Cent. Eur. J. Chem. • 7(1) • 2009 • 148-154
DOI: 10.2478/s11532-008-0102-8
Central European Journal of Chemistry
Development of efficient method for preconcentration
and determination of copper, nickel, zinc and iron ions
in environmental samples by combination of cloud point
extraction and flame atomic absorption spectrometry
Research Article
Mehrorang Ghaedi*a, Ardeshir Shokrollahia, Khodabakhsh Niknamb,
Ebrahim Niknama, Mustafa Soylakc
a
b
Chemistry Department, Yasouj University,
75914-353 Yasouj Iran
Chemistry Department, Bushehr University,
Bushehr Iran
Chemistry Department, University of Erciyes,
38039 Kayseri, Turkey
c
Received 21 August 2008; Accepted 22 September 2008
Abstract: A cloud point extraction procedure for the preconcentration of copper, nickel, iron and zinc ions in various samples has been described.
Analyte ions in aqueous phase are complexed with 3-((indolin-3-yl)(phenyl)methyl)indoline (IYPMI) and following centrifugation quantitatively extracted
to the aqueous phase rich in Triton X-114. The surfactant-rich phase was dissolved in 2.0 mol L-1 HNO3 in methanol prior to metal content determination
by flame atomic absorption spectrometry (FAAS). The effects of some parameters including, the concentrations of IYPMI, Triton X-114 and HNO3, bath
temperature, centrifuge rate and time were investigated on the recoveries of analyte ions. At optimum conditions, the detection limits of (3 SDb m-1) of
1.6, 2.8, 2.1 and 1.1 ng mL-1 for Cu2+, Fe3+, Ni2+ and Zn2+ along with preconcentration factors of 30 and enrichment factor of 48, 39, 34 and 52 for
Cu2+, Ni2+, Fe3+ and Zn2+ respectively, were obtained. The proposed cloud point extraction has been successfully applied for the determination of metal
ions in real samples with complicated matrix such as biological, soil and blood samples with high efficiency.
Keywords: 3-((indolin-3-yl)(phenyl)methyl)indoline (IYPMI) • Cloud Point Extraction • Triton X-114 • Flame Atomic Absorption Spectrometry
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1. Introduction
Monitoring trace element concentrations in biological
materials, particularly biological fluids, might be
considered a difficult analytical task [1,2], mostly due to
the complexity of the matrix and the low concentration
of these elements, which requires sensitive instrumental
techniques and often a preconcentration step [4,5].
The traditional liquid–liquid extraction and other
conventional separation methods are time-consuming
and labor-intensive approaches; require relatively large
amounts of high-purity and frequently toxic solvents,
which have to be disposed of properly. Cloud point
extraction (CPE) is based on the phase behavior
of surfactants in aqueous solutions, which exhibit
phase separation after an increase in temperature or
the addition of a salting-out agent [3,4]. Trace elements
can be extracted to the surfactant- rich phase usually
after formation of a hydrophobic complex with an
appropriate chelating agent [5]. This approach has been
successfully employed to extract and pre-concentrate
several trace elements from a variety of matrices
[6-14]. The technique is based on the property of most
surfactants in aqueous solutions to form micelles and
to separate into a surfactant-rich phase of a small
volume and a diluted aqueous phase when heated to
a temperature known as the cloud point temperature.
The small volume of the surfactant-rich phase obtained
with this methodology permits the design of extraction
schemes that are simpler, cheaper, more highly efficient
and more environmentally friendly than those extractions
that use organic solvents. Cloud point methodology
practical application of surfactants in analytical chemistry
has been used to separate and preconcentrate analyte
compounds as a step prior to their determination after
the formation of sparingly water-soluble complexes
[15-22].
* E-mail: m_ghaedi@mail.yu.ac.ir
148
M. Ghaedi, A. Shokrollahi, K. Niknam,
E. Niknam, M. Soylak
The efficiency of the CPE depends on the hydrophobicity
of the ligand and the complex, their apparent equilibrium
constants in the micellar medium and the kinetics of the
formation of the complex and their transference between
the phases. Triton X-114 was chosen as the non-ionic
surfactant because of its low cloud- point temperature
and high density of the surfactant rich phase as well as
its low cost, commercial availability and lower toxicity.
Since, our survey through literature did not show any
application of 3-((indolin-3-yl)(phenyl)methyl)indoline
(IYPMI) as complexing agent in CPE, therefore, we
decided to use it in the presence of Triton X-114 in CPE
as clean and simple procedure for preconcentration and
determination of Fe3+, Ni2+, Cu2+ and Zn2+ ions.
2.3. Test Procedure
2. Experimental Procedures
A typical cloud point experiment has been carried out
according to following procedure. An aliquot of 15 mL
of an aqueous solution containing 0.26 μg mL-1 of Cu2+,
Ni2+, Fe3+ and 0.13 μg mL-1 of Zn2+, 0.13 % (w/v) of Triton
X-114 and 0.66 mM of IYPMI in pH 5.5 was prepared.
The mixture was shaken for 1 min and left to stand for
20 min in a thermo-stated bath at 50°C. Separation
of the phases was achieved by centrifugation at
3500 rpm for 15 min. The whole system was cooled in
an ice bath for 15 min so that the surfactant-rich phase
would regain its viscosity and the bulk aqueous phase
was easily decanted. The remaining micellar phase was
dissolved in 0.5 mL of 2.0 M HNO3 in methanol and then
the analyte contents were determined by FAAS.
2.1. Reagents
2.4 Application
All solutions were prepared with deionized water.
Analytical-grade of acids, bases and other chemicals
used in this study were obtained from Merck, Darmstadt,
Germany. The calibration curve was established using
standard solutions prepared in 2 M HNO3 by dilution
from 1000 mg L-1 stock solutions (E. Merck, Darmstadt,
Germany). A 1% (w/v) Triton X-114 (E. Merck, Darmstadt,
Germany) was prepared by dissolving 1.0 g of Triton
X-114 in 100-mL volumetric flask with stirring. The
ligand, 3-((indolin-3-yl)(phenyl)methyl)indoline (IYPMI)
(scheme 1) was synthesized according to literature
[22].
The real samples were treated according to our previous
publication [23-27] as following.
The certified liver sample was also digested in the
manner described below. Triplicate samples (weight of
10 mg) were weighed in glass flasks and 5 – 10 mL
of concentrated HNO3 was added. The flasks were
capped and then digested at 60 – 70ºC for 1 – 2 h until
semi-dryness. The digests were treated with 5 mL nitric
acid and a few drops of H2O2, heated on a hot plate
at approximately 80°C until the color of the digestion
solution became bright yellow. Then the sample was
cooled, and diluted to a volume of 25 mL in volumetric
flasks with distilled water. Then the procedure given in
Section 2.3 was applied.
For digestion of orange juice samples, 5.0 g of the
orange juice samples were accurately weighed and
added with 5 mL of concentrated HNO3 in the digestion
vessel and a heating program was executed. After that,
1 mL of H2O2 (30% w/v) was added and heated for 2 h
in 650ºC. The residue was cooled and filtered and then
the procedure given in Section 2.3 was applied.
A 5 g of homogenized soil sample or 10 mL of blood
sample was weighed accurately in a 200-mL beaker.
In the digestion procedure, 10 mL concentrated HNO3
and 2 mL HClO4 70% were added and then heated
for 1 h. The content of beaker was filtered through
a Whatman No. 40 filter paper into a 250-mL calibrated
flask and its pH was adjusted to desired value and diluted
to mark with deionized water. Then the procedure given
in Section 2.3 was performed.
Lotus trees sample was purchased from Gachsaran,
Iran. Afterwards, they were taken in small mesh. A 40-g
sample was heated in silica crucible for 3 h on a hot plate
and the charred material was transferred to a furnace for
overnight heating at 650ºC. The residue was cooled and
Scheme 1. The structure of ligand
2.2. Instrumentation
A Shimadzu 680-AA-atomic absorption spectrometer
equipped with deuterium background correction
and copper, zinc, iron and nickel hollow-cathode
lamp as the radiation source using the resonance
wavelength of 324.8 nm for copper, 248.3 nm for
iron, 213.9 nm for zinc, 232.0 nm for nickel. The
instrumental parameters were adjusted according to the
manufacturer’s recommendations. A Hettich centrifuge
was used to accelerate the phase separation process.
A Metrohm 692 pH/ion meter furnished with a combined
glass-saturated calomel electrode was used for pH
measurements.
149
Development of efficient method for preconcentration and determination of
copper, nickel, zinc and iron ions in environmental samples by combination
of cloud point extraction and flame atomic absorption spectrometry
treated with 10.0 mL concentrated nitric acid and 3 mL
30% (w/v) H2O2 again kept in furnace for 2 h at the same
temperature so that no organic compound traces are left.
The final residue was treated with 3 mL concentrated
hydrochloric acid and 2 – 4 mL 70% perchloric acid
and evaporated to fumes, so that all the metals change
to respective ions. The solid residue was dissolved and
filtered and the pH was adjusted to 10.0 by addition
of KOH and diluted to 25 mL. Then, the procedure given
in Section 2.3 was performed.
3. Results and Discussion
3.2. Effect of IYPMI concentration
The concentration of IYPMI was evaluated over
the range 0.3 - 2.7 mM. The sensitivity of system as a
function of the IYPMI concentration is shown in Fig. 2.
At 0.66 mM of IYPMI concentration, the maximum
sensitivity of method was obtained. Therefore, 0.66 mM
of an IYPMI concentration was chosen for subsequent
experiments. At higher concentration of IYPMI, probably
due to formation of charge complexes, the sensitivity
of method will be reduced.
Since the IYMPI ligand possesses nitrogen donor
atoms and a conjugated p system, it should form stable
complexes with Cu2+, Ni2+, Fe3+, and Zn2+ ions. Thus,
we decided to examine its capability as a suitable
reagent for sensitive and extractive CPE of copper,
nickel, iron and zinc ions.
3.1. Effect of pH
The influence of pH on method sensitivity was carried
out by conducting a set of similar CPE experiments
by changing the pH of aqueous solution of sample
in the range of 2.0 to 9.0. Fig. 1 shows the effect of pH on
the sensitivity of the method. It was found that maximum
sensitivities were achieved at pH 5.5. Therefore,
a pH of 5.5 was selected for subsequent work.
In acidic medium, a weak complexation and uptake
occurs; which is explained by competition between
metal ions and hydrogen ions for the binding to
the ligand. On the other hand, by increasing the pH
the potential of active sites of ligand for metal
ion binding and consequently the metal ions uptake
were increased. The uptake capacities increased
with increasing pH, reaching plateau values at around
pH 5.5 (Fig. 1).
Figure 1. Effect of pH on cloud point extraction of analyte ions
150
Figure 2. Effect of ligand concentration on CPE of analyte ions
3.3. Effect of Triton X-114 concentration
The presence of active groups in the surfactant molecule
can be considered advantageous under certain
circumstances when electrostatic interactions are
favorable. The non-ionic surfactants, poly(-oxyethylene)7,5-(p-tert-octylphenyl)ether (Triton X-114) and
PONPE-7.5 are the most frequently used surfactant to
perform CPE experiments.
The preconcentration efficiency was evaluated using
Triton X-114 concentrations ranging from 0.04% to
0.27% (w/v). The results are demonstrated in Fig. 3.
The highest signal was obtained with 0.13% (w/v)
Triton X-114. By decreasing the surfactant concentration
to 0.04% (w/v) the recovery was reduced. The
method sensitivity also decreased for a higher Triton
X-114 concentration (0.27% w/v). This result might
be related to the presence of the high amount of
surfactant, resulting in an increase in the volume of the
surfactant-rich phase. In addition, the viscosity of
the surfactant-rich phase increases, leading to poor
sensitivity [28,29]. At lower Triton X-114 concentrations
(below 0.04% w/v), the preconcentration efficiency of
the complex was very low, probably due to assemblies
that were inadequate to quantitatively entrap the
hydrophobic complex [31,32]. A surfactant concentration
of 0.13% (w/v) for Triton X-114 was selected for all
subsequent experiments.
M. Ghaedi, A. Shokrollahi, K. Niknam,
E. Niknam, M. Soylak
to quantitatively transfer the rich phase from test tubes
to the graduated tubes and measuring the absorbance.
For larger volumes of acidified methanol, a dilution
was clearly the major reason for a gradual absorbance
reduction. A 0.5 mL of 2.0 M HNO3 in methanol was
therefore used throughout the remaining experiments.
3.6. Effect of temperature
Figure 3. Effect of surfactant concentration on CPE of analyte ions
3.4. Effect of NaCl concentration
The effects of NaCl as electrolyte were investigated
in the concentration range of 0.0 to 0.27 M (Fig. 4).
High sensitivity of method for Cu2+, Ni2+, Fe3+ and Zn2+
ions were obtained at 0.15 M NaCl concentration.
This effect might be explained by salting-out effect
or the additional surface charge when the NaCl
concentration is high, thus changing the molecular
architecture of the surfactant and consequently
the micelle formation process [31-35]. It is necessary
to emphasize that different blank solutions were also
evaluated and no significant signal was obtained.
In this way, 0.15 M of NaCl was used in all further
experiments.
The cloud point temperature of Triton X-114 is 23 - 26ºC
(approximately room temperature), which is preferred
for carrying out CPE procedure especially for thermally
labile compound and other analytical purposes. In order
to decrease the cloud point temperature of micellar solution
of Triton X-114, addition of NaCl solution is required.
It was desirable to employ the shortest incubation time
and the lowest possible equilibration temperature,
which cooperate for completion of the reaction
and efficient separation of phases. It was observed that
a temperature of 50ºC is adequate for analyte ions.
Higher temperatures lead to the decomposition of
IYPMI-ions complexes and the reduction of
analytical signal. At lower temperatures, the
separation of the two phases is not complete.
The dependence of method sensitivity upon
equilibration time was studied for a heating time
of 5 - 25 min. An equilibrium time of 20 min was chosen
as the optimal to achieve quantitative extraction.
3.7. Effect of centrifuge time and rate
Figure 4. Effect of salt concentration on CPE of analyte ions
For the best efficiency of the method, it is required to
preconcentrate trace amount of metal ions with high
sensitivity and in a short time. To test the influence
of centrifugation on the met hod sensitivity, CPE has
been carried out for a series of experiments, in which
a 15-mL aqueous sample containing 0.66 mM IYMPI,
0.26 μg mL-1 of Cu2+, Ni2+, Fe3+ and 0.13 μg mL-1 of
Zn2+, and 0.15 M NaCl has been heated to 50°C for
20 min and centrifuged at various rates and for different
lengths of time. The results indicate that at the optimized
reagent concentration, the best conditions for sample
centrifuging were 15 min at 3500 rpm.
3.5. Effect of methanol volume
3.8. Effect of foreign ions
Since the surfactant-rich phase obtained after the cloud
point preconcentration contains a high concentration
of Triton X-114 and, at the same time, the volume
obtained is rather small, 0.5 mL various concentration
of HNO3 in methanol was added to the surfactant-rich
phase after phase separation in order to facilitate
its introduction into the nebulizer of atomic absorption
spectrometer. There is an optimum volume of 0.5 mL
of 2.0 M HNO3 in methanol. Smaller volumes of methanol
were not tested because in this case it was not possible
In view of high selectivity attributed to flame atomic
absorption spectrometry, the only source of low
sample recovery must be the preconcentration step.
This problem may be attributed to the fact that cations
may react with ligand and anions may form stable
complex with metal ions and resulting in a decrease
in extraction efficiency. CPE procedures for Cu2+,
Ni2+, Fe3+ and Zn2+ ions determination in the high salt
content samples can be strongly affected by the matrix
constituents of the sample. To perform this study, 15 mL
151
Development of efficient method for preconcentration and determination of
copper, nickel, zinc and iron ions in environmental samples by combination
of cloud point extraction and flame atomic absorption spectrometry
of solution containing 0.26 μg mL-1 of Cu2+, Ni2+,
Fe3+ and 0.13 μg mL-1 of Zn2+, and interferents ion in
different interferents-to-analyte ratios in the presence
of 0.13% (w/v) Triton X-114, were subjected to the
complete procedure.Table 1 shows the tolerance limits
of the interferent’s ions (error < 5%). A relative error
of less than 5% was considered to be within the range
of experimental error. At the given level no significant
interference was observed in the determination of these
ions. Thus, the interference-free determination level of
present system indicates that high concentration of matrix
salts, have minimal effect on ions species relative to
matrix ions
The limit of quantification (LOQ) is the lowest level of
analyte that can be accurately and precisely measured.
The limits of quantification, defined as 10 times
the standard deviation (s) of the blank (n = 10), were
found to be 6.6, 11.8, 8.9 and 5.1 ng mL-1 for Cu2+, Fe3+,
Ni2+ and Zn2+. The precision of the proposed method was
evaluated by ten successive CPE with 0.26 µg mL−1 of
Ni2+, Fe3+ and Cu2+ ions and 0.13 µg mL−1 of
Zn2+ ion in 15 mL of sample solutions. The relative
standard deviations (RSD) were 3.6%, 2.0%, 2.7%,
2.1% for these ions, respectively.
Table 1. Effects of the interferences ions on the recoveries of
We have explored the feasibility of the CPE methodology
using preconcentration with IYMPI ligand in surfactant
media for the determination of concentration of Cu2+,
Fe3+, Ni2+ and Zn2+ ions in different matrices. A wide
variety of samples were tested by our method and these
included natural water, soil, blood, orange juice and
lotus tree leaves samples by addition method. Spiking
experiments checked reliability.The results are presented
in Table 2, 3 and 4. The quantitative recoveries of spiked
samples were satisfactory and were confirmed using
standard addition method. This indicates the robustness
of the system in the determination of analytes in real
samples.
the examined metal ions
Interfering ion to analyte weight
ratio
Interfering
Ions
Li+, Na+, K+, Al3+,
Ti+, NO3-, NO2-, Cl-
Ni
Cu
Zn
150
250
200
100
150
200
1000
Ba2+, Mg2+, Ca2+
600
Mn2+, SO42-
350
Co2+, Cd2+
250
Ag+
150
Pb2+
450
CH3COO-
900
2+
100
Hg
Fe
3.9. Characteristics of the method
Calibration graphs were obtained by preconcentrating
15 mL of a sample containing known amounts
of Cu2+, Fe3+, Ni2+ and Zn2+ ions under the experimental
conditions. Under the specified experimental conditions
the calibration curves Zn2+ ion were linear from
0.01 – 0.20 mg L−1 and for, Ni2+, Fe3+ and Cu2+ ions from
0.01 – 0.30 mg L−1. Limits of detection according to
IUPAC are also included. The limit of detection (LOD) of a
method is the lowest analyte concentration that produces
a response detectable above the noise level of
the system. The limits of detection (LOD) based
on three times the standard deviations of the blank
(N = 20, LOD = Xb +3s, where Xb is the blank value and
s is the standard deviation (SD) of the blank (n = 10), were
found to be (3 SDb/m) of 1.6, 2.8, 2.1 and 1.1 ng mL-1
for Cu2+, Fe3+, Ni2+ and Zn2+ along with preconcentration
factors of 30 and enrichment factor (slope of calibration
curve after preconcentration to slope of calibration curve
before preconcentration) of 48, 39, 34 and 52 for Cu2+,
Ni2+, Fe3+ and Zn2+ respectively.
152
3.10. Accuracy and applications
4. Conclusion
The micellar extraction of Cu2+, Fe3+, Ni2+ and Zn2+
ions with IYPMI into the phase of non-ionic surfactant
Triton X-114 has been investigated. The present cloudpoint extraction offers a simple, rapid, economical
methodology for preconcentration and separation of
analyte ions in real samples prior to their analysis
by flame atomic absorption spectrometry (FAAS).
The present method has numerous advantages over
reported methods [35-42]. Ligand used in this work is
distinct in terms of sensitivity, selectivity towards metal
ions. Optimum volume of the surfactant-rich phase and
low consumption of chemical reagents obtained by using
the present cloud-point methodology permitted to design
an extraction strategy presenting robustness, low cost,
good extraction efficiency and lower toxicity than those
using organic solvents. The proposed method gives very
low detection limits and good standard deviations.
M. Ghaedi, A. Shokrollahi, K. Niknam,
E. Niknam, M. Soylak
Table 2.
Recovery of trace elements from spiked blood and lotus
samples after application of presented procedure, samples (N = 3)
Ion
Fe
Cu
Zn
Ni
Blood
Lotus tree
Added
Found
RSD
Recovery
Found
RSD
Recovery
μg g-1
μg g-1
%
%
μg g-1
%
%
0.0
0.496
1.7
-
0.268
1.7
-
0.2
0.690
1.3
97.0
0.473
1.3
102.5
0.0
0.169
1.6
-
0.266
1.8
-
0.2
0.373
1.2
102.0
0.474
1.3
104.0
0.0
0.156
1.8
-
0.153
1.9
-
0.2
0.362
1.3
103.0
0.358
1.4
102.5
0.0
0.105
1.9
-
0.135
1.8
-
0.2
0.311
1.4
103.0
0.340
1.3
102.5
Table 3.
Recovery of trace elements from spiked environmental
samples after application of presented procedure
Ion
Added,
Found,
RSD,
Recovery,
µg mL-1
µg mL-1
%
%
BCR 185 R liver certified reference material ( mg kg-1)a
Fe
0
-
---
----
Cu
0
274.9
1.5
99.2
Zn
0
134.8
1.7
97.3
-----
---
Ni
0
---
Certified value is 277 mg kg for copper and 138.6 mg kg for zinc.
-1
-1
Table 4. Recovery of trace elements from spiked spinach and soil
samples after application of presented procedure
Ion
Added,
-1
References
Found,
RSD, %
-1
Recovery,
µg g
µg g
Zn
0
100
68.6
170.1
1.2
0.9
101.5
Ni
0
100
74.9
177.6
1.3
0.8
102.7
Cu
0
100
78.6
180.1
1.2
0.9
101.5
Fe
0
100
89.6
192.3
1.2
0.9
102.7
Zn
0
0.4
0.256
0.662
1.4
1.0
101.5
Ni
0
0.4
0.374
0.618
1.4
1.1
102.8
Cu
0
0.4
0.249
0.654
1.3
0.8
101.3
Fe
0
0.4
0.284
0.693
1.3
1.0
102.3
Zn
0
0.2
0.157
0.361
1.7
1.4
102.0
Ni
0
0.2
0.310
0.506
1.6
1.3
98.0
Cu
0
0.2
0.705
0.911
1.4
1.2
103.0
Fe
0
0.2
0.041
0.247
2.1
1.8
103.0
%
Spinache
Soil
Orange Juice
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