QUANTITATIVE THIN-LAYER CHROMATOGRAPHY .13. IDENTIFICATION OF RESIDUES ON OF DIFFUSELY REFLECTING LAYERS

A method is described for quantitative evaluation, by scanning densito metry, of unidentified contaminating residues present as small spots o n the surfaces of tablets, surfaces which act as diffusely reflecting layers, UV and visible spectra of the spots were compared with the spe ctra of different standards. Spectra were obtained dynamically at each selected wavelength a tablet is scanned and the difference between th e signal from the clean surface and that from the spot is taken as the absorption at the selected wavelength, This method is better for resi dues than subtraction of a spectrum taken in one run from a clean surf ace, from a spectrum taken from the spot. Semiquantitative results are obtained by comparison of peak areas From the tablets with calibratio n cur es from samples applied to a dry TLC plate, The limit of detecti on is from 1 to 5 ng per spot, depending on the substance measured.