12 Automatic Analysers in Oenology 665
12.4.2.4 Measurement Cell
The part of the cell traversed by the incident and reflected rays is made of quartz,
and the base of the cell may be composed of ceramic covered with a film of gold to
provide reflectance. The cell is maintained at a constant measurement temperature,
generally using a system based on the Peltier effect.
12.4.2.5 Detectors
The reflected rays are directed towards two lead sulphide photoelectric cells.
12.4.2.6 Computer
This component carries out the mathematical and statistical operations which enable
comparisons with the calibrated standards and determination of the analyte concen-
tration.
12.4.3 Operation
The installation and operation of a near infra-red analyser involves several steps.
12.4.3.1 Initial Calibration
This step involves calibration of the apparatus which will serve as a reference. It
consists of analysing the greatest number possible (minimum 50) wines or must
samples containing different and accurately known concentrations of each analyte.
The concentration points should be uniformly distributed over the probable scale
of measure for each analyte. The matricesshould mimic as accurately as possible
the wines and musts destined for analysis using that particular instrument. For each
calibration sample, a measurement iscarried out at a maximum number of wave-
lengths in the infra-red. Multi-linear regression is then carried out on the results
which enables the following relationship to be established:
C=K 0 +K 1 R 1 +K 2 R 2 +K 3 R 3 +...+KiRi
where
C is the concentration
K 0 is a constant characteristic of the individual instrument and varies according
to the analyte, independent of the wavelength
Kiis a constant of the instrument, a given compound at a specific wavelength
Riis an expression of the spectral measure at wavelength Li