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
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