Analytical Chemistry

aldehydes < amines < alcohols and phenols < acids. During the separation process there is competition
for adsorption sites between solute molecules and those of the mobile phase. Solute and solvent
molecules are continually being adsorbed and desorbed as the mobile phase travels through the system.
Solutes of low polarity spend proportionately more time in the mobile phase than those that are highly
polar. Consequently, the components of a mixture are eluted in order of increasing polarity (increasing
distribution ratio). Adsorption isotherms often show curvature (p. 82), unless the sample loading is very
low. This leads to pronounced tailing of solute peaks and a loss of resolution.

Stationary Phase

Almost any polar solid can be used, the most common choices being silica gel or alumina. A selection
of stationary phases is listed in Table 4.8 in order of decreasing adsorptive power. Silica gel and
alumina are highly polar materials that adsorb molecules strongly. They are said to be active adsorbents.
Activity is determined by the overall polarity and by the number of adsorption sites. In silica gel, the
adsorption sites are the oxygen atoms and silanol groups (—Si—OH) which readily form hydrogen
bonds with polar molecules. Adsorption sites of different types are present on the surface of alumina,
but, like silica gel, a proportion of them are hydroxyl groups. The amount of water present on the
surface has a profound effect on activity by blocking adsorption sites. If the water is progressively
removed by oven drying, the material becomes correspondingly more active.

Table 4.8 Adsorbents used in column

chromatography

Note: The positions of alumina and silica gel are

dependent on water content.

The choice of stationary phase and its degree of activity is determined by the nature of the sample. If
sample components are adsorbed too strongly, they may be difficult to elute or chemical changes may
occur. Weakly polar solutes should be separated on highly active adsorbents otherwise they may elute
rapidly with little or no resolution. Strongly polar solutes are better separated on adsorbents of low
activity. Silica gel can be prepared with a