Optical contact in ATR/FT-IR spectroscopy

Optical contact between an internal reflection element (IRE) and a sample i s crucial for obtaining a good spectrum in an attenuated total reflection ( ATR) experiment. When an air gap is present between the IRE and the sample, the spectrum severely deteriorates. Two techniques for determining the goo dness of contact are proposed-first, by making a comparison between polariz ed ATR spectra and, second, by making a comparison between ATR and transmis sion spectra. When the optical contact is achieved, the normalized p-polari zed ATR spectrum is exactly the same as the normalized s-polarized ATR spec trum. Moreover, the ratio between the normalized absorbance measured in the ATR mode and the normalized absorbance measured in the transmission mode e quals the ratio between the frequency being considered and the normalizatio n frequency. Theoretical and experimental investigations have been performe d in order to verify the proposed techniques.