3.2. Flow system
By considering the main interferences for nitrate determination
in waters, the flow system (Fig. 1) was designed to
separate nitrate from cationic and neutral species. In the injection
step, anions are retained in the ion-exchange column
and other species passed directly through the anionic column,
being removed by the carrier stream. In the alternative position
of the sliding bar injector, the column is inserted into
the eluent stream and the analyte is transported towards the
flow cell for measurement. It was observed with dye solutions
that an intense carryover between sample and previous
eluent zones occurred without employing the coil B. Thus,
nitrate retention was hindered by the presence of relatively
higher concentrations of perchlorate ions in the sample zone.
Carryover was minimized by inserting a 300-cm long coil
among the sample loop and the column and as showed in
Fig. 2, sensitivity and precision were improved in this condition.
On the other hand, results in Fig. 2 show that the
eluent concentration (initially 0.6 mol L-1) could be reduced
to 0.05 mol L-1, improving sensitivity, minimizing reagent consumption, and decreasing the acid concentration of the
residues. The effects of eluent and carrier flow-rates were
evaluated and the results are shown in Fig. 3. Higher signals
were obtained for lower flow-rates indicating that the kinetics
of nitrate retention and elution are slow. Flow rates of
1.7mLmin−1 and 4.3mLmin−1 were selected for the eluent
and carrier solutions, respectively, as a compromise between
sensitivity and sampling rate. It was further verified that the
precision was deteriorated by memory effects caused by the
incomplete removal of nitrate in the elution step. This drawback
was circumvented by increasing the resting time of the
injector in the elution position to 150 s.