Column efficiency and differences in adsorbabilities at equilibrium determine tile success of a chromatographic separation on a column. If the ratio between the equilibrium distribution coefficients, called the separation factor, is large, very primitive columns can give a cluantitativc separation. If, on the other hand, the distribution coefficients differ by only 10% or less, very cmcient columns must be used to achieve a satisfactory separation.
In ion-cschangc cllromatograplly the distribution coeficicnts and separation factors can be varied within wide limits. Only in favorable cases can the influcncc of changes in working conditions be predicted theoretically. With cxamplcs from metal separations, separations of organic acids by ion eschangc, and partition chromatography of sugars, the influence of some variables will bc discussed.
In the first successful metal separations on cation-cschange resins the separation factors were increased to intinity by a transformation of .onc of the metal cations itlto a non-adsorbable anion. The first application of such a separation, publishccl in I<)Jc), was the determination of alkali metals in the presence of vanadium, converted before the separation into vanadntc anions'. In other applications of the method complcxing agents were used to convert metal ions such as cobalt and iron into nonadsorbable anions to effect a quantitative separation by selective sorption2. In separations of very similar species this technique cannot be used. Here the elution method is of predominant importance. The great potentialities of this technique became generally known in 1947 after the publication of the results of investigations, carried out as part of the Plutonium Project, on the separation of the fragments formed during fission of the heavy elements 3. Striking results were the separations of rare earths and the first isolation of element 61 (Promethium).
Again, aclvantage was taken of complcsing agents such as citric acid to increase the separation factors of the elements to be separated.
Under certain conditions the order of elution is largely determined by the stability constants of the compleses. Simple theories, which account reasonably well for changes of complesing agent, eluant concentration, and 1~13 during the elution, were presented by TOMPKINS AND MAYEIZQ and by KETELLE AND UOYD~.
These changes affect the fraction of the sorbable ions in the esternal solution and can be accounted for by the calculation methods used in classical solution chemistry.
Although the application of complesing agents is a most convenient way to Anal.
C/rim. Acta, 38 (19G7) rG3-1G8