Determining suitable separation conditions

Determining appropriate conditions for flash column chromatography is carried out using TLC plates and varying the mobile phase until appropriate separation is achieved. The initial report of flash column chromatography discusses several factors for consideration when determining optimal separation as do many practical chemistry text books.² Some of the key considerations for the separation of two compounds include:

• Components need a separation Rf ≥0.15 (to avoid mixed fractions)

• Slowest moving component has an Rf ≥0.15 (otherwise you require very large amounts of solvent to run the column)

• Fastest moving component Rf ≤ 0.60 (to ensure the first compound has some retention on the column)

It is worth noting that the above are not hard and fast rules and with experience chemists develop an intuition as to what conditions will be successful and which will not work adequately.

There is also guidance provided in many of the text books to help determine the size of column and mass of silica to use for a given separation. These factors are not explored here, generally when undertaking a column in teaching labs, the column requirements will have been provided to you. The size of column and mass of silica also determine the appropriate fraction size to collect.

²M Casey, J Leonard, B Lygo and G Procter, Advanced Practical Organic Chemistry (1990) provides visual representations of suitable TLC plates for column chromatography. J R Mohrig, C N Hammond and P F Schatz, Techniques in Organic Chemistry (2006) discuss common problems encountered.