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

Abstract

Different spectroscopic methods are compared to quantitatively determine organic compounds adsorbed or grafted on silica. The studied example is a derivative 1 of a well known photosensitizer, benzophenone. Transmission FTIR, diffuse reflectance FTIR (DRIFT) and UV (DRUV) spectroscopy are used to measure the concentration of adsorbed 1 on silica. The diffuse reflectance spectra are treated according to the Kubelka–Munk theory. It is shown that for such a compound absorbing in the UV range, DRUV spectra actually display a linear correlation between the remission function F( R) and the concentration of adsorbed 1. As this method neither implies any sample preparation nor any problems arising from the supporting silica, it proves to be fast, efficient and with a low detection threshold. For the IR spectra, it is necessary to numerically substrate the spectrum of silica which absorbs the light in this range. Provided that some care is paid to this numerical treatment 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 preparation is much easier for DRIFT spectra as no pellets are made. Moreover, the DRIFT spectra of ground samples appear easier to process than the transmission spectra because of a better baseline and resolution. Although less resolved under 1500 cm −1, the DRIFT spectra of non-ground samples can also be processed and give satisfactorily and rapid results without any possible perturbation of the structural integrity of the sample.