A procedure in which part or all of the separated sample components are subjected to additional separation steps. It can be performed by conducting a particular fraction eluted from the first column into a second column or system that has a different separation characteristic. It includes techniques such as two-dimensional TLC using two eluent systems in which the second eluent is applied after rotating the plate through 90°. It also includes LC followed by GC and one LC mode followed by a different mode such as reversed-phase chromatography followed by SEC. See also multidimensional chromatography.