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

minimize detector saturation by Fe signal due to the
naturally strong presence of Fe in the soil samples. The
PCA calculations were made with the help of Pirouette
4.0, rev. 2 software (Infometrix, Bothell, USA). These
preliminary analyses were done for all collected samples
to determine the general tendency of the data
before applying quantitative measurements. Some of
these samples were randomly selected and analyzed
in duplicate.
2.4. re a g e n t s a n d s o l u t Io n s
All reagents were of analytical grade, and sub-boiling
distilled acids (HCl and HNO 3 ) were used to prepare
the aqua regia extraction solution. Deionized water
was used in all solutions. All flasks and glassware
were cleaned and then decontaminated by immersion
in HNO 3 (10 % v/v) overnight. Standard stock solutions
of the metals (1000 mg L -1 ) were used to prepare the
analytical curves.
2.5. aq u a r e g Ia extractIon f o r cd, cr, cu, fe, mn, nI,
pb a n d zn determInatIon
Extraction with aqua regia was applied instead of total
soil digestion to permit access to the maximum content
of elements actually available to plants [26]. The extraction
was performed according to the German norm,
DIN 38414-S7 [27]. Three samples of 300 mg dried soil
were weighed into digestor block tubes, and 3 mL of
aqua regia were added. The mixture was allowed to
stand at room temperature overnight. The tubes were
then transferred to a digestor block (Solab, Piracicaba,
Brazil), and a warming step of 3 hours at 120 ºC was
performed using a reflux apparatus. Finally, the extracts
were transferred to previously decontaminated tubes,
and the final volumes were made up to 10 mL.
2.6. applIcatIon o f t h e modIfIed bcr (co m m u n It y bu-
r e a u o f reference) sequentIal extractIon p r o c e d u r e
f o r cu, mn a n d zn determInatIon
The sequential extraction procedure (SEP) was performed
as described by Scancar et al.. [28]. Three samples
of 500 mg dried soil were weighed into centrifuge
tubes, and 10 mL of CH 3 COOH (0.11 mol L -1 , pH = 2.8)
was added. The mixture was shaken in a horizontal
shaker end-over-end (Barnstead, Dubuque, IA, USA)
overnight. The second step of the sequential extraction
was performed with 10 mL of NH2OH . HCI (0.5 mol L -1 ,
pH = 1.5), and the third step was performed with 1 mL
of H 2 O 2 (30% v/v, pH = 2) and 9 mL of CH 3 COONH 4
(1 mol L -1 , pH=2). Steps 1, 2 and 3 were achieved under
agitation using the same conditions over 10 h. In
the last step, an aqua regia extraction was performed
on the residues of the third stage. After each step, the
supernatant was separated by centrifugation and transferred
to decontaminated flasks.
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Guerra et al.
2.7. dtpa extractIon f o r cd a n d pb determInatIon
The DTPA extraction solution was prepared as 0.005
mol L -1 diethylenetriamine-pentaacetic acid (DTPA),
with 0.01 mol L -1 CaCl 2 and buffered at pH 7.30 with
triethanolamine [29]. Three samples of 1 g of each soil
sample were extracted with 5 mL DTPA solution in a
horizontal shaker end-over-end (Barnstead) over 2 h.
The supernatant was separated by centrifugation, and
it was transferred to previously decontaminated flasks.
2.8. spectrometrIc determInatIon o f metals b y aas
Aqua regia, DTPA and SEP extractions were performed
on the most superficial soil samples of the 15 sites, totaling
28 soil samples. Besides the samples collected
around the station for quantitative analyses, samples
further from Ferraz were also analyzed, but only up to
a depth of 20 cm. Thus, 14 samples of each site, both
around Ferraz and further from it, were quantitatively
analyzed.
Spectrometric determination of the elements Cd, Cr,
Cu, Mn, Ni, Pb and Zn in aqua regia, SEP and DTPA
extracts was performed using three different atomic
absorption-based methods: FAAS (Flame Atomic Absorption
Spectrometry), STAT-FAAS (Slotted Tube Atom
Trap - FAAS) and TS-FF-AAS (Thermospray Flame Furnace
Atomic Absorption Spectrometry). The more sensitive approaches,
STAT-FAAS and TS-FF-AAS, were used to reach
a high power of detection for Cd and Pb quantification.
All determinations were performed using a flame atomic
absorption spectrometer (AA240FS; Varian, Mulgrave,
Australia) equipped with a deuterium lamp for background
correction. Copper, Mn, Ni and Zn determination
in the aqua regia and SEP extracts was performed by
FAAS operation in sequential mode using an air-acetylene
flame at wavelengths of 324.7, 279.5, 232.0 and 213.9
nm, respectively. Chromium determination by FAAS
(357.9 nm) in the aqua regia extracts was performed
separately, because this analyte was determined using a
nitrous oxide-acetylene flame. Cadmium (228.8 nm) and
Pb (283.3 nm) determinations in the DTPA extracts were
performed using TS-FF-AAS. To enhance detectivity, a Ni
tube (10cm length) was placed above an air-acetylene
flame to increase the residence time of the free atoms
in the measurement zone. Using this approach sample
introduction was performed via a ceramic capillary (0.5
mm i.d.) achieving complete introduction [30, 31]. The
TS-FF-AAS arrangement was mounted as described by
Miranda and Pereira-Filho [32] and slightly modified by
Souza et al.. [33]. An important modification proposed
by Souza et al.. [33] was the use of a carrier solution
composed of a mixture of 0.5 % w/v EDTA and 0.1 %
v/v Triton X-100 to avoid capillary clogging.
The STAT-FAAS was applied for Cd and Pb determination
in the aqua regia extracts. The STAT-FAAS proposed
by Watling et al.. [34] consists of an atom trap
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