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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: fahmadi@kums.ac.ir

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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