652 M. Dubernet
The reliability of the FIA method reposes on the recording the signal according
to the following principles:
- The creation of an equilibrium in the analytical circuit such that return to baseline
is always obtained by the rinsing step andthat a given analyte concentration gives
a peak of constant height- The measurement method is comparative. All the measurements are made by
comparison with a calibration curve constructed from standards of accurately
known concentrationIf these principles are applied, any malfunctioning of the circuit translates into a
disruption in the recording (baseline drift, variation in peak shape or heights of the
calibration standard peaks etc.).
12.2.1.6 The Digital Interface
This component enables the analogue signal from the colorimeter to be converted
into a numerical value which may be directly expressed as analyte concentration. It
may often include a control module by which the functioning of the system may be
verified.
12.2.2 Commonly FIA Analytical Methods
According to Bouvier (1993) and Bouvier et al. (1995).
12.2.2.1 Determination of Volatile Acidity in Wine and Must (Grape Juice)
According to Pilone (1967), Sarris et al. (1970) and Dubernet (1976)
Principle
Volatile acidity is derived from the acids of the acetic series present in wine in the
free state and combined as salts.
Acids of the acetic series are isolated from wine by micro-distillation at 98◦C
under a stream of nitrogen. Sulphur dioxide is oxidised to sulphuric acid using
hydrogen peroxide and eliminated beforedistillation. Lactic acid is removed by
rectification during the distillation, and carbon dioxide does not interfere with the
determination. The distillate is mixed with a redox reagent (potassium iodide or
bromphenol blue) whose variations in colour intensity are proportional to the level
of volatile acidity.
Characteristics of the Method
Intra-laboratory reproducibility is 0.06g/L in acetic acid
Inter-laboratory reproducibility is 0.12g/L in acetic acid