2012_Redox Magnetohydrodynamics Enhancement of Stripping Voltammetry Of_puping Species

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Redox magnetohydrodynamics enhancement of stripping voltammetry of

lead(II), cadmium(II) and zinc(II) ions using 1,4-benzoquinone as an alternativepumping species

Ali A. Ensafi,a Z. Nazaria and I. Fritsch*b

Received 4th August 2011, Accepted 30th October 2011

DOI: 10.1039/c1an15700k

Differential pulse anodic stripping voltammetry (DPASV) coupled with redox-magnetohydrodynamics

(MHD) is used to enhance the anodic stripping voltammetry (ASV) response using a mercury thin film

glassy carbon electrode. The sensitivity increased to at least a factor of two (at 1.2 T) and is facilitated

by using 20.0 mmol L1 1,4-benzoquinone as an alternative pumping species to enhance ASV by redox-

MHD. The MHD force formed by the cross-product of ion flux with magnetic field induces solutionconvection during the deposition step, enhancing mass transport of the analytes to the electrode surface

and increasing their preconcentrated quantity in the mercury thin film. Therefore, larger ASV peaks

and improved sensitivities are obtained, compared with analyses performed without a magnet. The

influence of pH, 1,4-benzoquinone concentration, accumulation potential, and time are also

investigated. Detection limits of 0.05, 0.09 and 2.2 ng mL1 Cd(II), Pb(II) and Zn(II) were established

with an accumulation time of 65 s. The method is used for the analysis of Cd( II), Pb(II) and Zn(II) in

different water samples, certified reference materials, and saliva samples with satisfactory results.

Introduction

Plasma physics has been revolutionized by the discovery of

magnetohydrodynamics (MHD) waves and their application to

space physics and fusion research.1,2 Studies of MHD have also

been reported in the literature on the effects of external magnetic

fields on convection and therefore current in electrochemical

systems undergoing electrolytic reactions at electrode surfaces.39

MHD is a process in which the magnetic portion of the Lorentz

force can be used to propel a conductive fluid, including extensive

applications in pumping liquid or molten metals.1,10 MHD

pumps and mixers have been recently developed for propelling

redox containing electrolyte solutions in microsystems.1123 The

use of MHD in the field of microfluidics has been reviewed by

Qian and Bau.12 Redox species have been added to microfluidic

systems to avoid the problems of bubble formation and electrode

dissolution.2426

The MHD force, FB(N m3), as a body force, is produced by

the cross-product of ion flux j (C s1 m2) and magnetic flux

density B(tesla).5 The MHD effect refers to the convection

that results from MHD. In electrochemical systems where

a faradaic current is possible, the convection can affect the

concentration distribution near electrodes and thereby cause

changes in the limiting current under applied-potential condi-tions.2733 It is an enhanced mass transport-limited current in the

presence of the magnetic field that benefits the anodic stripping

voltammetry (ASV) measurements described herein.4,34,35 Parti-

cles and microbeads have been used to track redox-MHD

convective flows.26,3638 An overview of magnetoconvective

phenomena in general (which includes MHD), with a special

focus on redox systems and discussion of the equations that

govern the phenomena, is found in ref. 39.

ASV is an electrolytic method in which an electrode is held at

a reducing potential to deposit metal ions from solution at an

electrode. Often, the electrode is a mercury drop or a solid

conductor coated with a mercury film. In contrast to solid elec-

trodes, the surface of a mercury electrode is uniform, exhibitslarge hydrogen overpotential, and is reproducible, which make it

especially useful as the electrode material of choice for ASV.40 In

order to carry as much of the analyte metal ion(s) in the solution

as possible to the electrode for concentration into the amalgam,

convection is usually introduced, which can be accomplished by

stirring or moving the electrode through solution (e.g. rotating

disk electrode, hanging mercury drop electrode). After the ana-

lyte has accumulated for an adequate period of time, the

potential on the electrode is changed to reoxidize the analyte and

generate a current signal that is proportional to the concentra-

tion of the deposited metal.41,42 The mechanical convection

aDepartment of Chemistry, Isfahan University of Technology, Isfahan,8415683111, IranbDepartment of Chemistry and Biochemistry, University of Arkansas,Fayetteville, 72701, AR, USA. E-mail: [email protected]

This article is part of a web theme in Analyst and Analytical Methods onFuture Electroanalytical Developments, highlighting importantdevelopments and novel applications. Also in this theme is workpresented at the Eirelec 2011 meeting, dedicated to Professor MalcolmSmyth on the occasion of his 60th birthday.

424 | Analyst, 2012, 137, 424431 This journal is The Royal Society of Chemistry 2012

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during the preconcentration step may not be adequately efficient

or suit portable analysis or that of small-volume samples in

a micro-scale electrochemical cell. In contrast, MHD convection

does not need moving parts to be inserted into the electro-

chemical cell. The effectiveness of redox-MHD convection to

supply a predictable and uniform flow across the surface of the

electrode during the preconcentration step has been demon-

strated for the detection of the heavy metal ions Pb( II), Cd(II),

and Cu(II) in conjunction with the ASV technique.4,34,35 Largeranodic stripping peaks are observed (improving sensitivities and

detection limits), compared with analyses performed without

a magnet. Redox-MHD has also been used to enhance convec-

tion and control the structure of metal deposits in the related field

of metal electroplating.5,7,43

In the ASV studies that used redox-MHD in the preconcen-

tration step, the addition of high concentrations of Hg2+2 or

Fe3+4,34,35 to a sample solution was necessary in low magnetic

fields (


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with a pair of rare earth permanent magnets, separated by a 3.0cm gap, to generate fields of 0.5 T and 1.2 T. They were disk-

shaped NdFeB magnets having a thickness of 1.5 cm and

a diameter of 4.0 cm for the 0.5 T field and rectangular-shaped

NdFeB magnets having a thickness of 2.0 cm, a length of 4 cm,

and a width of 5 cm for the 1.2 T field. The working electrode

faced downward so that its surface normal was perpendicular to

the magnetic field direction to achieve a MHD force, inducing

a fluid flow that is mostly parallel to the electrode surface. The tip

of the reference electrode was positioned beside the working

electrode with the counter electrode residing on the other side of

the working electrode. The DPASV experiments with the

MFGCE were carried out in 10 mL of a solution containing 0.10

mol L1 KNO3 and 20.0 mmol L1 1,4-benzoquinone in universal

buffer at pH 2.0, spiked with Pb(II), Cd(II), and Zn(II) ions. The

deposition step lasted 65 s at 1.10 V. The stripping step was

initiated at 1.40 V and ended at 0.30 V. The instrumental

parameters used in the experiments were: a modulation time of

0.002 s or an interval time of 0.1 s, a modulation amplitude of 80

mV, and a step potential of 8 mV. This is equivalent to a scan rate

o f 8 0 m V s

1. Control experiments were performed in the absenceof magnets (0 T).

Results and discussion

Optimization of DPASV conditions using 1,4-benzoquinone and

redox-MHD

The electrochemical behavior of 1,4-benzoquinone at the surface

of the MFGCE was investigated using cyclic voltammetry

(Fig. 2A) in the absence of magnets. The cyclic voltammogram

exhibited a cathodic peak atEpc +0.09 V in the negative scan

corresponding to the reduction of 1,4-benzoquinone to hydro-

quinone. In the positive going potential sweep, an anodic peak atEpa +0.38 V appeared that belongs to the oxidation of

hydroquinone to 1,4-benzoquinone. The half-wave potential,

E1/2, was +0.26 V and the peak splitting,DEp, was +0.29 V. Thus,

1,4-benzoquinone could be used as a suitable reagent to enhance

deposition by redox MHD when the applied potential is more

negative than +0.26 V. As shown in Fig. 2B, DPASV of

1,4-benzoquinone removes much of the cathodic signal over

potentials more negative than 0.35 V. Therefore, rinsing away

1,4-benzoquinone or diluting it before the stripping step is not

necessary, in contrast with the ASV approach reported

previously.4,34,35

Table 1 Comparison of results obtained using the method described herein with those reported in other publications based on stripping voltammetry

Electrode Linear dynamic range Detection limit Ref.

Bismuth bulk electrode Zn(II) 10100 mg L1 93 ng L1 45Pb(II) 10100 mg L1 54 ng L1

Cd(II) 10100 mg L1 396 ng L1

Multiwall carbon nanotube electrode Zn(II) 58.4646.2mg L1 28.0mg L1 46Cd(II) 58.4646.2mg L1 8.4 mg L1

Pb(II) 58.4646.2mg L1 6.6 mg L1

Bismuth/poly(p-aminobenzene sulfonic acid) film electrode Cd(II) 1.00110.00 mg L

1 0.63mg L

1 47Zn(II) 1.00110.00 mg L1 0.62mg L1

Pb(II) 1.00130.00 mg L1 0.80mg L1

Disposable cartridge for preconcentration and carbon as an electrode Pb(II) 0.510 mg L1 0.15mg L1 48Screen-printed electrode Pb(II) 102000 mg L1 1.8 mg L1 49

Cd(II) 102000 mg L1 2.9 mg L1

Mercury film deposited on wax impregnated carbon paste electrode Pb(II) 1 105 to 5 109 mol L1 Not reported 50Cu(II) 1 105 to 5 109 mol L1 Not reportedCd(II) 1 105 to 5 109 mol L1 Not reported

Carbon paste electrode modified with a mercury film Cd(II) 0.010.16 mg dm2 0.25mg L1 51Pb(II) 0.020.45 mg dm2 0.07mg L1

Cu(II) 0.14 mg dm2 2.7 mg L1

Zn(II) 0.2810.36 mg dm2 0. 5 mg L1

Hanging mercury drop electrode Zn(II) 54.3482.2 mg kg1 0.69mg kg1 52Cd(II) 3.833.6 mg kg1 0.35mg kg1

Pb(II) 23.232.6 mg kg1 0.68mg kg1

Cu(II) 12.365.8 mg kg1 0.24mg kg1

Mercury thin film-glassy carbon electrode Cd(II) 0.0780.0 ng mL1 0.05 ng mL1 This workPb(II) 0.170.0 ng mL1 0.09 ng mL1

Zn(II) 4.5200.0 ng mL1 2.2 ng mL1

Fig. 1 The electrochemical cell with permanent magnets positioned in

a metal based U-shaped structure.


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TheE1/2value for 1,4-benzoquinone under these conditions is

quite positive of the reduction potentials for copper, lead, and

zinc ions. Thus, the 1,4-benzoquinone will not interfere by

reducing the metal ions in solution. Also, there will always bea significant cathodic current from the high concentration of 1,4-

benzoquinone when depositing all of these metals, contributing

to a large j, and therefore a large FBand sufficient convection.

The sensitivity for redox-MHD DPASV was optimized by

exploring the influence of chemical parameters and the accu-

mulation potential and time. A concentration of 40.0 ng mL1 Pb

(II) and 80.0 ng mL1 Cd(II) was used for the optimization

studies. Cd(II) and Pb(II) were chosen because they are heavy

metals of greater concern at low concentrations in the environ-

ment than Zn(II).

It is important to control pH in these studies because the 1,4-

benzoquinone redox potential depends on this parameter.

Therefore, the effects of different buffer types such as acetate (pH3.54.5) and universal (pH 2.04.5) buffers on the DPASV peak

currents for cadmium and lead were investigated. The results

showed that DPASV peak currents for cadmium and lead ions

decreased with increasing solution pH, presumably because the

free metal ions (hydrated) dominate in acidic media. In addition,

the loss of signal occurred more dramatically with pH in the

acetate buffer solution than in the universal buffer. Thus, the

universal buffer with a pH of 2.0 was selected as optimum

solution conditions.

The dependence of DPASV peak current on the scan rate

under the optimal solution conditions was investigated in the

range of 10200 mV s1 in the presence and absence of the

magnets (0.5 T). Fig. 3 shows that peak heights for cadmium (at

0.83 V) and lead (at 0.69 V) increased with increasing scan

rate until about 80 mV s1. At larger scan rates, the sensitivities

decreased. This behavior may be due to a decreased reversibility

of 1,4-benzoquinone at higher scan rates. A scan rate of 80 mV

s1 was therefore selected for quantitative studies. In addition,

the results confirm that the current amplitudes for lead and

cadmium are 1.7-fold with the magnets (0.5 T), compared tosignals in the absence of the magnets.

The influence of accumulation potential (Eacc) on DPASV

peak current for Cd(II) and Pb(II) was also investigated using the

optimized solution conditions and a scan rate of 80 mV s1 in the

presence and absence of the magnets (0.5 T). Fig. 4 shows that by

increasing the accumulation potential from 0.60 to 1.00 V,

the peak currents of Cd(II) (at 0.83 V) and Pb(II) (at 0.69

V) increased. Beyond 1.00 V, they began to level off, with the

exception of peak current for Cd(II) in the presence of magnets,

which continued to increase. Therefore, to provide the most

sensitive response for Cd(II), 1.10 V was selected as the opti-

mized accumulation potential for quantitation studies. (The

more negative deposition potential is also more desirable for Zn(II) because of its more negative standard electrode potential, as

demonstrated for real samples below.) Increasing the accumu-

lation potential to more negative values should increase the rate

of reduction of metal ions at the electrode surface. However,

more negative values did not affect the peak current, presumably

because of the mass transfer limit. In addition, at an accumula-

tion potential of 1.10 V, the signal in the presence of the

magnets is a factor of 1.7-times and 1.6-times that in the absence

of the magnets for Pb(II) and Cd(II), respectively.

Fig. 5 shows the influence of accumulation time (20 to 200 s)

on the DPASV currents for cadmium and lead ions in the pres-

ence and absence of the magnets (0.5 T). The peak currents

increased for both species up to 60 s in the presence of themagnets, providing no considerable additional improvement

Fig. 2 (A) Cyclic voltammetry response of 1,4-benzoquinone at the

surface of a MFGCE. Conditions: KNO3, 100.0 mmol L1; universal

buffer, pH 2.0; 1,4-benzoquinone, 20.0 mmol L1; 0.10 V s1. (B) DPASV

of the 1,4-benzoquinone under the same conditions, but with the added

parameters: deposition potential, 1.10 V; deposition time, 65 s; pulse

height, 100 mV. (These results were obtained in the absence of magnets.)

Fig. 3 Effect of the scan rate on the DPASV peak current for 40.0 ng

mL1 Pb(II) and 80.0 ng mL1 Cd(II). (a) and (c) are for Cd(II) in the

absence (open triangles) and presence (solid triangles) of magnets (0.5 T);

(b) and (d) are for Pb(II) in the absence (open circles) and presence (solid

circles) of magnets (0.5 T); conditions: KNO3, 100.0 mmol L1; 1,4-ben-

zoquinone, 20.0 mmol L1; universal buffer, pH 2.0; deposition potential,

1.10 V; deposition time, 65 s; pulse height, 100 mV.

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beyond that time. Thus, a deposition time of 65 s was selected for

subsequent experiments. At an accumulation time of 65 s the

signal in the presence of the magnets was a factor of 2.3-times

and 2.4-times that in the absence of the magnets for Pb(II)andCd

(II), respectively, confirming the enhancement due to MHD.

The influence of the 1,4-benzoquinone concentration from 0.1

to 40.0 mmol L1 on DPASV peak sensitivity was studied at 0.5

T. Fig. 6 shows the results for a solution containing 20.0 ng mL1

Pb(II) and 80.0 ng mL1 Cd(II). By increasing the 1,4-benzoqui-

none concentration from 0.1 to 20.0 mmol L1, the peak currents

of Cd(II) and Pb(II) increased. Higher concentrations of 1,4-

benzoquinone did not significantly affect the peak current.Increasing the concentration of the redox-MHD pumping species

increases the convection in the solution, hence increasing the rate

of arrival of the metal ions from the bulk of the solution to the

electrode surface. Thus, 20.0 mmol L1 of the redox-MHD

pumping species was selected for subsequent studies.

The effect of magnetic flux densities of 0.5 T and 1.2 T,

compared to 0 T, on DPASV using solutions containing 20.0 ng

mL1 Pb(II) and 150.0 ng mL1 Cd(II) was also studied. Fig. 7

shows an enhanced signal with increasing magnetic flux density

under the optimized conditions. This is due to an increase in

solution convection during the deposition step from the MHD

force. |FB| in a given location in solution is proportional to the

component of the B-field perpendicular to the ion flux there.

Leventis and Gao, who studied a similar orientation of a milli-

metre-sized working electrode relative to the magnet field

direction, but in a different redox solution, derived an empirical

formula with a dependence of the limiting current on |B|1/3.44 If

this dependence holds true during the deposition step, then thedependence of the DPASV peak current on |B| should be

similar. Our results under the optimized conditions suggest

a linear dependence (Fig. 7), but this is only based on two, non-

zero magnetic flux densities, and thus should not be over-

interpreted.

Fig. 4 Effect of deposition potential on the DPASV peak current for

40.0 ng mL1 Pb(II) and 80.0 ng mL1 Cd(II). (a) and (c) are for Cd(II) in

the absence (open triangles) and presence (solid triangles) of magnets (0.5

T); (b) and (d) are for Pb(II) in the absence (open circles) and presence

(solid circles) of magnets (0.5 T);conditions: KNO3, 100.0 mmol L1; 1,4-

benzoquinone, 20.0 mmol L1; universal buffer, pH 2.0; deposition time,

65 s; pulse height, 100 mV; scan rate, 80 mV s1.

Fig. 5 Effect of the deposition time on the DPASV peak current for 40.0

ng mL1 Pb(II) and 80.0 ng mL1 Cd(II). (a) and (c) are for Cd(II) in the

absence (open triangles) and presence (solid triangles) of magnets (0.5 T);

(b) and (d) are for Pb(II) in the absence (open circles) and presence (solid

circles) of magnets (0.5 T); conditions: KNO3, 100.0 mmol L1; 1,4-ben-

zoquinone, 20.0 mmol L1; universal buffer, pH 2.0; deposition potential,

1.10 V; pulse height, 100 mV; scan rate, 80 mV s1.

Fig. 6 Effect of different concentrations of 1,4-benzoquinone on redox-

MHD DPASV at 0.5 T of 20.0 ng mL1 Pb(II) (at 0.69 V) and 80.0 ng

mL1 Cd(II) (at 0.83 V).Conditions: KNO3, 100.0 mmol L1; universal

buffer, pH 2.0; deposition potential, 1.10 V; deposition time, 65 s; pulse

height, 100 mV; scan rate, 80 mV s1.

Fig. 7 Effect of different magnetic flux densities (0.0 T, 0.5 T, and 1.2 T)

on DPASV of solution containing 20.0 ng mL1 Pb(II) and 150.0 ng mL1

Cd(II). Conditions: KNO3, 100.0 mmol L1; 1,4-benzoquinone, 20.0 mmol

L1; universal buffer, pH 2.0; deposition potential, 1.10 V; deposition

time, 65 s; pulse height, 100 mV; scan rate, 80 mV s1.


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Figures of merit

Redox-MHD DPASV at 1.2 T was used for preparation of

calibration curves under the following optimum conditions: a pH

of 2.0 in universal buffer, an accumulation potential of1.10 V

for 65 s, and a scan rate of 80 mV s1 with a pulse amplitude of 80

mV. Calibration curves were constructed for 0.0780.0 ng mL1

Cd(II), 4.5200.0 ng mL1 Zn(II), and 0.170.0 ng mL1 Pb(II).

Typical DPASV responses of different concentrations of Cd(II

),Zn(II) and Pb(II) are shown in Fig. 8, confirming that there are no

interferences between those ions. Least squares analysis gave

equations of |DIp| 0.1655CCd(II) mA m L n g1 + 0.1474mA (R2

0.9947), |DIp| 0.0499CZn(II) mA mL ng1 + 0.03501 mA (R2

0.9929), and |DIp| 0.9198CPb(II) mA mL ng1 + 1.0820 mA

(R2 0.9982), for Cd(II), Zn(II), and Pb(II), respectively. The

results show that the system has a large linear dynamic range

with low detection limits for the ions. Detection limits (CLOD

3Sb/m, where Sb is the standard deviation for 6 replicate deter-

minations of the blank and m is the slope of the calibration curve)

were 0.05, 2.2 and 0.09 ng mL1 for Cd(II), Zn(II) and Pb(II),

respectively. The repeatability of the response of the method

using DPASV detection for Cd(II

), Zn(II

) and Pb(II

) was alsostudied. The relative standard deviations for determinations of

10.0 and 5.0 ng mL1 of Cd(II), Zn(II) and Pb(II) (n 6) were 2.8

and 1.7% for Cd(II),4.6 and 3.3%for Zn(II),and2.5 and 1.3%for

Pb(II), respectively.

Real sample analysis

To investigate the performance of the redox MHD-enhanced

DPASV to determine an unknown concentration of Cd(II),

Zn(II), and Pb(II), studies were carried out on the following

samples: NIST 1640 natural water standard, river water

obtained from Zayandeh-Roud river (Isfahan, Iran), lake water

from Maharloo Lake (Shiraz, Iran), synthetic sea water,human saliva, spring water, and tap water. Standard addition

calibration curves were used to obtain the unknown concen-

trations in the real samples. Amounts of 5 and 10 ng mL1 of

standard analyte were added. The slopes of the best fit lines

from the calibration curves were then used to convert the signal

to concentration for each sample containing added standard.

Then, recoveries were calculated from the difference of this

concentration value (with standard) minus the unknown

concentration (without standard), divided by the known

amount of standard added. The results are given in Table 2.

They show good agreement between added or present Cd(II),

Zn(II) and Pb(II) and the measured ones, as well as with

analysis performed by ICP, indicating the accuracy of themethod. Without further investigation, it is uncertain at this

time why the standard deviations for Zn(II) analysis are higher

than those for the other two metals. However, because this is

true for analysis by both redox MHD-enhanced DPASV and

ICP, the error is likely due to the chemistry of Zn(II) with the

sample matrix and sample preparation conditions rather than

to the detection method itself. Overall, the results confirm that

the real samples could be analysed at the ultratrace concen-

tration of the metal ions, with the inexpensive and simple

instrumentation and procedure, and having good accuracy and

precision.

Fig. 8 Redox-MHD DPASV responses of lead (0.68 V) ions,

cadmium ions (0.83V), and zincions(1.25V). (A) (a) 0.5 ngmL1;

(b) 2.0 ng mL1; (c) 10.0 ng mL1; (d) 20.0 ng mL1; (e) 28.0 ng mL1; (f)

33.0 ng mL1; (g) 38.0 ng mL1; (h) 50.0 ng mL1; and (i) 58.0 ngmL1 Pb

(II) at fixed concentrations of 40.0 ng mL1 Cd(II) and 25.0ng mL1 Zn(II).

(B): (a) 1.0 ng mL1; (b) 10.0 ng mL1; (c) 20.0 ng mL1; (d) 30.0 ng mL1;

(e) 40.0 ngmL1; (f) 50.0ng mL1; (g) 58.0ng mL1; (h) 62.0ng mL1; and

(i) 70.0 ng mL1 Cd(II) at fixed concentrations of 7.0 ng mL1 Pb(II) and

25.0 ng mL1 Zn(II). (C): (a) 6.0 ng mL1; (b) 10.0 ng mL1; (c) 25.0 ng

mL1; (d) 30.0 ng mL1; (e) 35.0 ng mL1; (f) 50.0 ng mL1; (g) 65.0 ng

mL1; (h) 70.0 ng mL1; and (i) 85.0 ng mL1 Zn(II) at fixed concentra-

tions of 40.0 ng mL1 Cd(II) and 7.0 ng mL1 Pb(II). Conditions: KNO3,

100.0 mmol L1; universal buffer, pH 2.0; deposition potential, 1.10 V;

deposition time, 65 s; pulse height, 100 mV; scan rate, 80 mV s 1; 1.2 T.


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Conclusions

Redox-MHD with a high concentration of the pumping species

1,4-benzoquinone was used for the first time in solutions con-

taining trace concentrations of analyte metals to produce

convection and therefore enhance deposition for a stripping

analysis. A significant advantage of 1,4-benzoquinone over

other pumping species used in past redox-MHD stripping

studies is that there is no need to remove it or dilute it before thedetection step, greatly simplifying the procedure. Redox-MHD

with 1,4-benzoquinone was shown here to enhance DPASV

signals of Cd(II), Pb(II) and Zn(II) ions for solutions containing

0.5 to 85 ng mL1 of those ions by inducing convection during

the preconcentration step. Detection limits as low as 0.05 ng

mL1 Cd(II) were measured with an accumulation time of only

65 s. This method is more sensitive and has better limits of

detection (from 0.05 to 2.2 ng mL1) than those found in

previously published papers4553 where DPASV was used to

determine these ions and a stirrer and stir bar provided solution

convection. In addition, the detection limit for Cd(II) here is

improved by a factor of30 from that reported previously for

redox-MHD, which had 1.5 times the magnetic flux density(1.77 T) for the same deposition period (60 s) using ASV and

a Fe3+ pumping fluid (at 5 times the concentration, 100 mmol

L1).34 More studies are needed to determine the reasons for the

dramatically enhanced detection limits over the earlier redox-

MHD studies. Possible explanations include the use of DPASV

instead of ASV, avoiding the rinsing/dilution step before strip-

ping, and the use of a pumping species that undergoes a 2-

electron transfer that is associated with high ion flux of H+.

Nevertheless, the results herein strongly encourage the use of

redox-MHD to achieve convection in portable trace metal

stripping analysis devices.

Acknowledgements

The authors are thankful to Isfahan University of Technology

Research Council (Iran) and the National Science Foundation

(Grant CHE-0719097, USA) for support of this work.

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Table 2 Determination of Cd(II), Zn(II) and Pb(II) in water samples

Sample

Added/ng mL1 Found/ng mL1 Recovery (%) Found by ICP methodb/ng mL1

Cd Zn Pb Cd Zn Pb Cd Zn Pb Cd Zn Pb

Tap water(Isfahan, Iran)

0.30(0.01) 36.4(2.2) 4.5(0.8) 0.3(0.1) 32.8(2.7) 4.9(1.0)5.0 5.0 5.0 5.8(0.4) 41.8(2.3) 10.2(1.5) 110 108 114 5.3(0.8) 41.8(3.5) 9.9(1.1)10.0 10.0 10.0 10.5(0.7) 46.8(2.9) 15.4(1.7) 102 104 109

Spring water 1.2(0.2) 186.4(3.5) 9.5(0.7) 1.0(0.2) 195.2(8.1) 10.1(0.9)

5.0 5.0 5.0 6.8(0.7) 191.3(3.3) 14.3(0.8) 112 98 96 10.0 10.0 10.0 11.6(1.1) 195.8(4.8) 20.2(1.0) 104 94 107

Human saliva 0.30(0.01) 47.5(1.1) 2.2(1.21) 5.0 5.0 5.0 4.8(0.3) 52.3(1.8) 7.7(0.58) 90 96 110 10.0 10.0 10.0 10.8(1.1) 57.6(1.6) 12.3(0.95) 105 101 101

Zayandeh-Roud River(Isfahan, Iran)

1.5(0.3) 143.0(2.8) 9.3(0.8) 1.3(0.4) 150.0(5.1) 9.0(1.0)5.0 5.0 5.0 6.4(0.5) 148.8(3.9) 14.8(1.0) 98 116 110 10.0 10.0 10.0 11.6(0.8) 152.8(4.1) 19.7(1.1) 101 98 104

Synthesized sea watera 6.6(0.5) 200.4(3.5) 15.5(1.1) 6.9(0.8) 211.1(8.3) 14.2(2.1)5.0 5.0 5.0 11.7(0.6) 206.8(3.4) 20.8(1.3) 102 128 106 10.0 10.0 10.0 16.7(1.2) 210.8(3.7) 26.1(1.2) 101 104 106

Lake Maharloo water(Shiraz, Iran)

9.5(0.6) 45.0(4.5) 45.8(1.3) 8.9(0.8) 45.6(5.5) 41.3(1.8)5.0 5.0 5.0 15.1(0.7) 50.8(5.2) 50.4(1.4) 112 116 92 10.0 10.0 10.0 20.3(1.4) 55.0(6.7) 55.3(1.3) 108 100 95

NIST 1640c 21.5(1.5) 54.7 (1.7) 26.7(1.0)

a Containing 5.5 and 16.0 ng mL1 Cd(II) and Pb(II), respectively. b Analysis was done after a 100-fold preconcentration of the sample by evaporation.

Standard deviations were obtained from triplicate analyses (N 3). c Cd(II), Zn(II) and Pb(II) was 22.79 0.96, 53.2 1.1 and 27.89 0.14 ng mL1,respectively.


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