Vnws

Normalizing the specific retention volume to 0°C (273.15 K) gives rise to the specific retention volume at 0°C (Vg):

Vg V g Tc Ws Tc where Tc is the column temperature.

The unretained peak is given by a substance that has no affinity for the stationary phase and therefore passes through the column at the same speed as the mobile phase. A substance that shows affinity for the stationary phase moves through the column more slowly than the mobile phase and is said to be retained. The ratio of the two velocities is known as the retardation factor (R):

rate of movement of retained peak rate of movement of mobile phase

A retained component spends time in both the mobile phase (tM) and the stationary phase (ts), and retention time tR is given by:

which is the fundamental equation for chromatography, neglecting the effects of nonlinearity of the sorption isotherm and band broadening.

since, in general, migration of the solute through the chromatographic column depends upon the equilibrium distribution of the solute between the stationary and mobile phases, retention is controlled by those factors that affect the distribution. In GLC, the distribution is essentially that for a two-component system; the sample (or solute) and the stationary phase (or solvent) (or the adsorbate and adsorbent respectively in gas-solid chromatography). The distribution then is a result of the molecular forces between the sample and the stationary phase and the effect of temperature and pressure on these interactions, although at the pressures normally used in GC the effect of pressure is negligible. All such forces are electrostatic in origin and are based on Coulomb's laws of attraction and repulsion between charges. The major forces involve those between charged ions (e.g. in ion chromatography), i.e. dipole-dipole interactions, dipole-induced dipole interactions, dispersion forces and hydrogen bonding forces. Dispersion forces are present in all atoms and molecules, but the other interactions depend on structural features in the molecule, i.e. ions (e.g. Cl"), polar functional groups (e.g. C-Cl, C-OH) and polarizable groups (e.g. aromatic and conjugated molecules).

In this case of GLC, assuming that the concentration of the sample (or solute) molecules in the mobile (gas) phase is very small, as is the case if small volume sample injections are made, then the solution of solute in the stationary phase may give rise to an 'ideal solution', and the vapour pressure (p) of the solute above the solution is given by Raoult's law:

The time spent in the stationary phase is dependent on the distribution coefficient (Kc) such that ts = KcVS. If Cs and CM are the concentrations of a component in the stationary phase and mobile phase, respectively, then the distribution constant is given by:

The rate of movement of a component through the column is inversely proportional to the distribution constant, i.e. a substance with a high concentration in the stationary phase (a high distribution coefficient) moves slowly through the column. Components of a mixture are, therefore, separated only if their distribution coefficients differ. Using volumes rather than times we can write:

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