Selection of the Stationary Phase in Liquid Chromatography (LC)

Once the LC mode and the type of column packing (porous, superficially porous) are selected, the choice of the stationary phase is determined by the requirements and the nature of the sample. In general, each stationary phase can have a unique selectivity towards sample components.

Selection of the Liquid-Solid Chromatography (LSC) Stationary Phase

Silica and alumina are the two most popular stationary phases (adsorbents). Silica is the preferred stationary phase mainly because of its availability, known performance and low cost. Silica has a lower reactivity, yields columns of better efficiency, and offers a higher linear capacity than alumina.

In general, silica and alumina are both used for the separation of the same type of compounds, although certain compounds tend to favor one over the other.

The following semiempirical relationships have been found by comparing the two adsorbents with regard to their selectivity for various functional groups:

• Moderately strong bases (pKb < 5) are preferentially adsorbed on silica

• Base-sensitive compounds should be separated on silica

• Acidic compounds are preferentially adsorbed on alumina

• Unsaturated molecules (olefins, aromatic) are preferentially adsorbed on alumina

• Halogen groups are preferentially adsorbed on alumina

Selection of the Liquid-Liquid Chromatography (LLC) Stationary Phase and Bonded Phase Chromatography (BPC)

All adsorbent materials recommended above for LSC can be coated with a liquid phase and used as LLC packing materials. The LSC adsorbents used in LLC have active surfaces and require a minimum liquid loading when used in LLC. However, column packings have been designed exclusively for LLC. There are several superficially porous supports consisting of a network of small spherical silica particles bonded to a solid glass bead. Their surfaces are relatively inactive and are used only for LLC. The porous surface provides a larger area on which thin films of liquid phase can be uniformly dispersed.

The stationary liquid phase is held on the support material by physical factors.  

 

Selection of the Ion Exchange Stationary Phase

Strongly acidic cation exchange resins (i.e. –SO₃^- H⁺) absorb both weak and strong cationic species. Similarly, strongly basic anion exchangers (i.e. –NR₄⁺Cl^-) absorb both weak and strong anions. Both strong cation and strong anion exchangers have the ability to split salts as the reaction shows below:

–SO₃^- H⁺  +  NaCl    =    –SO₃^- Na⁺  +   HCl