A student loaded a mixture, dissolved in 1cm3 of dichloromethane, CH2Cl2, onto a small flash chromatography column and then proceeded to elute with hexanes/ethyl acetate 10:1. He/she found that all of the mixture came off in the first 4 fractions, with no separation. What technical mistake did the student make?
The technical mistake that the student made is that they used an inappropriate solvent system for the flash chromatography.
Flash chromatography is a technique used for the separation of mixtures of compounds based on their different polarities. The principle is that compounds with higher polarity will interact more with the stationary phase (silica or other adsorbents), while compounds with lower polarity will move more quickly through the column.
In this case, the student used hexanes/ethyl acetate as the solvent system in a ratio of 10:1. Hexanes is a non-polar solvent, while ethyl acetate is a polar solvent. The 10:1 ratio implies that the solvent system is mostly non-polar.
Since the student loaded the mixture dissolved in CH2Cl2 onto the column and eluted with a non-polar solvent, all the compounds in the mixture, irrespective of their polarities, would elute quickly without separation. This is because the non-polar solvent is not effective at differentially interacting with the compounds based on their polarities.
To rectify this technical mistake, the student should have chosen a solvent system that provides a better range of polarity. An appropriate solvent system for flash chromatography typically consists of a mixture of non-polar and polar solvents in different ratios to achieve the desired separation. The choice of solvents and their ratios should be based on the known or estimated polarities of the compounds in the mixture. By selecting a suitable solvent system, the student would promote differential interaction of the compounds with the stationary phase, leading to better separation.