Chromatography, elution development

Elution development is by far the most common method of processing a chromatographic separation and is used in all types of chromatography. Elution development is best described as a series of absorption-extraction processes which are continuous from the time the sample is injected into the distribution system until the time the solutes exit from it. The elution process is depicted in Figure 1. 

A chromatographic separation can be developed in three ways, by displacement development, by frontal analysis, and by elution development, the last being almost universally used in all analytical chromatography. Nevertheless, for the sake of completeness, and because in preparative chromatography (under certain conditions of mass overload) displacement effects occur to varying extents, all three development processes will be described. 

It is important for the analyst to be able to select the best stationary phase to use for a particular chromatographic analysis. Silica gel can be used in two modes of chromatographic separations as a stationary phase in normal elution development or as a stationary phase in exclusion chromatography. 

A more efficient method of paper chromatography was developed by Beckman and Lederer making use of the formation of a technetiiun complex with thiourea in nitric acid. TcO is reduced by this reagent, in contrast to ReO". The paper is eluted with 2 N HCl. The R, values of technetium and rhenium have been found to be very different, namely 0.2 and 0.7, respectively. 

The progress of the reaction is monitored by thin layer chromatography, eluting with ethyl acetate (product Rf = 0.06, visualized and developed using UV light and KMn04 respectively). 

Figure 2.44. Comparison of (a) elution development and (b) frontal chromatography.

For separations involving large amounts of Am, Cm, or rare earths, displacement development provides a satisfactory first-cycle separation and yields Am and Cm products and a transcurium element fraction suitable for final separation by elution development. However, alternative methods for the first cycle (removal of the bulk of the lighter actinides and rare earths) are available besides displacement development chromatography, these include solvent extraction and the LiCl-anion exchange system. 

The latter system is used at TRU, while the SRL development program demonstrated the suitability of displacement chromatography. Both methods appear to be satisfactory. Until now, and for the foreseeable future, the quantity of transcurium elements has been too small to justify any process other than elution development for the final separation. 

Schadel, Trautmann, and Herrmann compared the HDEHP extraction chromatography system to ion exchange elution development for rare earth separations (32) and found the two methods to be about equally effective under optimum conditions. From their data for separating seven rare earths plus yttrium, they projected that all the rare earths could be separated in about 20 min. However, they did not include in their study those pairs most difficult to separate. 

Mass, infrared, ultraviolet, and mnr spectrometry have been highly developed as aids to identification. In some gas-chromatographic techniques some of the sample corresponding to a peak must be condensed and examined in others the effluent can be monitored continuously with an instrument such as the time-of-flight mass spectrometer, whereby mass spectra corresponding to each peak can be recorded as it is eluted. Developments in the mass-spectrometric analysis of gas chromatography effluents have been reviewed by McFadden. ... 

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