In situ Identification of Thin-Layer Chromatography Fractions by FT Raman Spectroscopy

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Abstract
The use of near-IR-FT Raman spectroscopy for the in situ analysis of fractions on TLC plates is described. The detection of additives in use in the plastics industry is used as an example. Reasonable-quality spectra were obtained from sample loadings equivalent to about 3 μg mm−2 in the most favorable case. Background fluorescence was not a problem, either from the adsorbent or the adsorbate, even when fluorescor was present to aid spot visualization. Similarly, staining with iodine to identify spot positions did not degrade the FT Raman spectra. When a mixture of three additives (200 μg of each) was eluted on a silica TLC plate, two of the additives gave good Raman spectra, sufficient for identification. The third component, a weaker scatterer, had spread over such a large area that the concentration was too weak to give a Raman spectrum visible above the background features from the TLC adsorbent. The concentration of the eluted spot is the limiting factor in this approach since the Raman experiment samples only about 1 mm2 of the total sample area. However, component detection is clearly feasible with the use of this technique, and advances in detector technology should substantially reduce the sample loadings required to effect identification, although it must be realized that background Raman features from the TLC adsorbent will ultimately obscure the spectrum of very dilute loadings of additives. In such cases, sample concentration on the plate will be necessary. A brief comparison with conventional Raman spectra obtained with 514-nm excitation is also made. While good spectra of one additive on silica were obtained, this approach could not be used with plates that contained added fluorescor or that had been stained with iodine for spot visualization, owing to intense background fluorescence. The approach was also prone to fluorescence form the TLC fraction itself and sample degradation.