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Applied Spectroscopy Reviews, 43: 67–92, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0570-4928 print/1520-569X online
DOI: 10.1080/05704920701723980
Sample Preparation for the Determination
of Metals in Food Samples Using
Spectroanalytical Methods—A Review
Maria das Graças Andrade Korn,1 Elane Santos da Boa
Morte,1 Daniele Cristina Muniz Batista dos Santos,1
Jacira Teixeira Castro,1 José Tiago Pereira Barbosa,1
Alete Paixão Teixeira,1 Andrea Pires Fernandes,1
Bernhard Welz,1 Wagna Piler Carvalho dos Santos,1,2
Eduardo Batista Guimarães Nunes dos Santos,3
and Mauro Korn3
1
NQA/GPQA, Instituto de Quı́mica, Universidade Federal da Bahia,
Campus de Ondina, Salvador, Bahia, Brazil
2
Centro Federal de Educação Tecnológica da Bahia, Barbalho,
Salvador-Bahia, Brazil
3
NQA/SONOFIA, Departamento de Ciências 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,
graphite furnace atomic absorption spectrometry, inductively coupled plasma optical
emission spectrometry, and inductively coupled plasma mass spectrometry. Among
the most frequently applied sample preparation techniques for food analysis are dry
ashing, usually with the addition of an ashing aid, and acid digestion, preferably
with the assistance of microwave energy. Slurry preparation, particularly with the
assistance of ultrasound, is increasingly used to reduce acid consumption and sample
preparation time. Direct analysis of solid samples is gaining importance in the field
of food analysis as it offers the highest sensitivity, avoids the use of acids and other
Address correspondence to Maria das Graças Andrade Korn, NQA/GPQA,
Instituto de Quı́mica, Universidade Federal da Bahia, Campus de Ondina, 40170-115
Salvador, Bahia, Brazil. E-mail: [email protected]
67
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68
M. das G. A. Korn et al.
aggressive reagents, makes possible the analysis of micro-samples, and can be applied
for fast screening analysis, e.g., of fresh meat.
Keywords: Food samples, sample preparation, trace element determination, atomic
spectrometry, inorganic mass spectrometry
INTRODUCTION
Elemental food composition data are important to both consumers and health
professionals, and recent food labeling legislation has highlighted this requirement. The determination of trace elements and contaminants in complex
matrices, such as food, often requires extensive sample preparation and/or
extraction regimes prior to instrumental analysis. Flame atomic absorption
spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GF
AAS), and inductively coupled plasma optical emission spectrometry (ICP
OES) are the main techniques used for the determination of trace element
contents in food analysis laboratories. The traditional techniques for sample
preparation are time consuming and require large amounts of reagents,
which are expensive, generate hazardous waste, and might contaminate the
sample with the analytes. Advances in sample preparation over the last few
decades have been propelled by the advance of microwave-assisted acid
digestion (1 – 3), ultrasound-assisted, extraction and slurry preparation (4),
and direct solid sampling analysis (5).
Quality control and safety in the food supply chain demands reliable
methodology that is both rapid and easily transferable. In order to minimize
the uncertainty in sample preparation a number of factors need to be considered. As statistically the degree of uncertainty in a method is directly
related to the number of stages involved, a minimization of that number
should reduce the uncertainty proportionally. Automation and mechanization
of processes also leads to a reduction in uncertainty. Automated procedures
are generally more reproducible than manual methods and will also
decrease the staff time spent on sample preparation, which is often the bottleneck in analytical laboratories (4, 5).
This review will discuss recent development in procedures for sample
preparation of food samples particularly under the above-mentioned aspects.
The discussion emphasizes analytes, samples, and effects on measurement
conditions using atomic and mass spectrometric techniques.
DRY-ASHING TECHNIQUES
Dry ashing is a sample preparation method generally convenient to be applied
for subsequent trace metal determination in food materials. Dry ashing or
oxidation is usually performed by placing 0.1 – 1 g of the sample in an open
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Determination of Metals in Foods
69
vessel and removing the organic matter from the samples by thermal
decomposition, normally in the presence of an ashing aid, using a muffle
furnace. Typical ashing temperatures are 450 to 5508C at atmospheric
pressure, and the ash residues are dissolved in an appropriate acid. The
degree of volatilization loss is a limiting factor and depends on (i) the
applied temperature, (ii) the form in which the analyte is present in the
sample, and (iii) the chemical environment in the ashing stage. Oxidizing
reagents may be used as ashing aids in order to prevent the volatilization of
analytes and also to speed up the ashing process. High-purity magnesium
nitrate and magnesium oxide are commonly used for that purpose (6).
Several papers have been published about dry ashing as a sample preparation method for metal determination in food samples. Tüzen et al. investigated the application of dry ashing to promote the decomposition of fish (7),
baby food (8), and honey (9). Approximately 1 g of sample was submitted to
dry ashing at 4508C for 4 – 16 h, depending on the matrix, and the residue was
dissolved in nitric acid. Aluminum, Cd, Co, Cr, Cu, Fe, Mn, Se, and Zn were
determined using GF AAS; recoveries were quantitative (95%) for all investigated elements.
Mindak et al. (10) developed a flow-injection hydride generation atomic
absorption spectrometry (FI-HG AAS) method for the determination of total
arsenic and selenium in food. A combination of microwave-assisted
digestion with nitric acid and dry ashing utilizing magnesium nitrate and
magnesium oxide as ashing aids was used to destroy the organic matrix
including refractory organometallic compounds present in many food
samples. Complete mineralization of these compounds is a pre-requirement
for the application of hydride generation for these elements. The resulting
ash was dissolved in hydrochloric acid and diluted to volume. The method
was validated using 21 food samples and nine reference materials.
The application of dry ashing methods is simple and large quantities of
food samples may be treated at the same time. This procedure permits the preconcentration of trace elements in the final solution, which is useful when very
low concentrations are to be determined. The ash is also completely free of
organic matter, which is a prerequisite for some analytical techniques. The
addition of an ashing aid, on the other hand, increases the content of
inorganic salts significantly, which might be a problem for the subsequent
determination of trace elements, and it might also contribute to contamination,
necessitating careful blank control.
WET-ASHING TECHNIQUES
Wet digestion methods include sample decomposition by an acid or mixtures
of acids, carried out in open vessels, in tubes, on a hot plate or in an aluminum
heating block or in closed vessels at elevated pressure (digestion bombs) with
thermal or microwave heating. Microwave-assisted digestion is an attractive
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M. das G. A. Korn et al.
method, especially for small samples. Extreme care should be exercised in
using sealed pressure vessels since there is much anecdotal evidence of
these vessels rupturing occasionally during conventional or microwaveassisted digestion of organic materials. The applicability of this technique is
strictly dependent on the type of food: carbohydrates are easily mineralized
with nitric acid at 1808C, while fats, proteins, and amino acids cause incomplete digestion due to the relatively low oxidation potential of nitric acid at
2008C; these materials require the addition of sulfuric and/or perchloric
acid with all the problems related to their use at high temperature and pressure.
The type of acid used in the preparation procedure can have important
consequences in the measurement step. It is commonly known that in all
atomic spectrometric techniques nitric acid is the most desirable reagent. In
spite of occasionally observed signal suppression in its presence (e.g., in
ICP OES), no severe analytical problems are encountered in practice with
nitric acid at concentrations up to 10%, sometimes higher, in all atomic spectrometric techniques as long as its concentration is similar in calibration and
sample solutions. Hydrogen peroxide, added in most mineralization procedures, is also rarely responsible for analytical problems (1). The presence
of hydrochloric acid is not troublesome in ICP OES analysis; however, its
exclusive use is kind of prohibited in GF AAS analysis because of the
possible formation of volatile and difficult-to-dissociate analyte chlorides
that could cause vapor phase and/or spectral interference (11). Because of
its high viscosity, utilization of sulfuric acid is usually avoided in spite of
its efficiency in digestion of organic matrices. Its presence is particularly undesirable in analytical techniques where the sample introduction is by nebulization (FAAS, ICP OES, ICP-MS).
Momen et al. (12) investigated two digestion procedures for the determination of essential (Cr, Cu, Fe, Mg, Mn, Zn) and non-essential (Al, Ba, Cd, Pb)
elements in nuts by ICP OES. The procedures included wet digestion with
HNO3/H2SO4 and HNO3/H2SO4/H2O2 in PTFE vessels and experimental
designs were used for optimization. The factors studies were HNO3, H2SO4,
and H2O2 volumes, digestion time, predigestion time, temperature of the hot
plate, and sample weight. The factors HNO3 and H2O2 volume and the
digestion time were found to be the most important parameters. The good
agreement between measured and certified values for all analytes (relative
error , 11%) in two certified reference materials (CRM), IAEA-331,
spinach leaves and IAEA-359, cabbage, indicates that the developed analytical method was working well.
In another study, Momen et al. (13) assessed four procedures for the determination of essential (Cr, Cu, Fe, Mg, Mn, Zn) and toxic (Al, Cd, Pb) elements
in legumes by ICP OES. These included wet digestion with HNO3/H2SO4 and
HNO3/H2SO4/H2O2 and dry ashing with Mg(NO3)2 and Mg(NO3)2/HNO3,
respectively. The precision, expressed as RSD for an aqueous standard containing 250 mg L21 of each analyte, was in the range of 1.5 –8.0%. The
accuracy, expressed as relative error was generally within the range of
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Determination of Metals in Foods
71
0.5– 10% for all analytes, and the quantification limits were lower than
2.5 mg g21. Although acceptable results were obtained with all procedures,
wet digestion with HNO3/H2SO4/H2O2 was recommended because of the
better recovery. The good agreement between measured and certified concentrations for IAEA-331 and IAEA-359 CRM indicates that the developed
analytical method is well suited for the determination of toxic and nutrient
elements in legumes and possibly similar matrices.
ICP OES with continuous hydride generation was used for the determination of As in seafood samples (14). The lyophilized samples were
digested with concentrated nitric and sulfuric acid. The reliability of the
developed method was checked by analyzing several CRM. Complete mineralization was obtained for an arsenobetaine-containing CRM with a mixture of
nitric and sulfuric acids followed by adding hydrogen peroxide in an open
digestion system and a digestion time of 4 h.
Studies on the transfer of chemical contaminants through the food chain
provide useful information for the development of surveillance programs
aimed at ensuring the safety of the food supply and minimizing human
exposure to toxic agents. Tinggi et al. (15) investigated two wet digestion procedures using acid mixtures of HNO3/H2SO4/HClO4 and HNO3/H2SO4 for
decomposition of food samples in Australian diet. The addition of hydrofluoric acid to the mixture of HNO3/HSO4 was also investigated for the determination of Cr. All the acid mixtures tested were found to be satisfactory but, for
safety reasons, HNO3/H2SO4 was the method of choice. Olivares et al. (16)
employed a wet digestion procedure using a mixture of nitric, perchloric,
and sulfuric acids for sample preparation of common Chilean foods for the
determination of Fe, Zn, and Cu by FAAS and assessed the intake of these
elements in a population living in Santiago, Chile. In another study, the
levels of essential elements, such as Cu, Cr, Fe, and Zn, and toxic elements
such as Al, Ni, Pb, and Cd were evaluated in a total of 40 samples of
legumes and 56 samples of nuts that are widely consumed in Spain (17).
These samples were mineralized in a digestion block with HNO3 and V2O5
and determined using GF AAS as the analytical technique. The reliability of
the procedure was checked by the analysis of a CRM; no matrix effects
were observed and aqueous standard solutions were used for calibration.
Kira et al. (18) developed a fast procedure for the determination of Ca, Cr,
Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk samples by ICP OES. This procedure
consisted of a partial digestion with hydrochloric acid on a hot plate. The results
were compared with two digestion procedures (dry ashing and microwaveassisted acid digestion). All the procedures showed similar levels of
precision, with coefficients of variation ,10% for the majority of the
elements. The accuracy was evaluated using a CRM, and the values were
within the confidence intervals for these products. Rodriguez et al. (19) used
a mixture of HNO3 and HClO4 (9:1 v/v) for the preparation of bovine milk
samples. Calcium, Cu, Fe, K, Mg, Na, Se, and Zn were determined by
FAAS, FAES, and fluorimetry. In another paper, Cava-Montesinos et al. (20)
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M. das G. A. Korn et al.
compared two sample preparation procedures for bovine milk samples for the
determination of Se and Te by HG AFS. The first digestion was in a
microwave oven using HNO3 and 30% v/v H2O2. The other procedure
consisted of many stages using a muffle furnace and a hot plate. A suspension
with 10% (m/v) Mg(NO3)2 . 6H2O and 1% (m/v), MgO as an ashing aid was
used for dry ashing of the sample in the muffle furnace. The ash was treated with
KBr and HCl before the quantification of the analytes using HG AFS. The
proposed method involved the use of a low-cost instrumentation, and
microwave-assisted sample pretreatment provides fast and accurate results.
Ferreira et al. (21) developed an acid digestion procedure for the determination of Cu in various food samples of animal and plant origin. A (3:1 v/v)
mixture of HNO3:HClO4 was used for digestion of the samples on a hot plate
until the total oxidation of the organic material. Copper was determined by
FAAS.
Santos et al. (22) proposed a fast and inexpensive wet digestion procedure
for beans samples. Essential (Ca, Cu, Fe, K, Mg, Mn, Ni, P, Zn) and nonessential (Al, Ba, Sr) elements were determined in bean digestates by ICP
OES. Experimental designs for five factors (HNO3 and H2O2 volume,
digestion time, block temperature, and particle size) were used for optimization of the digestion procedure, adopting a factorial experiment with 2521
design. The factor block temperature was found to be the most important
parameter and Doehlert designs were applied in order to determine the
optimum conditions. Digestion conditions were attained using 3.5 mL of concentrated HNO3 for 45 min. The accuracy of the results was demonstrated
using one CRM (spinach leaves NIST 1570a) and comparison with the
recommended official method.
Microwave-Assisted Digestion
Microwave (MW)-assisted digestion with nitric acid, nitric and hydrochloric
acids without or with the addition of hydrogen peroxide is a widely used
technique for the dissolution of food samples. Microwave heating has
several advantages over conventional heating on a hot plate, etc., as the
energy is generated in the digestion mixture and not transferred by conduction.
Among the key advantages of MW-assisted digestion are the much shorter
digestion times and the reduced need for aggressive reagents to obtain
complete digestion. There are two different systems available for MWassisted digestion, pressurized closed-vessel systems and open focused-MW
systems that work under atmospheric pressure. Microwave-assisted
digestion in closed vessels under pressure has gained popularity as a simple
and fast dissolution technique that minimizes acid consumption, the risk of
sample contamination, and loss of volatile elements. One of the limitations
is the time required for cooling before the vessels can be opened, which
may take hours, depending on the type of equipment used. The main
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Determination of Metals in Foods
73
advantages of focused-MW radiation are safety, versatility, control of
microwave energy released to the sample, and the possibility for programmed
addition of solutions during the digestion. However, loss of volatile elements
cannot be excluded in open-vessel digestion and results for low-level elements
might be affected by the high amount of reagents used and hence the increased
risk of sample contamination. This risk can be minimized by using vaporphase acid digestion, which has proven to be very effective in minimizing
the residual carbon content (RCC) (1, 2).
Table 1 gives an overview of more recent applications of MW-assisted
digestion for the analysis of food samples. Doner and Ege (23) evaluated
Table 1. Microwave-assisted dissolution for diverse food samples
Food
Biscuits
Bread and
biscuits
Beans
Flour
Herbal tea
Reagents
Technique
Ref.
HNO3, HCl
HNO3, H2O2
Fe, Zn
Al
FAAS
GF AAS
(23)
(24)
HNO3, H2SO4
HNO3, H2O2
HNO3, H2O2
Ca, Fe, Mg, Mn, Zn
Fe, Mn, Zn
Mg, Al, Ca, V, Cr,
Mn, Fe, Co, Ni, Cu,
Zn, Se, Sr, Sb, Ba,
As, Cd, Hg, Pb
Al, Ca, Mg, Mn
Cd, Cr, Fe, Ni, Pb
Cd, Cr, Fe, Ni, Pb
ICP OES
FAAS
ICP-MS
(25)
(26)
(27)
Al, Ca, Co, Cr, Cu,
Fe, K, Mg, Mn, Na,
Ni, Pb, Zn
Al, Ca, Co, Cr, Cu,
Fe, K, Mg, Mn, Na,
Ni, Pb, Zn
Ba, Ca, Cu, K, Mg,
Na, P, Zn
Al, Ca, Cu, Fe, Mg,
Na, Se, Zn
Al, Fe, and Zn
Al, Ca, Cu, Fe, Mg,
Mn, Zn
Cd, Cu, Ni, Pb, Zn
Hg
As
Cu, Zn
Ba, Ca, Cu, K, Mg,
Mn, P, S, Zn,
ICP OES, GF AAS (31)
Tea
Wheat grain
Durum wheat
flour
Edible oil
HNO3
HNO3, H2O2
HNO3, H2O2
Olive oil
HNO3, H2O2
Bovine milk
HNO3, H2SO4
Milk powder
HNO3, H2O2, TiO2
Yogurt
Bovine liver
HNO3, H2O2
HNO3, H2SO4
NaClO4, H2O2
HNO3
Fish
Fish
Fish
Milk powder
Vegetables
Analytes
HNO3, H2O2
TMAH
NH4NO3
HNO3, H2O2
ICP OES
(28)
ICP OES, ICP-MS (29)
ICP OES, ICP-MS (30)
ICP OES, GF AAS (32)
ICP OES
(33)
ICP OES, GF AAS (34)
FAAS
ICP OES
(35)
(36)
GF AAS
CV AFS
GF AAS
FAAS
ICP OES
(38)
(39)
(40)
(41)
(42)
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M. das G. A. Korn et al.
different digestion methods for biscuits prior to the determination of iron and
zinc using FAAS. In an initial study, MW-assisted digestion with hydrochloric
and nitric acids 3:1 v/v, 1808C, and 600 W provided an accuracy (spike
recovery 96– 102%), precision and digestion time comparable to dry ashing,
wet digestion (using different acid mixtures), and also to a simple acid
treatment at room temperature. The latter technique was further investigated
because of its simplicity and to reduce the digestion time. The addition of
ethanol was found necessary to digest the organic residue at room temperature. The method was validated by comparison of the data found for commercial biscuit samples using the proposed procedure and the AOAC official
spectrophotometric reference method. Jalbani et al. (24) investigated the
dietary intake of aluminum in bakery products consumed in the urban areas
of Hyderabad, Pakistan. Samples of different branded and non-branded
bread and biscuits were dissolved using MW-assisted and conventional wet
acid digestion prior to GF AAS analysis.
Costa et al. (25) used a factorial design for optimization of a focusedMW – assisted digestion of bean samples for the determination of Ca, Fe,
Mg, Mn, and Zn. A closed-vessel MW-assisted digestion was used to
certify the elemental compositions obtained after open digestion. The
accuracy was checked using the NIST SRM 8433 Corn Bran CRM. Results
were in agreement with certified values at the 95% confidence limit using a
Student’s t-test. Volumes of nitric and sulfuric acid, temperature, and the
interaction between the initial volumes of HNO3 and H2SO4 were significant
variables according to the P-values in the analysis of variance (ANOVA).
Santelli et al. (26) also used a Doehlert matrix response surface method to
optimize a focused-MW –assisted digestion of various food samples for the
determination of Fe, Mn, and Zn by FAAS. Three variables, irradiation
power, irradiation time, and composition of the oxidant solution
(HNO3 þ H2O2), were considered as factors in the optimization study. The
working conditions were established as a compromise between optimum
values found for each analyte, taking into consideration the robustness of
the procedure. These values were 260 W, 12 min, and 42% (v/v) for
irradiation power, irradiation time, and percentage of H2O2 in solution,
respectively. The accuracy of the optimized procedure was evaluated by the
analysis of CRM and by comparison with a well-established closed-vessel
MW-assisted digestion method.
Nookabkaew et al. (27) used MW-assisted digestion for three types of
popular herbal tea products, Gynostemma pentaphyllum, Camellia sinensis,
and Morus alba, which are widely consumed in Thailand and in the rest of
the world. The contents of Mg, Al, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se,
Sr, Sb, Ba, As, Cd, Hg, and Pb were determined by ICP-MS. Costa et al.
(28) investigated a focused-MW –assisted extraction of Al, Ca, Mg, and Mn
in tea leaves. The efficiency of extraction was evaluated using diluted acid
and alkaline solution of a tertiary amine in water. The extraction procedure
was implemented in 5 min. A hot plate digestion procedure was developed
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Determination of Metals in Foods
75
for comparison. A colorless digest was obtained for all the tea samples and the
total content for each analyte was used to calculate the efficiency of the extraction. The determination was carried out by FAAS and ICP OES; quantitative
recovery (80 – 100%) was obtained for Ca, Mg, and Mn, but for Al recoveries
were only 10– 80%, which is probably due to the strong interaction of this
element with the matrix. The results indicated that the percentages of extraction of the elements are directly related with the chemical form in which the
metal is present in the tea leaf.
Cubadda et al. (29, 30) used closed-vessel MW-assisted digestion for
determination of Cd, Pb, Fe, Ni, and Cr in selected food matrices by plasma
spectrometric techniques and to investigate the transfer of metal contaminants
through the food chain and the effect of food processing. Cadmium, Pb, Fe,
Ni, and Cr, were accurately determined in durum wheat grain and
derived products, wheat-based reference materials, and drinking water, used
both as an ingredient and for technology purposes in the industrial process.
The analytical determination was performed using ICP-MS. Changes in
the sample introduction system and complementary use of ICP OES
overcame the difficulties in determining the analytes in the food matrixes.
The benefits of ultrasonic nebulization in reducing spectral interferences
were demonstrated. Overall, a robust analytical method with high sample
throughput was developed.
The determination of trace elements in edible oils is important because of
both the metabolic role of metals and possibilities for adulteration detection
and oil characterization. Zeiner et al. (31) used an MW-assisted digestion of
the olive oil in closed vessels with a mixture of nitric acid and hydrogen
peroxide; the trace element content of olive oils was determined by ICP
OES and GF AAS. Recently, the quantification of selected metals in various
oils (olive, pumpkin seed, sunflower, sesame seed, hazelnut, grape, soy, and
rice oil) was carried out by ICP OES and GF AAS after MW-assisted
digestion (32). Differences in the metal concentrations for edible oils
obtained in this preliminary study were the basis for the development of an
additional analytical procedure applicable for oil characterization.
Santos et al. (33) developed a method for the determination of Ba, Ca, Cu,
K, Mg, Na, P, and Zn in whole and non-fat bovine milk after digestion in a
focused MW oven, using an alternate procedure based on gradual sample
addition to hot and concentrated acids. A two-level 23 full factorial design
experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the RCC and the final acidity of the
digestates. The best conditions were attained by adding small aliquots of
milk (ten additions of a volume of 0.5 mL during 5 min) to a digestion
mixture containing 3.0 mL nitric acid plus 1.0 mL sulfuric acid heated at
1058C. It was demonstrated that the digestion efficiency of the alternative
procedure was better than the conventional procedure; i.e., 98% compared
to 80% for the latter one. The accuracy was checked using two CRM,
whole and non-fat milk powder. This strategy expands the application of
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M. das G. A. Korn et al.
focused MW-assisted digestions and produces digestates more suitable for
measurement using instrumental techniques, such as ICP OES. A novel
MW-assisted high-temperature/high-pressure UV-TiO2 digestion procedure
was developed for the accelerated decomposition of biological samples such
as milk (34). The technique is based on a closed, pressurized MW digestion
apparatus. UV irradiation was generated by an immersed electrodeless
discharge lamp operated by the focused MW field in the single polymer
vessel. To enhance oxidation efficiency, a photo catalyst, TiO2, was added
to the MW-heated Teflon bomb. Measures of digestion completeness were
provided by the RCC and determination of trace and minor elements,
enabling a comparison of different digestion procedures and sample types.
Compared with other digestion systems, unusually low RCC of 1– 2% was
obtained, corresponding to a digestion efficiency of 98 –99%.
Yaman et al. (35) compared digestion methods, such as dry and wet
ashing and MW-assisted digestion for the treatment of yogurt samples prior
to the determination of essential nutrients by FAAS. Digestion in a
microwave oven was found to be an excellent technique in comparison with
dry and wet ashing for the determination of Al and Zn. Iron in this matrix
was not completely recovered after MW-assisted digestion using the
examined conditions. Aluminum concentrations in yogurt samples
fermented in Al containers were found to be significantly higher than in
plastic containers. Al concentrations of yogurt taken from the bottom of the
container were found to be higher than those from the center and top of the
Al containers.
An acid-vapor partial digestion procedure for bovine liver has been
proposed by Trevizan et al. (36) using a focused-MW oven and a laboratory-made PTFE support. The support was equipped with three cups of
approximately 4 mL volume each and the cups were adapted to the glass
reaction vessel of the MW oven. A mixture containing HNO3 and H2SO4
was heated to 1208C to generate the acid vapor. Bovine liver (50 – 90 mg)
were directly weighed into the cups followed by addition of a mixture containing NaClO4 þ H2O2. Samples were exposed to acid vapor during 15 –25 min
and then diluted with distilled and deionized water to a final mass of 3.0 g.
Recoveries of Al, Ca, Cu, Fe, Mg, Mn, and Zn were evaluated using an ICP
OES with axially viewed configuration. The advantages of using this
strategy are the reduced concentration of acid in the digestate, the possibility
of using a technical grade acid without any deterioration of analytical blank,
and the reduction of blank values due to the purification of reagents during
MW-assisted evaporation. The main attraction of the proposed procedure is
that all steps can be carried out in one single vessel, which improves the
capacity for trace analysis.
The determination of trace elements in seafood is of interest because of
nutritional and toxicological reasons. Nutritional because trace metals such
as Ca, Fe, Mg, Zn, Cu, Co, and Al are necessary for maintenance of
optimum health, and toxicological as metals such as Pb, Cd, As, and Hg are
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Determination of Metals in Foods
77
detrimental to health. Furthermore, trace elements are an important aspect of
environmental analysis because mussels are used as bioindicator organisms to
assess bioavailability of contaminant concentrations in coastal waters (37). An
MW-assisted digestion procedure was developed for the determination of Cd,
Cu, Ni, Pb, and Zn in fish samples from seven sampling stations of the Ria de
Aveiro (Portugal) by GF AAS (38). The accuracy of the analytical method was
evaluated through the analysis of two CRM (NIST-1577 band IAEA-V10);
good agreement was obtained between the experimental results and the
certified values. The authors concluded that the employed MW-assisted
digestion method could be considered as a fast procedure, since only 2 min
was required for a complete dissolution of the sample. Liang et al. (39)
proposed an automatic system, based on the on-line coupling of high-performance liquid chromatography separation, post-column MW-assisted digestion,
and cold vapor atomic fluorescence spectrometric (CV AFS) determination of
four mercury compounds in seafood samples. Post-column MW-assisted
digestion in the presence of potassium persulfate (in HCl), was applied in
the system to improve the conversion efficiency of three organic mercury
compounds into inorganic mercury. Parameters influencing the on-line
digestion efficiency and the separation were optimized. Dogfish muscle
(DORM-2) was analyzed to verify the accuracy of the method and the
result was in good agreement with the certified value. Serafimovski et al.
(40) proposed a simple and fast MW-assisted extraction of arsenic species
from fish tissue in tetramethylammonium hydroxide (TMAH, 0.075% m/v)
or in a water-methanol mixture (80 þ 20 v/v) that took only 20 min. Total
As was measured by GF AAS directly in the TMAH extract with Pd as a
modifier ensuring thermal stabilization of all extracted arsenic species.
Mesko et al. (41) proposed an MW-assisted sample combustion in the
presence of oxygen under pressure using ammonium nitrate as aid for
ignition. The system was adapted in a microwave oven with closed quartz
vessels. A quartz piece was used as a sample holder and to protect the cap
of the quartz vessel from the flame generated in the combustion process.
The sample was pressed into a pellet and placed on a disc paper in the
holder and 50 mL of 50% m/v ammonium nitrate solution was added. The
influence of the absorption solution (diluted or concentrated nitric acid or
water) on the recovery of Cu and Zn was evaluated. About 3 s of
microwave irradiation was necessary to start the combustion. The combustion
process was evaluated in relation to the influence of sample mass on the
ignition time, combustion time, and maximum operation pressure. Bovine
liver, milk powder, and oyster tissue CRM were used to evaluate the
accuracy of the procedure for determination of copper and zinc. Results
from the proposed procedure were also compared to those obtained with conventional digestion procedures, such as wet digestion in open vessels and
MW-assisted digestion in closed vessels. The advantages of this procedure
include the complete sample decomposition in shorter time than with other
procedures and the acid consumption was always lower than 2%. Other
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advantages are the low RCC of less than 1.4% without reflux and less than
0.3% with a reflux step and the possibility of using diluted acid as the
absorbing solution.
Araújo et al. (42) investigated the efficiency of MW-assisted acid digestion
of plants using different concentrations of nitric acid with hydrogen peroxide
(30% v/v) by measuring the RCC using ICP OES with axial viewing. Two
CRM, spinach leaves (NIST 1570a) and corn bran (NIST 8433), were used
for evaluating the accuracy attained when 2 mol L21 HNO3 was employed
for digestion. Under all experimental conditions RCC values were lower than
13% w/v, and even the highest concentration did not cause any interference
with element recovery. It has been observed that the high pressure that is
attained in closed-vessel operation improved the oxidative action of nitric
acid due to consequent temperature increase, even when this reagent was not
used at high concentration. The residual acid present in the digestates varied
from 1.2 to 4.0 mol L21, depending on the initial acid concentration. Hence,
for plant materials, MW-assisted acid digestion can be carried out under mild
conditions, which implies that digestates do not need extensive dilution
before introduction by pneumatic nebulization into an ICP OES. An additional
advantage is the lower amount of residue generated when working with less
concentrated acid solutions.
Ultrasonic Extraction
Some conjectural approaches keep up the application of ultrasound irradiation
to assist metallic species extraction from various solid samples, such as
intense disturbance imposed by acoustic wave propagation, disruptions
produced by microjets at the collapse of cavitation bubble, as well as the
products generated by volatile species sonodegradation. The application of
ultrasound to assist sample preparation points to some singularities that
align to the feature of expeditious preparation methods and low reagent consumption. Ultrasound speeds up sample preparation once it diminishes solvent
gradient concentration in the solid-liquid interface, yields unstable species
into the irradiated medium, and, sometimes, increases sample surface area
due to solid erosion.
Ultrasound has been employed for sample preparation in order to improve
analytical throughput (43); however, chemical information of samples
submitted to ultrasonic irradiation can be severely compromised since the
collapse of cavitation bubble results in a strong local temperature increase
and free radical production (4), which could provoke analyte loss and gross
analytical errors. Analyte losses were also observed for spectrometric determinations contrasting the results obtained for various metals in food samples pretreated with ultrasound devices with other sample preparation techniques and
certified materials (44 – 48).
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Nascentes et al. (44) proposed a fast and accurate method for the extraction of Ca, Mg, Mn, and Zn from vegetables using ultrasonic energy and dilute
acid. Optimized conditions for the ultrasonic bath were 1 L of water, 258C,
and 2% v/v detergent concentration. The best conditions for extraction
were 0.14 mol L21 HNO3, 10 min of sonication time, and a particle size
,75 mm. The accuracy of the proposed ultrasound-assisted extraction
method was assessed by using certified reference materials, as well as wet
digestion.
An ultrasound-assisted leaching procedure using diluted mixed acid
solution was developed for the determination of cadmium, copper, and zinc
in fish and mussel samples by FAAS (45). The effects of several parameters
such as nitric acid, hydrochloric acid, and hydrogen peroxide concentration,
volume of leaching solution, and sonication time were investigated. A 30-min
sonication, 568C operating temperature and 6 mL of a 1:1:1 mixture of 4 mol
L21 HNO3, 4 mol L21 HCl, and 0.5 mol L21 H2O2 were used for 0.5 g of
dried sample. The results from the proposed procedure were compared with
those obtained by microwave-assisted digestion, and the recovery obtained
with the leaching technique ranged from 92 to 114% for fish and from 88 to
103% for mussel samples. The accuracy of the developed method was investigated by analyzing a certified reference material (DORM-2). Melo et al. (46)
developed an ultrasound-assisted metal extraction with an aqueous solution of
tertiary amines (CFA-C reagent) in the presence of hydrogen peroxide for
improving metal solubilization in fish samples. The optimization was carried
out using a factorial design. The proposed procedure made possible the quantitative extraction of Ca, Cu, Fe, Mg, and Zn. Accuracy was evaluated by comparison with total acid digestion of the sample in a Parr bomb and using a CRM
(fish homogenate, MA-A-2, IAEA). All results were in agreement at a 95% confidence level according to a paired-t test.
Three different ultrasonic-based sample treatment approaches, automated
ultrasonic slurry sampling, ultrasound-assisted acid solid-liquid extraction
(ASLE), and enzymatic probe sonication (EPS), were compared for the determination 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 extraction) 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 ultrasound-assisted extraction procedure for the determination of major, minor,
and trace elements in lichen and mussel tissue as a possible alternative to conventional digestion methods. ICP-MS and ICP OES were used for the quantification of the elements. Parameters affecting extraction, such as extractant
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M. das G. A. Korn et al.
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. Closedmicrowave 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 determination 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.
Viñas et al. (51) developed a rapid and accurate procedure for the determination of Se, Cd, and Pb in different types of baby food using slurry
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Determination of Metals in Foods
81
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 biological 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 determination using FAAS. The statistical comparison by t-test (95% confidence
level) showed no significant difference between these results.
López-Garcı́a et al. (53) proposed a procedure for GF AAS determination
of phosphorus in honey, milk, and infant formulas using slurry sampling. Suspensions prepared in a medium containing 50% v/v concentrated hydrogen
peroxide, 1% v/v concentrated nitric acid, 10% m/v glucose, 5% m/v
sucrose, 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 corresponding 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
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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 nebulization. The recommended particle size was ,70 mm and the slurry concentration was 0.6% m/v, while the working slurry concentration could range from
0.3 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 transferring 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 online at 253.652 nm by axial view ICP OES. The first three procedures
produced results in agreement with the certified values. The two last procedures using nitric acid or TMHA could not be used for quantitative determination. 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 determination of Ca, Cu, Fe, Mg, and Zn in fish tissue samples by FAAS. In comparison 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
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Determination of Metals in Foods
83
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 determination, 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 determination 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 vaporization. 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 disadvantages 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 concentrations, 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
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M. das G. A. Korn et al.
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. Background 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 Zeemaneffect 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 temperature 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 determination was matrix independent using this technique. Even relatively high
amounts of chlorine, which are known for causing problems in mercury determination, 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
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Determination of Metals in Foods
85
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 determined 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 singleelement 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 sensitivity 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 homogeneity 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 Lücker 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).
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M. das G. A. Korn et al.
Ribeiro et al. (75) investigated the determination of Co in fish and other biological samples comparing direct SS-GF AAS and alkaline treatment with TMAH
and conventional line source GF AAS with HR-CS GF AAS. Method development 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 temperatures 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. Under
these 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 appropriate 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
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Determination of Metals in Foods
87
preparation procedure is of capital importance. It was observed that different
sample preparation procedures have been successfully applied for determination 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
Tecnológico, Brası́lia, Brazil) and FAPESB (Fundacão de Amparo a Pesquisa
do Estado da Bahia, Salvador, Brazil) for fellowships and financial supports.
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