Studies of solute retention in HPLC using hydrophobic pairing ions with octadecylsilica.
Following a general historical review of chromatographic development leading to the widespread use of bonded reversed phase ion—pairing chromatography, critical evaluations are made of current theories proposed for the retention mechanisms in such chromatography. In the light of the inadequacies shown by previous theories, experimental work is reported on the adsorption of several widely used anionic and cationic pairing ions on ODS-Hypersil as a typical capped C-18 stationary phase. Using substituted benzoic acids and aromatic amines as test solutes their retention as a function of appropriate pairing ion concentration is studied. For highly adsorbed pairing ions the effect of acetonitrile as organic modifier is also examined. Based on the results obtained, a model of retention involving both ion-exchange and desolvation is proposed and a quantitative relationship formulated between capacity factor and adsorbed pairing ion concentration. The agreement between the predicted capacity factors and those obtained by direct measurement is discussed. The above ideas have been applied to the separation of the acidic products formed during the autoclaving of D-glucose solutions. By comparison of the chromatographic behaviour of these products with that of standard acids, the acidic products are identified. In the light of results obtained the mechanism of decomposition of D-glucose solutions under autoclaving conditions is discussed. A systematic study of the chromatography of tricyclic antidepressant drugs is made. In this investigation it is shown that pairing ion alone is inadequate for resolution in certain cases. Optimum separations are shown to be obtained by the inclusion of organic basic counter ions in addition to conventional pairing ion. The results of a systematic study of different counter ions are shown and interpreted as an extension of the ion—exchange—desolvation ideas previously suggested. The general applicability of organic counter ion ideas is demonstrated for the antimalarial drug proguanil and its metabolites. An improved method of serum determination of these compounds is suggested. A discussion of the difficulties in attempting to relate chromatographic behaviour using bonded reversed phase silica to the chemical structure of the solute is made utilising experimental data from the literature.
High-performance liquid chromatography (HPLC), Pairing ions, Solute retention, Octadecylsilane Hypersil (ODS-Hypersil), Acetonitrile, Methyl cyanide, Cyanomethane, D-glucose, Dextrose, Tricyclic antidepressants (TCAs)
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