Chromatography in More than One Dimension

the gaseous mobile phase must be confined in a column, so that a pressure gradient can cause it to flow past the stationary phase and eventually elute the separated bands out the effluent end of the colunm. This is inherently a ID separation, along the column, from one end to the other. This dimensionality applies even shonld the column be coiled to fit in a GC oven rather than vertically straight, like Tswett s gravity flow Uqnid mobile phase column. However, unlike gases, liquids as mobile phases do not always reqnire confinanent to move in a desired direction or retain their volume. If they are in contact with porons beds of small particles or fiber mats, surface forces (capillary attraction) can often indnce than to flow. Thns it is possible to carry out the LC process on a stationary phase arrayed as a thin surface layer, usually a planar 2D surface. Examples include the matted cellulose fibers of a sheet of paper or a thin laya of silica gel or alumina particles on a planar support (e.g., a pane of glass). 

The radial separation in the earlier example is not the most efficient way to perform surface chromatographic separations. A square planar thin-layer chromatography (TLC) plate (not to be confused with the theoretical plates discussed previously ) may have a line of spots containing sample mixtures and reference standard materials deposited just above one edge. Submerge that edge in solvent to a level just below the line of spots, and capillary attraction 

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