Gradient-elution method, continuous

Significant number of modern HPLC separation methods are based on gradient elution or continuous variation of the eluent composition during the analytical experiment. Theoretical description of the gradient separation is very complex and, to large extent, nonexistent. It is possible to use basic HPLC theory for phenomenological explanation of gradient retention dependencies. 

Fractional extraction242 (dissolution) may also be employed, as was demonstrated for a polyxylose, using batchwise extraction with aqueous ethanol.141 A more convenient procedure would involve adsorption of the polyglycose onto an inert, solid support (such as cellulose), followed by extraction using a procedure already described.243 The continuous nature of this process, the opportunity for solvent to reach all particles of the polymer, and the possibility of using gradient elution make this method attractive, particularly if the examination of a range of polymeric distributions is desired. 

Continuous radioactivity flow monitoring has very significantly reduced the cost of development of separation methods and production of useful amounts of high purity phos-phonate radiochemicals. It has also permitted simple application of powerful LC gradient elution techniques to fractionation of hard-to-separate radioactive mixtures. 

The increasing technological significance of lanthanides will continue to pave the way for development of rapid and efficient separation methods with sensitive detection. Several reviews on lanthanide analysis indicate the importance of these materials and the need for the development of new and better techniques. Many HPLC based techniques have been developed for the separation of individual lanthanides using cation exchange, dynamic ion-exchange methods, etc. The selection of gradient elution techniques has been... 

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