separation involves the placing of a sample onto a liquid or solid stationary
phaseand passing a liquid or gaseous mobile phasethrough or over it, a
process known as elution. Sample components, or solutes, whose distribution
ratios (vide infra) between the two phases differ will migrate (be eluted) at
different rates, and this differential rate of migrationwill lead to their separa-
tion over a period of time and distance.
Chromatographic techniques can be classified according to whether the sepa-
ration takes place on a planar surfaceor in a column. They can be further sub-
divided into gasand liquid chromatography, and by the physical form, solid or
liquid, of the stationary phase and the nature of the interactions of solutes with
it, known as sorption mechanisms(vide infra). Table 1lists the most important
forms of chromatography, each based on different combinations of stationary
and mobile phases and instrumental or other requirements.
Paper chromatography (PC)is simple and cheap but lacks the separating
power and versatility of thin-layer chromatography (TLC)which has largely
replaced it. Both require only inexpensive equipment and reagents, and, unlike
the various forms of column chromatography, comparisons can be made
between a number of samples and standards chromatographed simultaneously.
Gas (GC)and high performance liquid chromatography (HPLC)are comple-
mentary techniques best suited to the separation of volatile and nonvolatile
mixtures, respectively. Both these techniques are instrumentally-based and
computer-controlled, with sophisticated software packages and the ability to
separate very complex mixtures of up to 100 or more components. HPLC is
particularly versatile, having several alternative modessuited to different types
of solute. For example, ion-exchange (IEC)and ion chromatography (IC)are120 Section D – Separation techniques
Table 1. A classification of the principal chromatographic techniques
Technique Stationary Mobile Format Principal
phase phase sorption
mechanism
Paper chromatography (PC) Paper (cellulose) Liquid Planar Partition (adsorption,
ion-exchange,
exclusion)
Thin-layer chromatography (TLC) Silica, cellulose, Liquid Planar Adsorption (partition,
ion-exchange resin, ion-exchange,
controlled porosity exclusion)
solid
Gas chromatography(GC)
Gas-liquid chromatography (GLC) Liquid Gas Column Partition
Gas-solid chromatography (GSC) Solid Gas Column Adsorption
Liquid chromatography(LC)
High-performance liquid Solid or bonded-phase Liquid Column Modified partition
chromatography (HPLC) (adsorption)
Size-exclusion chromatography (SEC) Controlled porosity Liquid Column Exclusion
solid
Ion-exchange chromatography (IEC) Ion-exchange resin or Liquid Column Ion-exchange
Ion chromatography (IC) bonded-phase
Chiral chromatography (CC) Solid chiral selector Liquid Column Selective adsorption