Question

Why do you start with the least polar solvent/solvent mixture and progress to increasing polar solvent/solvent mixtures when eluting the ferrocene compounds from the column rather than starting with more polar solvent system and progressing to less polar solvents?

Answer 1

In normal phase column (as opposed to reverse phase column) the packed silica gel inside the column is polar. It will interact with the more polar ferrocene compounds strongly and it will interact with the less polar ferrocene compounds weakly.

Thus if you start with the least polar solvent/solvent-mixture, then the less polar among the ferrocene compounds will elute first as it will be held weakly by the silica gel. Then as you increase the polarity of the solvent/solvent mixture, the more polar ferrocene compounds will start eluting. This way, by tuning the solvent polarity (from least polar to the more polar) better separation among the compounds can be achieved. This way you will get pure products.

On the other hand, if you start with more polar solvent/solvent-mixture in the beginning, then almost all of the ferrocene compounds will elute simultaneously from the column resulting in poor seaparation. You will end up with mixture of ferrocene compounds.