application note hr-cs aas – flame technique

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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,

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 Gracas Andrade Korn, NQA/GPQA,Instituto de Quımica, Universidade Federal da Bahia, Campus de Ondina, 40170-115

Salvador, Bahia, Brazil. E-mail: [email protected]

Applied Spectroscopy Reviews, 43: 67–92, 2008

Copyright # Taylor & Francis Group, LLC

ISSN 0570-4928 print/1520-569X online

DOI: 10.1080/05704920701723980

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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 require-

ment. The determination of trace elements and contaminants in complex

matrices, such as food, often requires extensive sample preparation and/orextraction 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 con-

sidered. 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 bottle-

neck 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

M. das G. A. Korn et al.68

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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 prep-

aration method for metal determination in food samples. Tuzen et al. investi-

gated 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 inves-

tigated 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 pre-

concentration 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

Determination of Metals in Foods 69

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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 microwave-

assisted 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 incom-

plete 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 spec-

trometric techniques as long as its concentration is similar in calibration and

sample solutions. Hydrogen peroxide, added in most mineralization pro-

cedures, 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 unde-

sirable in analytical techniques where the sample introduction is by nebuliza-

tion (FAAS, ICP OES, ICP-MS).

Momen et al. (12) investigated two digestion procedures for the determi-

nation 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 analyti-

cal method was working well.

In another study, Momen et al. (13) assessed four procedures for the deter-

mination 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 con-

taining 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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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 concen-

trations 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 determi-

nation 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 miner-

alization was obtained for an arsenobetaine-containing CRMwith 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 pro-

cedures using acid mixtures of HNO3/H2SO4/HClO4 and HNO3/H2SO4 for

decomposition of food samples in Australian diet. The addition of hydrofluo-

ric acid to the mixture of HNO3/HSO4 was also investigated for the determi-

nation 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 microwave-

assisted 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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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 determi-

nation 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 non-

essential (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 optimiz-

ation 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 con-

centrated 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 MW-

assisted 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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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 vapor-

phase 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 Reagents Analytes Technique Ref.

Biscuits HNO3, HCl Fe, Zn FAAS (23)

Bread and

biscuits

HNO3, H2O2 Al GF AAS (24)

Beans HNO3, H2SO4 Ca, Fe, Mg, Mn, Zn ICP OES (25)

Flour HNO3, H2O2 Fe, Mn, Zn FAAS (26)

Herbal tea HNO3, H2O2 Mg, Al, Ca, V, Cr,

Mn, Fe, Co, Ni, Cu,

Zn, Se, Sr, Sb, Ba,

As, Cd, Hg, Pb

ICP-MS (27)

Tea HNO3 Al, Ca, Mg, Mn ICP OES (28)

Wheat grain HNO3, H2O2 Cd, Cr, Fe, Ni, Pb ICP OES, ICP-MS (29)

Durum wheat

flour

HNO3, H2O2 Cd, Cr, Fe, Ni, Pb ICP OES, ICP-MS (30)

Edible oil HNO3, H2O2 Al, Ca, Co, Cr, Cu,

Fe, K, Mg, Mn, Na,

Ni, Pb, Zn

ICP OES, GF AAS (31)

Olive oil HNO3, H2O2 Al, Ca, Co, Cr, Cu,

Fe, K, Mg, Mn, Na,

Ni, Pb, Zn


Bovine milk HNO3, H2SO4 Ba, Ca, Cu, K, Mg,

Na, P, Zn

ICP OES (33)

Milk powder HNO3, H2O2, TiO2 Al, Ca, Cu, Fe, Mg,

Na, Se, Zn


Yogurt HNO3, H2O2 Al, Fe, and Zn FAAS (35)

Bovine liver HNO3, H2SO4

NaClO4, H2O2

Al, Ca, Cu, Fe, Mg,

Mn, Zn

ICP OES (36)

Fish HNO3 Cd, Cu, Ni, Pb, Zn GF AAS (38)

Fish Hg CV AFS (39)

Fish TMAH As GF AAS (40)

Milk powder NH4NO3 Cu, Zn FAAS (41)

Vegetables HNO3, H2O2 Ba, Ca, Cu, K, Mg,

Mn, P, S, Zn,

ICP OES (42)


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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 tempera-

ture. The method was validated by comparison of the data found for commer-

cial 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 focused-

MW–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

(HNO3þH2O2), 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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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 extrac-

tion. 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 extrac-

tion 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 exper-

imental 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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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 labora-

tory-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 contain-

ing NaClO4þH2O2. 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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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-perform-

ance 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 (80þ 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 con-

ventional 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.

Araujo 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 con-

sumption. 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 determi-

nations contrasting the results obtained for various metals in food samples pre-

treated 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 extrac-

tion 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 investi-

gated 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 quanti-

tative extraction of Ca, Cu, Fe, Mg, and Zn. Accuracy was evaluated by com-

parison 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% con-

fidence 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 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


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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


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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


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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


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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


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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


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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).


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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


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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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application note hr-cs aas – flame technique

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