664 M. Dubernet
12.4 Near Infra-Red Analyser
According to Cabanis et al. (1983), Bouvier (1986) and OIV (1994).
12.4.1 Principle
Most organic compounds when subjected to infra-red radiation present characteris-
tic absorption bands. The spectral data are compared to those obtained for standard
reference wines used to calibrate the instrument. The concentration of the analytes is
calculated using multiple linear regression; the apparatus is computerised and can be
linked to an automatic sampler. The analyte should exhibit strong absorption bands
for the method to be exploitable, and it is thus only suitable for major wine or must
constituents, essentially ethanol and sugars. The major qualities of this technique
are simplicity of operation, high samplethroughput and the lack of necessity for
sample preparation – the only required sample re-treatment is the removal of carbon
dioxide from musts in fermentation.
12.4.2 Material
The IR equipment used in oenology operates by reflectance. The circulation cell
containing the sample is equipped with a reflector at its base which reflects the
incident IR radiation; this essentially means that the radiation traverses the sample
twice. The various components of the system are described in the following sections.
12.4.2.1 Sample Pumping System
In general this is composed of a peristaltic pump which is responsible for filling the
cells. The instrument is usually equipped with a thermostatted bath to control the
temperature of analysis.
12.4.2.2 Light Source
This is a tungsten polychromatic light source producing light in the spectral range
320–2500 nm. It is essential that the power supply is perfectly stable to ensure con-
stant light intensity.
12.4.2.3 Wavelength Selector
The instruments used in oenological analysis are equipped with interference filters
of fixed wavelength or monochromators to select the characteristic wavelengths of
the analytes.