Vibrational Spectrometry of Pesticides and Related Materials on Thin Layer Chromatography Adsorbents

Abstract

Methods of measuring the vibrational spectra of species adsorbed on thin layer chromatography (TLC) plates have been studied by Fourier transform infrared (FT-ir) and Raman spectrometry. Previous measurements of the transmittance spectra of species separated by TLC (with the adsorbent deposited on I plates) showed poor signal/noise ratio at high frequency due to scattering by the adsorbent. Scattering has been minimized in these experiments by treating the TLC plate with Fluorolube, an infrared mulling oil whose refractive index closely matches that of the adsorbent. Spectra of chlorinated hydrocarbon pesticides on silica gel and alumina adsorbents measured in this way show differences which can be correlated with the mechanism of adsorption. FT-ir spectra of compounds on TLC plates can be obtained from microgram quantities of sample but Raman spectrometry is not as sensitive, requiring at least 200 µg of sample. The Raman bands of adsorbed pesticides are weak and considerably broadened with respect to the corresponding bands in the spectra of the pure samples.