Instant Notes: Analytical Chemistry

stand for several minutes to allow the atmosphere in the tank to become

saturated with the solvent vapor;

● small volumes of liquid samples and standards, or solutions, are spotted

onto the sorbent surface of a TLCplate along a line close to and parallel with

one edge (the origin). The plate is then positioned in the tank with this edge

in contact with the mobile phase and the cover replaced (Fig. 1(a));

● the mobile phase is drawn through the bed of sorbent from the edge of the

plate, principally by capillary action, and this developmentprocess is halted

shortly before the solvent frontreaches the opposite side of the plate. Sample

components and standards migrate in parallel paths in the direction of flow

of the mobile phase, separating into discrete zones or spots;

● the plate is removed from the development tank, dried in a current of warm

air, and solute spots located by appropriate methods (vide infra);

● each solute is characterized by the distance migrated relative to the solvent

front, i.e. its Rfvalue, which will lie between 0 and 1 (Topic D2), and

unknowns are identified by comparisons with standards run simultaneously.

Figure 1(b)illustrates a developed and visualized TLCplate with Rfvalues

shown alongside. Note that the shapes of some spots have become slightly

elongated in the direction of flow of the mobile phase, which is an example of

tailing(Topic D2). This is caused by slow desorption as the solute migrates, or

saturation of adsorption sites by high concentrations of the solute leading to a

convex sorption isotherm, and is most likely to occur where adsorptionis the

principal chromatographic sorption mechanism(Topic D2).

Stationary phase Stationary phases used in TLC are microparticulate sorbents with particle
diameters of between 10 and 30mm. The smaller the mean particle size and the
narrower the size range, the better the chromatographic performance in terms of
band spreading(efficiency) and resolution(Topic D2). Thin-layer chromatog-
raphy plates are prepared by coating sorbents onto rectangular plastic,
aluminum or glass sheets in adherent uniform layers approximately 250mm

132 Section D – Separation techniques

TLC plate

coated with

absorbent

Origin line

Mobile phase

Lid

Developing

tank

Sample spots

1.00

0.59

0.27

0

Rf

(a)

(b)

Fig. 1. TLC plates (a) during and (b) after development and visualization; Rfvalues are
shown alongside. a, Reproduced from R.J. Hamilton & S. Hamilton, Thin-Layer
Chromatography, 1987. b, Reproduced from R.M. Smith, Gas and Liquid Chromatography
in Analytical Chemistry, 1988. © John Wiley & Sons Ltd. Reproduced with permission.