Fig. 2. Analysis of artificial seawater standard solutions with the ammonia and nitrite levels indicated.
3. Results and discussion
3
.
1
. E6aluation of the FA system
’
s performance The potential chart recordings of the FA system in operation when samples of
different ammonia and nitrite concentrations are analyzed are presented in Fig. 2. In Fig. 2A the system’s response to the ammonia levels in standard solutions is
presented, while in Fig. 2B the response of the system to the nitrite content of the standard solutions is demonstrated. The range of detection presented is between 0.1
and 10 ppm of NH
3
and NO
2 −
. Furthermore, in Fig. 2A and B the response of the system to the primary species, NH
3
and NO
2 −
in the presence of NO
2 −
and NH
3
, respectively as interfering species is demonstrated. The measured concentration of
0.5 ppm NH
3
by the presented system in the presence of 5 ppm NO
2 −
in the sample was 0.57 ppm NH
3
, while the measured concentration of 5 ppm NH
3
in the presence of 0.5 ppm NO
2 −
was 5.11 ppm NH
3
. Similarly, the measured concentra- tion of 0.5 ppm NO
2 −
by the presented system in the presence of 5 ppm NH
3
in the sample was 0.56 ppm NO
2 −
, while the measured concentration of 5 ppm NO
2 −
in the presence of 0.5 ppm NH
3
was 4.95 ppm NO
2 −
. In Fig. 3, the linear range of detection for ammonia and nitrite is presented to be 0.5 – 10 ppm, while the
detection limit of the system based on a signal to noise ratio of 3 is 0.01 ppm NH
3
and 0.04 ppm NO
2 −
. The sensitivity of the system is 41 mVppm and 58 mVppm, respectively for the nitrite and the ammonia in artificial seawater samples. In Table
1, the analysis times of artificial seawater samples varying the ammonia and nitrite content is presented. The sample analysis time varies from 0.5 to 3.0 min depending
on the sample analyzed, resulting in a minimum sampling rate of 20 samplesh. The reproducibility of the system is shown in Table 2 where the time between two
measurements is 1.5 h. Finally, in Fig. 4 the signal stability of the system during the
monitoring of an artificial seawater sample containing 2.5 ppm NH
3
and 2.5 ppm NO
2 −
is presented. The signal drift of the system is 0.02 mVmin for the monitoring of both ammonia and nitrite substances for the time of 3 h indicating the
measurement of 2.5 9 0.12 ppmh NH
3
and 2.5 9 0.17 ppmh NO
2
.
3
.
2
. Analytical results In order to evaluate the performance of the FA system described, seawater
samples containing ammonia and nitrite were measured and the values were compared to the results obtained using the well-established colorimetric methods.
Initially, seawater sample with low ammonia and nitrite content was used to complete the recovery studies. The recovery studies were carried out by the
additions of appropriate amounts of the stock ammonia and nitrite solutions to the filtered seawater sample with low ammonia and nitrite content. The filtration of the
Fig. 3. Calibration curve of the FA System towards the NH
3
and the NO
2 −
content of the artificial seawater standard solutions.
Table 1 Analysis time of artificial seawater samples varying the concentration of ammonia and nitrite under a
single run NH
3
ppm NO
2
ppm T min
t min 3.0
0.10 0.10
1.8 2.9
0.50 0.50
3.3 1.00
1.00 3.2
3.1 2.8
2.50 2.50
3.3 2.3
5.00 5.00
2.3 1.5
10.0 1.8
10.0
Table 2 Signal reproducibility of the FA system
a
E
2
mV
b
E
3
mV
c
Average E mV
d
E
1
mV Sample type
Concentration ppm 86.0
83.2 NH
3
85.2 9 1.7 0.5
86.4 50.0
47.2 47.5
48.2 9 1.5 2.5
28.0 29.8
29.3 9 1.1 5.0
30.0 12.0
13.0 13.0
12.7 9 0.6 10
NO
2
42.0 0.5
45.7 43.9 9 1.9
44.0 72.0
71.8 72.0
71.9 9 0.1 2.5
84.0 5.0
82.0 83.5
83.2 9 1.0 10
97.5 95.0
96.2 96.2 9 1.3
a
The time between each measurement of each sample is 1.5 h.
b
E
2
is the potentiometric response of the system 1.5 h after E
1
.
c
E
3
is the potentiometric response of the system 3 h after E
1
.
d
Average E mV is the average of the individual potentiometric responses E
1
, E
2
and E
3
.
Fig. 4. Signal stability of the FA system during the analysis of an artificial seawater sample containing 2.5 ppm NH
3
and 2.5 ppm NO
2 −
. The signal drift of the FA System was measured to be 0.02 mVmin.
seawater samples was held for the ammonia and nitrite determination by the colorimetric technique and also for the comparison of the results obtained for the
filtered and non-filtered seawater samples using the FA system. The determination of the ammonia and nitrite content in filtered and non-filtered
spiked seawater samples was carried out in order to evaluate the performance of the FA system when non-filtered seawater samples are analyzed. The results demon-
strated in Table 3 for both the filtered and non-filtered spiked seawater samples indicate that the filtration step during the analysis of spiked seawater samples with
the FA system can be omitted.
264
E .A
. Moschou
et al
. Aquacultural
Engineering
22 2000
255 –
268
Table 3 Determination of ammonia in filtered and non-filtered seawater samples from rearing medium with FA system
Non-filtered sample concentration ppm R
Sample type Expected sample concentration
Filtered sample concentration ppm R
ppm NH
3
93 0.28
0.30 103
0.31 104
1.40 1.49
106 1.46
5.00 100
5.00 5.17
103 0.66
106 NO
2
100 0.62
0.62 104
1.54 1.60
104 1.60
5.35 104
5.00 97
5.16 R, the values obtained by the recovery studies calculated as — R = experimental concentrationexpected concentration100.
265 E
.A .
Moschou et
al .
Aquacultural
Engineering
22 2000
255 –
268
Table 4 Determination of ammonia and nitrite primary species in non-filtered spiked seawater samples in the presence of nitrite and ammonia interfering
species
a
Interfering concentration ppm Resulting concentration
Primary species Interfering species type
Primary concentration ppm type
Colorimetry FA system
C ppm R
C ppm R
1.14 104
1.00 91
1.10 NH
3
– –
1.10 100
109 1.20
1.00 NH
3
1.10 NO
2
97 1.50
1.63 109
NO
2
1.40 1.45
NH
3
91 1.90
1.85 97
NO
2
1.80 1.72
NH
3
5.10 102
– –
NH
3
– –
5.00 5.52
NH
3
108 5.10
NO
2
5.00 –
– 9.85
98 –
10.0 –
NH
3
– –
– 10.1
10.0 99
NO
2
10.0 –
NH
3
1.04 100
1.04 100
NO
2
1.04 –
– 1.05
101 1.09
105 NO
2
1.04 NH
3
1.00 1.24
118 95
0.960 NO
2
5.00 NH
3
1.00 NO
2
1.66 1.84
90 –
– 1.72
93 1.67
91 92
1.70 1.80
NO
2
1.84 NH
3
– 5.04
5.48 109
NH
3
5.00 –
NO
2
NO
2
10.0 10.04
100 –
– –
– 10.0
100 10.0
– 10.04
NH
3
NO
2
–
a
Comparison of the data obtained with FA system and colorimetric technique.
The evaluation of the presented FA system was performed by the comparison of the determined NH
3
and NO
2 −
levels in spiked seawater samples to that obtained with the standard colorimetric methods. The photometric indophenol-blue method
was used for the NH
3
determination while the Griess method was held for the measurement of the NO
2 −
content of the samples. The seawater samples containing 0.1 ppm NH
3
and 0.04 ppm NO
2
were filtered through a 0.4 mm filter in order to be used for the colorimetric technique. On the contrary, the samples analyzed with
the FA system were not filtered. The seawater samples were spiked with different amounts of ammonia andor nitrite and the analysis results are shown in Table 4.
The samples containing ammonia andor nitrite levels greater than 2 ppm could not be analyzed with the specific colorimetric methods due to the limitations in the
range of detection. From Fig. 4 it can be seen that the presented FA system can be used for the accurate monitoring of non-filtered seawater samples with low and
high ammonia and nitrite content.
4. Conclusions
