What factors determine the selection of methods of the separation of a mixture?

When approaching a separation problem, the first order of business is to identify what is different between the molecules (or ions) that you wish to separate. For example, if all of the molecules are hydrophobic but differ in the magnitude of their hydrophobicity, then a separation method based on hydrophobic interaction is a good choice. Separation of triglycerides falls into this category. On the other hand, if all of the compounds of interest are chemically related and vary only in terms of polar substituents, a separation mood that involves retention of the polar components will be a better option. For example, a drug and its decomposition products will often be closely related so a separation method that retains compounds of interest via their polar substituents will likely give the best separation. Likewise, if your mixture consists of different ionic components, the best separation will probably be based on an ion-exchange process whereas if the only thing different between the ionic components is their hydrophobicity, a hydrophobic separation mechanism might be the best option.

Next, you need to consider the physical properties of the compounds of interest. If the compounds of interest are volatile, generally gas chromatography will be the preferred separation mode. You can often produce derivatives that will render a non-volatile compound volatile but it’s generally preferred to avoid such steps unless absolutely necessary. On the other hand, if the compounds of interest are thermally labile, liquid chromatography will be a better choice.

Finally, you need to consider how you will detect the mixture components you wish to separate. If you are using gas chromatography, the flame ionization detector will detect most analytes but it won’t detect molecules such as water or carbon dioxide. In the latter case, you’ll need to use a different detector such as the thermal conductivity detector. Likewise in the case of liquid chromatography, many compounds can be detected using a UV detector. Assuming the analytes in your mixture falls into that category, the UV detector is an obvious choice. But if your compounds of interest do not absorb in the UV, you may need to use a refractive index detector or an evaporative light scattering detector. If you have a compound that can be separated using either gas chromatography or liquid chromatography, the availability of a suitable detector may drive your decision as to which mode to use.
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