Comment on the order of elution of the following solutes in (a) normal phase and (b) reversed phase liquid chromatography. Give reasons why.
Solute X - non-polar
Solute Y- Polar, a dipole
Solute Z - Polar, more polar than solute Y
I understand that polarity is the reason of the difference in elution order, but can anyone help with how the order is determined and why? thanks
In the case of chromatography, like attracts like. So, the more polar compound will elute slower than the compounds not as polar because the plate is polar.
Highest Retention Time: X > Y > Z :Lowest Retention Time
In the case of reverse phase chromatography, it is reversed.
In normal phase liquid chromatography, the elution order is based on the solute's polarity. The more polar the solute, the stronger the interaction it will have with the polar stationary phase. Consequently, polar solutes will elute later than non-polar solutes.
In this case, solute X is non-polar, meaning it does not have any dipole or charge. Therefore, it will have weak interactions with the polar stationary phase, resulting in early elution. Solute Y is polar and has a dipole moment, making it more polar than solute X. Due to the dipole-dipole interactions between solute Y and the polar stationary phase, it will elute after solute X.
Solute Z, on the other hand, is even more polar than solute Y. It may have additional functional groups or increased polarity, resulting in stronger interactions with the polar stationary phase. As a result, solute Z will elute last in the normal phase liquid chromatography due to its stronger affinity for the stationary phase compared to solute Y.
In reversed phase liquid chromatography, the elution order is reversed compared to normal phase. Here, the stationary phase is non-polar and the mobile phase is polar. The more polar the solute, the stronger its interaction with the polar mobile phase. Therefore, in reversed phase liquid chromatography, the elution order will be opposite to that in normal phase.
In this case, solute X, being non-polar, will have weak interactions with the polar mobile phase and will elute last in reversed phase chromatography. Solute Y, being polar with a dipole moment, will elute before solute X, but after solute Z due to its intermediate polarity. Solute Z, being the most polar of the three solutes, will have the strongest interaction with the polar mobile phase and elute first in reversed phase liquid chromatography.
In summary, the order of elution in normal phase chromatography is determined by the increasing polarity of the solutes, with non-polar solutes eluting first and polar solutes eluting later. In reversed phase chromatography, the order of elution is reversed, with the most polar solute eluting first and non-polar solutes eluting last.