adsorptive cathodic stripping voltammetry determination of ultra trace of lead in different real...
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Adsorptive Cathodic Stripping VoltammetryDetermination of Ultra Trace of Lead inDifferent Real SamplesLaleh Hosseinzadeh a , Shahriar Abassi a & Farhad Ahmadi ba Department of Chemistry, Ilam University, Ilam, Iranb Faculty of Pharmacy, Pharmaceutical Chemistry Department, KermanshahUniversity of Medical Science, Kermanshah, IranVersion of record first published: 06 Nov 2007.
To cite this article: Laleh Hosseinzadeh , Shahriar Abassi & Farhad Ahmadi (2007): Adsorptive CathodicStripping Voltammetry Determination of Ultra Trace of Lead in Different Real Samples, Analytical Letters,40:14, 2693-2707
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ELECTROCHEMISTRY
Adsorptive Cathodic StrippingVoltammetry Determination
of Ultra Trace of Lead in DifferentReal Samples
Laleh Hosseinzadeh and Shahriar Abassi
Department of Chemistry, Ilam University, Ilam, Iran
Farhad Ahmadi
Faculty of Pharmacy, Pharmaceutical Chemistry Department,
Kermanshah University of Medical Science, Kermanshah, Iran
Abstract: In the present work, an adsorptive cathodic stripping voltammetric method
using a hanging mercury drop electrode (HMDE) was described in order to determine
the ultra trace of lead ions with carbidopa in different real samples. The method is based
on accumulation of lead metal ion on mercury electrode using carbidopa as a suitable
complexing agent. The potential was scanned to the negative direction and the differ-
ential pulse stripping voltammograms were recorded. The instrumental and chemical
parameters were optimized. The optimized conditions were obtained in pH of 8.4,
carbidopa amount of 1.0 � 1026 M, accumulation potential of 0. 0 V, accumulation
time of 100 s, scan rate of 100 mV/s and pulse height of 50 mV. The relationship
between the peak current versus concentration was linear over the range of
2.4 � 10210–4.8 � 1027 M. The limits of detection were 5.8 � 10211 M and the
relative standard deviation at 4.8 � 10210, 2.1 � 1028, and 2.4 � 1027 M of lead
ion were obtained 3.2, 2.9, and 2.7%, respectively (n ¼ 7).
Keywords: Carbidopa, lead, adsorptive cathodic stripping voltammetry
Received 5 July 2007; accepted 10 July 2007
Address correspondence to Farhad Ahmadi, Faculty of Pharmacy, Pharmaceutical
Chemistry Department, Kermanshah University of Medical Science, Kermanshah,
Iran. E-mail: [email protected]
Analytical Letters, 40: 2693–2707, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0003-2719 print/1532-236X online
DOI: 10.1080/00032710701588069
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INTRODUCTION
Lead is one of the most toxic metal ions both in the environment and the human
body. In the environment, it arises from both natural and anthropogenic
sources. Generally, human exposure to lead comes from the following main
sources: using leaded gasoline; using lead-based paint; having lead pipes in
water supply systems; and exposure to industrial sources from processes
such as lead mining, smelting, and coal combustion. Additional sources of
lead include soldered seams in food cans, ceramic glazes, batteries, and
cosmetics (Silbergeld 1996). In humans exposure to lead can result in a
wide range of biological effects depending on the level and duration of
exposure. A wide variety of symptoms, including memory loss, irritability,
anemia, muscle paralysis, and mental retardation have been attributed to
lead poisoning (Chen and Huang 2002). Also, exposures can cause impair-
ments in intellectual functioning, kidney damage, infertility, miscarriage,
and hypertension (Silbergeld 1996). Lead is a special hazard for young
children. So, due to the toxicity of lead, selective signaling of lead is very
important for the detection and treatment of the toxic metal ion in various
chemical and biological systems (Xia et al. 2002; Khan and Alam 2005; Wu
et al. 2006; Daftsis and Zachariadis 2007; Barbosa et al. 2007; Ghaedi et al.
2007). The commonly used method for lead analysis is atomic spectrometry
including atomic absorption spectrometry (AAS) and atomic emission spec-
trometry. These methods have high sensitivity and excellent selectivity;
however, they have the intrinsic drawbacks, such as the requirement of com-
plicated and expensive instruments, high cost, not for in situ measurement,
etc. Despite these methods, electrochemical method is attractive for in situ
determining of lead since it exhibits high sensitivity, good selectivity, rapid
response, and low cost. What is more, the instrument employed in electroche-
mical method is relatively simple and conveniently miniaturized for in situ and
automated detection. One of important electrochemical procedure is adsorp-
tive cathodic differential pulse stripping voltammetry collection and has
been widely used for the determination of trace level of metal ions because
it possesses very high sensitivity (Vanden Berg 1991; Wu and Batley 1995).
A few ligands have been reported for the sensitive determination of lead
(Yokoi et al. 1995; Abollino et al. 1999; Mousavi et al. 2001; Zayats et al.
2002), for the method with 8-quinolinol (Vanden Berg 1986), the selectivity
is not good and excess copper shows a large interferent., for the method
with o-cresol phthaloxene (Wang et al. 1993), a large overlapping signal in
the presence of excess molybdenum has been reported for the method with
xylenol orange (Ensafi et al. 2006) and other method with Pyrogallol red
(PGR) (Ensafi et al. 2003) a large detection limit has been reported.
Carbidopa is administrated in association with levodopa in pharma-
ceutical preparation for Parkinson’s disease in order to achieve better thera-
peutic effect and lower toxicity. To the best of our knowledge, there has
been no report of the use of carbidopa as ligand for monitoring of metal
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ions with electrochemical methods. The main objective of the current work is
to develop a sensitive and convenient electrochemical method and introduc-
tion of derivative catecholamine (carbidopa, Fig. 1) as a suitable and
selective reagent for the determination of selective ultra trace of lead in
environmental, food, and biological samples.
EXPERIMENTAL
Reagents and Materials
All solvents and reagents, obtained from Merck (Darmstadt, Germany) or
Aldrich (Milwaukee, WI) were of the analytical grade and were used
without further purification. Three distilled water was used throughout.
Working lead ion solutions were prepared from 1000 mg l21 atomic absorp-
tion standard solutions after appropriate dilution with water. A 1.0 M of
NH3/NH4Cl buffer (pH 8.2) stock solution, prepared by mixing the appropri-
ate amounts of NH3 and HCl, was used to prepare solutions of the supporting
electrolyte. Stock standard solutions of carbidopa employed in the exper-
iments were prepared according to literature (Socorro et al. 2006) in
1.0 � 1023 M nitric acid and stored in dark and cooled place. More dilute
solutions were obtained from the stock solution just before the measurements.
Whole blood samples from adults were provided by the local public hospital in
a blood bag with anti-coagulant agent citrate phosphate dextrose adenine
solution (CPD-A) at 14% (w/v).
Instruments
Electrochemical experiments were carried out using a polarographic processor
model 746 VA (Metrohm) in combination with a polarographic stand model
747 VA (Metrohm). This electrode stand consists of a hanging mercury
drop electrode (HMDE) as working electrode, an Ag/AgCl as reference
electrode (3 m KCl) and a platinum wire as an auxiliary electrode. A
rotating Teflon rod stirred solution in the voltammetric cell.
Absorbance spectra were recorded using an hp spectrophotometer
(Agilent 8453) equipped with a thermostated bath (Huber polystat cc1). For
Figure 1. Structure formula of carbidopa.
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complexation studies, the temperature of the cell holder was maintained at
25 + 0.18C. The FT-IR measurement was carried out with a Shimadzu (IR
Prestige-21). A microwave oven (model WL 5001, 1000W, China)
equipped with Teflon high-pressure microwave acid-digestion vessel was
used for digestion of food and blood samples. A Chem Tech Analytical
model CTA-2000 atomic absorption spectrometer (Shimadzu) was used for
lead determination. The pH values of the solution were adjusted employing
a Metrohm model 780 using a combined glass electrode.
General Procedure
Ten milliliters of the supporting electrolyte solution (0.01 M of ammonium
buffer; pH ¼ 8.4), containing 1.0 � 1026 M of carbidopa was pipetted into
the voltammetric cell and solution was purged with pure nitrogen
(99.999%) for 3 min. An accumulation potential of 0.0 mV was applied to a
fresh mercury drop, while the solution was stirred, for an accumulation time
100 s. After the accumulation step finished, the stirring stopped after 10 s.
After optimization to be described below, the voltammogram was recorded
by applying a differential pulse negative-going scan terminating at 21.0 V
versus Ag/AgCl. All the results in this paper were obtained at room tempera-
ture, with a nitrogen atmosphere maintained over the solution surface.
Sample Preparations
In order to demonstrate its application in practical analysis, the procedure was
employed to detect lead ions in different samples that was prepared as follows:
Sample of Foods and Sugar Treatment
1.0 gr of each sample was accurately weighed and transferred into a 25 ml
Teflon high-pressure microwave acid-digestion vessel. A 4.5 ml of portion
of concentrated nitric acid and a 6.0 ml of 30% H2O2 were added. The
vessel were tightly sealed and then positioned in the carousel of microwave
oven. The system was operated at full power for 3.0 min. The digest was evap-
orated to dryness. The residue was dissolved with 10 ml of 5% nitric acid and
was filtered. After cooling, the solution was neutralized using NH3 1.0 M, and
transferred to a volumetric flask and diluted to 25 ml with water (Sancho et al.
1997; Qiong et al. 2006).
Blood Sample
Exactly 2.0 ml of the whole blood sample was transferred to each test tube.
The samples were spiked with the appropriate volume of standard solution
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(Table 3) of lead and left to equilibrate in a water bath for 1 h at 378. After
that, a small amount of water (1 ml) was used to quantitatively transfer the
spiked samples into Teflon high-pressure microwave acid-digestion vessels
for the digestion. A 2.0 ml of portion of concentrated nitric acid and a
4.0 ml of 30% H2O2 were added. After digestion of blood samples the
closed vessels were cooled, the digests were quantitatively transferred to
10 ml volumetric flasks, neutralized with NH3 1.0 M and diluted to volume
with distilled water.
Preparation of Lead-Carbidopa Complexe
To 5.0 ml of 0.01 mmol hot water solution (608C) of carbidopa (0.0226 g of
carbidopa), quietly, 1.0 ml hot water solution (608C) of 0.005 mmol of lead
ions (0.01656 g of Pb(NO3)2. 2H2O) were added. The resulting mixture
was stirred for 30 min. Then, the mixture was allowed to stand in dark
place at room temperature for 48 h for perfect loose of complete precipitation.
The obtained crystals were filtered off, washed several times with water then
pentane, collected, dried in vacuum over anhydrous calcium chloride and
stored in dark and in dry conditions (Mohamed et al. 2004).
RESULTS AND DISCUSSION
Spectrophotometric Study of Interaction of Lead with Carbidopa
In order to prove the formation of lead-carbidopa complex, we carried out the
uv-vis spectrometry experiments. The absorption spectra of carbidopa
(5.0 � 1025 M) in buffer solution of ammonium (pH ¼ 8.4, 0.01 M) are illus-
trated in Fig. 2a. Carbidopa produced the maximum adsorption peak in
Figure 2. UV–Vis spectra of (a) carbidopa and lead; and (b) mole ratio plots of lead-
carbidopa complex at 300 nm.
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285 nm and lead produced no peak in buffer solution. When we added lead
with different concentrations, an adsorption peak was produced in 300 nm
and absorbance increased with the increase in the concentration of lead,
while the maximum adsorption wavelength remained without change. The
stoichiometry of the resulting complex was determined from the absor-
bance-mole ratio plot at maximum wavelength of the corresponding
complex in studied system (Fig. 2b). A distinct inflection point at mole ratio
about 0.50 strongly supports the formation of ML2 complexes in between
lead-carbidopa.
Determination of Mode of Bonding by IR Spectroscopy
A substantial part of the knowledge concerning the mode of bonding in metal
complexes can be gained by applying IR spectroscopy. A verification of the
structure of the complexes of the organic ligands can be easily achieved
comparing the IR spectra of the free ligands with those of the complexes.
The IR spectra of carbidopa reveal a band at 3120 cm21, which attributed
to OH group (Mohamed et al. 2004). This band is hidden upon complexation
with lead. The indicate of coordination mode through the phenolic OH of
catechol moiety is difficult. This is also attributed to the presence of -OH
group of the carboxylate, the amino group, aliphatic OH group or even NH
group, which may appear in the range of 3500–3000 cm21. The involvement
of the phenolic catechol in complex formation is confirmed by the red shift of
the frequency of the n(C-O) from 1150 cm21 in the free carbidopa to the
Figure 3. Differential pulse voltammograms of: (a) the carbidopa (dote line), (b) the
lead ions in the absence of carbidopa (solid bold line) and (c) lead ions with carbidopa.
Conditions: pH ¼ 7.5; scan rate 20 mV/s; pulse height, 50 mV; accumulation time,
30 s; accumulation potential, 20.20 V; carbidopa concentration, 1.0 � 1026 M; lead
2.4 � 1027 M.
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30–50 cm21 in the complexe (Zaki and Mohamed 2000). The IR spectra of
both complexes show bands at 875 cm21 for lead-carbidopa comlex, charac-
teristic of water of coordination (Zaki et al. 1998).
Formation of Stripping Peak
The potential reduction of lead is 20.48 V, so the strong peak current in
20.50 V (vs. Ag/AgCl) could not be the reduction peak current of pure
lead (Fig. 4). Furthermore, there was no peak 20.50 V observed when
carbidopa existed without lead ions. Finally the peak(c) indicated that
lead-carbidopa complex produced the peak current in 20.50 V. The lead-
carbidopa complex has strong adsorption at a mercury electrode and
produces the reductive peak current. All the above indicate that lead and
carbidopa really produced a new complex, and this complex was electro
active and it can be produced by a reduction current in 20.50 V (vs. Ag/AgCl).
Optimization of Parameters
Effect of pH
The effect of pH of the supporting electrolyte was also studied by varying the
pH in the range 5.0–11. The peak current of lead as a function of pH is shown
in Fig. 4a. Maximum peak current was obtained at pH 8.4. Below pH 8.0, the
sensitivity is slightly lower. This could be because of only slight dissociation
of protons from the carbidopa. At pH higher than 9.0, the reduction peak
current decreased, that may be due to formation of hydroxyl species of lead.
Thus, pH 8.4 was chosen as optimal for measurement of lead.
Effect of Carbidopa Concentration
The effect of carbidopa concentration on the sensitivity of proposed method
was also studied. The obtained results (Fig. 4b) show that the cathodic
stripping peak current of lead-carbidopa complex increased with increasing
the carbidopa concentration up to 1.0 � 1026 M, and leveling off at higher
concentrations because the electrode is standard. An optimum carbidopa con-
centration of 1.0 � 1026 M was selected for further experiments.
Effect of Electrolysis Potential
The effect of electrolysis potential on the peak current of lead was examined
over the range þ100 to 2600 mV. As can be seen in Fig. 4c, in the potential
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range of þ100 to 0.0 mV the peak current of lead increased with changing the
accumulation potential. The cathodic stripping peak current decreased with
changing the accumulation potential from 0.0 to 2600 mV. An accumulation
potential of 0.0 mV was used for the optimized analytical procedure.
Figure 4. Optimization of parameters for determination of 2.4 � 1027 M of lead;
(a) effect of pH on the peak currents of lead (conditions are as in Fig. 3); (b) effect
of carbidopa concentration on the peak current of lead in pH ¼ 8.4; (c) influence of
accumulation potential on the peak current of lead in pH ¼ 8.4; and carbidopa concen-
tration of 1.0 � 1026 M; (d) effect of accumulation time on the peak current at opti-
mized condition; and (e) effect of scan rate on the sensitivity of proposed method.
1026 M of carbidopa.
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Effect of Preconcentration Time
The dependence of the cathodic peak current on the accumulation time was
also studied (Fig. 4d). The peak current was found to increase with increasing
the accumulation time, indicating an enhancement of lead uptake at the
electrode surface. Normally, the increase in the response current continued
until a maximum signal level (presumably corresponding to either saturation
or an equilibrium surface coverage) is attained. The obtained results indicate
that the attainment of a steady-state accumulation level of lead at the electrode
surface requires an exposure time of 100 s for 2.4 � 1027 M of lead. An
Table 1. Analytical data of calibration curves of lead in optimum conditions
Analyte
Linear
equation
(�1029 M) L.R (M) R2RSD (n ¼ 7)
2.4 � 1027 M LOD(M)
Lead Y ¼ 1.025X
þ 2.067
2.4 � 10210
–4.8 � 10270.999 3.2 5.8 � 10211 M
Figure 5. Differential pulse cathodic stripping voltammograms of lead on hanging
mercury drop electrode (HMDE) under optimum conditions and after 100 s accumu-
lation time. Lead concentration (�1029 M): (a) 0.0; (b) 0.24; (c) 24; (d) 120; (e)
170; (f) 190; (g) 240; (h) 280; (i) 380; (j) 430; (k) 480.
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accumulation time of 100 s was used for further studies. However, extending
the accumulation period can further increase the sensitivity.
Effect of Scan Rate on the Lead Stripping Peak
Based on the Randles–Sevcik (Equation (1)), (Bard and Faulkner 2003), the
stripping peak current (ip) increases with the square root of scan rate (v1/2).
iP ¼ ð2:69 � 105Þn3=2ACD1=2n1=2 ð1Þ
Therefore, an optimum scan rate providing a maximum ‘analyte signal’ is
desirable (Thompson et al. 2006). For this purpose, the stripping voltammo-
grams were recorded for 2.4 � 1027 M of lead and 1.0 � 1026 M of
carbidopa solution with various scan rates of electrode potential ranging
between 20 and 140 mV/s, while accumulation potential and accumulation
time were 0.0 V and 100 s, respectively. Based on the results obtained,
Table 2. Tolerance limit to foreign ions on the recovery of 2.4 � 1027 M of lead
Tolerance limit
Ions MForeign ion/MPb
Alkaline and alkaline earth metal 1500
Ag(I), Hg(II), Al(III), Mo(VI), Co(II), Ni(II), Rh(III), Fe(II) 500
Cu(II), Cr(III), Pd(II), Cd(II) 300
C2O422, SO3
22, I2 400
Fe(III), U(VI) 5
Tlþ 40
Bi(III), Ga(III) 100
Cr(VI) 8
Table 3. Determination of lead in blood samples (n ¼ 3)
Sample No.
Proposed method
Added (ng/ml) Found (�1029 M)
1 0.0 5.0 + 0.22
100 14.4.0 + 0.54
200 24.5 + 0.14
2 0.0 ,LOD
100 9.5 + 0.48
200 19.7 + 0.29
3 0.0 5.69 + 0.35
100 10.29 + 0.42
200 25.1 + 0.15
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the maximum value for the analyte signal appeared at a scan rate of
100 m Vs21 (Fig. 4e).
Linear Range, Detection Limit, and Reproducibility of the Method
To verify the linear relationship between peak current and analyt concen-
tration, a calibration graph was constructed under optimum conditions. The
result of this study was shown in Table 1. The differential pulse cathodic
stripping voltammogram at different concentrations of lead after 100 s
accumulation time are also shown in Fig. 5. The detection limits (three
Table 5. Determination of lead in water samples (n ¼ 3)
Pb2þ
Sample Added (�1029 M) Found (�1029 M)
Persian gulf 0.0 0.80 + 0.1
0.82 1.72 + 0.05
4.10 5.16 + 0.09
Paveh river 0.0 .LOD
0.82 0.90 + 0.16
4.10 4.01 + 0.12
Mineral Water 0.0 .LOD
0.82 0.73 + 0.01
4.10 4.10 + 0.09
Table 4. Determination of lead ions in food samples (n ¼ 3)
Sample
Lead
Added (ng/g) Found (ng/g)
Rice 0.0 0.0
50.0 49.6 + 0.28
100.0 101.0 + 0.27
Flour 0.0 0.0
50.0 51.5 + 0.29
100.0 99.8 + 0.37
Soya 0.0 6.2 + 0.26
50.0 60.6 + 0.18
100.0 104.1 + 0.21
Sugar 0.0 30.2 + 0.4
50.0 79.3 + 0.32
100.0 128.8 + 0.27
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Table 6. Comparison of proposed work with the some published works
LOD (M) Linear range (M)
Tolerance limit for Pb(II) Mion/MPb(II) Ref
Cr(VI) Cd(II) Cu(II) Zn(II) Fe(III) Ag(I) Co(II) Ni(II) Al(III)
This
work
5.8 � 10211 2.4 � 1021024.8 � 1027 5.0 300 300 100 5.0 500 500 500 500
5.0 � 1027 7.0 � 1027–5.6 � 1026 1 1 1 1 1 1 — 1 — 28
2.8 � 10210 4.8 � 10210–1.4 � 1027 — 250 100 400 100 1000 500 250 — 18
6.0 � 1029 2.0 � 1028–4.0 � 1026 — 50 10 1000 1000 — 1000 — 1000 29
8.0 � 1029 1.0 � 1027–2.5 � 1025 — 1 1000 1000 1000 1000 1000 1000 — 30
4.7 � 1029 2.4 � 1028–7.2 � 1027 20 1 1 — 1000 — 20 20 — 19
3.8 � 10210 1.4 � 1029–3.8 � 1027 10 10 10 10 — — 10 10 10 31 L.Hossein
zadeh
etal.
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times signal to noise) with a 100 s preconcentration time are also reported in
Table 1. For 7.0 successive determinations of 4.8 � 10210, 2.1 � 1028, and
2.4 � 1027 M of lead ion the standard deviation were obtained 3.2%, 2.9%,
and 2.7%, respectively.
Interference Studies
The most significant characteristic of the proposed method is its high selectiv-
ity. In fact, the selectivity achieved due to selective lead-carbidopa interaction
is further enhanced by the medium exchange following the accumulation of
the analyte. Coexisting metal ions can interfere with the accumulation of
lead if they compete for the binding sites on the electrode surface. Possible
interference by other metals with the cathodic stripping voltammetry of lead
was investigated by the addition of the interfering ion to a solution containing
2.4 � 1027 M of lead under the optimized conditions with 100 s preconcen-
tration time. The tolerance limit was defined as the concentration, which
gave an error of 5.0% or less in the determination of 2.4 � 1027 M of lead.
The results of this study are summarized in Table 2. From the result, it is
concluded that the method is free from interferences of foreign ions.
Among the ions tested only more than five-fold excess U(VI) and Fe(III)
and eight-fold excess of Cr(VI) were possible interfering ions on the determi-
nation of lead. The other cations such as zinc, cobalt, nickel, and mercury were
not interfering, are of particular significance considering their major interfer-
ence in most of previously reported systems.
Analytical Applications
The proposed method was successfully applied to determination of lead ions
in different samples, and their preparations were mentioned in the section:
Sample Preparations. The water samples such as seawater, river water, and
underground water were filtered and were subjected to UV digestion for 2 h
before determination. After adjusting the pH sample by ammonium buffer
to 8.4, it was stored in cooled place. The results are given in Tables 3–5.
CONCLUSION
The present study demonstrates that the adsorptive stripping analysis of lead in
the presence of carbidopa is an excellent and selective method for determi-
nation of trace amounts of this cation. To conclude, the above system offers
a practical potential for determination of lead, having special advantages of
high sensitivity, high selectivity, simplicity, and speed that have not been
present together in the previously reported system (Table 6).
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