Determination of cadmium and lead in fruit juices by stripping
chronopotentiometry and comparison of two sample
pretreatment procedures
F. Lo Coco
a,*
, P. Monotti
b
, F. Cozzi
a
, G. Adami
c
a
Department of Economic Sciences, Environmental and Agroindustrial Section, University of Udine, Via Tomadini 30/A, 33100 Udine, Italy
b
Consultant of the Chemical Laboratory of Steroglass, Via Romano di Sopra 2/C, 06079 S. Martino in Campo, Perugia, Italy
c
Department of Chemical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
Received 3 April 2005; received in revised form 20 June 2005; accepted 27 June 2005
Abstract
A method for the determination of cadmium and lead in apple and pear fruit juices by stripping chronopotentiometry (SCP) is
described. The results obtained after previous digestion of the sample with concentrated sulphuric acid and dry-ashing (sample pre-
treatment procedure ‘‘A’’) and after a treatment with concentrated hydrochloric acid followed by heating in hot water bath (sample
pretreatment procedure ‘‘B’’) were compared. The sample pretreatment procedures were tested on real fruit juice samples and all data
confirmed that the efficiency of procedure A was greater than procedure B. In particular, for samples treated with procedure A, good
linearity was obtained in the range of examined concentration as is shown by the determination coefficients that were 0.998 (n = 4) for
cadmium and 0.996 (n = 4) for lead. Recoveries of 86–104% for cadmium and of 87–102% for lead were obtained from a sample spiked
at different levels. The accuracy was also evaluated by means of a matching reference sample of spiked skim milk powder (BCR 150—
Community Bureau of Reference) to prove the reliability of the method. The detection limits were 2.0 ng g
1
for cadmium and
4.8 ng g
1
for lead. The relative standard deviations (mean of nine determinations), evaluated on a real sample, were 7.8% and
6.5% respectively. The average content was in the range not detectable–3.0 ng g
1
for cadmium and 8.2–21.3 ng g
1
for lead.
2005 Elsevier Ltd. All rights reserved.
Keywords: Stripping chronopotentiometry; Cadmium; Lead; Fruit juices
1. Introduction
The absorption of heavy metals with the diet occurs
both in inorganic forms, through the corresponding
salts, and as constituents of organic molecules (proteins,
fats, carbohydrates and nucleic acids). Some heavy met-
als (i.e. zinc, copper, iron and selenium) are essential
nutrients for health, whereas others (i.e. mercury, cad-
mium, lead and tin) are toxic or have no known benefi-
cial effects. Even the heavy metals with beneficial effects
are dangerous if assumed in large quantities (
). Heavy metals may be present in
foods either naturally, or by the result of human activi-
ties (manuring practices, industrial emissions, exhausted
gases, etc.), or by contamination during industrial pro-
cesses, preservation and cooking (
). There
is a relationship between the long-term effects on health
and the presence of heavy metals in foods and so it is
essential, in the interest of public health, to hold them
at toxicologically acceptable levels (
).
Among heavy metals, cadmium and lead have a primary
importance owing to their metabolic inertness (
). Food and Agriculture Organization of the United
Nations/World Health Organization (FAO/WHO) fixed
weekly intake limits of 7 lg kg
1
of body weight for
0956-7135/$ - see front matter
2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodcont.2005.06.015
*
Corresponding author. Tel.: +39 0432 249337.
E-mail address:
(F. Lo Coco).
www.elsevier.com/locate/foodcont
Food Control 17 (2006) 966–970
adults for cadmium (
World Health Organization, 1992
)
and of 25 lg kg
1
for lead (
). The Regulation
466/2001 of European Committee of 8 March 2001 fixes
the maximum limits in foodstuffs for some contami-
nants. Maximum level for lead in fruit juices as defined
in article 1 of Council Directive 90/642/EEC is 50 ng g
1
(
). The Regulation is linked by
the Directive 2001/22/EC of European Committee of 8
March 2001 that lays down the sampling methods and
the methods of analysis for the official control of the
levels of Cd, Pb and other contaminants in foodstuffs
and beverage (
Even if the concentrations of metals in beverages
are normally low, a significant contribution to the in-
crease of the metal quantity assumed by man may
derive, owing to the potential great consumption of
beverages.
The determination of heavy metals in fruit juices and
beverages can be investigated by spectrometric tech-
niques, in both absorption and emission (
) and by electroanalytical
techniques (
Baldo, Bragato, & Daniele, 1997; Fomints-
eva, Zakharova, & Pikula, 1997; Mannino, 1982
). In
the last years stripping chronopotentiometry (SCP) has
been employed for trace and ultra-trace metal determi-
nations in food and beverage matrices (
). SCP is the current name for the technique for-
merly known as potentiometric stripping analysis
(PSA) (
). SCP is a versatile electro-
analytical technique first proposed by Bruckenstein and
Bixler (
) and further devel-
oped by other authors (
). SCP is a two-step technique: the first step,
named ‘‘preconcentration’’, is an electrolysis of the solu-
tion containing the metal ions which are amalgamated
on a mercury-coated glassy carbon electrode. The sec-
ond step, named ‘‘stripping’’, is a chemical reoxidation
of the deposited metals (
Estela, Tomas, Cladera, & Cer-
). Potential and time data are digitally derived
and E is plotted versus dt dE
1
in order to increase both
sensitivity and resolution of the analysis.
In this work SCP was used for the determination of
cadmium and lead in apple and pear fruit juices, after
previous digestion with sulphuric acid and dry-ashing
of the sample (pretreatment procedure ‘‘A’’) and after
a treatment with concentrated hydrochloric acid fol-
lowed by heating in hot water bath (sample pretreat-
ment procedure ‘‘B’’). The microwave digestion was
not tested due to limitations of sample size (low concen-
trations of cadmium and lead in fruit juice need a min-
imum sample size of 25 g). A simple filtration of sample
was excluded because various matrix effects complicate
the determination of metal (
). Procedure A requires a longer time of anal-
ysis, but results are the more accurately and precise, as
demonstrated by our data.
2. Materials and methods
2.1. Reagents and standards
All glassware was rinsed with 10% (v/v) ultra-pure
nitric acid (C. Erba, Milan, Italy). Ultra-pure water
obtained by the Pure lab RO and the Pure Lab UV
system (USF, Ransbach Baumbach, Germany), ultra-
pure and certified hydrochloric acid (C. Erba, Milan,
Italy), pure sulphuric acid for analysis, analytical grade
mercury(II) chloride (C. Erba, Milan, Italy), standard
solutions of cadmium and lead of 1000 mg L
1
(Panreac
Quimica, Barcelona, Spain) were used. A 2 M hydro-
chloric acid solution, a solution containing 0.5 ng lL
1
of cadmium and a solution containing 1.0 ng lL
1
of
lead were obtained by dilution with ultra-pure water.
A certified reference material BCR 150: ‘‘Trace elements
in a spiked skim milk powder’’ (Community Bureau of
Reference, Brussels, Belgium) was used for accuracy
evaluation.
2.2. Instrumentation
Determinations were carried out by a PSA ION
3
potentiometric stripping analyser (Steroglass, S. Mar-
tino in Campo, Perugia, Italy) connected to an IBM-
compatible personal computer. The analyser operated
under the control of the NEOTES software package
(Steroglass). A three-electrode system consisting of a
3 mm diameter glassy carbon working electrode, a plat-
inum wire counter electrode and a silver/silver chlo-
ride
ksaturated potassium chloride reference electrode
(Steroglass) was used for all measurements. The electro-
chemical cell consisted of a 40 mL vessel supplied with
an electrical spiral stirrer. The electrochemical proce-
dure was performed at the preconcentration step under
stirring conditions and at the stripping step under quies-
cent conditions.
Atomic
absorption
spectrometric
measurements
were carried out by a Spectra 110 spectrometer equipped
with a graphite furnace (Varian, Victoria, Australia).
Cadmium was determined using the following instru-
mental parameters: drying, 30 s at 125
C; ashing, 30 s
at 500
C; atomising, 10 s at 1900 C; wavelength
228.8 nm. Lead was determined using the following
instrument parameters: drying, 30 s at 125
C; ashing,
30 s at 500
C; atomising, 10 s at 2770 C; wavelength
283.3 nm.
2.3. Sample pretreatment procedures
2.3.1. Procedure A
Twenty five gram of each representative sample of
apple or pear fruit juices collected as reported in annex
I of Directive 2001/22/EC were exactly weighed in a
50 mL quartz crucible and dried at 120
C for approxi-
F. Lo Coco et al. / Food Control 17 (2006) 966–970
967
mately 12 h. (1–2 mL) Sulphuric acid was added to the
sample to wet all the mass. The sample was completely
carbonised on a hot plate, then transferred in a muffle
oven and the temperature was slowly increased up to
500
C. The sample was dry-ashed for 12 h until white
ashes were obtained. If carbon particles remained, the
crucible was cooled at room temperature, the residue
was moistened with a few drops of water and 0.5–
1.0 mL of concentrated nitric acid and the crucible was
kept again in a muffle oven for 30–60 min at 500
C.
The crucible was then cooled at room temperature and
the ashes were dissolved with 10 mL of 2 M hydrochlo-
ric acid, gently heating on a hot plate. Then the solution
was cooled at room temperature and quantitatively
transferred to a 50 mL volumetric flask. The volume
was filled up to the mark with 2 M hydrochloric acid.
The same treatment was used for the preparation of five
blanks and of the samples for graphite furnace atomic
absorption spectrometry determination (GFAAS). This
long sample treatment was necessary for a slow dry-
ashing to prevent volatilisation losses as described by
several authors, as reported by Crosby (
).
2.3.2. Procedure B
Twenty five gram of each representative sample col-
lected as reported in annex I of Directive 2001/22/EC
were exactly weighed and treated with 10 mL of concen-
trated HCl. The solution was heated in hot water bath
for 30 min, then cooled at room temperature. After cen-
trifugation for 5 min at 3000 rpm, the solution was
quantitatively transferred to a 50 mL volumetric flask
and the volume was filled up to the mark with 2 M
hydrochloric acid. The same treatment was used for
the preparation of five blanks.
2.4. Determination of cadmium and lead
A 10 mL volume of the solution obtained as de-
scribed in Section
was introduced into the electro-
chemical cell together with 10 mL of water and 1.0 mL
of 1000 mg L
1
of mercury(II) in 1 M hydrochloric acid.
Before analysis, the working electrode was coated with a
thin mercury film at
0.9 V against the reference elec-
trode by electrolysing for 1 min a mercury(II) chloride
solution of a concentration equal to that added to the
sample. For the subsequent determination, the electrol-
ysis time was 300 s at the potential of
0.9 V; the poten-
tial of the electrodes was monitored every 300 ls.
Quantitative analysis was carried out by the method of
standard additions by adding both 100 lL of a solution
containing 0.5 ng lL
1
of cadmium and 100 lL of a
solution containing 1.0 ng lL
1
of lead.
All the operational conditions (potentials, times, con-
centration of standards, etc.) were defined after several
optimisation studies on real fruit juice samples (
).
2.5. Statistical analysis
Paired StudentÕs t-test was used to determine whether
significant differences existed between results obtained
by SCP and GFAAS and for accuracy evaluation
(
3. Results and discussion
3.1. Comparison of the two sample pretreatment
procedures
First of all, the two sample pretreatment procedures
A and B were tested on ten real apple and pear fruit
juices samples and results confirm that the efficiency of
procedure A was greater than procedure B (see
).
3.2. Stripping curves, calibration curves
and performance of method
The detailed results obtained with the more efficient
pretreatment procedure (procedure A: previous diges-
tion of the sample with concentrated sulphuric acid
and dry-ashing) suggested for SCP determination of
Cd and Pb in fruit juice are reported.
In
the stripping curves for cadmium and lead
for a real sample of fruit juice are plotted. Cadmium
and lead were oxidised at approximately
0.61 V and
0.42 V, respectively, under the conditions described,
and peak areas (ms) relative to the sample and three
standard additions were measured.
These areas were plotted versus total amounts of cad-
mium and lead. A good linearity was obtained in the
range of the examined concentrations, as is shown by
both the equations of the lines Y = 5.9
· 10
7
X + 972
for cadmium (slope error: ±0.2
· 10
7
; intercept error:
±184) and Y = 6.1
· 10
7
X + 3.8
· 10
3
for lead (slope
Table 1
Concentrations of cadmium and lead (ng g
1
) determined in 10
different commercial samples of fruit juices (pear and apple) by
stripping chronopotentiometry (SCP) after digestion with sample
pretreatment procedure A and after treatment with sample pretreat-
ment procedure B
Sample
Cadmium
Lead
SCP-A
SCP-B
SCP-A
SCP-B
Pear 1
not detectable
not detectable
8.9
6.9
Pear 2
2.8
2.2
8.2
6.6
Pear 3
3.0
2.4
12.3
10.3
Pear 4
2.6
2.1
15.6
12.6
Pear 5
not detectable
not detectable
18.8
15.8
Apple 1
not detectable
not detectable
21.3
16.3
Apple 2
not detectable
not detectable
17.2
14.2
Apple 3
2.9
2.3
18.4
14.6
Apple 4
2.4
2.0
17.8
14.8
Apple 5
not detectable
not detectable
21.4
17.4
Values are the average of three determinations. Each determination
was repeated three times.
968
F. Lo Coco et al. / Food Control 17 (2006) 966–970
error: ±0.3
· 10
7
; intercept error: ±0.5
· 10
3
), where Y is
the integrated area (ms) and X is the analyte mass (mg).
The determination coefficients (R
2
) were 0.998 (n = 4;
t
exp
> t
crit
: 22.3 > 4.3) for cadmium and 0.996 (n = 4;
t
exp
> t
crit
: 15.8 > 4.3) for lead (
Committee, 1988; Miller & Miller, 2000
The accuracy for lead and cadmium was evaluated by
adding at the same time of the addition of 1–2 mL of
sulphuric acid appropriate volumes of a cadmium and
lead solution to a sample of fruit juice. Both the spiked
and the unspiked samples were analysed three times
(three independent treatments on the same sample) by
the proposed method and each analysis was repeated
three times. Results are reported in
; as may be
seen, recoveries of 86–104% for cadmium and 88–
102% for lead were obtained.
The accuracy was also evaluated analysing a match-
ing reference sample, BCR 150: ‘‘Trace elements in a
spiked skim milk powder’’ (Community Bureau of Ref-
erence, Brussels, Belgium) to prove reliability of the
method. The cadmium (21.9 ± 1.4 ng g
1
dry weight,
mean ± standard deviation) and lead (1178 ± 83 ng g
1
dry weight) results agreed with the cadmium (21.8 ±
1.4 ng g
1
dry weight) and lead (1000 ± 40 ng g
1
dry
weight) certified values (t
exp
< t
crit
; 0.1 < 4.3, n = 5,
P = 0.95 for cadmium and 3.7 < 4.3, n = 5, P = 0.95
for lead).
The repeatability of the method was evaluated by car-
rying out three independent treatments on the same
sample of fruit juice and each solution was analysed
three times. The obtained values were statistical ana-
lysed by employing the same software running all the
analytical steps. The average concentrations were
3.0 ng g
1
for cadmium, with a relative standard devia-
tion of 7.8% and 12.3 ng g
1
for lead, with a relative
standard deviation of 6.5%.
By using the working conditions stated above, the
detection limits were 2.0 ng g
1
for cadmium and
4.8 ng g
1
for lead by considering three times the stan-
dard deviation(s) of five blanks and by utilizing the
expression 3 s S
1
, where S is the slope of the linear
regression of calibration curve (
). Analytical data of the proposed method were
obtained considering performance criteria and quality
control laid down in annex 2 of the Directive 2001/22/
EC of European Committee of 8 March 2001 (
).
3.3. Data for fruit juice real samples and comparison
with GFAAS measurements
The method was applied to cadmium and lead deter-
minations in 10 different commercial samples of fruit
juices. The results were compared with those obtained
by GFAAS and are shown in
. A paired
StudentÕs t-test showed that the mean values (t
exp
< t
crit
;
0.3 < 2.2, for cadmium and 1.9 < 2.2 for lead) not signif-
icantly differ.
The average values found for the analysed samples
were in the range not detectable–3.0 ng g
1
for cadmium
and 8.2–21.3 ng g
1
for lead.
Table 2
Ranges of recoveries (ng g
1
) of cadmium and lead added to a sample
of fruit juice
Originally
present
Added
Found
(minimum–maximum)
Range (%)
(minimum–maximum)
Cadmium
3.0
1.5
3.9–4.5
86–100
3.0
3.0
5.2–6.2
87–103
3.0
4.5
6.6–7.8
88–104
Lead
12.3
5
15.3–17.5
89–101
12.3
10
19.5–22.3
88–100
12.3
15
24.4–28.0
90–102
Values are the average of three determinations. Each determination
was repeated three times.
Fig. 1. Stripping curves relative to cadmium and lead determination in a sample of fruit juice; (a) sample; (b-d) sample added with one, two and three
aliquots of both 100 lL of a solution containing 0.5 ng lL
1
of cadmium and 100 lL of a solution containing 1.0 ng lL
1
of lead.
F. Lo Coco et al. / Food Control 17 (2006) 966–970
969
4. Conclusions
The determination of cadmium and lead in apple and
pear fruit juices by stripping chronopotentiometric anal-
ysis (SCP) can be considered a sensitive and relatively
expensive analytical procedure for these beverage
samples. This method works also for orange or grape
fruit juices due to similar matrices.
Our data confirm that the sample pretreatment pro-
cedure with concentrated sulphuric acid followed by
dry-ashing is the more efficient and we suggested this
treatment for the SCP determination of Cd and Pb in
fruit juice. Sample pretreatment require a slow dry-
ashing to prevent volatilisation losses, but for several
reasons is preferred to other procedures. SCP can be
regarded as an alternative or complementary technique
with respect to GFAAS technique and can be considered
a good choice for small-medium laboratories. A short
time of analysis, also in comparison to GFAAS, is
required, the cost and size of the instrumentation are
comparatively low and require only a moderate space.
Furthermore, the extensive and flexible software sup-
porting the instrumentation makes it possible not only
to automate the analysis fully, but also to present the
results digitally and graphically, and to store them for
possible future processing and statistical treatment.
The time required for the sample pretreatment proce-
dure and electrode and matrix interferences are the
major drawbacks of the reported SCP method.
Acknowledgements
The authors thank Mr Victor Tosoratti, Department
of Chemical Sciences and Technologies, University of
Udine, for technical support.
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Table 3
Concentrations of cadmium and lead (ng g
1
) determined in 10
different commercial samples of fruit juices (pear and apple) by
stripping chronopotentiometry (SCP) and by graphite furnace atomic
absorption spectrometry (GFAAS)
Sample
Cadmium
Lead
SCP
GFAAS
SCP
GFAAS
Pear 1
not detectable
not detectable
8.9
9.4
Pear 2
2.8
3.1
8.2
8.0
Pear 3
3.0
2.8
12.3
10.4
Pear 4
2.6
3.0
15.6
15.1
Pear 5
not detectable
not detectable
18.8
17.9
Apple 1
not detectable
not detectable
21.3
20.6
Apple 2
not detectable
not detectable
17.2
16.2
Apple 3
2.9
2.5
18.4
19.4
Apple 4
2.4
2.1
17.8
16.8
Apple 5
not detectable
not detectable
21.4
20.8
Values are the average of three determinations. Each determination
was repeated three times.
970
F. Lo Coco et al. / Food Control 17 (2006) 966–970
Document Outline
- Determination of cadmium and lead in fruit juices by stripping chronopotentiometry and comparison of two sample pretreatment procedures
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