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.
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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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