chelating

1. Talanta 59 (2003) 1227/1236
A new chelating sorbent for metal ion extraction under high
saline conditions
D. Prabhakaran, M.S. Subramanian *
Department of Chemistry, Indian Institute of Technology, Chennai 600 036, India
Received 13 August 2002; received in revised form 26 December 2002; accepted 3 January 2003
Abstract
A new chelating polymeric sorbent was developed by functionalizing Amberlite XAD-16 with 1,3-dimethyl-3-
aminopropan-1-ol via a simple condensation mechanism. The newly developed chelating matrix offered a high resin
capacity and faster sorption kinetics for the metal ions such as Mn(II), Pb(II), Ni(II), Co(II), Cu(II), Cd(II) and Zn(II).
Various physio-chemical parameters like pH-effect, kinetics, eluant volume and flow rate, sample breakthrough
volume, matrix interference effect on the metal ion sorption have been studied. The optimum pH range for the sorption
of the above mentioned metal ions were 6.0/7.5, 6.0/7.0, 8.0/8.5, 7.0/7.5, 6.5/7.5, 7.5/8.5 and 6.5/7.0, respectively.
The resin capacities for Mn(II), Pb(II), Ni(II), Co(II), Cu(II), Cd(II) and Zn(II) were found to be 0.62, 0.23, 0.55, 0.27,
0.46, 0.21 and 0.25 mmol g1 of the resin, respectively. The lower limit of detection was 10 ng ml1 for Cd(II), 40 ng
ml1 for Mn(II) and Zn(II), 32 ng ml1 for Ni(II), 25 ng ml1 for Cu(II) and Co(II) and 20 ng ml1 for Pb(II). A high
preconcentration value of 300 in the case of Mn(II), Co(II), Ni(II), Cu(II),Cd(II) and a value of 500 and 250 for Pb(II)
and Zn(II), respectively, were achieved. A recovery of /98% was obtained for all the metal ions with 4 M HCl as
eluting agent except in the case of Cu(II) where in 6 M HCl was necessary. The chelating polymer showed low sorption
behavior to alkali and alkaline earth metals and also to various inorganic anionic species present in saline matrix. The
method was applied for metal ion determination from water samples like seawater, well water and tap water and also
from green leafy vegetable, from certified multivitamin tablets and steel samples.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Polymeric sorbent; Saline; Resin
1. Introduction
The use of metal ions has increased substantially
over years due to their importance as valuable
intermediates in industries as catalyst and in
electric and electronic devices as semiconductors
[1], etc. resulting in contamination of water
resources by industrial effluents which has been a
serious issue in the recent past. As a result, the
determination of trace concentrations of metal
ions in man made and natural water resources is of
current trend of interest in order to have a check
* Corresponding author. Fax: /91-44-2257-8241.
E-mail address: mssu@rediffmail.com (M.S. Subramanian).
www.elsevier.com/locate/talanta
0039-9140/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-9140(03)00030-4

2. 1228 D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236
on the eco-system. Several incidents of metal ion
toxicity to humans and aquatic lives were re-ported.
Hence it requires a time-to-time analysis
of these trace metal ions in environmental as well
as in bio-fluid samples in order to sustain and
preserve the eco-system. But the determination
process are not that simple as the metal ions are
surrounded and encapsulated by a variety of
complex matrix species which interfered, thus
forcing for the need of an extraction technique,
which could selectively extract the analytes of
interest. This has paved way to the development
of solid phase extraction (SPE) technique.
Chelating polymeric matrices termed as poly-chelatogens
(PC) [1] have been used in SPE
technique and have highlighting features such as
high degree of selectivity, versatility, durability
and a good metal loading capacity, enhanced
hydrophilicity [2], but follow second order kinetics
[3,4]. Organic ligand moiety containing active
functional groups that can selectively chelate the
metal ions of interest are incorporated into a
polymeric support via chemical modification.
Several chelating supports have been used [5/
19], of which the use of polystyrene-divinylbenzene
(PS-DB) resins were found to be fruitful. These
resins can be functionalized as well as loaded with
a variety of organic ligands having various func-tional
groups [2,3,18]. These PS-DB resin beads
are commercially available as Amberlite XAD
series, which differ in their degree of cross linkage,
bead size, pore diameter, etc., directly influencing
the rate and degree of metal ion sorption. Several
papers on Amberlite XAD series [20/40] as
chelating polymeric resin have been reported in
the recent past. Amberlite XAD-16 was found to
be the most promising polymeric support, which
has a greater surface area for immobilising 1,3-
dimethyl-1-3-aminopropan-1-ol, an organic ligand
moiety of small molecular size. The synthesis of
this chelating resin, and its characteristic sorption
for Mn(II), Pb(II), Ni(II), Co(II), Cu(II), Cd(II)
and Zn(II) are the subjects of the present paper.
The preconcentrating capability of this chelating
resin was ascertained by applying to the analysis of
various water samples, steel and vitamin samples
and as well as to green leafy vegetable.
2. Experimental
2.1. Instrumentation
A varian model spectrAA-20 flame atomic
absorption spectrometer was used for the determi-nation
of metal ion concentration, under condi-tions
as prescribed in Table 1. A Digisun DI-707
model digital pH-meter was used for pH measure-ments.
An Orbitek-DS model mechanical shaker
with 200 rpm was used for batch equilibration
studies. Infra red (IR) spectra and far infra red
(FIR) spectra for the characterization of the
chemically modified matrix were recorded using
a Bruker model IFS 66V Fourier Transform
spectrometer. TGA data were obtained using
Perkin Elmer thermal analyzer.
2.2. Chemicals and reagents
All the reagents used were of analytical grade.
Standard metal ion stock solutions (1000 mg
dm3) were prepared by dissolving the appropri-ate
amounts of analytical reagent grade Pb(NO3)2,
NiSO4 / 6H2O, Co(NO3)2 / 6H2O, ZnSO4 / 7H2O,
3CdSO4 / 8H2O, CuSO4 / 5H2O and MnCl2 /
4H2O, in double distilled water containing small
amounts of the corresponding acid. The pH
adjustments were carried out using chloroacetic
acid/acetic acid (pH 1/3), sodium acetate/acetic
acid (pH 4/6) ammonium acetate/ammonia solu-tion
(pH 7/8) and ammonium chloride/ammonia
(pH 9/10).
2.3. Synthesis of functionalized chelating resin
Amberlite XAD-16 resin (surface area 825 m2
g1, pore diameter of 20/50 mesh and bead size
0.3/1.2 mm) was obtained from Fluka chemicals
(Switzerland). It was washed thoroughly by soak-ing
in 4 M HCl overnight and then washed with
double distilled water to neutral pH and finally
with ethanol, filtered and dried in vacuum before
use. The dried Amberlite XAD-16 beads (10 g)
were subjected to the process of nitration followed
by amination as per literature procedure [33]. The
aminated resin was dried and vacuumized for 48 h
and was refluxed with acetyl acetone (11 ml) in

3. D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236 1229
absolute ethanol (50 ml) with 4 A°
molecular sieves
(2 g) for 3 h at 90 8C. The imine resin was washed
thoroughly with excess of hot ethanol, dried and
further reduced with 4 g of sodium borohydride in
dry methanol under ice-cold conditions. The
resulting greyish/brown colored beads were
washed with plenty of distilled water to neutral
pH, filtered and vacuum dried. The synthesized
chelating resin was characterized using IR, FIR,
TGA and CHN analysis. Synthetic scheme depicts
the route for synthesizing the chelating resin
(Scheme 1).
2.4. Adopted methods for the preconcentration and
determination of Mn(II), Pb(II), Ni(II), Co(II),
Cu(II), Cd(II) and Zn(II)
2.4.1. Batch method
2.4.1.1. Metal ion sorption as a function of solution
pH. Each metal ion solution (50 ml, 10 mg dm3)
was shaken with 100 mg of the resin in the pH
ranging from 2 to 10 for a period of 60 min. The
resin samples were filtered through Whatman No.
1 filter paper and the filtrates were collected. The
sorbed metal ions were stripped with different
concentrations of hydrochloric acid. The concen-tration
of metal ions of interest present in both the
filtrate samples and the stripped samples were
analyzed by FAAS.
2.4.1.2. Kinetic studies on the metal ion sorption.
The time duration required for the complete
sorption of the metal ion of interest was studied
using 100 mg resin, with metal ion solution (50 ml,
9 mg dm3) in the case of Pb(II), Mn(II), Zn(II)
and (50 ml, 8 mg dm3) for Cd(II), Ni(II), Cu(II)
and (50 ml, 6 mg dm3) Co(II) under optimum
pH conditions for various intervals of time.
2.4.1.3. Loading capacity of the functionalized
chelating resin. The maximum metal loading
capacity of the resin beads were studied by
saturating 100 mg of the resin beads with an
excess metal ion solution (50 ml, 100 mg dm3) in
the case of Pb(II), Mn(II), Ni(II) and (50 ml, 60
mg dm3) for Cu(II), Cd(II), Zn(II), Co(II) under
Table 1
Operating parameters in FAAS
Element Wave length (nm) Lamp current (mA) Slit width (nm) Acetylene/air flow rate (l min1)
Mn(II) 279.5 5.0 0.2 3/4.5
Pb(II) 217.0 7.0 1.0 3/4.5
Ni(II) 232.0 3.5 0.2 3/4.5
Cu(II) 324.7 3.5 0.5 3/4.5
Cd(II) 228.8 3.5 0.5 3/4.5
Co(II) 240.7 7.0 0.2 3/4.5
Zn(II) 213.9 5.0 1.0 3/4.5
Scheme 1.

4. 1230 D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236
optimum pH conditions for a duration of 6 h. The
amount loaded was estimated using FAAS.
2.4.1.4. Tolerance limits of electrolytes. Effect of
diverse ions on the recovery of metal ions was
tested by batch equilibration method using 50 mg
resin equilibrated with metal ion solution (40 ml,
1.25 mg ml1) in the presence of various concen-trations
of electrolyte species up to which 0% loss
in the analytical signal were observed.
2.4.1.5. Limit of metal ion detection (LOD). LOD
for various metal ions were studied by equilibrat-ing
metal ion solution (125 ml, 0.01/0.1 mg dm3)
with 100 mg resin under optimum conditions for
10 min and then determining their concentration
by FAAS.
2.4.1.6. Resin reusability test. To test the resin
reusability, 100 mg of the resin was shaken with
(50 ml, 100 mg dm3) of the metal ions and their
concentrations in both aqueous and sorbent phase
were determined. Thereafter, the sorption and
desorption of metal ions were repeated on the
same resin beads after washing them with plenty of
water till neutral pH.
2.4.2. Column method
The chelating polymeric resin beads were pre-conditioned
(1 g) by soaking in buffer solution for
2 h and packed in a glass column (0.635 cm) by
slurry method. A suitable aliquot of the solution
containing the analyte of interest in the concentra-tion
range (0.01/0.20 mg dm3) was passed
through the packed column. The flow rate was
optimized using a Mariotte type apparatus. The
sorbed metal ions were eluted with 10 ml of eluting
agent at a flow rate of 1 ml min1 and the metal
ion concentrations were determined by FAAS.
In order to ensure quantitative sorption of the
metal ion from large volumes of sample solutions,
breakthrough volume studies were performed
using sample volumes ranging from 500 to 5000
ml containing 100 mg of the metal ion of interest
under optimum flow rate through a precondi-tioned
column. After desorption the resin bed was
subjected to water wash till neutral pH and
subsequently reused.
The interference of diverse ions was then studied
using synthetic mixture of seawater as per the
literature [41]. The results were further confirmed
by spiking with trace amount of analyte of interest
(3 l, 0.03 mg ml1). The concentration of metal ion
in the solid phase in each case was determined by
FAAS.
3. Results and discussion
3.1. Characterization of the functionalized
polymeric resin matrix
3.1.1. IR and FIR spectra
The IR spectra for the vacuum dried functiona-lized
resin showed bands in the region ranging
from 3600 to 3200 cm1 corresponding to /OH
and /NH stretching frequencies. Further, addi-tional
bands at 1505.7 and 1380.6 cm1 originat-ing
due to C/N stretching frequency confirmed
the presence of the chelating ligand moiety
attached to the polymeric backbone.
IR studies of the metal ions (Mn(II), Pb(II),
Ni(II), Co(II), Cu(II), Cd(II) and Zn(II)) chelated
resin showed a red shift in the range 15/20 cm1
for C/N and /OH stretching frequencies indicat-ing
the metal chelation with the active sites present
in the polymer matrix. This was evident from FIR
data studies in the region 500/100 cm1, which
showed spectral bands in the region 500/400 and
290/120 cm1 indicating nNMn and nOMn
vibrational frequencies of the chelating active sites.
3.1.2. CHN elemental and thermal analysis
The data obtained from CHN elemental analy-sis
C 76.84%, H 8.37%, N 6.9% were comparable
with the theoretically calculated values C 76.97%,
H 9.27%, N 6.83%. The CHN analysis confirmed
the presence of one ligand moiety per repeat unit
of the polymeric matrix. Thermo gravimetric
analysis (TGA) data showed a gradual weight
loss of about 8% up to 110 8C indicating the
presence of water molecules in the pores of the
polymeric matrix, thereby ensuring a better hydro-philicity
character to the resin beads.

5. D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236 1231
3.2. Optimized experimental parameters using
batch method for the metal extraction
The influence of solution pH on the sorption of
metal ions by batch equilibration technique is
shown in Fig. 1. The results depict the sorption
process to be more favorable in near neutral
conditions, which is also reflected on the relatively
low acidity of the chelating matrix.
The rate of metal ion sorption under optimum
pH conditions can be seen from Fig. 2. Complete
sorption to the sorbent phase was ensured in less
than 25 min for all the metal ions (shown in Fig. 2)
highlighting its high degree of kinetic exchange
with modified polymeric support and the t1/2
values (half loading time) were in the range 2.5/
3.3 min for all the metal ions.
The metal loading capacity of the resin beads is
shown in Table 2. The difference in loading
capacity values for different metal ions under
study was attributed due to variation in the
stability constants of the metal chelates formed
by the active sites with the metal ions under study.
The theoretical value taking into account the
number of functional groups has been calculated
to be 4.877 mmol g1 of the resin. But the
experimental resin capacity values for individual
metal ions were below the theoretical values, which
may be due to many exchange sites being inacces-sible
to the metal ions. The metal loading capacity
studies were repeated by column method and the
results obtained were in good agreement with
batch equilibration method.
The concentrations of the diverse ions were
increased up to which 0% loss in analytical signal
was observed is shown in Table 3. The experi-mental
results show a high degree of tolerance
towards alkali and alkaline earth metal ion species,
in targeting analytes of interest. The above ob-servation
was also confirmed by using synthetic
sea water mixture in column studies. The results
also show 100% recovery of the analyte ion of
interest.
The sensitivity of the developed method is
reflected by the LOD studies, defined as the lowest
concentration of metal ion of interest below which
quantitative sorption of the metal ion by the
chelating matrix is not perceptibly seen. LOD for
various metal ions were studied under optimum
conditions and the values are indicated in Table 2
ensuring a high degree of selectivity and precon-centrating
ability by the developed method.
Durability and reusability nature of the chelat-ing
matrix was tested with metal ion solutions by
batch equilibration method and the metal ions
concentrations were determined. Thereafter, the
Fig. 1. Effect of pH on metal ion sorption.

6. 1232 D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236
Fig. 2. Kinetic studies on metals ion sorption.
sorption and desorption of metal ions were
repeated on the same resin beads. The capacity
of the resin was found to be practically constant as
shown in Fig. 3 for more than 30 cycles showing
the feasibility of multiple use of the chelating
polymeric resin without any loss in its physical and
chemical properties.
3.3. Column studies for the preconcentration of
trace concentration of metal ions
From sample breakthrough volume studies, the
% recovery obtained for various sample volumes
and the corresponding preconcentration factors
are shown in Table 4, reflecting the enhanced
metal ion exchange ability and selectivity nature in
species targeting, which is one of the salient
features of the developed chelating matrix. The
optimum flow rates for various metal ions under
study were found to be in the range 3/6 ml min1
(as shown in Table 2), which, however, can further
be improved by using a micro column.
4. Applications
4.1. SPE of trace metal ions from water samples
Three liters of sea/well/tap water collected near
different zones of Chennai, Tamil Nadu, India,
Table 2
Optimum conditions for the sorption and desorption of metal ions on the chelating polymeric matrix
Experimental parameters Mn(II) Pb(II) Ni(II) Cu(II) Co(II) Cd(II) Zn(II)
pH range 6.0/7.5 6.0/7.0 8.0/8.5 6.5/7.5 7.0/7.5 7.5/8.5 6.5/7.0
t1/2 (min) 3.0 2.8 2.5 3.0 2.9 3.3 2.7
Metal sorption capacity (mmol g1) 0.62 0.23 0.55 0.46 0.27 0.21 0.25
Concentration of HCl (M) 4 4 4 6 4 4 4
Optimum flow rate (ml min1) 3/4 4/5 3/4 5/6 5/6 4/5 4/5
Average % recovery 99.9 100.2 100.0 98.2 98.6 99.6 99.1
Lower limit of detection (ng ml1) 40 20 32 25 25 10 40

7. D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236 1233
after filtration through 0.45 mm membrane filter,
was passed through the resin bed preconditioned
earlier and the water samples were pH optimized
for metal ion sorption and the recovered metal
ions were determined by FAAS. The validity of the
results was tested by standard addition method, by
spiking a known amount (10 mg) of individual
metal ions except for Pb(II) wherein 60 mg of the
metal ion, was spiked to the water sample. The
results pertaining to the analysis of trace amount
of metal ion of interest from sea water are shown
in Table 5, confirms the satisfactory recovery of
the analytes.
4.2. Recovery of metal ions from certified steel
samples
The steel sample (0.1 g, BCS grade No.491) was
dissolved in aqua regia and was heated to aid
dissolution, followed by evaporation to dryness.
The residue was extracted with water and the pH
was optimized and passed through the precondi-tioned
resin column. The results obtained are
shown in Table 6, which reflect the ability of the
resin system to extract and preconcentrate metal
ions, present in both micro and macro levels
quantitatively.
Table 3
Tolerance limits for diverse ions
Metal ions Tolerance limits (mol dm3) up to which 0% loss in analytical signal
NaCl NaF Na2SO4 Na3PO4 KNO3 KCl Ca2 Mg2
Mn(II) 0.21 0.32 0.09 0.03 0.14 0.12 0.31 0.52
Pb(II) 0.71 0.51 0.28 0.05 0.51 0.54 0.51 0.82
Ni(II) 0.43 0.63 0.18 0.07 0.33 0.25 0.11 0.51
Cu(II) 0.34 0.48 0.14 0.04 0.15 0.22 0.38 0.63
Co(II) 0.20 0.12 0.04 0.01 0.06 0.05 0.06 0.10
Cd(II) 0.34 0.42 0.12 Ia 0.21 0.71 0.01 0.02
Zn(II) 0.13 0.18 0.06 0.02 0.07 0.14 0.20 0.31
a Interfered.
Fig. 3. Reusability of the resin.

8. 1234 D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236
4.3. Determination of Ni(II) and Mn(II) in
certified multi-vitamin/multimineral tablets
Multi-vitamin/multimineral tablets obtained
from Nature made Nutritional Products and
CVS Pharmacy, Inc., USA were used for analysis.
Two tablets were dissolved and decomposed by
heating in aqua regia thrice. The residue was
dissolved in minimum volumes of dil. HCl and
the pH was adjusted to 5, filtered and the filtrate
was adjusted to optimum pH and then passed
through the preconditioned resin bed. The metal
ion concentration in the solid phase was deter-mined
by FAAS after leaching with 4 M HCl and
the results are given in Table 6.
4.4. Analysis of Co(II) and Ni(II) in green leafy
vegetable
The green leaves were air dried followed by
heating to 110 8C in air-oven for 3 h and then
powdered. Five grams of the sample was decom-
Table 4
Sample breakthrough volume
Sample volumea (ml) % Recovery
Mn Pb Ni Cu Co Cd Zn
500 100 100 100 100 100 100 100
1000 100 100 100 100 100 100 100
1500 100 100 100 100 100 100 100
2000 100 100 100 100 100 100 100
2500 100 100 100 100 100 100 100
3000 100 100 100 100 100 100 95
3500 93 100 95 100 87 84 81
4000 83 100 91 100 71 80 80
5000 76 100 86 96 65 75 73
Preconcentration factor 300 500 300 400 300 300 250
a Sample contains 100 mg of metal ions of interest.
Table 5
Analysis of water samples
Analytes Method Concentration (mg l1)
Sea water R.S.D. (%) Tap water R.S.D. (%) Well water R.S.D. (%)
Mn(II) Direct 5.01 2.12 15.61 3.13 17.61 1.81
S.D. 4.95 2.74 14.98 2.76 17.50 1.42
Pb(II) Direct / / 27.60 0.62 24.11 1.43
S.D. 60.11 1.10 26.92 1.71 23.78 2.36
Ni(II) Direct 9.90 2.81 10.31 1.73 24.41 1.31
S.D. 10.14 1.36 9.90 2.14 23.78 2.41
Co(II) Direct 2.81 3.23 3.71 1.84 4.91 1.21
S.D. 2.34 2.42 3.63 2.10 5.03 1.69
Cu(II) Direct 18.08 2.84 27.80 0.89 16.80 1.14
S.D. 17.75 1.23 27.20 1.83 17.12 1.01
Cd(II) Direct 1.79 1.73 2.41 1.69 0.91 3.12
S.D. 1.67 1.24 2.50 1.54 0.88 2.93
Zn(II) Direct 116.11 1.03 140.14 2.12 80.29 1.15
S.D. 121.03 1.17 139.20 1.69 79.18 1.92
Values are based on triplicate analysis.

9. D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236 1235
Table 6
Metal ion concentration in green leafy vegetable, certified steel samples and multivitamin tablets
posed with conc. H2SO4/H2O2 mixture repeatedly
until colorless. The solution was diluted and the
pH was raised to 4.5 using 3Msodium acetate and
filtered. The filtrate was adjusted to optimum pH
and was passed through the resin bed and the
sorption of metal ions was studied by standard
addition method and results are shown in Table 6.
5. Conclusions
The newly developed chelating resin, Amberlite
XAD-16 functionalized with 1,3-dimethyl-3-ami-nopropan-
1-ol was found to have a superior resin
loading capacity and a high preconcentration
factor for the metal ions Mn(II), Pb(II), Ni(II),
Co(II), Cu(II), Cd(II) and Zn(II) when compared
with other chelating matrix reported in literature.
Further more, the half loading time (t1/2) of this
chelating resin was also very short and the kinetic
studies ensured faster exchange kinetics in the
process of metal ion uptake. On using a micro
column, the time of analysis can be still decreased
substantially. High selectivity for metal ions was
ensured under saline conditions and the reusability
of the resin was as high as greater than 30 cycles
without any loss in its sorption behavior. The
system was also successful in preconcentrating
metal ions from large sample volume. The column
and batch methods were in good agreement with
low R.S.D. values reflecting the validity and
accuracy of the method when applied to water,
multivitamin, green leafy vegetable and steel
samples. Simultaneous preconcentration of all
metal ions under study could be possible provided
the overall metal ion concentrations do not exceed
over the total metal load capacity value of the resin
matrix, with separation favorable under relatively
low pH conditions.
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Samplesa Method Analytes
Mn(II) R.S.D. (%) Ni(II) R.S.D. (%) Co(II) R.S.D. (%)
Green leafy vegetable (mg g1) Direct / / 30.72 3.12 9.83 2.78
S.D. / / 31.21 2.81 9.97 2.91
Multivitamin tablet-I (mg per tablet) Theoretical 2000 / 10.00 / / /
Experimental 1980 2.3 9.82 2.92 / /
Multivitamin tablet-II (mg per tablet) Theoretical 3.50 / / / / /
Experimental 3.47 2.11 / / / /
Steel (mg per 100 mg) Theoretical 16.10 / 0.005 / / /
Experimental 16.07 3.12 0.005 0.41 / /
a Values are based on triplicate analysis.

10. 1236 D. Prabhakaran, M.S. Subramanian / Talanta 59 (2003) 1227/1236
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