12 Automatic Analysers in Oenology 667
12.5.1 Material and Principle of the Method
The apparatus consists of an interferometer using Fourier transform infra-red spec-
trophotometry covering the range 2000–10000 nm which corresponds to part of the
mid infra-red range of the electromagnetic spectrum. After calibrating the instru-
ment for different organic compounds, spectral analysis of the data enables the
simultaneous determination of several analytes in must or wine.
12.5.2 Interferometry and Fourier Transform
Interferometry is an alternative method toclassic techniques of spectral acquisi-
tion, which tend to be onerous and time-consuming if fine resolution is required. It
enables all wavelengths emitted by a single infrared source to be treated simulta-
neously in real time. As preliminary wavelength selection is not required, complete
spectra may be obtained in less than1s. The first step (Fig. 12.1) consists of produc-
ing an interferogram of the sample to be analysed. The interferogram is based on the
separation of polychromatic infra-red light (emitted by an incandescent filament) on
a blade. Before arriving at the detector the two components of the split signal follow
different pathways – one part traverses the sample directly while the other part is
reflected against a mobile mirror before passing through the sample.
Each elementary wavelength of the infra-red radiation arrives at the detector as
two component parts with a phase difference ofp. By the action of the mobile mir-
ror, this phase difference will vary continually during measurement. The final signal
Fig. 12.1Simplified schematic diagram of a Michelson interferometer