Cent. Eur. J. Chem. • 12(3) • 2014 • 386-390
DOI: 10.2478/s11532-013-0381-6
Central European Journal of Chemistry
Lead and cadmium atomic absorption
determination in solid carbonized food
samples using flame-furnace atomizer
Research Article
Kostyantyn S. Lugovyy1, Olexander M. Buhay2,
Antonina S. Alemasova1*
1
Chemistry faculty,
The Donetsk National University,
83000 Donetsk, Ukraine
2
The Institute of Applied Physics,
National Academy of Science of Ukraine,
40030 Sumy, Ukraine
Received 8 July 2013; Accepted 11 November 2013
Abstract: Direct atomic absorption solid sampling analysis using flame-furnace atomizer enables a significant decrease in the analysis duration,
to avoid sample pollution and to exclude toxic reagents. The selection of the chemical modifiers decreasing the detection limit and
improving the results repeatability is based on the analyte’s free atoms formation mechanism. The developed kinetic approach has
allowed to determine pre-exponential factors k0 and apparent activation energies Ea of atomization processes for Pb(II) and Cd(II)
compounds and to propose effective chemical modifiers sodium N,N-diethyldithiocarbamate and urea for food samples. The express
and precision technique for lead and cadmium determination in food, using proposed chemical modifiers and carbonization techniques
was developed.
Keywords: Atomic absorption analysis • Solid samples • Flame-furnace atomizer • Kinetic study • Direct analysis
© Versita Sp. z o.o.
1. Introduction
Current methods of high-temperature processes
quantitative description in semi-closed electrothermal
atomizers in atomic absorption (AA) spectroscopy are
mainly based on thermodynamic [1-3] and kinetic [4-9]
approaches. Studies and descriptions of atomization
processes for direct AA analysis of solid samples have
been paid little attention. Several types of atomizers
are usually used for solid sampling analysis [10]. We
consider flame-furnace atomizer to be the most suitable
for routine analyses, because disassembling of the
atomizer is not needed.
Our previous work was devoted to the kinetic study
of Pb and Cd atomization from solid soil samples [11].
In this work we have been studying the atomization
process from solid carbonized food and have proposed
express and precision technique for food AA analysis
using chemical modifiers.
Chemical modifiers selection for solid food samples
atomic absorption analysis can be based on the numeral
values of kinetic parameters of free atoms formation in
flame-furnace atomizer.
Plant and animal foodstuffs were selected for
analysis. Cottage cheese is ae significant protein source
and one of the most popular and available foods. Plant
foods accumulate toxic metals which remain in the
finish product. In addition cottage cheese unlike other
foodstuffs contains significant amounts of fat.
2. Theoretical procedure
Taking into account the atomizer construction and
experiment conditions, the atomization processes in
AA spectrometry analysis of solid samples in the flamefurnace atomizers can be presented by the scheme:
* E-mail: [email protected]
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K. S. Lugovyy, O. M. Buhay, A. S. Alemasova
k
k1
k2
gr
X →
B →
A →
Z,
(1)
where X is the vapor-forming sample; B are the atoms,
diffusing through the carbonized sample; A is the atomic
vapor; Z are the atoms removed from the analytical zone;
k1 is the apparent rate constant of free-atom formation;
kgr – effective constant of the diffusion rate through the
carbonized sample; k2 is the rate constant of dissipation
(removal) of free atoms from the analytical zone.
Since the absorbance A(t) varies proportionally
with the amount of atomic vapor, and the free atoms
dissipation rate is constant [11], Scheme 1 can be
described by a set of equations
,
(2)
where X(t) is the amount of atoms in the source at a
moment t.
After mathematical transformations we get
Eq. 3, which describes the atomic absorption signal, i.e.,
the dependence of absorbance on the time for flamefurnace atomizer.
(3)
Analysis of the Eq. 3 shows that it is hard to design
any experiment that could allow experimental separating
of two processes – diffusion and atomization. We
propose to use an effective rate which is equal for both
processes: k1(t)=kgr(t)=keff(t).
Furthermore, an atomic absorption signal is formed
at increased temperature of atomization cell and the
apparent rate constant of free-atom formation, k1 and
kgr, are temperature- and, accordingly, time-dependent:

Ea 
,
k eff (t ) = k 0 exp −

 RT (t ) 
(4)
where k0 is the pre-exponential factor; Ea is the effective
energy of the free-atom formation; R is the universal gas
constant and T(t) is a time function of temperature.
General atomization process can be influenced by
several simultaneous processes. Thus, the apparent
rate constant of atomization is a numeral characteristic
of some overall rate but not of some certain process
rate. In this case the second equation in system (Eq. 2)
is reduced to
.
(5)
The following approach is usually used to find kinetic
and diffusion parameters of experimental signals. Using
various hypotheses and applying the transformation of
equations which describes atomic absorption signals
(within the definite model frame), we attempt to get the
expression like an Arrhenius equation. With the help of
this equation, necessary kinetic parameters are obtained
by different methods. In our case the equations which
describe an atomic absorption signal are non-linear and
they cannot be expressed as an Arrhenius-like equation.
Therefore another approach was applied. Experimental
atomic absorption signals were approximated by model
curves, obtained by the substitution of numerical values
of kinetic parameters in the equation, which describes
the absorbance dependence on time. The numerical
values of kinetic parameters were selected by means
of the least-squares procedure, using the Levenberg–
Marquardt iterative algorithm [12-14].
3. Experimental procedure
3.1. Apparatus
AA spectrophotometer was «Saturn-3» (Ukraine)
equipped with flame-furnace atomizer and deuterium
background corrector. The source of resonance radiation
was a hollow-cathode lamp LT-2 (Russia). Resonance
signal was recorded and treated using personal computer
IBM PC. Dosing unit for solid samples was a patented
device (dosing accuracy of 3%) from our Laboratory [15],
aliquots of the standard solutions were sampled by the
dosing unit UNIPIPETTE 2000 with an accuracy within
±0.4%. The structure of dry residue on the graphite rod
after atomization was studied using electron microscope
(JEOL JSM-820, Japan) equipped with the system for
energy-dispersion X-ray microanalysis (Link AN10/85S).
The fire gas was acetylene (Type B, second-class quality,
volume concentration 99.0%). The model mixture was
investigated by X-ray diffraction method. X-ray powder
diffraction patterns of the samples were recorded at room
temperature, using a powder diffractometer DRON-2
with the Ni-filtered copper radiation. The scanning rate
was 1° min-1.
3.2. Reagents
The reagents were analytical or higher grade. Standard
solutions of Pb(II) and Cd(II) nitrates were prepared
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Lead and cadmium atomic absorption determination
in solid carbonized food samples using flame-furnace atomizer
Table 1.
Analyte
Cd
Pb
Kinetic parameters of Pb and Cd atomization from solid carbonized food (atomization temperatures were 1873 and 1673 K respectively).
Carbonized sample
and modifier
Apparent activation
energyEa (kJ mol-1)
Pre-exponential
factor k0 (s-1)
Rate constant of
dissipation k2 (s-1)
Cocoa
114
7.0×105
2.0
Cocoa + DEDTK
128
4.0×106
1.9
5
2.0
Cocoa + urea
116
1.8×10
Cottage cheese
156
1.7×106
3.7
Cottage cheese + DEDTK
149
1.8×10
6
3.8
Cottage cheese + urea
150
2.1×106
3.7
from the standard reference solutions available from
SKTB OP PCI NAN of Ukraine, Odessa.
3.3. Laboratory Analyses
Absorbance of Pb and Cd was recorded at 283.3 nm and
228.8 nm, respectively. Standard solutions of lead(II)
(5 µL, 10 µg mL-1) and cadmium(II) (5 µL, 1 µg mL-1)
were sampled on the surface of graphite (type MPG-6)
rod followed by drying at 373 K during 20 s. Pyrolysis
was carried out during 40 s at a gradual temperature
rise from 373 K to 873 K or 623 K for lead and cadmium,
respectively. The heating program was then stopped,
gas burner with stoichiometric acetylene-air flame was
placed under the graphite rod and atomization process
was then carried out at 1873 K and 1673 K for Pb and Cd,
respectively. Absorption signals were measured twice in
each experiment. Absorbance impulse was recorded
by personal computer in units of integral absorbance,
the signal was digitized and then treated with software
package MathCad 14 to give apparent activation energy
Ea, pre-exponential factor k0, and the rate constant k2 for
dissipation of analyte from the analytical zone.
Corn grits, cottage cheese and cocoa powder were
chosen for analysis. Taking into account the fact that
the direct bringing of solid samples into the atomizer
led to spraying of samples at the stage of pyrolysis and
irreproducible results, we decided to use carbonization
(incomplete mineralization) of foodstuffs samples with
the following direct brining of solid carbonized food into
the atomizer. The samples of corn grits and cottage
cheese were held in the muffle burner at 723 K for 15
and 70 minutes respectively, cocoa samples – at 573 K
for 40 minutes. The received carbonized samples were
pounded and weighed.
The solid carbonized samples of 4 mg were sampled
directly on a surface of the graphite rod in the flamefurnace atomizer. In all cases the lead and cadmium
absorbance was within 0.4 s.
To decrease detection limit and improve the
repeatability of lead and cadmium AA determination
in food samples, chemical modifiers – sodium N,Ndiethyldithiocarbamate (DEDTK) and urea (mixed with
carbonized sample 1:10) were used. The results were
used for kinetic parameters determination of the free
atoms formation in the presence of the modifiers.
The
distribution
of
carbonized
sample
macrocomponents on the surface of the graphite rods
was studied by the electron microscopy after five
atomization cycles for each sample.
4. Results and discussion
Table 1 gives the kinetic parameters of Pb and Cd
atomization from solid carbonized food.
The physical meaning of effective energy of free
atom formation, Ea, is a minimum additional energy
needed for the start of their transition to the free state.
A pre-exponential factor, k0, is also responsible for the
transition rate. According to Absolute Rate Theory [16],
the rate of free-atom formation can be expressed as
,
where ΔS is the entropy of activation and Z is the
proportionality coefficient.
The possibility of atoms energies internal
redistribution between the degrees of freedom is
described by the entropy factor, exp(ΔS/R). The entropy
factor values are the same for atomization from the
surface of the graphite rod and from the pores in it.
Z and k0 values will be dependent on the free atoms
formation mechanism.
Table 1 shows that the presence of chemical
modifiers DEDTK and urea do not lead to considerable
change of kinetic parameters Ea, k0 и k2. This suggests
that the atomization mechanism has not changed. Free
lead and cadmium atoms are formed as a result of
dissociation or reduction of the same compounds. These
compounds may be sulphides of elements defined. The
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K. S. Lugovyy, O. M. Buhay, A. S. Alemasova
a
b
Figure 1. Electron microscopy graphite rod photographs after 5 atomization cycles of carbonized corn grits sample without a chemical modifier
(a) and with DEDTK (b).
evidence of it is a model experiment, where the graphite
powder, standard solutions of lead and cadmium and
DEDTK were mixed and this mixture was held in the
muffle burner under the condition of the carbonization
samples. X-ray analysis of dry residue proves the
formation of PbS and CdS under these conditions.
The observed increase of an analytical signal with the
modifiers is explained by the shift of thermodynamical
balance “pre-atomization compound ↔ free atoms” to
the free atoms formation when the chemical modifiers
are present.
The addition of chemical modifiers was found to
improve the results repeatability. We might suggest that
the adding of a chemical modifier influences the analyte
distribution on the surface of the graphite rod. To confirm
this, electron microphotos of the graphite rod surface of
atomizer flame-furnace were made after 5 atomization
cycles of a carbonized corn grits sample without a
chemical modifier and with DEDTK (Fig. 1).
Electron microscopy photographs (Fig. 1) show
that chemical modifiers affect the distribution of
sample dry residue on the surface of graphite rod.
Without the modifiers (а) the surface of the rod is a
three-dimensional structure with deep pores. With a
chemical modifier the graphite rod surface has “flatter”
and more uniform structure. It suggests more uniform
distribution of a sample on the graphite rod surface of
the flame-furnace atomizer with chemical modifiers.
Thus, the use of chemical modifiers results in the same
vaporization conditions of the solid particles from the
surface of graphite rod into the analytical zone. It better
explains result repeatability with our proposed chemical
modifiers.
Based on the above-mentioned findings, an
express technique for Pb and Cd determination in
the food samples with the chemical modifier, DEDTK
was developed. The apparatus was calibrated by the
standard additions technique. An aliquot of the standard
lead(II) or cadmium(II) solution was sampled to the rod
surface. Carbonized samples were weighed, then mixed
with chemical modifier DEDTK (10:1 by weight) and
sampled to the flame-furnace atomizer. After that, the
complete heating cycle was carried out.
The measurements were checked by comparison
with those of the standard flame atomic absorption
technique involving a dry mineralization procedure [17].
The results are given in Table 2.
The results of the two methods agree reasonably with
each other. Pb content in corn grits exceeds maximum
permissible concentration (MPC). The technique
developed here has satisfactory repeatability, with Sr
being no more than 6%. Detection limits of Pb and Cd
are considerably lower than their MPC (0.1 mg kg-1 and
0.05 mg kg-1, respectively). The analysis proceeds for
no longer than 2 hours, whereas the dry mineralization
procedure requires no less than 20 hours and can be
complicated by the loss of volatile lead and cadmium
compounds and contamination of samples.
5. Conclusions
The kinetic model of atomization of solid samples for
AA analysis in a flame-furnace atomizer was developed.
The procedure provides determination of the kinetic
parameters of free atoms formation in atomizer,
ascertaining a predominant atomization process. It
allows us to understand if the use of chemical modifiers
leads to an atomization mechanism change.
Efficient chemical modifiers – sodium N,Ndiethyldithiocarbamate and urea - were proposed for
improvement in precision of AA determination of lead
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Lead and cadmium atomic absorption determination
in solid carbonized food samples using flame-furnace atomizer
Table 2. Weight fraction of Pb and Cd in food (n=4; P=0.95).
Sample
Analyte
Direct measurement in flame-furnace
atomizer; modifier – DEDTK
Corn grits
Cottage
cheese
Cocoa
Pb
Cd
Standard procedure
(EN 14082:2003)
c ± δ (mg kg-1)
RSD (%)
c ± δ (mg kg-1)
RSD (%)
MPC
[18]
(mg kg-1)
1.1±0.1
5.0
1.1±0.1
3.7
0.5
0.26±0.02
5.7
0.25±0.01
2.3
0.3
0.080±0.006
4.6
0.080±0.004
3.2
1.0
0.24±0.02
5.5
0.24±0.02
3.4
0.5
RSD – relative standard deviation
and cadmium in the solid samples of carbonized food.
It has been shown that modifiers do not change the
atomization mechanism, which mainly adds up to lead
and cadmium sulphide pyrolysis or reduction.
Precision express methods of lead and cadmium
determination in the solid samples of carbonized food
with the chemical modifiers was developed. Time
needed for analysis does not exceed 2 hours. The
relative standard deviation is within 6%.
Acknowledgements
We acknowledge the Ministry of Education and Science
of Ukraine for financial support of our work (grant No
0112U002704).
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