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Fields of Application / Industry:
Chemistry / Polymer Industry
Clinical Chemistry / Medicine /
Hygiene/ Health Care
Electronics
Semi-Conductor Technology
Energy
Environment / Water / Waste
Geology / Mining
Food / Agriculture
Metallurgy / Galvanization
Refineries / Petrochemistry
Pharmacy
Cosmetics
Material Analysis
Others
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Sample Preparation for the Determinationof Metals in Food Samples Using
Spectroanalytical Methods—A Review
Maria das Gracas Andrade Korn,1 Elane Santos da BoaMorte,1 Daniele Cristina Muniz Batista dos Santos,1
Jacira Teixeira Castro,1 Jose Tiago Pereira Barbosa,1
Alete Paixao Teixeira,1 Andrea Pires Fernandes,1
Bernhard Welz,1 Wagna Piler Carvalho dos Santos,1,2
Eduardo Batista Guimaraes Nunes dos Santos,3
and Mauro Korn3
1NQA/GPQA, Instituto de Quımica, Universidade Federal da Bahia,
Campus de Ondina, Salvador, Bahia, Brazil2Centro Federal de Educacao Tecnologica da Bahia, Barbalho,
Salvador-Bahia, Brazil3NQA/SONOFIA, Departamento de Ciencias Exatas e da Terra,
Universidade do Estado da Bahia, Salvador, BA, Brazil
Abstract: The present article gives an overview of recent publications and modern
techniques of sample preparation for food analysis employing atomic and inorganic
mass spectrometric techniques, such as flame atomic absorption spectrometry,
chemical vapor generation atomic absorption and atomic fluorescence spectrometry,
(ASLE), and enzymatic probe sonication (EPS), were compared for the determi-
nation of Cd and Pb by GF AAS in biological reference materials (47). The
sample mass chosen to perform the analysis was 10 mg and the liquid volume
was 1 mL of 1 mol L21 nitric acid. Optimum performance (total metal extrac-
tion) of ultrasound-assisted ASLE for Cd was only achieved in two of the four
materials investigated, and total Pb recovery was only possible in three of the
five samples. Total extraction with the enzymatic probe sonication was only
obtained for Cd in oyster tissue. Neither ASLE nor EPS were able to extract
Cd or Pb from spruce needles. Pb concentration obtained after EPS was
found to be highly dependent on sample centrifugation speed and time.
Krishna and Arunachalam (48) investigated the application of an ultra-
sound-assisted extraction procedure for the determination of major, minor,
and trace elements in lichen and mussel tissue as a possible alternative to con-
ventional digestion methods. ICP-MS and ICP OES were used for the quanti-
fication of the elements. Parameters affecting extraction, such as extractant
Determination of Metals in Foods 79
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concentration, sonication time, and ultrasound amplitude, were optimized to
get quantitative recovery of elements. These studies indicate that the
method is fast (within 15 min including centrifugation time) and simple for
the determination of Na, K, Ca, Mg, Cr, Mn, Co, Ni, Cu, Zn, Ge, As, Se,
Rb, Sr, Zr, Ag, Cd, In, Sb, Cs, Ba, Pb, and Bi. Quantitative recoveries were
obtained for most of the elements for which certified concentrations were
available using a 1% (v/v) HNO3 as extractant and metal solubilization
could be achieved within 4-min sonication time at 40% sonication
amplitude and 100 mg sample weight. An overall precision of better than
10% could be achieved for many elements in multiple extractions. Closed-
microwave digestion method was also used for the estimation of various
elements in lichen and mussel samples for comparison.
A new sample preparation procedure for elemental characterization,
involving acid extraction of the analytes from onion cultivar samples by
means of an ultrasonic bath, was proposed by Alvarez et al. (49). The
technique of total reflection X-ray fluorescence was successfully applied for
the simultaneous determination of Ca, K, Mn, Fe, Cu, and Zn. The procedure
was compared with wet and dry ashing procedures for all the elements using
multivariate analysis and the Scheffe test. The FAAS technique was
employed for comparison purposes and accuracy evaluation of the proposed
analytical method. Good agreement between the two techniques was found
when using the dry ashing and ultrasonic leaching procedures.
Slurry Sample Preparation
Slurry sampling was considered to have certain advantages over direct solid
sampling, since it is possible to change the slurry concentration by simple
dilution, hence combining the advantages of solid and liquid sampling.
Another advantage that has been claimed is that aqueous standards may be
used for calibration. However, the stabilization of the slurry, its homogeneity,
particle size, and sedimentation also have to be considered.
Li and Jiang (50) used an electrothermal vaporization dynamic reaction
cell ICP-MS to determine trace elements in rice slurry samples. The
influence of instrument operating conditions and slurry preparation on the
ion signals was investigated. Since the sensitivities of Cr, Cu, Cd, Hg, and
Pb in the rice flour slurry and aqueous solution were quite different,
standard addition and isotope dilution methods were used for the determi-
nation of these elements in NIST SRM 1568a rice flour CRM and two rice
samples purchased from the market. The analytical results for the CRM
agreed with the certified values. The results for the rice samples, for which
no reference values were available, were also found to be in good
agreement between the isotope dilution and standard addition methods.
Vinas et al. (51) developed a rapid and accurate procedure for the deter-
mination of Se, Cd, and Pb in different types of baby food using slurry
M. das G. A. Korn et al.80
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sampling and GF AAS. Suspensions prepared in a medium containing 0.1%
(w/v) Triton X-100, 30% (v/v) concentrated hydrogen peroxide, 1% (v/v)concentrated nitric acid and a chemical modifier (0.5% (w/v) nickel for Se,0.2% (w/v) nickel plus 1% (w/v) ammonium dihydrogenphosphate for Cd,
and 1% (w/v) ammonium dihydrogenphosphate for Pb) were introduced
directly into the furnace. Calibration with aqueous standard solutions was
used for the determination of Se and Pb, while the standard addition
technique was used for Cd determination. The reliability of the procedures
was established by comparing the results obtained with those found for five
fish-based baby foods after MW-assisted digestion and by analyzing six bio-
logical CRM. The results showed that no previous sample mineralization was
necessary because the experimental procedure was simple, which reduces the
risks of contamination and loss through volatilization.
Silva et al. (52) developed a method to determine Mn and Zn in powdered
chocolate samples by slurry sampling FAAS. The optimized conditions,
which were established using univariate methodology, were a sample mass
of 150 mg, 2.0 mol L21 nitric acid solution, sonication time of 15 min, and
a slurry volume of 50 mL. The analytical results were compared with those
obtained after open vessel and acid bomb digestion procedures and determi-
nation using FAAS. The statistical comparison by t-test (95% confidence
level) showed no significant difference between these results.
Lopez-Garcıa et al. (53) proposed a procedure for GF AAS determination
of phosphorus in honey, milk, and infant formulas using slurry sampling. Sus-
pensions prepared in a medium containing 50% v/v concentrated hydrogen
peroxide, 1% v/v concentrated nitric acid, 10% m/v glucose, 5% m/vsucrose, and 100 mg L21 of potassium were introduced directly into the
furnace. Calibration was performed using aqueous standards prepared in the
same suspension medium and the analytical curve was linear between 5 and
80 mg L21 P. The reliability of the procedure was checked by comparing
the results obtained by the proposed method with those found with a
reference spectrophotometric method after mineralization and by analyzing
several CRM. Cava-Montesinos et al. (54) developed a simple and fast
procedure for the determination of As, Sb, Se, Te, and Bi in milk samples
by HG AFS. Samples were treated with aqua regia for 10 min in an ultrasound
water bath and pre-reduced with KBr for total Se and Te determination or with
KI and ascorbic acid for total As and Sb; the determination of Bi was possible
with or without pre-reduction. Slurries of samples, in the presence of
Antifoam A, were treated with NaBH4 in HCl medium to form the correspond-
ing hydrides, and the calibration solutions were prepared and measured in the
same way as samples. Results obtained by the developed procedure compared
well with those found after MW-assisted digestion of samples. The proposed
method is simple and fast, and only 1 mL of milk is required.
Anthemidis and Pliatsika (55) developed a simple on-line slurry formation
and direct nebulization system for multi-element analysis of cocoa and coffee
powder samples by ICP OES. A laboratory-made microchamber with a
Determination of Metals in Foods 81
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magnetic stirrer was used for on-line slurry formation in a dispersant solution of
0.5% v/v Triton X-100 and 1% v/v HNO3. A Babington-type nebulizer
combined with cyclonic-type spray chamber was adopted for on-line slurry neb-
ulization. The recommended particle size was ,70 mm and the slurry concen-
tration was 0.6%m/v, while the working slurry concentration could range from0.3 to 3.3% m/v with proportional sensitivity. Excellent agreement was found
between the standard addition calibration procedure and calibration against
aqueous standard solutions for almost all of the investigated elements. The
reliability of the proposed method was confirmed by comparing it with FAAS
and GF AAS after wet digestion and no significant differences were observed
between the two methods.
A simple method combining slurry sampling after cryogenic grinding and
the use of a permanent modifier was proposed for the determination of Cd and
Pb in foods by GF AAS (56). The potentialities of cryogenic grinding were
evaluated for different materials that are difficult to homogenize, such as
high-fat and high-fiber tissues. Animal and vegetal samples were cut into
small pieces and ground in liquid nitrogen for 2 min. Slurries were prepared
directly in the autosampler cup by transferring an exact amount of ground
material (5–20 mg) to the cup, followed by 1.00 mL of 0.2% (v/v) HNO3
containing 0.04% (v/v) Triton X-100 and sonication for 30 s, before transfer-
ring onto the platform that has previously been coated with 250 mg W and
200 mg Rh. No statistical differences were found by the paired t-test at the
95% level between the results for Cd and Pb in foods slurries and those
obtained with digested samples.
Santos et al. (57) tested five different slurry preparation procedures for
fish tissue samples after grinding the solid samples to a particle size of
53 mm: (1) using aqua regia plus HF, 30 min of sonication, standing time of
24 h followed by another 30 min of sonication; (2) same as the previous
one, except that the standing time and the second ultrasound treatment were
omitted; (3) same as the previous one, except that HF was not used; (4)
same as the previous one, except that the aqua regia was replaced by nitric
acid; (5) same as the previous one, except that the nitric acid was replaced
by tetramethylammonium hydroxide (TMAH). The Hg vapor was generated
in a continuous-flow system and the emission signal intensity measured on-
line at 253.652 nm by axial view ICP OES. The first three procedures
produced results in agreement with the certified values. The two last pro-
cedures using nitric acid or TMHA could not be used for quantitative determi-
nation. For practical reasons, Procedure 3, with a detection limit (3 s, n ¼ 10)
of 0.06 microgram per gram for a sample mass of 20 mg in a final volume of
15 mL was recommended, because it was simple, rapid, and robust.
Bugallo et al. (58) developed a slurry sampling method for the determi-
nation of Ca, Cu, Fe, Mg, and Zn in fish tissue samples by FAAS. In compari-
son with microwave-assisted digestion, the proposed method was simple,
required only a short time, and eliminated total sample dissolution before
analysis. The suspension medium was optimized for each analyte to obtain
M. das G. A. Korn et al.82
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quantitative recoveries from fish tissue samples without interferences.
However, Fe recoveries were not higher than 46%. Treatment of samples
suspended in nitric acid by MW irradiation for 15–30 s at 75–285 W
permitted achieving efficient recoveries for Ca, Fe, Mg, and Zn. Further
reduction of matrix effects for iron determination was accomplished by the
use of an additional short step of MW-assisted slurry treatment. However,
use of the standard addition technique was required for Ca and Cu determi-
nation, and hydrochloric acid had to be used as suspension medium for the
last one. The standard deviations obtained using slurry sampling method
and MW-assisted digestion were not significantly different, and the mean
relative standard deviation of the overall method (n ¼ 3) of the slurry
sampling method for different concentration levels was less than 12%.
DIRECT SOLID SAMPLING ANALYSIS
Direct solid sampling (SS) analysis is the oldest technique for the determi-
nation of metals by spectrometric techniques using arc or spark emission
and, together with X-ray fluorescence spectrometry, it is still the most
widely used technique in metallurgical laboratories nowadays. Among the
techniques that can be used for direct SS in combination with AAS, ICP
OES, and ICP-MS are laser ablation and electrothermal atomization or vapor-
ization. From these alternatives, GF AAS has been shown to be the most
attractive technique for the direct analysis of solid samples, mainly because
of the absence of a nebulizer system, which simplifies the introduction of
the solid material into the atomizer. Direct SS analysis offers a number of
advantages, such as the reduced sample preparation time and hence a faster
analysis; higher accuracy, as errors due to analyte loss and/or contamination
are dramatically reduced; higher sensitivity due to the absence of any dilution;
and the absence of any corrosive or toxic waste. Another advantage is the long
residence time of the sample in the GF AAS atomizer, which usually makes
possible complete volatilization of the particles independent of their size
and complete atomization of the analyte. Moreover, it shows quite low
limits of detection, which is highly desirable in trace analysis. Most of the dis-
advantages that have been mentioned for direct SS analysis using GF AAS are
actually no longer valid. There are reliable tools available nowadays both for
manual and automatic introduction of solid samples into the graphite furnace,
and it has been shown that in most cases aqueous standards can be used for
calibration also in direct SS analysis. The only limitations that have to be
mentioned are the relatively short linear working range of AAS, which
usually limits direct SS analysis to the determination of low trace concen-
trations, and the imprecision of the results, which is typically of the order
of 10% due to the inhomogeneity of natural samples (59).
Flores et al. (60) developed a new device to introduce solid biological test
samples directly into the flame of an AAS instead of the traditional
Determination of Metals in Foods 83
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introduction systems and also to avoid prior sample combustion-vaporization.
Copper was determined directly in bovine liver samples by FAAS without the
need of extraction, digestion, or slurry preparation. Between 0.05 and 0.50 mg
of the test sample was weighed directly into a small polyethylene vial
connected to a glass chamber. A flow of air carries the test sample as a dry
aerosol to a T-shaped quartz cell positioned in the optical path above the
burner. The atomic vapor generated produced a transient signal of less than
3 s duration; integrated absorbance was used for signal evaluation. The
results were compared with those obtained after a conventional sample
digestion and there was no statistical difference between the results from
the proposed system and those obtained after digestion and determination
by conventional FAAS. No excessive grinding of the samples was required
and samples with particle size less than 80 mm were used throughout. Back-
ground signals were always low and a characteristic mass of 1.5 ng was
found for Cu. The proposed system allows the determination of 60 test
samples in 1 h and it can be easily adapted to conventional atomic absorption
spectrometers.
Detcheva and Grobecker (61) developed direct SS methods for GF AAS
and applied these to the determination of Hg, Cd, Mn, Pb, and Sn in seafood.
All elements except for Hg were measured using a third-generation Zeeman-
effect AAS combined with an automatic solid sampler. The calibration range
was substantially extended using the three-field and dynamic mode and high
analyte masses could be determined without laborious dilution of solid
samples. The measurements were based on calibration with CRM of
organic matrices. In case solid CRM were not available, calibration with
aqueous standard solutions was proved to be an alternative. No matrix
effects were observed under optimized conditions and results were in good
agreement with the certified values. Direct SS-GF AAS with Zeeman-effect
background correction proved to be a reliable, rapid, and low-cost method
for the control of trace elements in seafood.
Grobecker and Detcheva (62) validated the determination of total
mercury by direct SS-GF AAS with Zeeman-effect background correction
and a specially designed furnace using CRM of different origin. The tempera-
ture program provided only one stage; atomization of mercury and pyrolysis
of the matrix was performed at a constant temperature in the range of 900–
10008C. A calibration curve established using aqueous solutions and solid
CRM; all points were covered by one line, indicating that mercury determi-
nation was matrix independent using this technique. Even relatively high
amounts of chlorine, which are known for causing problems in mercury deter-
mination, did not influence analytical results. The accuracy of the method
became evident when comparing certified and experimental values. The
precision of the measurements in a range from 0.5 to 50 ng Hg did not
exceed 3% RSD.
Oleszczuk et al. (63) developed a method for the determination of cobalt,
copper, and manganese in green coffee using direct SS-GF AAS. The authors
M. das G. A. Korn et al.84
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used a number of botanical CRM and preanalyzed samples of green coffee for
method validation, and ICP OES after MW-assisted acid digestion of the
samples was used as reference method. Manganese and cobalt could be deter-
mined using aqueous standards for calibration, but calibration with solid CRM
was necessary for the determination of copper. No significant difference was
found between the results obtained with the proposed method and certified or
independently determined values. Seven samples of Brazilian green coffee
were analyzed, and there was no significant difference between the values
obtained with SS-GF AAS and ICP OES for Mn and Cu. Hence, a single-
element technique that does not require any sample preparation besides
grinding of the coffee beans appears to be an attractive alternative to the
multi-element techniques that have been used up to now. The much better sen-
sitivity of this technique is an additional advantage in the determination of
trace elements such as cobalt and others that might be of importance.
Oliveira et al. (64) investigated systematically sample preparation and
micro-homogeneity for the determination of Cd and Pb in bovine liver using
direct SS-GF AAS. Two different procedures for sample preparation have been
investigated: (a) drying in a household microwave oven followed by drying in
a stove at 608C to constant mass and (b) freeze drying; ball and cryogenic
mills were used for grinding. Particle size, sample size, and microsample hom-
ogeneity were investigated. All samples showed good homogeneity (He , 10)
even for low sample mass, but samples dried in a microwave oven/stove and
ground in a ball mill presented the best homogeneity. The results obtained
with both methods of sample preparation indicated the possibility to produce
bovine liver of reference for determination of Cd and Pb by SS-GF AAS.
A very interesting series of studies about direct SS-GF AAS has been
carried out by Lucker et al. (65–70) between 1987 and 1999, investigating
the possibility of analyzing fresh meat for contaminants as kind of a
screening method to be carried out directly in the slaughterhouse. This idea
has been picked up recently by Damin et al. (71) in order to find out if this
technique could be used within the Brazilian program of residue control in
products of animal origin. The authors investigated the determination of Cd
and Pb in fresh meat, which was weighed directly onto the SS platform
using palladium and magnesium nitrates as a mixed modifier. The results
were in good agreement with those obtained after acid digestion, taking into
account the average humidity of 27+ 2% of fresh meat. Aqueous standards
could be used for calibration and the limits of detection of 0.13 mg kg21 for
Cd and 1.9 mg kg21 for Pb as well as the average RSD of 14% were more
than adequate for the purpose.
The recently introduced technique of high-resolution continuum source
(HR-CS) AAS (72–74) appears to offer even greater advantages for direct
SS-GF AAS, as the entire spectral environment of the analytical line becomes
visible at high resolution. This feature makes it possible to detect and avoid
spectral interferences and the system also offers new possibilities to correct
for spectral interferences and is greatly facilitating method development (72).
Determination of Metals in Foods 85
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Ribeiro et al. (75) investigated the determination of Co in fish and other biologi-
cal samples comparing direct SS-GF AAS and alkaline treatment with TMAH
and conventional line source GF AAS with HR-CS GF AAS. Method develop-
ment was found to be much easier using HR-CS AAS, and the best LOD of 5 ng
g21 was obtained with direct SS and HR-CS GF AAS.
Borges et al. (76) used HR-CS GF AAS for the determination of Pb in fish
and meat CRM using direct SS analysis. Ruthenium was used as a permanent
modifier, and the 217.001-nm resonance line was used for the determination
because of its better signal-to-noise ratio with HR-CS AAS. Under
optimized conditions the electron excitation spectrum of the PO molecule
with rotational fine structure could be separated in time from the Pb absorption
signal, avoiding any spectral interference. A limit of detection of 10 ng g21
could be obtained and the precision was typically better than 10% RSD.
The values obtained in seven CRM were in agreement with the certified
values according to the t-test for a 95% confidence level.
Da Silva et al. (77) investigated the determination of Hg in fish and meat
CRM using direct SS analysis with HR-CS GF AAS. Initial experiments
indicated that it was not possible to use a chemical modifier for this kind of
analysis, as in this case the Hg absorption peak would coincide with the
excessive background absorption caused by the organic matrix. It was also
found that without a modifier Hg from fish samples was already lost at temp-
eratures around 1008C, as it is mostly present as methyl mercury in this matrix,
which is much more volatile than inorganic Hg. The authors finally used a
temperature program without a pyrolysis stage, using only a drying stage of
3 s at 1008C, followed directly by the atomization stage at 11008C. Underthese conditions the Hg signal appeared before the background and could
be separated because of the superior background correction capabilities of
HR-CS AAS. Aqueous standards were used for calibration, which had to be
stabilized with potassium permanganate in order to avoid losses of Hg in
the drying stage. Good agreement was found between determined and
certified values for six CRM according to the t-test for a 95% confidence
level. The precision, expressed as RSD, was typically around 5% and the
detection limit was determined as 0.1 mg g21 Hg in the solid sample.
CONCLUSION
Assured information about metal concentration in food samples is essential
from the society from the nutritional, technological and toxicological point
of view. Atomic and inorganic mass spectrometric techniques , after appropri-
ate sample preparation, are most frequently used in order to obtain the required
and reliable information about metals in foods, particularly at trace levels.
Ultratrace species need particular laboratorial structures.
In this perspective, the integrity of chemical information is strongly
dependent on the prior analytical steps and an adequate selection of sample
M. das G. A. Korn et al.86
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preparation procedure is of capital importance. It was observed that different
sample preparation procedures have been successfully applied for determi-
nation of a wide variety of elements in diverse food samples and the trends
are to minimize sample handling and reagent consumption in order to
reduce sample contamination and to improve analytical throughput. In this
sense, direct solid analysis and slurry analysis have obtained special interest
of analytical chemists since they cover the mentioned sample preparation
trends.
ACKNOWLEDGMENTS
The authors are grateful to the CNPq (Conselho Nacional de Desenvolvimento
Tecnologico, Brasılia, Brazil) and FAPESB (Fundacao de Amparo a Pesquisa
do Estado da Bahia, Salvador, Brazil) for fellowships and financial supports.
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