Question

Answer the following about chromatography

a. A normal phase column chromatography for the separation of a polar and a nonpolar compound calls for a sequential elution of the following two solvents:

Methylene chloride & Hexanes

Which solvent should be used first? Please explain

b. A student loaded a mixture onto a small silica chromatography column in 1 mL of methanol, then proceeded to elute with hexanes/ethyl acetate/methanol gradient. The student found that all of the mixture came off in the first fraction, with no separation. What technical mistake did the student make? Please explain.

Answer 1

A. Hexane should use first which is non polar solvent than Methylene chloride. In a normal phase column chromatography non polar compounds will be eluted inititally followed by polar compounds. If we use Methylene chloride initially polar compounds will be eluted along with all the non polar compounds.

B. We should not disslove crude compound in methanol to absorb compound onto silica. We should try to dissolve the crude compound in Mehtylene chloride which is low boiling solvent can be easily removed. And % of polar solvents (ethylacetate and especially methanol) should be increased according to elution of non polar compounds. We should keep on hold the same % of polar solvents until the completion of one particular compound, after that we can gradually increase the % of polar solvent. Same principle has to be applied to every compound until finish the column.