Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples RAA Munoz and ES Almeida, Universidade Federal de Uberlaˆndia, Uberlaˆndia, Brazil L Angnes, Universidade de Sa˜o Paulo, Sa˜o Paulo, Brazil ã 2013 Elsevier Inc. All rights reserved.

Introduction Electroanalytical Methods Stripping Analysis Sample Preparation Methods Traditional and Microwave-Assisted Digestion Methods Extraction Methods Microwave-assisted and ultrasound-assisted extractions Conclusions and Outlook Acknowledgments References

1 1 2 3 3 5 5 9 9 10

Introduction The development of an analytical methodology commonly includes sampling, sample pretreatment, detection, calibration, and evaluation of the final result in order to act over a specific problem. The sample preparation step should provide the analytes in an adequate medium (typically aqueous solution) to be detected and quantified. At this step, sample homogeneity is required and the interference from sample matrix on the employed detection system has to be eliminated. In this way, the sample preparation brings the most significant error source in the analytical method development and is the most time-consuming step especially when solid samples are analyzed. For this reason, this step has fundamental importance in the overall analytical method development, mainly when electroanalytical methods are applied. Electrochemical stripping analysis is the most popular analytical technique for trace-element determination. The main advantages of stripping analysis over spectrometric methods are the low-cost of instrumentation and analysis, portability of the equipment (on-site analysis), easy miniaturization, high sensitivity (comparable with electrothermal atomic absorption spectrometry), and capability of chemical speciation. On the other hand, electrochemical detection may be more susceptible to interference from residual organic species and thus special attention to sample preparation should be give when organic samples are analyzed by stripping analysis. Sample preparation reviews for electroanalysis are scarce. Brainina and collaborators1,2 presented two reviews for sample preparation, the last one including 230 references about environmental and food analysis by stripping voltammetry, and the other one related to electrochemical sample preparation techniques for aqueous solution containing organic compounds. Both reviews are organized in accordance with the different electrochemical sensors employed in each analysis. Dry ashing methods for stripping voltammetric determination of trace elements in biological samples were also reviewed.3 Oppositely, a great variety of reviews describe sample preparation techniques mainly for spectrometric determinations. Reviews include microwave based4–6 and ultrasound-assisted methods7,8 focused on spectrometric elemental determinations in environmental, food, and biological samples. This text focuses exactly on different strategies adopted for the treatment of organic and inorganic samples to be analyzed by electrochemical stripping techniques aiming determinations of trace elements. All the possible sample pretreatments for petroleum-based oils, liquid fuels, edible oils, soil, sediment, plant, and biological samples, from the traditional dry ashing to the fast ultrasound-assisted extractions, will be presented and compared. Hence, the text organization is based on the different sample preparation strategies, but also mentioning both electrode and electrochemical technique for each cited work.

Electroanalytical Methods To the non-electrochemist readers, a brief introduction into the electroanalytical methods is described herein. For a broader overview on electroanalytical methods and a more detailed description and explanation of concepts involved in each electrochemical technique we recommend the following textbooks.9–11 The measurement of electrical quantities, such as current, potential, and charge, and their correlation with the chemical characteristics of a sample is the basis of electrochemical methods of analysis. Electrochemical processes occur at the electrodesolution interface and therefore a small amount of molecules present in the bulk solution contributes on the process to generate the electrical response.

Reference Module in Chemistry, Molecular Sciences and Chemical Engineering

http://dx.doi.org/10.1016/B978-0-12-409547-2.05750-4

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Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

There are basically two types of electroanalytical measurements, potentiometric and potentiostatic modes. The first one is carried out at “zero” current and the potential variation between indicator and reference electrodes is proportional to the concentration of the target species. The reference electrode is kept at constant potential during the whole measurement. The indicator also called working electrode is responsible for sensing the presence of the target species in solution. The potentiostatic mode is based on impute an adequate potential on the working electrode to promote an electron-transfer reaction between electrode and electroactive species, producing a current variation proportional to those species. The potential magnitude is relative to the energy required to promote an electron-transfer reaction, and the generated current is related to the concentration of the target species (the resulting plot of current vs. potential is a voltammetric recording). The controlled-potential electroanalytical techniques are generally more sensitive and selective, and offer a wider linear ranger than the potentiometric technique. The different manners of potential application, such as ramp, potential steps, pulses, and waves, distinguish the controlled-potential techniques. These techniques can be separated in the linear sweep techniques (including the linear sweep voltammetry and the cyclic voltammetry which is mainly used for the investigation of electrochemical processes occurred at the electrode-solution interface) and the step and pulse techniques, such as normal pulse voltammetry, differential pulse voltammetry, and square-wave voltammetry. The objective of applying pulses of potential (instead of the linear sweep) is to decrease the non-faradaic current (capacitive current contribution related to the charging of the double electric layer at the electrode surface). Free of the background current, the faradaic current (related to the electrontransfer of the target species) appears more sensitively in the voltammetric recording, which results in an increase of sensitivity especially important for analytical applications. These techniques were initially developed for the dropping mercury electrode with the intention of reducing the capacitive current contribution by current sampling at the end of drop life. The discovery of polarography by Heyrovsky in 1922 (voltammetry employing the dropping mercury electrode) definitively propelled the popularity of electroanalytical methods. After more than 90 years, mercury electrodes are still among the best sensors for electroanalytical measurements. The exceptional high hydrogen overvoltage on mercury electrodes (electrochemical processes involving the formation of gas molecules at the electrode are limited by the electron-transfer) offers the possibility of extending the window potential range to very negative potentials as much as 1.5 V (vs. Ag/AgCl in KCl saturated medium). As a result of it, the electrochemical reduction of certain molecules which requires very high cathodic potentials, such as zinc cations, is achievable at mercury electrodes. An additional property of mercury electrodes is their capacity of interaction with different metals generating metallic amalgams. This feature made possible the accumulation of very low amounts of metallic cations into the mercury drop after the application of sufficient energy (potential) at the electrode in order to promote the electrochemical reduction of the metallic species. In a following step, the accumulated metallic species are reoxidized (by applying a linear sweep potential) and consequently stripped away from the electrode (as cationic species). The electrochemical oxidation of different metallic species preaccumulated at the electrode occurs at specific potential regions, which can be used for the identification of metallic cations. The resulting graphic representation of current versus the applied potential is called voltammogram. Analogous to a ultra-violet (UV)–vis spectrum where each band corresponds to a chemical form present in the sample, a voltammogram presents wave-shaped peaks correspondent to different metallic species previously electrodeposited at the electrode.

Stripping Analysis Electroanalytical methods, which present a prior accumulation step under convective mass transport of the target analytes to the electrode followed by a measuring step at which the target species are ‘stripped away’ from the electrode generating a current magnitude equivalent to their concentration, are known as stripping analytical methods. This two-step technique also named ‘stripping analysis’ can be performed by applying different manners of deposition and stripping of the analytical element. The works referenced during this text used predominantly anodic stripping voltammetry (ASV), stripping chronopotentiometry (SCP), and potentiometric stripping analysis (PSA). The difference among those techniques is related to the stripping step, more specifically to the manner that the electroactive species are stripped from the electrode and to the correspondent registered signal. Stripping voltammetry consists in applying a potential (linear or pulsed scan) to the electrode whilst the current is registered; the direction of the stripping potential distinguishes between anodic or cathodic stripping voltammetry. On the other hand, potentiometric stripping methods (developed by Jagner in 1976) measure the potential variation during the stripping process, which can be performed either by applying a constant current (cathodic or anodic SCP) or by the action of an oxidant agent in solution such as dissolved oxygen (PSA). Potentiometric stripping techniques are less susceptive to organic compounds generally present in solution than voltammetric ones, which can be tremendously important for the analysis of complex samples such as biological fluids with minimal sample treatment (simple sample dilution). After the discovery of preconcentrating electroactive species on the mercury electrode drop, polarography ascended in popularity for analytical purposes attributable to the huge increase of sensitivity attained by accumulation step. This preconcentration advent was extended to other electrodes allowing increments of sensitivity of 100–1000 times and improvement of the detection limits (concentrations lower than 10 10 mol l 1).9 Additional to the remarkable sensitivity, advantages such as low-cost, simple and portable equipment for on-site analysis, rapid and reliable methods, as well as the easy miniaturization of electroanalytical systems put in evidence electroanalysis in many fields of research. Mercury film coated on glassy carbon electrodes (GCEs) appeared as an attractive alternative to mercury drop electrodes for trace metal determination employing stripping analysis. Around 30 elements of the periodic table can be determined by employing stripping analysis at mercury electrodes.9 Complementary to mercury electrodes, bare gold electrodes are the most suitable sensors

Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

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for the determination of mercury, arsenic, selenium, and copper by using stripping analysis. Carbon paste electrodes (CPEs, manufactured by mixing carbon powder, agglutinating oil, and a chemical modifier agent) represent another type of electrode applied in electroanalysis since their discovery in 1960s.12 The typical chemical modifier is a ligand which links specifically to the target species (similar strategy adopted in the adsorptive stripping analysis where the chemical modifier molecules surround the mercury drop). The simplicity of construction and the possibility of renewing the electrode surface by simple mechanical polishing are the main positive points of CPEs; nevertheless, the same mechanical polishing causes lack of reproducibility between measurements. An additional type of electrode widely used for analytical applications is the screen-printed electrode (SPE). Printer machines can produce large amounts of SPEs using ceramic plates as substrates. The common three electrode system (working, counter, and reference electrodes) can be printed at the same substrate by applying commercial-available inks (carbon ink for working and counter electrodes and silver chloride ink for the reference one). Chemical modifiers can also be incorporated in the carbon ink in order to produce electrodes with improved selectivity and sensitivity. SPEs have found massive importance as source of disposable sensors for portable devices such as the glucose sensor for home-diagnostic of diabetes patients. Additionally, SPEs were successfully applied for stripping analysis determination of heavy metals as shown in a review.13 The more recent interest of electrochemists and analytical electrochemists is the development of new materials, especially carbon based materials such as pyrolytic graphite, boron-doped diamond,14 and carbon nanotubes.15 Low potential detection (more selective), increase of sensitivity (lower detection limits), and widening of the window potential range (investigation of very high potential processes not before studied) are examples of promising features obtained by exploiting those new materials. New sources for sensor development especially attending the green chemistry concept are also a demanding direction of research by analytical electrochemists. The elimination of toxic materials and reagents from standard analytical procedures such as the highly toxic mercury has recently received elegant solutions. An example is the bismuth film electrode which presented excellent performance for the electrochemical stripping analysis determination of heavy metals with similar performance of mercury electrodes.16

Sample Preparation Methods The present section is organized in the different sample preparation methods developed for the stripping analysis of samples, which can be classified in liquids and solids, or in organic and inorganic. Liquid samples can occur in aqueous solution (e.g., natural water, beverages, and biological fluids) but also in organic medium (oils and fuels). Solid samples can present predominantly organic nature (e.g., food, plants, and biological tissues) or predominantly inorganic nature (e.g., soils and sediments). Frequently, aqueous samples can be analyzed directly by electroanalytical techniques for total element determinations and especially for trace-element speciation (in this case, minimal sample handling is desired). Sample dilution can be applied based on the analyte concentration and on the interference of sample matrix without compromising trace-element determinations due to the remarkable sensitivity of stripping analysis. A sample dilution step can be advantageous by decreasing the concentration of sample matrix in the electrochemical cell measurement, for example, in the analysis of biological fluids and fermented beverages by PSA.9 Stripping determination of metals in organic liquid samples such as gasoline, edible oils, and biodiesel has been demonstrated using oil–alcohol– water emulsions (high dilution ratio of sample in the emulsion).17 Although few reports have demonstrated the use of stripping determinations of metals in liquid organic samples, several procedures for sample preparation (e.g., biological fluids, edible oils, and gasoline) have been reported in the literature, from traditional and microwave-assisted digestion methods to extraction procedures, which are discussed in details in the following subsections. The main purpose of sample treatment is to assure metal release from the organic matrix and eliminate (partial or completely) sample matrix, which may interfere on the electrochemical detection. One exception of organic liquid sample that can be directly analyzed by stripping techniques without any sample preparation (just electrolyte addition) is the case of hydroethanolic samples such as distilled beverages and fuel ethanol.17

Traditional and Microwave-Assisted Digestion Methods Dry ashing was initially used for sample preparation due to its easy operation and effective destruction of organic materials.3 Organic compounds are ignited and oxidized by air at elevated temperatures (500  C) and at atmospheric pressure. The carbonaceous residues (ashes) are dissolved in diluted acid solutions. For stripping analysis, a substantial dilution (0.5 g per 100 ml) may be necessary in order to circumvent the interference from sample ash on the electrochemical detection. Aiming the analysis of liquid hydrocarbons such as gasoline, Wickbold combustion (using an oxygen–hydrogen flame) was introduced in the 1960s for the determination of chloride and sulfur. After some improvements on the Wickbold combustion system, platinum determination was carried out in gasoline. The total time of combustion was 5 just minutes; however, UV decomposition (for hours) of residual organic matter was required for accurate stripping voltammetric determinations.18 In order to diminish the sample preparation time, to work at lower temperatures and consequently reduce losses by volatilization (typically occurred in dry ashing procedures), wet digestion processes were an attractive alternative to the dry ashing methods for several samples such as petroleum-based products, food, biological tissues and fluids, soils, sediments, and fertilizers. However, in some cases dry ashing is preferred than wet digestion processes. A dry ashing procedure for the analysis of medicinal plants was compared with wet digestion methods aiming the stripping voltammetric of cadmium and lead.19 Well-defined voltammetric peaks were only verified after dry ashing procedure, which indicated that residual organic matter may have originating from the wet digestion processes and affected the stripping voltammetric detection.19

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Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

Acid decomposition was extensively addressed for food samples such as milk, meat, curd, meal, flour, and fish. These digestion procedures used moderate-temperature (not higher than 150  C) for decomposition of the organic matrix. The employment of such low temperatures was necessary for the determination of volatile elements as mercury, arsenic, and selenium. Lambert and Turoczy20 performed a critical evaluation of four different of sample preparation for selenium determination in canned fish: (1) open and closed digestions recommended by the association of analytical chemists (AOAC), (2) UV irradiation in the presence of HNO3/H2O2, (3) pressure-aided HNO3 and HNO3/H2O2 digestion, and (4) oxygen bomb combustion were evaluated. Voltammetric determinations were affected by the reminiscent residual organic matter after the UV irradiation method and pressure-aided HNO3 and HNO3/H2O2 digestions. Loss of selenium was verified when the open AOAC and oxygen combustion methods were applied. The closed AOAC method, which involves only HNO3 but required larger digestion times (18 h of predigestion and 2 h heating at 150  C), was the best sample decomposition procedure.20 Nevertheless, wet digestion processes for organic sample preparation are time-consuming (hours) and require constant and close operator attention. To overcome this troublesomeness, microwave ovens have been applied for sample heating and dissolution. The first wet digestion assisted by microwave was performed in 1975 employing domestic equipment. After this work, microwave ovens specifically for sample preparation were developed and their use is constantly increasing as reviews indicate.4–6 The development of new materials of high chemical resistance, such as Teflon (Polytetrafluoroethylene) and perfluoroalcoxy, allowed the performance of digestion processes at high-pressure closed vessels and consequently at higher temperatures. Under this condition, the HNO3 boiling point is increased as well as its oxidant power, which facilitates the decomposition of organic samples.5 The great disadvantage of using closed systems is the additional time required for cooling and depressurization of vessels for the addition of reagents during the microwave heating. Contrarily, focused microwave ovens using borosilicate, quartz or Teflon vessels operating at atmospheric pressure are also employed for sample decomposition. Figure 1 presents a scheme of a focused microwave oven used for sample preparation. An adaptor at the top of the digestion vessel allows gas vapor condensating, which decreases losses of reagents and sample, and continuous reagent additions during the digestion process. Therefore, larger samples can be digested and the overpressure due to gas formation is easily relieved through the adaptor. Otherwise, the use of concentrated H2SO4 is required to elevate the digestion temperatures and consequently improve the HNO3 oxidant power.5 Different microwave heating programs were developed for the decomposition of environmental, petroleum-based products, fat-rich food, and biological samples employing different mixtures of nitric, hydrochloric, and perchloric acids, and hydrogen peroxide at pressurized vessels.5 Focused microwave ovens (operated at atmospheric pressure) were also applied for the decomposition of organic samples (oils).21 The latter microwave oven has been shortly employed for sample preparation due to the high concentration of acid in digested samples.5,21 In order to reduce the high content of residual acid (H2SO4), the sample was added through the adaptor of the focused microwave oven to the acid which was microwave-preheated in the digestion vessel. This proposed procedure was successfully applied for the decomposition of diesel fuel and milk decreasing dramatically the volume of concentrated H2SO4.5 An important parameter to be evaluated when stripping techniques are utilized is the residual organic matter from the digestion process, which may interfere on stripping voltammetric determinations. The influence of residual organic matter from digested petroleum-based products on ASV and PSA determinations of copper, lead, mercury, and zinc was evaluated.22 Diesel fuel and

Figure 1 Scheme of a focused microwave oven used for sample preparation.

Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

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crude oil samples were decomposed in a focused microwave oven (atmospheric pressure) and in a closed-vessel microwave (using pressurized vessels) oven. In this study, samples decomposed in the closed-vessel microwave oven presented more interferences on ASV determinations (adsorption processes on the working electrode), which indicated the presence of organic matter in the respective digested samples.22 Therefore, even after developing and optimizing an acid digestion procedure assisted by microwave (mass amount, volume and concentration of acids and oxidants, and type of microwave oven) especially for highly organic samples such as food, edible oils, and petroleum-based products, it is worth noting to evaluate the more adequate electrochemical stripping technique for metal determination in order to reduce interferences from residual organic matter. In this way, the complete digestion of such samples is preferred for stripping analysis. On the other hand, extraction methods are also an important alternative to wet digestion procedures assisted or not by a microwave oven as will be discussed further.

Extraction Methods Extractions of analytes from solid or immiscible liquid samples to an aqueous phase, which contains acids or oxidants, have been widely attended. The main advantage of such extractions is the simplicity of operation, which does not demand either drastic pressure and temperature or sophisticated equipments. Otherwise, some extractions are time-consuming and when organic samples are treated, the residual organic matter in the aqueous extractor is not negligible and consequently affects electrochemical determinations. Then, carbon-retaining columns were necessary to be used prior stripping determinations of metals in the aqueous extractor.23 Removal of such species was also attained by organic solvent extraction.24 Extraction performance close to 100% was attained employing hot (50–90  C) concentrated HCl23 or a mixture of concentrated HCl with 35% (m/v) hydrogen peroxide25,26 for metal determination in oily petroleum-based products. Edible oils from peanut, sunflower, soy, maize, rice, grape-seed, and hazelnut were also treated with a mixture of concentrated HCl with 35% (m/v) hydrogen peroxide prior the PSA determination of cadmium, copper, lead, and zinc at a mercury-based electrode (Hg-plated GCE).27 Extraction of Cr(VI) from crude oils was carried out with MgSO4 in phosphate buffer.28 Iodine monochloride was extensively used for lead-alkyl extraction from gasolines and the use of stripping voltammetry for inorganic lead determination was performed.17 Undoubtedly, extraction methods have been mainly developed for metal determinations in solid environmental samples (soils, sediments, plants, fertilizers, and particulate matter).1,2 Hydrofluoric acid is commonly applied for soil samples in order to reach the complete extraction of metals which may occur in silicates. Retention of Cd, Cr, Cu, Mn, Pb, and Zn in solid residue which remains undissolved in nitric acid extraction from several materials such as coal, meals, brewer’s yeast, and industrial compost was studied.29 Decomposition with HNO3 and HF was required for quantitative release of metals (especially for Cr) into solution.29

Microwave-assisted and ultrasound-assisted extractions Extractions can be improved if an external energy is introduced such as the increase of temperature of the sample-extractor medium. Following this strategy, microwave irradiation has been explored to accelerate extractions with the additional advantage of employing softer conditions of heating and reagents than the ones demanded in digestions. Microwave-assisted extraction of metals from soils was reported for metal determination by stripping techniques.30 Additionally, ultrasound has been addressed for sample preparation especially to assist extractions methods. Reviews on this topic7,8 emphasize the simple and safe operation conditions and the minimal waste of reagents, since extraction procedures are performed at atmospheric temperature and pressure and the use of strong acids and oxidants are reduced or even avoided, which consequently decreases loss of volatile analytes. The ultrasound irradiation in aqueous solutions induces the acoustic cavitation phenomenon, which involves the formation, grown, oscillations, and implosions of numerous gas micro-bubbles. At the center of each collapsed bubble, temperature around 5000 K and 100 atmospheric pressures are produced, which results in the formation of hydroxyl radical and hydrogen peroxide (sonolysis of water).31 The main ultrasound devices employed for analytical purposes are the ultrasonic horn, which focuses its energy on a localized region providing more efficient cavitation, and the ultrasonic bath, which distributes its energy along the bath. Despite the lack of uniformity in the distribution, the regions over the piezoelectric crystals of the bath (source of ultrasound) and their immediate vicinity can be explored with relatively high efficiency of ultrasound process.32 Therefore, higher analytical frequency can be obtained exploring those regions of the bath, if the sample preparation is the determinant step. Considering a routine analysis laboratory, this ultrasound unit offers important advantages, such as higher sample throughput and the low-cost of the equipment. Although the main application of both ultrasonic units have been addressed metal extraction at room temperature from soil samples,33,34 other environmental samples such as petroleum-based products25,26 were ultrasound-treated for metal extraction to an aqueous phase. Nevertheless, few extraction procedures assisted by either microwave or ultrasound units were applied for the determination of metals by stripping techniques. Spectrometric techniques are usually selected for element determinations since they are less susceptive to interference of organic residues from the sample matrix. When extraction procedures were carried out for stripping analysis of organic samples, additional treatment was required to eliminate residual organic matter, such as filtration23 or even UV digestion.35 The inconvenient introduction of an additional step in sample preparation is always avoided when an analytical methodology is developed since this step can be a source of contamination. Consequently, the development of sample preparation strategies for stripping determination of metals is crucial in order to eliminate or reduce interference of organic residues.

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Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

Table 1

Sample preparation procedures for petroleum-derivative focusing on metal determination by stripping techniques

Sample

Detected analyte

Procedure

Technique

Electrode/ substrate

References

Lubricating oil

Cu(II) Pb(II) Cu(II) Hg(II) Pb(II) Zn(II) Pb(II)

MW-assisted digestion with HNO3/H2SO4/H2O2

PSA

Hg-plated GCE

21

MW-assisted digestion with HNO3/H2O2

SW ASV SCP PSA

Au-Cdtrode Hg-plated GCE

22

Hot HCl extraction (temperature not mentioned)

PSA

Hg-plated GCE

23

Extraction with its morpholine-4-carbodithioate onto microcrystalline naphthalene US-assisted extraction with HCl/H2O2 at room temperature

DP AdSV

HMDE

24

SW ASV

Au-Cdtrode

25

SW ASV

HMDE

26

LS AdSV DP ASV

HMDE Hg-plated GCE

28 36

Diesel fuel Crude oil

Naphta and gasoline Crude oil Lubricating oil

As(III)

Lubricating oil

Cu(II) Pb(II) Zn(II)

Crude oil Gas oil

Cr(VI) Pb(II)

US-assisted extraction with HCl/H2O2 under heating at 90  C at room temperature Extraction with MgSO4 in phosphate buffer at 90–95  C Wet digestion with HNO3

DP, differential pulse; SW, square-wave; LS, linear sweep; ASV, anodic stripping voltammetry; AdSV, adsorptive stripping voltammetry; PSA, potentiometric stripping analysis; SCP, stripping chronopotentiometry; US, ultrasound; MW, microwave; GCE, glassy carbon electrode; HMDE, hanging mercury drop electrode; Cdtrode, electrode obtained from compact disks.

Tables 1 and 2 summarize analytical methodologies for the determination of metals in petroleum-based products and solid environmental samples (soils, sediments, and particulate matter) applying electrochemical stripping techniques, once extraction procedures are often applied for such samples. Mercury electrodes (Hg-plated, and hanging mercury drop electrode) were the most selected sensor for metal determinations while gold electrodes were employed for determinations of mercury, arsenic, and selenium. The use of bismuth-based electrodes has been preferred for metal determination in various samples as an environmental-friendly alternative to Hg-based electrodes.45,46,49,51 A few examples of other types of chemically modified electrodes were reported, such as Nafion-coated electrodes41 and a self-assembly monolayer modified-gold electrode.52 The function of these chemical modifiers is to eliminate the interference from interfering molecules to the electrochemical detection. Figure 2 summarizes the current scenario of sample preparation strategies for stripping determination of metals in environmental, biological, and food samples, highlighting the aid of microwave ovens and ultrasonic horns or baths into wet digestion and extraction procedures. Acid digestion methods have been preferred especially when crude oil and petroleum-based products were analyzed due to the extremely high organic content. However, the complete decomposition of these matrices requires extreme conditions (high temperature and pressure in the presence of concentrated acids and oxidants) and thus extraction methods at room temperature with the aid of ultrasound have been proposed.25 Extraction methods have been the predominant option for solid environmental samples. Selection of the acid solution is based on the type of sample and how analyte is present in the sample matrix. For example, Maroulis et al. verified that HNO3 is the preferred choice for the phosphorites and HClO4 for the fertilizers.38 Soil samples are commonly treated with solutions in a different pH range in order to obtain metal lability in different chemical environments. Extractions with nitric acid, acetic acid/ acetate buffer, and CaCl2 solutions of sediments were performed and the stripping voltammetric determination of Cd, Cu, Pb, and Zn in each extract solution was carried out.54 Stripping voltammetry is able to measure selectively hydrated cations and labile complex (labile metal ions) present in aqueous solutions. Diluted nitric acid extraction releases metal ions associated with acidsoluble matrix such as carbonates, amorphous hydrous oxides, and organic acid colloids. Diluted acid solutions in the presence of reducing agents (e.g., hydroxylamine hydrochloride) releases metal ions associated with hydrous oxides of iron, manganese, and aluminum and eliminates colloidal species that can interfere on the ASV determinations. Weak acid (pH 5) extraction liberates metal ions bound to ion-exchange sites or associated with carbonate minerals and CaCl2 solution (pH 6–8) extraction releases metal ions that occupy ion-exchange sites.54 Based on the different procedures described in the literature for metal extraction from soils and sediments, the European Union’s Community Bureau of Reference established a three-step sequential extraction scheme named the BCR protocol.52 In the first step, soil extraction with 0.11 mol l 1 acetic acid liberates water and acid soluble species, which are the ones with highest bioavailability and thus highest toxicity. In the second step, the solid residue is extracted with 0.1 mol l 1 hydroxylamine hydrochloride (pH 2), which releases metal species bound to reducible matter (iron and manganese oxyhydroxides). In the last step, the solid residue is extracted with hydrogen peroxide that releases metals bound to organic matter. To assess the total amount of metal in soils and sediments, extractions with concentrated strong acids and as well as acid digestion eventually with HF41 are necessary as is shown in Table 2.

Table 2

Sample preparation procedures for environmental samples focusing on metal determination by stripping techniques

Sample

Detected analyte

Procedure

Technique

Electrode/substrate

References

Medicinal plants

Cd(II) Pb(II) Zn(II) Cd(II) Pb(II) Cu(II) Pb(II)

Wet digestion with HNO3/H2O2 for 2 h at 120  C versus dry ashing (2.5 h) MW-assisted extraction with NH4NO3, and preservation of the extract in HNO3

DP ASV

HMDE

19

DP ASV and DP AdSV

HMDE

30

US-assisted extraction with HNO3

LS ASV

SPE

33

Wet digestion with HCl/HNO3 followed by UV irradiation and addition of H2O2 Wet digestion with HNO3/HCl

DP ASV

Au rotating-disk

35

PSA

Hg-plated GCE

37

Wet digestion with five different acids: HCl, HNO3, H3PO4, H2SO4, HClO4, and HCl/HNO3

SW ASV

38

Wet digestion with HNO3

SW ASV and SW AdSV (for Co, Ni, Mo, and Cr)

Hg-plated on wax-impregnated graphite electrode Hg-plated GCE

39

Wet digestion with HNO3/HClO4 under hot plate at 100  C Wet digestion with HF/HCl at 180  C for 4 h

SW ASV

BDDE

40

SCP

41

SW AdSV

Nafion-modified Hg-plated GCE BDDE

DP AdSV

Hg-plated Ir

43

Soil

Paint, dust wipes, soil, and air River sediment Tobacco leaves, soil, and cigarettes Phosphorites and phosphate fertilizers Soil and airborne particulate

Hg(II) Zn(II) Cd(II) Pb(II) Cd(II) Pb(II)

Sediment river

Cu(II) Pb(II) Cd(II) Zn(II) Co(II) Ni(II) Mo(VI) Cr(VI) Pb(II)

Soil

Sb(V)

River sediment

Pb(II)

Soil

Cr(III) U(IV) As (III) Pb(II) Cd(II) Pb(II) Cd(II) Hg(II)

Soil Soil Soil Soil

Pittosporum tobira leaves Marine sediment

Cd(II) Cu(II) Pb(II) Mn(II)

Soil

Cu(II)

Soil Soil

Pb(II) Cd(II) Cu(II)

Soil

Al(III)

Sediment

Cd(II) Pb(II) Cu(II) Zn(II) As(III)

Soil

US-assisted digestion with HNO3/HClO4 (4:1) for 30 min MW-assisted digestion with HNO3 for 10 min followed by filtration MW-assisted digestion with HNO3/HCl Extraction with acetic acid and shaked for 16 h at room temperature Extraction with acetic acid

DP AdSV SW ASV

HMDE Bi-plated CPE

44 45

SCP

46

Extraction with HNO3/H2O2

SW ASV

Extraction with HCl/H2O2 at 90  C for 45 min

SCP

Bi2O3 bulk-modified SPE Au-plated on Ir microelectrode array Hg-plated GCE

48

Extraction with HCl/HNO3 followed by addition of NH4NO3 under stirring for 30 min Extraction with acetic acid under stirring for 16 h at 22  C followed by centrifugation Extraction with acetic acid

SW AdSV

Bi-plated BDDE

49

LS ASV

Au

50

SCP

Bi-plated SPE

51

BCR protocola

LS ASV

52

Extraction with water, centrifugation, filtration through an oxine microcolumn Three extraction solutions based on different pH lability: HNO3, acetic acid/acetate buffer, and CaCl2 solution

DP ASV

Au-modified with MAA monolayer SPE-modified alizarin

53

DP ASV

HMDE

54

DP AdSV

HMDE

55

Extraction with H3PO4 for As(III) and As(V) and with boiling HCl/HNO3 for total As

BCR, European Union’s Community Bureau of Reference; BDDE, boron-doped diamond electrode; CPE, carbon paste electrode; MAA, mercaptoacetic acid; SPEs, screen-printed electrodes. a BCR protocol: sequential extraction with acetic acid, hydroxylamine chloride (pH 2), and H2O2.

42

47

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Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

Figure 2 Scheme of the current scenario of sample preparation techniques strategies for stripping determination of metals in petroleum products, food, biological, and environmental samples.

Figure 3 Scheme indicating the evaluated regions of the ultrasonic bath based on the extraction of copper and lead from lubricating oils. The location of piezoelectric crystals is indicated by the two circles.

Ultrasonic baths and horns are also an alternative for the same purpose but still scarcely employed for electroanalysis. An ultrasonic horn was applied to accelerate the acid digestion of river sediments prior stripping voltammetric determination of lead.42 Ultrasonic baths (with optional heating up to 90  C) is a low-cost apparatus easily accessible to any laboratory, can treat dozen of samples simultaneously, and have been applied for metal extraction from lubricating oils,25,26 dust wipe, particulate matter, and soil samples.34 The main disadvantage of ultrasonic baths is lack of homogeneity of ultrasound propagation over the metallic recipient. A common ultrasonic bath (240 mm  140 mm  100 mm, 20 kHz) was evaluated based on the extraction of copper and lead from lubricating oils.25 Polypropylene tubes (containing sample and 2 ml of 1:1 (v/v) concentrated HCl/H2O2) were placed on different regions of the bath in accordance with Figure 3. Efficient extractions (close to 100%) of copper and lead after 30 min exposure were only obtained in the 5a and 5b regions, which are located exactly over the piezoelectric crystals responsible for ultrasound propagation. Efficient extractions of lead (close to 100%) were attained in a wider region of the bath (4a, 4b, 5a, and 5b) including the region between the two piezoelectric crystals.25 Therefore, the efficiency of sonication of ultrasonic baths must be evaluated prior use for sample preparation and different approaches can be found in the literature.32 Figure 4 presents SW (square-wave) ASV recordings at disposable gold electrodes obtained from compact disks (Au-Cdtrodes) for the determination of copper and lead in lubricating oils after US-assisted extraction. In combination with commercially available portable instrumentation for electroanalysis, ultrasonic baths can be easily transported to the field and be supplied by a battery or a generator if no source of electricity is available. In this context, the application of disposable electrodes for metal determination in environmental samples is essential for on-site analysis. Ashley et al.33,34 demonstrated the feasibility of performing on-site analysis of environmental samples using field-portable ultrasonic

Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

9

Figure 4 SW ASV recordings for the determination of copper and lead after US-assisted extraction from lubricating oils using a disposable gold electrode (Au-Cdtrode). Adapted from Munoz, R. A. A.; Oliveira, P. V.; Angnes, L. Talanta 2006, 68, 850–856.

extraction and lead determination at disposable SPEs. Other examples of disposable electrodes applied for metal determination are gold electrodes obtained from compact disks22,25 and unmodified or modified SPEs.34,46,51,53 The association of an ultrasonic horn with a conventional three electrode system improved the mass transport of electroactive species to the electrode surface and this feature was explored to increase the preconcentration efficiency in ASV with consequent increase of sensitivity. Furthermore, Mandigan et al. employed a high-power horn (475 W) into an electrochemical cell to perform the ASV detection of copper extracted in situ from lubricating oil particles dispersed in the aqueous electrolyte.56 The application of ultrasound was carried out simultaneously to the preconcentration step. This work pioneered the development of sonoelectroanalysis, which consists in the combination of an ultrasonic horn with stripping techniques for the metal determination in difficult real samples. Hence, sample preparation can be performed in the electrochemical cell simultaneously to metal determination. This is possible because the ultrasonic horn activates and cleans the working electrode avoiding the electrode passivation by surfaceactive compounds, produces emulsions involving organic samples-aqueous electrolyte, and extracts analytes from the sample matrix to the aqueous phase. Using this technique, biological, environmental, and food samples were analyzed as presented in a review.57 Lead determination in gasoline is one example of the application of sonoelectroanalysis for petroleum-based products which generally requires a prior sample treatment. The above mentioned benefits of the ultrasonic horn were applied for gasoline analysis by just diluting the sample in the aqueous electrolyte (HNO3 solution) without any prior treatment.

Conclusions and Outlook The use of high-sensitive stripping techniques for trace determination strongly depends on the sample preparation step especially when the analysis of organic materials is demanded. The obtainance of efficient, fast, and cheap sample preparation procedures has been reached by the evolution of techniques as observed in this text. The combination of microwave and ultrasound with acid digestions or extractions brings important advantages such as the reduction of reagents and samples and the decrease of analysis time. On the other hand, this combination has not been widely reported for the analysis of soils and sediments especially if the main purpose of the work is speciation analysis, for which special conditions of extraction are required (e.g., the BCR protocol in Table 2). The effects of residual organic matter on stripping determinations are still concerning electroanalytical chemists. PSA has overcome these problems under specific conditions and samples. The introduction of an ultrasonic horn in the electroanalytical system is a promising methodology to fulfill the analysis of complex samples in a single step (sample preparation, electrode cleaning, and stripping analysis occurring simultaneously in the electrochemical cell). The use of ultrasonic baths for metal extractions is probably the simplest solution accessible to any laboratory that can be directed to environmental, biological, and food samples. On-site analyses are possible using portable battery-charged ultrasonic baths and potentiostats coupled with disposable electrodes. Even though, the challenge of miniaturize sample preparation methods is still in development and the combination with lab-on-a-chip systems is of great interest for the confection of disposable analytical systems.

Acknowledgments The authors thank the support from Brazilian Foundations (CNPq process 305227/2010-6 and 306504-2011-1), Capes, and State Foundations (FAPEMIG PPM-00236-12 and FAPESP).

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Sample Preparation Techniques for the Electrochemical Determination of Metals in Environmental and Food Samples

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