Direct Optical Evaluation Above 400 nm

When TLC scanners are used for absorption measurements in the 400-800 nm range, illumination is by tungsten filament lamp. To determine the optimum wavelength for measuring a substance (the absorption maximum), the absorption spectrum is measured at the centre of the spot obtained by chromatography. This spectrum can then be kept in a hbrary of spectra and later used for identifications of other samples. 

Dyes that absorb in the visible range, e.g. those foimd in the analysis of writing inks, can be quantified. 

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