FACTORS INVOLVED IN THE HIGH-PRESSURE LIQUID-CHROMATOGRAPHIC SEPARATION OF ALKENES BY MEANS OF ARGENTATION CHROMATOGRAPHY ON ION-EXCHANGERS- OVERVIEW OF THEORY AND NEW PRACTICAL DEVELOPMENTS

The literature on the use and theory of silver(I) complexation (argent ation) chromatography is reviewed. Apolar isomeric alkenes, for exampl e numerous terpenes and fatty acids, can be well resolved by means of high pressure liquid chromatography on cation exchange materials coate d with silver(I) ions, but unfortunately the mechanism on which these separations are based is not fully understood. As part of an attempt t o provide a predictive model for the separation of volatile alkenes, t he retention of more than 40 C2-C10 olefins besides a number of C14-ph eromones and volatile terpenes is given and the results discussed. The separation is mainly governed by the type and number of double bonds (isolated, conjugated or aromatic), their steric accessibility (number and size of substituents) for complexation with silver(I) ions, the r elief of inherent strain upon complexation and electron donating or wi thdrawing groups, or polar groups close to the double bond. Polar grou ps such as hydroxyls also complex weakly with silver. Other functional groups, including carbonyls, and the overall polarity of the solute p lay only a minor role. Similarly most solvents have only a limited inf luence on the selectivity and speed of separation, and function more a s a transportation medium as in gas chromatography. A comparative mode l which gives semi-quantitative estimations of the retention time of m ost volatile olefins in methanol is presented. Substituent parameters (Ag-s) which quantitatively forecast the influence of different alkyl side chains are part of this model.