A transmission infrared technique for detecting bands due to adsorbed species on silica gels in the region 4000 to 200 cm-1 is described. The region below 1300 cm-1 contains strong Si-O bulk modes that dominate the much weaker bands due to adsorbed species. Silica gels are strong infrared scatterers, and adsorbed molecules lead to changes in the spectral artifacts in the region containing the bulk modes. These artifacts dominate and mask out the much weaker bands due to adsorbed species. By embedding the silica gel in a pliable polymer film of similar refractive index, one minimizes the artifacts, enabling detection of bands attributed to adsorbed compounds.
J. K. Cox and C. P. Tripp, "Method for Acquiring Low-Frequency Transmission Infrared Spectra of Adsorbed Compounds on Silica Gels," Appl. Spectrosc. 54, 144-147 (2000)