Method for running a dry flash column

Under vacuum, pre-elute the column with a solvent which will give a tic Rf of about 0.2 for the least polar constituent of the sample. If the silica has been packed correctly, the solvent should run down the column with a horizontal front, but if it channels, the column should be sucked dry and repacked. Keep the surface of the silica covered with solvent while preeluting until solvent starts collecting, then suck it dry. 

Load the sample as a solution, in the same solvent as used for pre-elution, in an even layer onto the surface of the silica. Alternatively, if the sample is insoluble in the pre-elution solvent, it can be pre-adsorbed onto a small quantity of silica (see Section 11.5.1, loading a flash column)) which is then spread on the surfece of the silica in the funnel. 

Sequentially add solvent fractions to the funnel according to the quantities indicated in Table 11.1, sucking the silica dry in between each fraction, and keeping fractions separate. For each successive fraction increase the solvent polarity by increasing the proportion of the more polar solvent by about 5-10% (e.g. from 50% EtOAc/50% petrol to 55% EtOAc, for a 10% increase). Analyse the fractions by tic to determine the locations of the components of interest, but as a rough guide, the solvent mixture which would give the compound a tic Rf value of about 0.5 will probably elute it. 

As with any chromatographic technique expertise will only come with experience, but given that, you should be able to separate quite closely running compounds quickly using this technique. 

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Method for running a dry flash column rctlumn packing... 

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