The Mechanism of Chromatographic Retention - Mixed Phases > Page 20

 

The arginine also provides potential ionic interactions with the free amine group. The overall character of the heptapeptide is, therefor, polar, and thus the peptide would be described as hydrophilic or lyophilic in nature. As already stated, hydrophilic and hydrophobic are terms used to describe the overall interactive character of a large molecule as opposed to the individual group interactions. Nevertheless they are basically alternative terms that have been adopted to describe the polar and dispersive interactive character of a molecule respectively.

Mixed Phases

The interactive mechanism involved in mixed phases has been the subject of much controversy and, to some extent, at the time of writing this book, still is. Originally the distribution coefficient of a solute, which has been shown to be directly proportional to its retention volume (see book 6), was thought to be an exponential function of the concentration of solvent in the mixed phase.

The reason for this is uncertain, but appears to have arisen from the apparent approximate relationship between the logarithm of the retention volume and solvent composition for aqueous solvents such as methanol/water mixtures and tetrahydrofuran/water mixtures. There appears to be no rational physical chemical explanation given for this relationship but was assumed from the result of an arbitrary curve fitting procedure. This approximate fit appears, in retrospect, to be a fortuitous result of the strong association between the water and the solvent.

The true interactive character of mixed phases was disclosed by the pioneering work of Purnell et al.(4-6), who carried out some very simple GC experiments to identify he relationship between phase composition and retention.