liquid or solid in nature and can range in complexity from a simple

is far less than that of the front. The reduction in retention is by far the greatest for the over loaded peak. It is also clear that in chromatography, column over load is a very effective way of increasing the throughput and by adjusting the selectivity (using temperature, selected stationary phases, or gradient elution) very large sample loads can be tolerated. This approach should always be considered first for moderate loads before contemplating large scale column design. Preparative Chromatography Apparatus Very large sample loads will necessitate the use of large scale chromatographic equipment. However, the conventional preparative chromatograph, although certainly more massive, is generally less complex than the analytical chromatograph. Although gradient elution has been used in preparative chromatography, it should be avoided, if possible, due to the cost of solvents and the complication involved in solvent recovery

plate surface. A reference photo cell (not shown) is placed close to the lamp to compensate for changes in light intensity that may arise from variations in lamp emission. Due to the very short path length (the thickness of the film of column eluent) the detector has the required low sensitivity and low flow impedance. The detector can operate very satisfactorily at concentrations as great as 10-2 g/ml (1% w/w), which is ideal for preparative chromatography. Another advantage of the device for preparative work, is its very low flow impedance and thus can easily cope with the high flow rates used in preparative LC. The film thickness does depend, among other things, on the column flow rate and thus fairly precise flow control is necessary for the satisfactory performance of this detector. Fraction Collectors Fraction collection in preparative chromatography is usually achieved using a multi-port valve and a number of collection vessels. The selector valve should be programmable