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Reversed-phase liquid chromatography (RPLC) is one of the most popular and powerful analytical separation methods. A recent survey has shown that approximately 60% of all commercial high performance liquid chromatography columns sold are reversed-phase columns . Yet many practitioners of RPLC view this technique as "black magicn because the retention mechanism is not well understood, especially at the molecular level.
We have tested against an extensive data base of almost 350 sets of experiments and in agreement with theory have found that the mobile phase contribution to retention can be described by the binary interaction constants of solutes with solvents. And, we examine the effects of octadecyl alkyl chain bonding density on partitioningand retention of small nonpolar solutes.
From 0 to about 3.1 umol/m2, In this region, solute retention increases linearly with increasing alkyl surface density because at low alkyl densities, configurational constraints are very small and chain packing has no effect on solute retention. At high densities, increasing the bonding density should lead to increasingly severe packing constraints of the stationary phase chains. Thus the theory predicts that the process of solute transfer into the stationary phase will be opposed by increasingly negative changes in the conformational entropy (increasingly positive free energy), due to increased ordering of the interphase chains, and this will disfavor partitioning. The result showed that they decrease in very small increments for bonding densities ranging from 3.15 to 4.07 umol/m2. However, at these same bonding densities, Vs has increased 21.3% in this range, while Vm has remained essentially constant, decreasing only 3.6%. Therefore, the cumulative effect of these two changing parameters was for k' to increase very slightly; this increase was so slight that k’essentially remains constant over this bonding density range. This is the same sort of “leveling off’ behavior.
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