Quantitative assessment of organic compounds adsorbed on silica gel by FTIR and UV-VIS spectroscopies: the contribution of diffuse reflectance spectroscopy

Different spectroscopic methods are compared to quantitatively determine or ganic compounds adsorbed or grafted on silica. The studied example is a der ivative 1 of a well known photosensitizer, benzophenone. Transmission FTIR, diffuse reflectance FTIR (DRIFT) and UV (DRUV) spectroscopy are used to me asure the concentration of adsorbed 1 on silica. The diffuse reflectance sp ectra are treated according to the Kubelka-Munk theory. It is shown that fo r such a compound absorbing in the UV range, DRUV spectra actually display a linear correlation between the remission function F(R) and the concentrat ion of adsorbed 1. As this method neither implies any sample preparation no r any problems arising from the supporting silica, it proves to be East, ef ficient and with a low detection threshold. For the IR spectra, it is neces sary to numerically substrate the spectrum of silica which absorbs the ligh t in this range. Provided that some care is paid to this numerical treatmen t which takes the sample heterogeneity into account, transmission and DRIFT spectra also display a linear correlation between the absorbance or the F( R) function and the concentration of adsorbed 1, However, sample preparatio n is much easier for DRIFT spectra as no pellets are made. Moreover, the DR IFT spectra of ground samples appear easier to process than the transmissio n spectra because of a better baseline and resolution. Although less resolv ed under 1500 cm(-1), the DRIFT spectra of non-ground samples can also be p rocessed and give satisfactorily and rapid results without any possible per turbation of the structural integrity of the sample. (C) 2001 Elsevier Scie nce B.V. All rights reserved.