Instant Notes: Analytical Chemistry

mobile phases for TLCand HPLCare to be found listed in Table 1, Topic

D6.

The more polar the solute, the more tenaciously it will be adsorbed onto

the surface of an adsorbent. Nonpolar solutes (e.g. saturated hydrocarbons)

have little or no affinity for polar adsorbents, whilst polarizable solutes (e.g.

unsaturated hydrocarbons) have weak affinities arising from dipole/induced

dipole interactions. Polar solutes, especially those capable of hydrogen-

bonding, are adsorbed strongly and require fairly polar mobile phases to

elute them. An approximate order of increasing polarity and therefore order

of elution (increasing distribution ratio) among classes of organic solutes is

alkanes<alkenes<aromatics<ethers<esters<ketones and

aldehydes<thiols<amines and amides<alcohols<phenols<acids

Adsorption-based chromatography is particularly useful for separating

mixtures of positional isomers such as 1,2-, 1,3- and 1,4-di-substituted

aromatic compounds with polar substituents, whereas members of a homolo-

gous series have similar polarities and cannot generally be separated at all.

Chiral chromatography, a mode of HPLC, depends on differences in the

adsorptive interactions of two or more enantiomers of a compound with a

chiral stationary phase. Gas solid chromatography(GSC) is an adsorption-

based mode of GC.

● Partitionis a sorption process analogous to solvent extraction (Topic D1),

the liquid stationary phase being thinly coated or chemically bonded onto

an inert solid. Where the liquid is bonded to the supporting solid, it is

debateable as to whether it behaves as a liquid and whether the sorption

process should be described as modified partition, because adsorption may

also be involved. In true partition, solutes are distributed according to their

relative solubilities in the mobile and stationary phases, but the exact

mechanism for bonded phasesis not clear. The use of bonded phases has

become widespread in all forms of chromatography, and a pure partition

mechanism probably occurs only in gas liquid chromatography(GLC)

where the stationary phase is not chemically bonded to the column wall

(Topic D4).

Bonded phases are described in more detail in Topics D4 and D6.

● Ion-exchangeis a process whereby solute ionsin the mobile phase can

exchange with counter-ionscarrying the same charge and associated with

oppositely charged groups chemically bound to the stationary phase. The

stationary phase is a permeable polymeric solid, such as an insoluble organic

resin or a chemically modified silica, containing fixed charge groups and

D2 – Principles of chromatography 123

Table 2. Adsorbents for chromatographic separations

(listed in order of decreasing polarity)

Alumina (most polar)

Charcoal

Silica gel

Molecular sieve

Magnesium silicate

Cellulose

Polymeric resins (least polar)

(styrene/divinyl benzene)