Question

If 3 spots on a TLC plate have the following RF’S, A=0.43, B=0.89 and C=0.53 what would be the order that the spots came off a column? What would happen if you increased the polarity of the solvent?

Answer 1

If 3 spots on a TLC plate have the following RF’S, A=0.43, B=0.89 and C=0.53 what would be the order that the spots came off a column? What would happen if you increased the polarity of the solvent?

On TLC plate retention factor (R.F) is defined as,

RF = (Distance travelled by compound)/(Distance travelled by solvent front)

For a mixture of compound distance travelled by solvent front is constant.

Hence, as distance travelled by compound increases RF value increases (In fraction denominator is constant, so with increase in numerator, fraction value increase)

The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatographic conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate.

Hence for the given compound increasing order of polarity (lowest first) is given as,

B(RF=0.89) < C(RF=0.53) < A(RF=0.43) …………………. (order reverse of RF value)

In column chromatographic separation:

A polar compound is used as mobile phase.

More polar the solvent (mobile phase) more it dissolves the polar compound in. This means the more polar compound move further.

Hence for A, B, C compounds the spots order from up to down is,

Same as above polarity order,

B(RF=0.89) < C(RF=0.53) < A(RF=0.43).

With increasing polarity we can have effective separation specially of B and C (which differs less in RF and polarity).

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