Characterization of indoles by thin-layer chromatography and in situ fluorometry

The characterization of microgram quantities of a number of naturally occurring and synthetic indoles through a combination of thin-layer chromatography and in situ fluorescence spectroscopy is reported. Instrumental detection limits of 0.03–0.05 μg of the indoles are possible using the native fluorescence of the indoles in the ultraviolet range, with excitation maxima in the range 285–310 nm and emission maxima in the range 345–360 nm. Spraying with a dilute acid solution (0.1 N H₂SO₄ in methanol) produces an additional pair of maxima, with excitation at about 350 nm and emission at about 450 nm. The presence of a polar compound such as sulfuric acid or dimethyl sulfoxide in the spray produces an enhancement of the indole fluorescence. The procedure should find application in the determination of indoles in biological samples.