American Journal of Anal yt ical Chemistry, 2011, 2, 626-631
doi:10.4236/ajac.2011.25071 Published Online September 2011 (
Copyright © 2011 SciRes. AJAC
Preconcentration of Lead in Sugar Samples by Solid
Phase Extraction and Its Determination by Flame
Atomic Absorption Spectrometry
Saied Saeed Hosseiny Davarani1,*, Neda Sheijooni-Fumani2, Amin Morteza Najarian1,
Mohammad-Ali Tabatabaei1, Siavash Vahidi1
1Department of Chemistry, Faculty of Science, Shahid Beheshti University, GC, Tehran, Iran
2Department of Marine Living Resources, Iranian National In stitute for Oceanography, Tehran, Iran
Received May 21, 2011; revised June 27, 2011; accepted July 4, 2011
A simple and sensitive solid phase extraction utilizing C18 filled cartridges incorporated with dithizone for
preconcentration of lead and its subsequent determination by flame atomic absorption spectrometry (FAAS)
was developed. Several parameters such as type, concentration and volume of eluent, pH of the sample solu-
tion, flow rate of extraction and volume of the sample were evaluated. The effect of a variety of ions on
preconcentration and recovery was also investigated. At pH = 7.4 and 1.0 mol·L–1 HCl eluting them, lead
ions were recovered quantitatively. The limit of detection (LOD) defined as 3Sbl was determined to be 8.1 μg
L–1 for 500 mL of sample solution and eluted with 5 mL of 1.0 mol·L–1 HCl under optimum conditions. The
accuracy and precision (RSD %) of the method were >90% and <10%, respectively. In the end, the proposed
method was applied to a number of real sugar samples and the amount of lead was determined by spiking a
known concentration of lead into the solution.
Keywords: Solid Phase Extraction, Lead, Dithizone, Flame Atomic Absorption Spectroscopy (FAAS), C18
Modified Cartridges
1. Introduction
In the past several years, environmental pollution caused
by contamination by an assortment of heavy metal ions
has become a global problem. Of the heavy metals most
harmful for human is lead. Exposure to excessive am-
ounts of lead may cause irreversible neurological damage
as well as renal disease and cardiovascular effects [1].
On the top of that, the aforementioned pollutions have
found their way in the food-chain and have greater en-
dangered public health [2]. Hence, accurate and sensitive
determination of lead in food samples is of prime impor-
Various methods and techniques are utilized for de-
termination of heavy metals in food samples. It is most
often done by ICP-OES, ICP-MS, GFAAS and FAAS
[3]. Due to its low cost and simplicity, the most com-
monly used technique for determination of metals in
various samples is FAAS. However, FAAS has its own
limitations and is particularly circumscribed by its char-
acteristic low sensitivity [4-8]. Preconcentration tech-
niques play a pivotal role should FAAS is to be utilized
as it improves analytical detection limit, increases sensi-
tivity by several orders of magnitude and enhances ac-
curacy of results [9-10].
Recently a great deal of work has been devoted to
solid phase extraction (SPE) as a preconcentration tech-
nique. It offers advantages such as short extraction time,
low cost, high enrichment factors and recoveries and low
consumption of non environment-friendly solvents. SPE
can easily be used in tandem with FAAS without much
trouble and is generally considered to be a simple
method [2,11-16].
Currently, the most common and widely accepted
method for determination of lead in sugar samples is
ICUMSA’s, which is based upon a colorimetric proce-
dure and is suitable for white and raw sugar, as well as
low-grade products with lead contents not exceeding 0.5
mg Pb/Kg. Although widely accepted and utilized in
analysis laboratories, the method involves consumption
of considerable amounts of highly toxic and dangerous
potassium cyanide and perchloric acid. It also consists of
procedures that require highly skilled operators. The
shortcomings of the current methods and FAAS along
with the ever decreasing allowable amount of lead in
food products prepare the grounds to develop other
methods for determination of lead in sugar samples [12].
The most pivotal step in SPE is considered to be choo-
sing the sorbent material as it gives the method its char-
acteristic properties such as selectivity and capacity to-
wards various metal ions. C18 filled cartridges are still
widely used as a sorbent. However, due to its limited
ability to absorb metal ions quantitatively at trace and
ultra-trace levels, its surface is treated with chelating
agents. Besides having the appropriate chelating proper-
ties to bond, dithizone has proved to be highly selective
towards lead. It also offers high capacity and sensitivity
[13,14] and is used for modification of sorbent in this
The objective of this paper is to prepare dithizone
modified C18 SPE cartridges and investigate its ability to
absorb lead by means of FAAS. Despite the fact that
rival methods utilizing inductively coupled plasma (ICP)
and electrothermal (ET) sample introduction yield slightly
better detection limits, this method offers advantages
such as simplicity, low cost as well as ease of operation
in comparison with those mentioned above. Conditions
such as pH, eluent volume, concentration, flow rate of
elution, and sample volume were optimized. Finally, the
amount of lead in real sugar samples is determined.
2. Experimental
2.1. Apparatus
Concentration of Pb(II) ions were determined by an AA-
680 Shimadzu (Kyoto, Japan) flame atomic absorption
spectrometer (FAAS) in an air-acetylene flame, accord-
ing to the user’s manual provided by the manufacturer. A
lead Hollow cathode lamp was used as the radiation
source with wavelength set at 217.0 nm. pH adjustments
were carried out by a Metrohm model-627 (Herisau,
Switzerland) pH-Meter. Sep-Pak C18 cartridges pro-
duced by Waters (USA, Florida) containing 500 mg of
octadecylsilane served as solid phase and a Rocker 600
(Todays, Taiwan) was also utilized in the process.
2.2. Reagents and Materials
All reagents were of the analytical-reagent grade. The
stock standard solution (1.000 g·L–1) of Pb2+ was ob-
tained from E. Merck (Darmstadt, Germany) and was
diluted with deionized water. A 50 g·L–1 solution of lead
was prepared and working standard solutions of lead
were prepared by diluting a proper amount of the afore-
mentioned solution. Dithizone was dissolved in chloro-
form; both reagents were purchased from E. Merck
(Darmstadt, Germany). 1.0 mol·L–1 solution of HCl was
prepared by appropriate dilution of stock solution of
concentrated hydrochloric acid in deionized water.
Buffer solutions with different pH were prepared by dis-
solving appropriate amounts of suitable salts salt pur-
chased from E. Merck (Darmstadt, Germany) and Fluka
(Switzerland) in deionized water. Sugar was acquired
from the grand market of Tehran.
2.3. Cartridge Modification
Each cartridge was washed with 5 mL of ethanol, 10 mL
of water followed by 5 mL of 1.0 mol·L–1 HCl and then
another 10 mL of deionized water in order to remove all
the potential contaminants as a result of the manufactur-
ing process. The cartridge was then dried by passing air
through it for a few minutes. 2.0 mL of dithizone solu-
tion (1000 mg·L–1 in chloroform) was left in the cartridge
and allowed to penetrate inside the pores of the solid
phase. The solvent was allowed to evaporate at 60˚C for
30 minutes. Subsequently, air was passed through the
cartridge for a several minutes to ensure that it is thor-
oughly dried.
2.4. General Procedure
The extraction process was carried out by passing solu-
tions containing Pb2+ ions through the cartridge. Back-
extraction was done by eluting the cartridge with 5.0 mL
of 1.0 mol·L–1·HCl. At the end, the lead content of the
samples were determined by FAAS.
2.5. Application to Sugar Samples
50.000 grams of sugar was dissolved in 500 mL of am-
monium acetate solution (pH = 7.4). Solid phase extrac-
tions were performed on the samples by passing the so-
lution through the modified cartridge. Back-extractions
were performed by eluting the lead content by 5.0 mL of
1.0 mol·L–1·HCl.
3. Results and Discussion
3.1. Adsorption Properties of Cartridge and
Solutions of known concentration of lead were passed
through the column which contained only C18. The re-
sults indicated that less than 5% of Pb2+ ions were re-
Copyright © 2011 SciRes. AJAC
tained by the cartridge. After modifying the C18 support
by dithizone, the tendency of the cartridge to absorb the
ions increased considerably. This increase can be attrib-
uted to the complex formation between the ligand and
the lead ion (Figure 1). In order to maximize the absorp-
tion capability of the modified cartridge, the SPE condi-
tions were optimized one at a time.
3.2. Type of Eluent
Due to the intrinsic properties of ligand, an acidic solu-
tion is required for the lead ions to be desorbed. An
acidic solution protonates the chelating sites and elutes
the cartridge of Pb2+ ions. The criteria for eluent selec-
tion were strength of acid, its availability in laboratories
and green chemistry considerations. Nitric acid (HNO3),
hydrochloric acid (HCl), acetic acid (CH3COOH) and
ethanol were investigated to be utilized as eluent. Being
an organic solvent, elution by ethanol lead to recoveries
over 100%. However, in addition to Pb2+ ions, it eluted
the ligand as well. Among acid solutions, acetic acid
exhibited substantially lower recovery than the other two.
Among nitric acid and hydrochloric acid, hydrochloric
acid yielded higher recoveries. Hence; HCl was chosen
as eluent.
3.3. Concentration and Volume of Eluent
In order to determine the optimized eluent concentration,
100.0 mL aliquots of samples solu tions containing 0.05
mgL –1 of Pb2+ were passed through the cartridge and
were eluted with 5 mL of HCl having different concen-
trations. As indicated in Figure 2, recovery of Pb2+ in-
creased up to 1.0 mol·L–1 but decreased with further in-
crease of concentration as more concentrated acid de-
stroys and hydrolyses the solid support. Thus 1.0 mol·L–1
HCl was chosen as the optimum concentration of eluent.
Also the volume of the eluent was optimized in the
process. The range from 1.0 mL to 10.0 mL of eluent
volume was investigated and the results indicated that no
considerable increase in recovery was achieved by in-
creasing the eluent volume beyond 5.0 mL. So the vol-
ume of 5.0 mL was employed in this work. (Figure 3)
Figure 1. Structure of ligand and the coordination com-
pound it forms.
Figure 2. Effect of concentration of HCl on the recovery of
Pb+2. Conditions: Sample volume: 100 mL, amount of dithi-
zone: 2.0 μg, sample flow rate: 15 mL·min–1, eluent flow rate:
15 mL·min–1, eluent volume: 5 mL, pH of sample solution: 7.4.
Figure 3. Effect of volume of HCl on the recovery of Pb+2.
Conditions: Sample volume: 100 mL, amount of dithizone:
2.0 μg, sample flow rate: 15 mL·min–1, eluent flow rate: 15
mL·min–1, eluent concentration: 1.0 mol·L–1, pH of sample
solution: 7.4.
3.4. Effect of pH
Since adsorption of lead ions occurs as a result of forma-
tion of a coordination compound, pH has a profound
effect on the retention of the ions by the ligand. C18 un-
dergoes acidic hydrolysis at pH under 2 and its silica
structure is prone to dissolution at pHs above 8 (Dong,
2006) thus pH = 1 was discarded during optimization.
Optimization of pH was carried out (Figure 4) and based
on the results pH = 7.4 were chosen as optimum pH.
Dithizone is a bidentate chelating agent and possesses
two coordination sites; sulfur and nitrogen. Thus, when
the concentration of hydronium ion increases, the more
susceptible site to protonation, i.e. nitrogen, is unable to
bond to lead ions, hence reduced retention. Higher values
of pH causes the problem of precipitation of lead in the
form of Pb(OH)2. The aforementioned pH value was
adjusted by addition of NH3 and HCl solutions and was
later stabilized using a 1.0 mol·L–1 ammonium acetate
Copyright © 2011 SciRes. AJAC
Figure 4. Effect of pH of sample solution on the recovery of
Pb+2. Conditions: sample volume: 100 mL, amount of dithi-
zone: 2.0 μg, sample flow rate: 15 mL·min–1, eluent flow
rate: 15 mL·min–1, concentration of eluent: 1.0 mol·L–1,
eluent volume: 5 mL.
3.5. Flow Rate Optimization
The potential implications caused by the flow rate of the
solution through the cartridge and flow rate of elution
were examined. Solid phase extraction was conducted on
solutions with known quantities of lead while altering the
flow rate in the range of 5 - 25 mL/min. Retention de-
pressed at least 20% with 5 mL/min increments of flow
rate beyond 15 mL/min in both cases. Thus, the fastest
flow rate with the best recovery was determined to be 15
3.6. Dilution and Kinetic Effect
One of the parameter to be optimized is the kinetic effect
of adsorption on the ligand. By diluting the solution,
more time is being given to the ions to be adsorbed on
the ligand. To study this effect, 10 μg of Pb2+ was dis-
solved in volumes of ammonium acetate solution in the
range of 50 - 1000 mL. A sharp improvement in recovery
was detected in the early stages but no considerable im-
provement was observed upwards of 100 mL mark. Al-
though volumes as high as 500 mL can be used to yield
high enrichment factors, a sample volume of 100 mL can
be utilized with no sizeable decrease in recovery.
3.7. Interferences
To assess the application of the proposed solid phase
extraction to real samples, effects of some interfering
ions which may enter sugar merchandise during the
course of production such as arsenic, iron, magnesium,
cobalt, zinc, and copper were investigated under the op-
timized conditions. A fixed amount of Pb (II) ions was
taken with a known concentration of the aforementioned
foreign ions and their effect on the recovery of Pb(II)
was investigated. Those effects on solid phase extraction
were totally negligible and were also suitable for atomic
absorption spectrometric determinations and did not
cause any type of difficulty or implication.
3.8. Figures of Merit
The analytical performance of the method was evaluated
by plotting a calibration curve with 6 standard solutions.
The calibration curve was linear in the range of 27 - 200
μg·L–1 with a correlation coefficient equal to 0.9910. The
standard deviation of the method was determined by
conducting eight replicate analysis and proved to be
smaller than 10%. The limit of detection (LOD) and limit
of quantification (LOQ) of the method, defined equal to
three and ten times the standard deviation of the blank
and when preconcentration factor equals 100, were de-
termined to be 8.1 μg·L–1 and 27 μg·L–1, respectively.
Values as low as 4.2 μg·L–1 and 14 μg·L–1 are achievable
should a preconcentration factor of 200 is applied.
3.9. Application to Sugar Samples
The proposed method was applied to sugar samples.
50.000 grams of sugar was weighted and dissolved in
500.0 mL of ammonium acetate solution. The samples
were passed through the cartridge. After elution by 5 ml
of 1.0 mol·L–1 HCl, the lead concentration in the samples
was determined by FAAS. To establish the accuracy of
the peaks, sugar samples were spiked with known
amounts of lead and the proposed SPE method was ap-
plied. The results (Table 1) showed good agreement with
the ones obtained from the simple standard technique.
4. Conclusions
The proposed method provides an effective approach
towards preconcentration of Pb2+ ions in sugar samples.
The method is simple, rapid and reliable in comparison
with rival methods (Table 2). The analytical perform-
ance and figures of merit, namely; relative standard de-
viation, LOD, LOQ and recovery for this method are
either better or comparable to other methods and in-
volves minimal usage of solvents and chemicals that are
Table 1. Determination of lead in sugar samples by the pro-
posed method.
Amount added
Pb2+ (ppb)
Amount found Pb2+
(ppb) (±%RSD)
aSugar - < 20 -
Sugar 50 45 (±3) 90 (±6)
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Table 2. Figures of merit of comparable methods for determination of lead.
Reagent LOD (μg·L–1) Maximum enrichment factor T echnique Ref e rences
3-Aminopropyltriethoxysilane modified silica gel 4.00 - GFAASa [16]
8-Hydroxyquinoline immobilized on controlled pore glass 8.27 - ICb [17]
Cellulose sorbent with phosphoric acid groups (Cellex P) 1.8 197 FAAS [18]
Silica gel functionalized with methylthiosalicylate 15.30 41 ICP-OESe [19]
Silica Gel (Pb-02) 5.0 52 FAAS [20]
Dithizon e 4.4 200 FAAS Present Work
aGraphite furnace atomic absorption spectroscopy; bIon chromatography; cFlame atomic absorption spectroscopy; dElectro thermal atomic absorption spectroscopy;
eInductively coupled plasma optical emission spectroscopy.
not environmentally friendly. The high preconcentration
factor and precision of this method as well as its satis-
factory reproducibility makes it applicable to sugar sam-
ples in which their lead content is below the detection
limit of FAAS.
5. Acknowledgements
Financial support for this work by the Research Affairs,
Shahid Beheshti University, is gratefully acknowledged.
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