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The order in which components of a mixture are eluted from a column is related to their relative polarty. Thus with a mixture of two components of differing polarity, e.g. a hydrocarbon and a ketone, separation is achieved because the more polar ketone is adsorbed more strongly on the adsorbent and hence the hydrocarbon may be eluted with a relatively non-polar solvent; the ketone is then eluted by changing to a more polar solvent. The ease of elution of the adsorbate may be broadly in the following order:
Saturated hydrocarbons > alkenes, alkynes, aromatic hydrocarbons >esters, aldehydes and ketones > amines, alcohols, thiols > phenols, carboxylic acids.
In a comprehensive study of a mixture having unknown chromatographic characteristics it is frequently desirable to be initially guided in the selection of adsorbents and solvents from information obtained by t.1.c. analysis using alumina or silica gel on microscope slides. Only if these prove unsatisfactory
would recourse be made to the other adsorbents.
It should be noted that the resolution obtained on a t.1.c. plate is rather better than would be obtained on a conventional adsorption column (see however other column techniques below) and hence further trials should be made with the various activity grades and with controlled solvent composition changes before the bulk of material is submitted to this type of separation. It is in these trials that careful attention to the chromatographic profile obtained from suitable analysis of the eluate fractions, and to the total recovery of material from the column, is so important.