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multaneous determination of Zn, Cu, Pb and Cd in wine
samples, (ii) flame atomic absorption spectrometry (FAAS)
for individual determination of Zn and Cu content, and
(iii) Zeeman graphite furnace atomic absorption spectrometry
(ZGFAAS) for individual determination of Pb and Cd
content. ZGFAAS method was selected for determination
of Pb and Cd due to its sensitivity, accurate background
correction (Zeeman-effect background correction), and because
this method provides detection limit low enough to
measure the usually low content of Pb and Cd in wines.
Simultaneous determination of Zn, Cu, Pb and Cd content
in wines were carried out by flow-through stripping chronopotentiometry
(FTSCP) method. FTSCP is a two-step
analytical method. In the first step, the analyte species are
collected at a working electrode, which is set to a suitable
deposition potential or at a suitable deposition current. After
a short quiescence period, in the second step the deposit
was stripped by a constant current, whereas the change of
the potential of the working electrode during the dissolution
is registered. The potential-time dependence gives the
duration of the dissolution (chronopotentiometric stripping
time) which is according to the Faraday’s lows of electrolysis
proportional to the analyte concentration. The
original S-shape of potential-time dependence is converted
(transformed) to a peak-like signal containing the stripping
peaks of the deposited species. The compact flow system
operates fully automatic; it contains computer controlled
electromagnetic valves for switching either the carrier electrolyte,
sample or standard solutions to the flow. The electrolyte,
solutions or samples are driven through the system
and cell by a peristaltic pump. The hearth of the system is
the compact flow-through electrochemical cell with porous
flow-through working electrode.
Electrochemical FTSCP measurements were performed on
a fully automated computer controlled electrochemical analyser
EcaFlow Model 150 GLP (Istran Ltd., Bratislava,
Slovakia) equipment with two solenoid inert valves, a peristaltic
pump, 1 mm inner diameter PTFE tubing and microprocessor
controlled potentiostat/galvanostat. The compact
three-electrode flow-through electrochemical cell of
type 104 (Istran Ltd., Bratislava, Slovakia), equipment
with Pt auxiliary, Ag/AgCl reference and vitreous carbon
macro porous working electrode (E104L, Istran Ltd.) was
used.
The determination of Zn and Cu by FAAS was carried out
on Perkin-Elmer (PE) Model 1100 spectrophotometer, and
the experimental equipment used for Pb and Cd determination
by ZGFAAS was Zeeman atomic absorption spectrophotometer,
Perkin-Elmer (PE) Model 4100ZL with
Zeeman graphite furnace, pyrolytic graphite tubes (HGA)
with L’vov platforms and autosampler PE Model AS-71.
Instrumental conditions
Operation parameters for FTSCP were as follows: The
deposition of metal ions from wine samples on carbon porous
working electrode is performed by applying a suitable
deposition potential, i.e. in the potentiostatic mode at –
1800 mV. The deposit was stripped galvanostatically by
applying a stripping current of 200 μA, whereas stripping
chronopotentiogram is recorded and evaluated. Others parameters
were: starting potential I, –1800 mV; starting potential
II, –1400 mV; end potential, 100 mV; quiescence
time I 5 s; quiescence time II 30 s; sample volume, 1 ml;
flow rate, 6 ml/min.
The instrumental conditions for ZGFAAS measurements
were: resonance wavelength, 283.3 nm for Pb, and
228.8 nm for determination of Cd; slit with, 0.7 nm; signal
processing parameter, peak-area mode; injection volume,
20 μl. The temperature and gas programmes were as follows:
For determination of Pb: step 1: temperature 110 °C,
1 s ramp time, 50 s hold time, argon flow 250 ml/min; step
2 temperature 500 °C, 5 s ramp time, 30 s hold time, argon
flow 250 ml/min; step 3: temperature 1900 °C, 0 s ramp
time, 5 s hold time, argon flow stop; step 4: temperature
2400 °C, 1 s ramp time, 2 s hold time, argon flow 250 ml/
min. For determination of Cd the conditions were: step 1:
temperature 110 °C, 1 s ramp time, 50 s hold time, argon
flow 250 ml/min; step 2: temperature 400 °C, 30 s ramp
time, 30 s hold time, argon flow 250 ml/min; step 3: temperature
700 °C, 10 s ramp time, 20 s hold time, argon flow
250 ml/min; step 4: temperature 1400 °C, 0 s ramp time, 5 s
hold time, argon flow stop; step 5: temperature 2400 °C, 1 s
ramp time, 2 s hold time, argon flow 250 ml/min.
The instrumental conditions for flame atomic absorption
spectrometry (FAAS) were: resonance wavelength,
324.7 nm for Cu and 213.8 nm for Zn; slit width, 0.7 nm;
signal processing parameter, peak-area mode; flame type,
air/acetylene flame.
Reagents
Analytical-reagent grade chemicals were used in all experiments.
Al electrolyte and solutions were prepared with ultrapure
(double-distilled deionised) water obtained from a
Millipore Milli-Q purification system (specific resistance of
ultrapure water was greater or equal to 18 MΩ cm).
The carrier electrolyte for FTSCP measurements was solution
0.01 mol dm -3 CH 3 COOH + 0.01 mol dm -3
CH 3 COONa + 0.2 mol dm -3 NaCl. The electrolyte for
preparation (dilution) of wine samples and preparation of
standard solutions of heavy metals was solution 0.1 mol
dm -3 HCl. The standard solution of heavy metals for additions
to the wine sample, concentration of: 400 μg/l Zn;
10 μg/l Cd; 20 μg/l Pb and 40 μg/l Cu, was prepared from
certified reference materials (Istran Ltd.) by simple diluting
with 0.1 mol dm -3 HCl.
For the ZGFAAS and FAAS measurements a stock solutions
of 1000 mg/l of Pb, Cd, Cu and Zn, (Merck) were
used as the reference standard. Working standards of different
concentration range (depending on the element and
the method of determination) were prepared from the stock
(reference) standard by dilution with 0.15 % nitric acid solution
(Suprapur, Merck).
» November/Dezember 2008 | DLR