ATR Spectroscopy

Attenuated Total Reflection Spectroscopy

Infrared spectroscopy on biological systems is usually performed in transmission mode. This means that the IR beam of the spectrometer is passing through the sample and the transmitted IR intensity is measured. Owing to the high IR absorptivity of water, IR samples are very thin, usually only a few micrometer optical pathlength. The water content in transmission samples is therefore quite low, which may in some cases constitute a major problem. Moreover, it is quite difficult to change sample conditions during a measurement, e.g. by adding of ligands, changing the pH or ion concentration.

An alternative to transmission mode experiments is offered by the attenuated total reflection (ATR) technique. In this technique, the IR beam is guided in an IR transparent crystal by total reflection. Due to quantum mechanical properties of the IR light, the electromagnetic field may extend beyond the crystal surface for about one micron as a so-called evanescent field. By applying e.g. a protein sample directly onto the surface of the crystal, it is sensed by this evanescent wave and contributes to the absorption of the IR beam.