Introduction - Determination of Anions in Natural and Treated Waters - page 11

Flow injection analysis is based on the introduction of a defined volume of sample into
a carrier (or reagent) stream. This results in a sample plug bracketed by carrier (Fig. 1(a)).
The carrier stream is merged with a reagent stream to obtain a chemical reaction between
the sample and the reagent. The total stream then flows through a detector (Fig. 1.1(b)).
Although spectrophotometry is the commonly used detector system in this application,
other types of detectors have been used, namely fluorometric, atomic absorption emission
spectrometry and electrochemical, eg ion selective electrodes.
The pump provides constant flow and no compressible air segments are present in the
system. As a result the residence time of the sample in the system is absolutely constant.
As it moves towards the detector the sample is mixed with both carrier and reagent. The
degree of dispersion (or dilution) of the sample can be controlled by varying a number of
factors, such as sample volume, length and diameter of mixing coils and flow rates.
When the dispersed sample zone reaches the detector, neither the chemical reaction nor
the dispersion process has reached a steady state. However, experimental conditions are
held identical for both samples and standards in terms of constant residence time,
constant temperature and constant dispersion. The sample concentration can thus be
evaluated against appropriate standards injected in the same manner as samples (Fig. 1.1
(c)).
The short distance between the injection site and the merging point ensures negligible
dispersion of the sample in this part of the system. This means that sample and reagent
are mixed in equal proportions at the merging point.
The mixing technique can be best understood by having a closer look at the
hydrodynamic conditions in and around the merging point (Fig. 1.1(d)). In Fig. 1.1(d) the
hydrodynamic behaviour is simplified in order to explain the mixing process. Let us
assume that there is no axial dispersion and that radial dispersion is complete when the
sampler reaches the detector. The volume of the sample zone is thus 200µg after the
merging point (100µL sample+100µL reagent as flow rates are equal). The total flow rate
is 2.0ml min −1 . Simple mathematics then gives a residence time of 6s for the sample in
the detector flow cell. In reality, response curves reflect some axial dispersion. A rapid
scan curve is shown in Fig. 1.1(e). The baseline is reached within 20s. This makes it
possible to run three samples per minute and obtain baseline readings between each
sample (no carry-over), ie 180 samples per hour.
The configuration of an FIA system is shown schematically in Fig. 1.1(f). The
(degassed) carrier and reagent solution(s) must be transported in a pulse-free transport
system and at constant rate through narrow Teflon (Du Pont) tubing.
In a practical FIA system, peristaltic pumps are usually used since they have several
channels, and different flow rates may be achieved by selection of a pump tube with a
suitable inner diameter.

Table 1.2 Equipment for flow injector analysis

Supplier Model Features Detectors available

Advanced LCG 1 Relatively low-cost instrument, recorder Colorimeter (other detectors