The reason for operating a FIA system in
the stopped-flow mode is to increase the measurement
sensitivity by increasing the residence
time, and therefore the conversion of the measured
species. The rate of the resazurin-sulfide
reaction under investigation was relatively
slow. It was impossible to detect the catalytic
effect of trace levels of lead with continuousflow
FlA. Stopping the flow to ensure sufficient
time for the reaction was necessary. Fig. 7
shows the relationship between stopping time
and sensitivity. It was found that with longer
stopping times, the sensitivity increases and the
linear range decreases. For lead concentrations
higher than 100ppb, stopping for 15s provided
sufficient sensitivity, with straight-line
correlations between 1 and 1000ppb tested.
For the samples with lead concentrations below
100pbb, which is the case for drinking
water, longer stopping times were needed. A
s I
I
4~
i
2 r
k
0 ~
0
1 I
4 6
Sodium Hydroxide (M)
10
• 200 ppb Lead e Blank
Fig. 6. Optimization of the pH.
M. Li, G.E. Pacey / Talanta 42 (1995) 1857-1865 1863
12
W t~ °i
10
8
[]
6
4
2 '~
0
0
[]
[]
I
200
[] •
[]
400 600 800 1000
Lead Concentration (ppb)
120
• 20see e30sec. •40sec. []50sec.
Fig. 7. Optimization of the time.
stopping time of 70 s was selected since it provided
high sensitivity and a good linear range.
Since the determination of 1 ppb of lead was
the objective of this work, a very small amount
of sample dispersion is desired. However the
peak must reach a wide enough plateau to
allow for a reproducible stopping point. In
order to have the smallest dispersion possible,
the following experiment was conducted. With
the manifold in fig. 1, the ideal dispersion
coefficinet would be 3.10, that is, when the
sample is used as the carrier stream and there is
only dilution from mixing with reagents. A
small sample volume would produce too large
a dispersion coefficient, e.g. a 9.80 pl sample
injection volume gave a dispersion coefficient
as large as 5.14 and a large sample volume
would reduce the sample throughput. Small
sample volumes produce narrow peaks. On
increasing the injection volume to 100 lal, the
dispersion coefficient was reduced to 3.72 and
the peak had a 26.4 s plateau. The peak started
at 10.2 s after injection, after 22 s the peak
platform formed and lasted for 26.4 s, and the
peak returned to baseline in 30 s. Therefore, a
volume of 100 lal was selected and run-stop
modes for pump I and pump II, and injection
and filling times for the injector were established.
The sample throughput of the system
was 30 injections per hour.
The computer system employed in this work
was able to provide information concerning the
peak height, area, and its slope. Both height
and area were proportional to the lead concentration,
but the linear range and precision were
different. The peak height was a good representation
of the lead concentration for a short
stopping time (such as 7 s) for concentrations
higher than 100 ppb. With a longer stopping
time, the linear range was limited and the
precision was poor. Area measurement gave
better precision and the calibration lines were
linear up to 1000ppb for all the stopping
times. Therefore, area measurements were used
for this method.