12 Automatic Analysers in Oenology 673
12.5.8 Usual IR Methods
The number of parameters that may be determined by FRIR is constantly increasing
due to the continuous development of new calibrations. The most important methods
currently available may be divided into three distinct categories.
12.5.8.1 “Direct” Methods
These are methods involving the determination of organic compounds for which
there is a direct relationship between molecular absorbance and concentration. Rou-
tinely used methods are presented in Table 12.1.
12.5.8.2 “Indirect” Methods
In oenology two different cases may arise:
- The target compound haslittle or no absorbance in the mid-IR either because of
its native structure or because it ispresent at low concentrations
- The target compound itself is not determined; rather an index reflecting a group
of organic compounds is measured
The determination of potassium is an example of the first case. As it obviously does
not absorb in the IR, compounds to which its concentration is correlated are detected
and the correspondence obtained by chemometric approaches.
An example of the second case is the determination of pH or titratable acidity,
which are reliably determined by the measurement of the complex equilibrium of
the organic compounds that determine their values. Routinely used methods are
presented in Table 12.2.
12.5.8.3 “Biological” Methods
These methods involve the measurement of indicative values based on biological
data identifiable in the mid IR.
Routine application are as follows.
Table 12.1Usual methods of determination in oenology by near IR analysers
Intra-laboratory Inter-laboratory
Analyte Measurement scale reproducibility reproducibility
Glucose and fructose 0–400 g/L−− 1.3g/L
Malic acid 0.3–6 g/L 0.15g/L−− 0.6g/L−−
Lactic acid 0.3–6 g/L−− 0.15g/L 0.6g/L
Carbon dioxide 60–1300 mg/L−− 95mg/L−− 230mg/L
Glycerol 0.2–10 g/L 10%
Tartaric acid 1–15 g/L−− 10%
Gluconic acid 0.1–15 g/L 10%