Brazilian Journal of Analytical Chemistry - BRJAC - Brazilian Journal ...

ChemometriC a n d a n a l y t iC a l strategies f o r t h e s t u d y o f s o i l s f r o m m a r i t i m e an t a rC t iC a
use of an autosampler that spends at least 10 s for each
sample. The STAT-FAAS became an interesting strategy
for fast sequential determination of Cd and Pb, because
it combines high analytical throughput (the same
observed by FS-FAAS) and good LODs (limits of detection,
defined by IUPAC as three times the standard deviation
of the blank (n=10) divided by the slope of the
calibration curve) with intermediate values between
the conventional FAAS and TS-FF-AAS. The poorer performance
of TS-FF-AAS in the fast-sequential mode, in
relation to analytical throughput, for Cd and Pb determination,
is due to the signal stabilization time (60 s for
each sample) before starting the measurements.
3.4. chemometrIc treatment o f s o Il c h e m Ic a l a n a l y s e s:
metal c o n c e n t r a t Io n s a n d s o Il c h a r a c t e r I s t I c s
For the HCA, the data were organized in a matrix
with 28 rows (number of samples) and 29 columns
(investigated variables, expressed as the average of
three replicates). The variables investigated were the
pseudototal concentrations of Cd, Cr, Cu, Mn, Ni, Pb
and Zn; the bioavailable concentrations of Cd and Pb;
the percentage at each step of the SEP and the sum
of the three initial fractions related to the labile portion
of Cu, Mn and Zn; and finally, soil characteristics
(pH, TOC and granulometry, including the percentages
of sand, silt and clay), totaling 29 variables. The data
were autoscaled and, for dendrogram calculation, the
Euclidean distance and incremental linkage methods
were used.
The HCA results are shown in Fig. 3. The dendrogram
exposed remarkable differences between soil samples
around the station (especially those impacted by the oil
tanks) and the samples further from Ferraz, as seen by
the formation of two groups of samples at each sampling
region. The control points around Ferraz Station,
points 1 and 6 (see fig. 1), had chemical characteristics
similar to those points further from Ferraz, as seen from
their position in the dendrogram (Fig. 3). These results
confirm the preliminary analysis performed with the
combination of SR-XRF and PCA, which identified differences
between the samples of the 2 sites.
For PCA application, some of the variables were discarded
to facilitate the analysis interpretation. In the
same fashion as for HCA, fractions 1, 2 and 3 of SEP for
each element (Cu, Mn and Zn) were combined. All data
were then organized in a matrix with 28 rows (number
of samples) and 17 columns (related to the variables).
This matrix was autoscaled to give the same importance
to all variables, and the PCA was applied. The scores
and loadings of the PCA plot are shown in Fig. 4a and
4b. A large separation was observed between the sampling
sites (Fig. 4a, scores plot) and the variables related
to this behavior, as seen in Fig. 4b. Points directly impacted
by the oil tanks had the highest concentrations
of Cr, Ni and Pb.
Table III confirms that the points directly affected by
Ferraz oil tanks had higher concentrations of Cr, Ni and
fi g u rE 3. hCa (d E n d ro g ra m) w i t h all d a t a o B t a i nE d f ro m thE quantitativE d E t E r m i n a t i o n o f m E t a l s a n d s o i l CharaCtEristiCs.
154 Br J Anal Chem