Sunday, July 24, 2011

CHROMATOGRAPHIC METHODS

In principle these methods are similar to capillarity methods the main differences being that in capillarity the liquid and its solute rise by surface tension through a micro porous system whereas in chromatographic analysis they fall by gravity through such a system ,chromatographic adsorbent may be selected to be strongly polar and chemically reactive towards one component of a mixture to be tested.

The manipulative technique consists in packing a tube of perhaps an inch diameter with an adsorbent powder such as finely divided alumina and pouring the liquid on the top of the column of adsorbent . the larger aggregates tend to be retarded in their rate of flow as compared to the smaller molecules and the aggregates or molecules of polarity opposite to that of the adsorbent tend to be further retarded or adsorbed and fixed .

The components in the solution therefore become separated in space and the spacial sepatation may be "developed" by washing with pure solvent or elutriant.The chief advantage of the method is that by proper development components may be isolated as separate fractions . In certain cases of biological importance component which are extremely difficult to estimate quantitatively by chemical methods are very easily separated in almost a pure form by chromatography.

The properties of the adsorbent depend largely on its method of preparation . Alumina silica gel politer starch magnesium carbonate (Ponds and indeed any finely divide material may be used .
     The technique is still largely empirical but general principles are emerging and mathematical investigations have commenced to be made .In general good separation is obtained of two components if one has a much bigger molecule or is more aggregated than the other , or if one component carries an electrical charge of opposite sign to that of the adsorbent whilst the other is electrically inert or of the same charge as the adsorbent.

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