Multiple development constant composition

Multiple developments can be classifled into nniform mnltiple developments (UMD, also called nnidimensional multiple developments) for methods in which the migration distance and mobile phase composition are constant and incremental multiple development (IMD) with mostly ascending migration distances. However,... 

Unidimensional multiple development is the repeated development of a plate over the same distance with an eluent of constant composition after careful drying between development steps. 

Any model of the composition of matter had to explain two widespread observations known as the law of mass conservation and the law of definite (or constant) composition. As you ll see, an atomic theory developed in the early 19 century explained these mass laws and another now known as the law of multiple proportions. 

Although the different types of multiple development (38) are also rarely used for preparative purposes, the advantage of the method may be undestood. The location of the compounds to be separated, and hence the ARf values, can be influenced by the number of developments using the unidimensional multiple development (UMD) technique. UMD means the repeated development of the chromatographic layer over the same development distance with a mobile phase of constant composition. Perry et al. reported (39) that using UMD, if the Rf after the first development is /iyi the values of the multiply developed solute can be predicted by use of the equation ... 

Two models have been proposed to describe the process of retention in liquid chromatography (Figure 3.3), the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968 and 1983). Both these models assume the existence of a monolayer or multiple layers of strong mobile-phase molecules adsorbed onto the surface of the stationary phase. In the solvent-partition model the analyte is partitioned between the mobile phase and the layer of solvent adsorbed onto the stationary-phase surface. In the solvent-competition model, the analyte competes with the strong mobile-phase molecules for active sites on the stationary phase. The two models are essentially equivalent because both assume that interactions between the analyte and the stationary phase remain constant and that retention is determined by the composition of the mobile phase. Furthermore, elutropic series, which rank solvents and mobile-phase modifiers according to their affinities for stationary phases (e.g. Table 3.1), have been developed on the basis of experimental observations, which cannot distinguish the two models of retention. 

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