Instant Notes: Analytical Chemistry

mobile counter-ions. Both cationicand anionicion-exchangers are available,

the exchange processes being represented by the following equations

cation-exchange: nR-H++Xn+= (R-)nXn++ nH+

anion-exchange: nR+Cl-+Yn-= (R+)nYn-+ nCl-

where Rrepresents the polymeric resin or silica, and Xn+and Yn-are solute

cationsand anionsrespectively of valency n.

The factors affecting ion-exchange equilibria and selectivity are described

in Topic D7.

● Exclusiondiffers from the other sorption mechanisms in that no specific inter-

actions between solute species and the stationary phase are necessary or desir-

able. The stationary phase is a controlled-porosity silica or polymer gel with a

range of pore sizes, and solutes remain in the mobile phase throughout the

separation, merely diffusing through the porous structure to different extents

depending on their size and shape. Solutes whose size exceeds the diameter of

the largest pores are entirely excluded from the structure and migrate at the

same rate as the mobile phase. Solutes smaller than the diameter of the smallest

pores can diffuse throughout the structure and have the slowest rate of migra-

tion. Solutes of an intermediate size can diffuse through some pores but not

others, migrating at rates between those of the largest and smallest species.

Size-exclusion chromatography(SEC) is a mode of HPLC(Topic D7) and

is also a classical technique employing large columns of silica or polymeric

gel particles and gravity flow of the mobile phase. It is sometimes described

as gel permeationor gel filtration chromatography.

During a chromatographic separation, individual solutes develop a symmetrical

or Gaussian concentration profile(Topic B2) in the direction of flow of the

mobile phase. The profiles, known as bandsor peaks,gradually broaden and

often become asymmetrical as the solutes continue to migrate through the

stationary phase. The principal underlying reasons accounting for the peak

shapes and the observed broadening can be summarized as follows:

● continual sorption and desorption of a solute between a mobile and a

stationary phase inherently produces a Gaussian concentration profilewhich

broadens as the solute migrates further. (This can be demonstrated by a math-

ematical treatment of a solvent extraction procedure to separate mixtures,

developed in 1952, and known as Craig Countercurrent Distribution);

● solute species travel slightly different total distances through a particulate

stationary phase, causing concentration profiles to broaden symmetrically,

this being known as the multiple-path effect;

● solute species spread by diffusionin all directions when they are in the

mobile phase. Diffusion in both the direction of flow of the mobile phase and

directly counter to it (longitudinal oraxial diffusion) contributes to the

symmetrical broadening of the peak profile;

● sorption and desorption, or mass transfer, between the stationary and

mobile phases, are not instantaneous processes, and are sometimes kineti-

cally slow. Because the mobile phase moves continuously, a true equilibrium

distribution of a solute is never established, and the concentration profile in

the stationary phase lags slightly behind that in the mobile phase causing

further peak broadening. Slow desorption can also result in the peak

becoming asymmetricalor skewed(vide infra);

Peak profiles and
band broadening

124 Section D – Separation techniques