Automatic analytical methods for environmental monitoring and control

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  dialysers and gas diffusion units (Figure III.2.15.a) in which the separation of analytes from a donor stream to an acceptor stream is carried out using a permeable membrane that separates the two streams;
  
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  solvent exaction units (Figure III.2.15.b) that contains two special components:
  

- a separator for the organic phase.

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  packing reactors in which the packing material may be an ion-exchange resin used for analyte pre-concentration or/and for removing the interfering species; immobilized enzymes used for analyte selective degradation; oxidizing or reducing agents that act directly on the analyte or generate reagents “in-situ” that react with the analyte.
  

(a) (b)
Figure III.2.15. (a) Scheme of a dialyser/gas diffusion unit used in FIA manifold. (b) Scheme of a solvent extraction unit used in FIA manifold (i) organic phase segmentor, (ii) phase separator.

Generally, chromatographic detectors with cell volumes smaller then 20 L are well suited for FIA work. On the other hand, conventional detectors may be furnished with flow-through cells provided that the system will accommodate and handle flow cells of sufficiently small holdup volumes and apertures. Two types of flow-cells used for spectrophotometric measurements are shown in Figure III.2.16. Any detection technique that is compatible with a flowing stream is suitable for flow injection.

Spectrophotometry, nephelometry, fluorescence, chemiluminescence, atomic absorption, flame photometry, potentiometry with ion-selective/modified electrodes or field effect transistors, amperometry with sensors and biosesors and voltammetry with wire-type or rotating disk electrodes are the most important detection techniques used in FIA.

1. 
   (b)
   

The most important feature of FIA that distinguishes it from the other continuous flow techniques is the well-defined concentration gradient formed when an analyte is injected into the carrier stream. The gradient approach yields the capability to perform procedures not feasible by conventional continuous flow analysis. Many gradient techniques have been developed, including titration, gradient dilution and calibration, gradient scanning, FIA stopped-flow technique and simultaneous injection of two zones, which completely or partially overlap permitting the execution of selectivity studies and standard addition procedures.

A FIA signal represents a variation of concentration from zero to C^max and there is no single element of fluid with the same concentration as the neighboring one. Any element of fluid along the gradient corresponds to a fixed dispersion (D) and they can be related to a fixed delay time elapsed from the moment of injection. In gradient techniques, one of these elements is selected to take the measurement in continuous or stopped-flow mode.

The principle of gradient dilution is based on selecting for the measurement any point, other than the peak maximum. The advantage of this technique is that of obtaining the readouts after pre-selected delay times (Figure III.2.17.a), manual pre-dilution of the sample can be avoided and a large concentration range can be accommodated within the dynamic range of the detector used.

The gradient calibration technique is an extension of the above described one. Its main goal is to avoid the usual repetitive calibration by means of a series of diluted solutions, as the information sought is in fact already contained within some of the segments of fluid originating from a sample zone of the most concentrated standard sample solution (Figure III.2.17.b).

The FIA stopped-flow approach is based on a combination of stopped-flow measurements and of gradient dilution. It is based on the increase of residence time, keeping the reactor short and decreasing the flow-rate of the carrier stream. Choosing appropriate stop-go time intervals, the reaction time will increase and the reaction rate will be proportional to the concentration of the analyte (Figure III.2.17.c), for a pseudo-zero-order reaction. This technique allows a fine adjustment of the reagent/sample ratio by selecting a corresponding delay time.

(b) (c)