Coloured spots are easily observed on developed chromatograms. However, different approaches need to be adopted when colourless components are to be observed. It is convenient to classify such methods as specific or non-specific.
Iodine chamber – the developed plate is suspended in a closed jar containing a few crystals of iodine for about a minute. In presence of iodine vapour most organic compounds appear as brown spots.
UV viewing cabinet – majority of colourless compounds can be viewed under illumination with UV light in a UV viewing cabinet. Commonly the cabinets are equipped with a long wavelength (366 nm) and short wavelength ( 254 nm) light sources. Under exposure of selected source coloured spots appear on a dark background and under exposure to light from short wavelength source dark spots are seen against a bright background. In either case the spots should be marked with a pencil while observing in the cabinet for Rf calculations.
Amino acids ,primary and secondary amines –A 0.2% ninhydrin solution in water saturated with butanol is sprayed on briefly heated paper chromatogram when deep blue or purple spots begin to appear.
Alkaloids – Dragendroff’s agent gives orange or orange – red precipitates in presence of alkaloids
Phenols – Ferric chloride solution spray results in appearance of different coloured spots – read, blue, green or purple when phenols are present.
Aldehydes and Ketones – 2,4 – dinitro phenyl hydrazine in a mixture of methanol and sulphuric acid (Brady’s reagent) spray results in bright orange or yellow colour spots in presence of aldehydes and ketones.
Paper chromatography is a versatile analysis technique and has wide range of applications which will be taken up in a subsequent article