The flow system comprised two flow cells (path length
50 mm), which were nested in tandem aiming to improve sensitivity,
thus to attain this goal the measurement system depicted
in Fig. 2 was designed. The signal generated by the phototransistors
(Det1, Det2) were converted to potential difference (mV)
by the operational amplifiers OA1 and OA2 and sent to the operational amplifier OA3 that was configured as an analog signals
summing. Under this configuration, it would be expected that the
output signal (So) should present a linear relationship concerning
to adding the signals coming from the operational amplifiers
OA1 and OA2. In this sense, some experiments were carried out
to confirm this assumption using a set of nitrite standard solution
with concentration within the range of 50–400g L−1. The
corresponding linear equation, which were deduced from the
obtained results are shown in Table 2. Comparing the slops of
the analytical curves we can observe that an increase in signal
was achieved in the adding condition, nevertheless it was 30%
lower than the sum of the two slops. The sensitivity improvement
was less than the expected value, nevertheless standard
deviation and linearity were similar to that achieved using a single
channel, and therefore this arrangement could be used to
improve sensitivity