DRIFTS

The term DRIFTS ( English diffuse reflectance infrared fourier transform spectroscopy , German »diffuse reflection Fourier transform infrared spectroscopy «) describes a method of infrared spectroscopy in which the diffuse reflection of a sample is measured.

construction

Infrared light from a thermal source (see e.g. Globar ) is passed through an FTIR spectrometer for spectral resolution and then focused on a sample. There are two effects that cannot be easily separated experimentally when the sample surface is rough:

• Some of the radiation is reflected in a specular way on surfaces.
• Another part of the radiation has penetrated the sample, is mostly scattered several times and, if it was not absorbed on the way, exits again through the surface. This radiation emitted in a large solid angle is focused on the detector of the spectrometer with a suitable reflector .

application

In addition to the high signal intensity of the Fourier spectroscopic method, diffuse reflection offers advantages over a transmission measurement. For example, it allows the investigation of highly scattering or absorbing samples such as textiles, layers of lacquer, foams, and paper. In addition, no sample preparation is required, which is why powders can be measured directly - on catalyst pellets (e.g. made of zeolites ), sample preparation would falsify the reaction conditions.

evaluation

Even if the reproducibility is given with careful sample preparation, an exact quantitative evaluation is not possible. However, various semi-quantitative models have been developed. Most often the Kubelka-Munk theory is used, in which the spectral reflectance of the sample is traced back to two wavelength-dependent material constants, the absorption and the scattering coefficient. The theory is based on a one-dimensional system, so that the measured coefficients only allow conclusions to be drawn about microscopic quantities under further assumptions that are rarely met in practice.