Two-Dimensional Development on Single Layers

a 2-D separation can be seen as 1-D displacement operating in two dimensions. The 2-D TLC separation is of no interest if selection of the two mobile phases is not appropriate. With this in mind, displacement in either direction can be either selective or non-selective. A combination of two selective displacements in 2-D TLC will lead to the application of different separating mechanisms in each direction. As an extreme, if the solvent combinations are the same (5ti = 5t2 5vi = va) or very similar (5ti 5vi 5ya), the compounds to be separated will be poorly resolved or even unresolved, and as a result a diagonal pattern will be obtained. In such circumstances, a slight increase in resolution might occur, because of an increase by a factor of V2 in the distance of migration of the zone (4). 

The point at which the sample is spotted can be regarded as the origin of a coordinate system (9). The process of development is performed in two steps the first in the direction of the x-axis to a distance L. After evaporation of the solvents used, the second development will be performed in the direction of the y-axis to a distance L,. The positions of the compounds after development in the x-direction depend on the 

The point at which the sample is spotted can be regarded as the origin of a coordinate system (9). The process of development is performed in two steps the first in the direction of the x-axis to a distance Lz. After evaporation of the solvents used, the second development will be performed in the direction of the y-axis to a distance Ly. The positions of the compounds after development in the x-direction depend on the ST and Sy values of the first mobile phase being applied. Similarly, the migration distances of the individual compounds also depend on the total solvent strength and total selectivity of the second mobile phase. After development in the x-direction, the ordinates of all compounds are zero. After development in the y-direction, their abscissa values follow from their positions on the x-axis after the first development. The final positions of the spots are thus determined by the coordinates x(i) and y(t ), which can be expressed as follows  

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