Figure 12.6
Schematic representation of an automated colorimetric analysis.regular intervals prevents diffusion and cross-contamination between reagents and samples. A glass coil
inserted into the stream provides a delay long enough for mixing to be completed within the separated
sections of the solution. If interfering materials are present and cannot be masked, a separator unit is
incorporated prior to the measurement stage. For a colorimetric method, the colour reagent is added and
a reaction coil to allow for mixing and colour development is inserted. This coil may be immersed in a
suitable thermostat bath. Finally the coloured solution is fed to the detector via a 'debubbler' which
removes the air. The most convenient detection devices are filter photometers and spectrophotometers,
which are readily operated as 'flow instruments', although emission instruments may be used. An
Autoanalyser can be concurrently processing a number of samples and analysis rates of 150 samples per
hour have been reported. The major disadvantage is the inability to handle solid, or high viscosity
samples without a prior manual dissolution step. Figure 12.7 summarizes the automated determination
of magnesium in urine by means of its reaction with o,o'-dihydroxyazobenzene, and fluorimetric
detection.
A more recent development is a technique known as flow injection analysis, in which a discrete volume
of a liquid sample is injected into a carrier stream. Reagents required for the development of the
analytical property of the analyte, e.g. colour developing reagents for spectrophotometry, are already
present in the stream. The stream then flows straight to the detector and the technique depends upon the
controlled and reproducible dispersion of the sample as it passes through the reaction zone. Thus the
reaction does not necessarily need to develop to completion,