3. Results and discussion3.1. Respo

3. Results and discussion
3.1. Response characteristics
To study the response characteristics, the analysis conditions
were fixed according to details given in Section2. Firstly, the
SPE-ASVmethodwas investigated for thedetectionofmetallic
pollutants contained in seawater samples. For that, the first
step was to check the redox behaviour and the experimental
conditions for the electrochemical determination of the target
heavy metals[21].Fig. 1a shows a multi-element ASVat a scan
range between−1 and 0V for the determination of 100gL
−1
Zn, Cd, PbandCu ions depositedat the SPE andusing seawater
as supporting medium. As it can be seen, good peak shapes, at
−0.9,−0.55,−0.4 and−0.2V for Zn, Cd, Pb andCu, respectively
were obtained without overlapping. The obtained ASV curve
indicated the possibility of using the SPE for the simultaneous
determination of these metals in a single ASV run.
As expected, the stripping currents of Cd and Pb increased
following the addition of increasing volumes of the standards
(Fig. 1b).Fig. 1c and d shows single Pb and Cd voltammo-gramswithoxidationpeaks that approximatelyappear at−0.5
and−0.65V, respectively. Moreover, in these figures it can be
observed that the peak current variations between standards
(metal concentrations ranging from 10 to 90gL
−1
) are pro-portional, showing its feasibility for the calibration.
3.2. Matrix effect
Hydrochloric acid is commonly used as a supporting elec-trolyte for electrochemical analysis of heavy metals [21].
Moreover, in order to maintain the screen-printed Ag pseudo-reference electrode potential, a constant level of chloride ions
is needed. For this reason, first experiments were carried out
by addingHCl to give a final concentration of 0.1M at the mea-suring solutions prepared in doubled distilled water. Under
these conditions, it was observed a deterioration of the peak
shape over the time resulting in a poor repetitivity, which was
attributed to the damaging of the mercury film and thus these
measuring conditions were discarded.
Considering the high salinity of the seawater samples,
mainly due to NaCl, the use of a 0.6M NaCl solution at pH
8 (synthetic solution resembling seawater[9]), as a support-ing electrolyte was also checked. For this, several standard
solutions of Pb and Cd prepared in this medium were anal-ysed.Fig. 2a shows the calibrationcurves for Pbobtainedusing
either synthetic (0.6M NaCl solution at pH 8) or the Cantabric
Sea seawater as supporting electrolytes.Fig. 2b shows simi-lar calibration curves obtained for Cd. As it can be observed,
good linear responses were obtained for both metals in both
mediums. However, a clear matrix effect can be observed,
since responses obtained when using standards prepared in
synthetic seawater were more sensitive than in real seawater
medium (0.0153 and 0.0103A/gL
−1
for Pb and 0.0095 and
0.0062A/gL
−1
for Cd, respectively). In order to check the
differences between seawaters matrices from different ori-gins (Mediterranean, Cantabric and North Sea), the slopes of
linear regressioncurveswerecomparedusing thestatisticStu-dent’st-test. No significant differences between the slopes (5%
significance level) were observed.
Thecomplexityof seawatermatrices,i.e. diversityof anions
andcations andconductivityamongother physical andchem-ical parameters, let us tochoose the standardadditionmethod
for the analysis of the different samples.