Comment on the order of elution of the following solutes in (a) normal-phase and
(b) reversed-phase liquid chromatography. Give reasons for the difference.
Solute X nonpolar
Solute Y polar, a dipole
Solute Z polar, more polar than Solute Y
In normal-phase liquid chromatography, the elution order is based on the polarity of the solutes, with the least polar solutes eluting first. Since solute X is nonpolar, it will likely have the least affinity for the polar stationary phase, resulting in early elution. Solute Y being polar, with a dipole, will have moderate affinity for the stationary phase and elute after solute X. Solute Z, being more polar than solute Y, will have the highest affinity for the stationary phase and will elute last.
In reversed-phase liquid chromatography, the elution order is opposite to normal-phase chromatography. This is because the stationary phase consists of a nonpolar material, while the mobile phase is polar. Nonpolar solutes have higher affinity for the nonpolar stationary phase and tend to elute later. On the other hand, polar solutes have less affinity for the nonpolar stationary phase and elute earlier.
Therefore, in reversed-phase liquid chromatography, the elution order would be reversed compared to normal-phase chromatography. Solute Z, being the most polar, will elute first, followed by solute Y, and solute X, which is nonpolar, will elute last.
The difference between the elution orders in normal-phase and reversed-phase chromatography is due to the differences in the polarity of the stationary and mobile phases. In normal-phase chromatography, the stationary phase is polar, and the elution order is based on solute polarity. In reversed-phase chromatography, the stationary phase is nonpolar, and the elution order is determined by the solute's affinity for the nonpolar stationary phase in the presence of a polar mobile phase.