Instant Notes: Analytical Chemistry

VRVM 1 D  (4)

or VRVMDVs (5)

where Dhas been substituted for kusing the relation

kD (6)

and the volumes of the stationary and mobile phases in the column are VS

and VM, respectively.

Equation (5) is regarded as a fundamental equation of column chromato-

graphy as it relates the retention volume of a solute to its distribution ratio.

● Planar separations(PC andTLC). Separations are normally halted before the

mobile phase has travelled completely across the surface, and solutes are char-

acterized by the distance they have migrated relative to the leading edge of the

mobile phase (solvent front). A solute retardation factor, Rf, is defined as

Rf (7)

The maximum value of Rfis 1, which is observed for a solute having a distri-

bution ratio and retention factor of zero, and therefore migrating at the same

velocity as the mobile phase. Solutes whose Dand kvalues are greater than

zero are proportionately retarded, the minimum value for Rfbeing zero,

which is observed when the solute spends all of the time in the stationary

phase and remains in its original position on the surface. Practical values of

Rfare evaluated from the ratio

Rf= (distance moved by solute) / (distance moved by the solvent front)

Sorptionis the process whereby solute species are transferred from the mobile

to the stationary phase, desorptionbeing the reverse process. These processes

occur continually throughout a chromatographic separation, and the system is

therefore described as being in a state of dynamic equilibrium. A solute is

repeatedly re-distributed between the phases as the mobile phase advances, in

an attempt to maintain an equilibrium corresponding to its distribution ratio, D.

There are four basic sorption mechanisms, and it is common for two or more

to be involved simultaneously in a particular mode of chromatography, viz:

adsorption; partition; ion-exchange; exclusion.

● Adsorptionis a surface effect, not to be confused with absorption, which is

a bulk effect. Surface adsorption involves electrostatic interactions such as

hydrogen-bonding, dipole–dipole and dipole-induced dipole attractions.

Solute species compete with the mobile phase for a limited number of

polar sites on the surface of the adsorbent of which silica gelis the most

widely used. Its surface comprises Si-O-Siand Si-OH(silanol) groups, the

latter being slightly acidic as well as being polar, which readily form

hydrogen bonds with slightly-polar to very-polar solutes. Water in the

atmosphere can de-activate an adsorbent surface by itself being adsorbed,

thereby blocking adsorption sites. This can be overcome by drying the

adsorbent if a more active material is required, although reproducibility

may be difficult to achieve unless ambient temperature and humidity are

carefully controlled. Some common adsorbents are listed in Table 2. Suitable

Sorption
processes

1

1  k

VS

VM

VS

VM

122 Section D – Separation techniques