Page 74 - Practical book PC406-MNU-2025

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL II INSTRUNENTAL ANALYSIS (PC 406)

B) Thin-layer chromatography (TLC)

I In nt tr ro od du uc ct ti io on n

 Thin-layer chromatography is one of the easiest of the many chromatographic

techniques.

 A thin layer of a suitable solid substance is coated on a sheet of glass, metal or

plastic.

 A very small sample of the mixture to be analyzed is “spotted” onto the sheet.

 By immersing one edge of the sheet in an appropriate liquid developing solvent (mobile

phase), the solvent is drawn up the sheet by capillary action, and the compounds of

interest are carried along at different rates, effectively separating the components.

 This is commonly called “developing” the plate. After the plate has been developed, it is

examined under ultraviolet (UV) light, which allows you to note the location of the

spots.

 Experimental conditions in TLC include the solvent system used to dissolve the

compounds, the adsorbent coated onto the TLC plate, the thickness of the adsorbent

layer, and the relative amount of the material spotted onto the plate.

 Under an established set of experimental conditions, a given compound always travels a

fixed distance relative to the distance the solvent front travels.

 This ratio of the distance the compound travels to the distance the solvent front

travels is called the Rf value.

 The symbol Rf stands for “retardation factor,” or “ratio-to-front,” and it is expressed as

a decimal fraction.

The equation is shown below:

The distance travelled by the substance
R f =
The distance travelled by the solvent

When the conditions of measurement are completely specified, the Rf value is constant for

any given compound, and it corresponds to a physical property of that compound.

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