Wine Chemistry and Biochemistry

12 Automatic Analysers in Oenology 651

12.2.1.2 The Peristaltic Pump

The peristaltic pump is at the heart of the FIA analyser; containing 20–30 channels

in general, it regulates all liquid movements in the analytical circuit. Each channel

is constituted of a tube, composed generally of PVC which is pressed by a series

of rotating rollers against a circular surface to create pressurised flow. The flow

rate within each tube depends on its internal diameter which varies from 0.005 to

0.110cm. In this way the rhythm of introduction of the sample and different reagents

may be controlled with a high degree of precision.

12.2.1.3 The Analytical Cassette

This component contains all the elements of the analytical circuit. It is here that the

sample may be subjected to different treatments according to the requirements of

the analysis, i.e.

Dilution

Dialysis

Addition of reagent

Mixing

Distillation

Passage through a water bath (variable temperature and duration)

Part of the circuit may be thermostatted

From the time it exits the pump the liquid flow in the analytical circuit is seg-
mented regularly by air bubbles every 1 or 2cm; these bubbles are formed when

the needle passes from the rinsing vial to the sample vial. Due to surface tension

forces, the air bubbles prevent the diffusion of compounds in solution from one

section of the liquid column formed between two air boluses and the preceding or

succeeding column. In addition, the air bubble wipes the internal wall of the glass

or PVC tube, thus minimising the problem sample-to-sample cross contamination.

A second generation of FIA technology has been proposed, based on micro-flow

techniques which obviates the need for segmentation. This technology has remained

rather marginal due to problems with clogging of the tubes which can be difficult

to clear. Otherwise, the analytical principles remain the same as for segmentation-

based instruments.

12.2.1.4 The Detector

The most widely-used detectors in FIA methods are colorimetric.

12.2.1.5 The Recorder

A property of an analyte in the sample is converted by the detector into a signal

whose intensity is proportional to the analyte’s concentration. The signal is recorded

in the form of peaks whose height is proportional to the concentration.