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tionary phases themselves. Thus if our sample mixture contains closely related isomeric solutes, the mixing of two stationary phases will not resolve in general the components if neither of the phases separated the species in the first phase. [If the solvent blending approach is used, it is possible under certain circumstances for the mixed solvent to resolve closely related solutes which neither phase can separate alone. If the solvents mix nonideally, then a new solvent may form whose characteristics differ from each single solvent. Such a new solvent may be able to resolve the given mixture. Purnell's case (4) (42) under complexation equi libria.] For example, enantiomers will not be resolvable on optically inactive liquid phases, no matter how the phases are mixed together. Consequently, it is necessary to search for single liquid phases with the desired selectivity for given so lute mixtures. The advantage of mixed liquid phases, as illustrated in Figure 8, is in providing a means of changing relative retention when the multi-component mixture con tains solutes of different families. Conclusion
The purpose of this paper has been twofold. First, we have at tempted to show the thermody namic approach to selectivity in GLC. As we have pointed out, much work remains to be done in this area; however, definite devel opments have been discerned. Sec ond, we have cited several new types of selective liquid phases for GLC. While many highly interest ing applications have occurred, it has been necessary to be selective in the choice of topics in order to remain within the confines of this paper. Our selection has been based on the potential these ad vances possess for the solution of highly difficult separation problems. We have omitted from discussion such developments as the use of fused salts as liquid phases (e.g. 108) and the separation of isotopic molecules (109) and metal chelates (110). Time has also prevented us from discussing the use of such parameters as the Kovats Index (111) in retention and thus selec tivity. I t is hoped, however, that
