microwave-assisted sample preparation for trace element analysis || microwave-assisted extraction

231Microwave-Assisted Sample Preparation for Trace Element Determination http://dx.doi.org/10.1016/B978-0-444-59420-4.00008-8Copyright © 2014 Elsevier B.V. All rights reserved.

Microwave-Assisted Extraction

Fabio A. DuarteUniversidade Federal de Santa Maria, Departamento de Química, Santa Maria, RS, Brazil

Pedro V. OliveiraUniversidade de São Paulo, Instituto de Química, São Paulo, SP, Brazil

Ana R. A. NogueiraEmbrapa Pecuária Sudeste, São Carlos, SP, Brazil

8.1. INTRODUCTION

A wide variety and complex nature samples are presented for analysis, and fre-quently, most of these samples are in the solid state. Depending on the analytical technique, these samples need a treatment for total or partial digestion/decom-position or dissolution to extract the elements of interest from the sample to the solution. Most of the analytical techniques used for routine trace element deter-mination, such as volumetric analysis, flame atomic absorption spectrometry (F AAS), electrothermal atomic absorption spectrometry (ET AAS), inductively coupled plasma optical emission spectrometry (ICP OES), inductively coupled plasma mass spectrometry (ICP–MS), atomic fluorescence spectrometry (AFS), ultraviolet–visible spectrophotometry, chemiluminescence, and electroanalytical techniques require the analytes to be present in liquid medium (normally aque-ous solution) before determination [1]. Particularly, for the F AAS, ICP OES, and ICP–MS techniques, which in general use nebulization for sample introduction, the conversion of the sample into an aqueous solution can be mandatory [2].

The term digestion/decomposition of organic matrices represents a complex process of sample transformation that is usually performed under high tempera-ture and/or at high pressure in combination with oxidant reagents. Dissolution can be generally defined as a more simple step for dissolving a substance in a suitable liquid at a relatively low temperature, with or without a chemical reaction [3]. Currently, the total conversion of solid samples in an aqueous solution has been carried out by dry ashing, mainly for samples with a high content of organic compounds (e.g., food, plants, and biological materials), fusion for refractory inorganic matrices (e.g., soil, sediment, geological materials, and technological products), and wet digestion methods, one of the oldest and still most frequently

Chapter 8

232 Microwave-Assisted Sample Preparation for Trace Element Determination

used process for sample preparation of organic and inorganic matrices. Wet diges-tion methods have been typically performed with open or closed systems, which require the use of acids (HNO3, HCl, HF, HClO4, or H2SO4) or a mixture of acids usually in combination with oxidant reagents (e.g., H2O2, K2S2O8). In this case, instrumentation involves conventional heating (e.g., burner, hot plate, and sand bath) or microwave radiation that is the most innovative and up to date approach with a wide range of applications for sample preparation [1].

Microwave heating is faster in comparison to conventional heating pro-cesses, for both open and closed systems. However, even using strategies such as microwave heating with a high pressure and temperature, and a mixture of concentrated acids, some samples with complex matrix, such as silicates, alu-minates, carbides, or a mixture of several refractory materials are very difficult to dissolve, making the sample preparation one of the most challenging tasks [1,3–5]. In general, sample preparation methods are frequently time consuming and require special attention to avoid systematic and random errors that will decrease the accuracy and precision of results. In addition, these errors can be more evident for trace and ultratrace element determination [1,3].

Special attention has been given to the simplification of sample preparation methods, and the main motivation is to minimize or even avoid total digestion, espe-cially for inorganic matrices. In this context, methods based on microwave-assisted extraction (MAE) for leaching of elements without the necessity of performing a complete sample digestion or dissolution have been considered as a viable alterna-tive to conventional digestion methods. In this way, sample preparation methods with this purpose include MAE, supercritical fluid extraction, accelerated solvent extraction, ultrasound-assisted extraction, and solid-phase extraction [3].

Since the introduction of the first commercial microwave oven dedicated to analytical laboratories, the main applications are designated for sample prep-aration based on the total digestion or dissolution for both organic and inor-ganic matrices, respectively. However, the use of MAE without the necessity of total sample digestion or dissolution has been successfully used as an effort to improve the efficiency of element extraction and to reduce the overall time of the analytical process. The special characteristics of microwave heating, which include low processing time, direct and selective heating, and more controllable heating process, are important characteristics for the potential implementation of MAE methods for trace element determination [5,6]. The theory of microwave heating is presented in Chapter 2 of this book and, in the present chapter, the principles and a summary of the studies using MAE for the quantitative leaching of elements for the total and/or partial determination are described.

8.2. PRINCIPLES AND TECHNOLOGIES

Extraction consists of the migration of the analytes (organic or inorganic) by desorption, diffusion, and/or solubilization process to a suitable solvent. The interaction between the solvent and the analytes must be greater than that between the analytes and the matrix in order to achieve a quantitative extraction.

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This general definition can be applied for all kinds of extraction methods dedi-cated to organic compounds, elements, organometallic and organometalloid species from solid matrices [6,7].

The conventional approach for the extraction of organic compounds (e.g., pes-ticides, polycyclic aromatic hydrocarbons, and phenols) from solid samples (e.g., soils, sewage sludges, vegetables, plants, and inorganic materials) has historically been carried out using Soxhlet extraction or mechanical shaking [4]. Microwave radiation has been employed for extraction of organic compounds aiming to accelerate the sample preparation, making possible to treat many samples simul-taneously, possible automation and demanding less volume of extracting solvents and producing less quantity of residues. Due to these characteristics, MAE has been considered in agreement with green chemistry recommendations [8].

The MAE has been applied for inorganic, organometallic, and organometal-loid analytes for leaching from environmental, geological, food, biological, tech-nological, and industrial samples [6]. Many factors should be considered to choose the extraction strategy to be applied in a sample preparation method. However, for element extraction using closed vessel systems, the main factors are the extraction efficiency and reproducibility, time, cost, and safety. Efficiency and reproduc-ibility have become the most critical factors when the extraction process must be quantitative for many samples and need to guarantee a low variation between rep-licates. In this way, the particle size of solid samples is a key parameter that must be considered, and this can be achieved by a suitable grinding process. In general, a small particle size enhances the extraction efficiency [9].

The choice of the grinding method is very dependent on the properties of the sample matrix, especially related to its hardness, fiber, and fat contents. The cryo-genic grinding method (brittle fracture) relies on an increase in the hardness of all materials and insertion of failures in the crystal structure for reduction to small pieces [10,11]. Other mechanical mills such as mortar and pestle, blend, ball and mortar, and disk mills have been also widely applied to grind samples [10].

The first report of using a microwave oven as a heating source for sample digestion and element determination was in 1975 [12]. This pioneering work has encouraged new investigations, and about 10 years later, the first use of a microwave-assisted system for extraction of organic compounds was proposed, and it has been successfully used up to now [13]. Nowadays, many applications for extraction can be performed in microwave ovens with the same equipment used for wet sample digestion. In a few cases, the replacement of the original rotor with specific accessories for desired sample extraction can be required.

The MAE can be applied using closed vessel systems (high pressure) or using a focused-microwave oven (atmospheric-pressure system). Although focused-microwave systems present advantages, such as handling up to 10 g of sample, safety, possibility to perform addition of reagents during the pro-cess, operation at atmospheric pressure, and allowing simultaneous processing, it has a relatively small number of applications for element extraction. On the other hand, closed vessel systems operating at high pressure are preferred for this purpose. The main advantages of closed vessel systems are related to the


possibility of working at temperatures higher than the boiling temperature of reagents allowing a high sample throughput, reduced risk of analyte losses and contamination, lower reagent consumption, and a better control of the micro-wave distribution inside the cavity, improving the repeatability. Many factors should be taken into account to choose an extraction method to be applied such as the efficiency and reproducibility of extraction, facility of operation, cost, time, degree of automation, and safety [6,7].

Several commercial microwave ovens for operation in closed vessel are currently available, consisting of one or two magnetrons capable of delivering power up to 1600 W. These very flexible systems allow the treatment of 8–48 samples by replacing the sample carrying rotor. Vessels (volume of 15–100 ml) are usually made of highly pure and nonreactive materials [e.g., polytetrafluo-roethylene (PTFE), perfluoroalkoxy, PTFE-perfluoropropylvinylether (PTFE–TFM), and quartz] minimizing contamination risks and possible chemical and/or mechanical attack using different acids or solvents. In some equipments, the reaction temperature can be controlled by means of one or two sensors: (1) a temperature probe immersed inside a reference vessel or (2) an infrared sensor below the oven cavity, which measures the temperature of each vessel in the rotor base. In general, the maximum operating conditions within the vessels range from a pressure of 20 bar (290 psi) to 200 °C for the 25 ml volume vessels to 100 bar (1430 psi) and 320 °C for the 50 to 100 ml volume vessels.

8.3. CURRENT APPLICATIONS

This section presents some examples of MAE dedicated for inorganic analytes in several kinds of samples. This section was divided into categories of sample kinds such as environmental, biological and food, and miscellaneous (polymers, crude oil, airborne particulate matter, and pharmaceuticals), and examples of application for each type will be presented along with tables.

8.3.1. Environmental Analysis

The accumulation of metals and metalloids in sediments, soils, sludge, and other environmental samples may cause several problems related to element transfer to the aquatic medium, and consequently to the food chain [14]. There is no doubt that the determination of the total element concentration, unlike speciation analysis, does not provide suitable information about bioavailability, mobility, or toxicity. The environmental behavior of the potentially toxic ele-ments depends critically on the form in which they occur. The manner in which an element is bound to the matrix of environmental samples, such as soils or sediments, influences the mobility and, ultimately, the bioavailability and tox-icity of the element to organisms. As a result, there is considerable interest in improving the understanding of element–solid associations in natural and pol-luted systems [15].


The extraction of elements from environmental samples could be performed using sequential extraction methods. The pioneer work was proposed by Tessier et al. [16], which was based on a five-stage extraction/fractionation of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn in river sediments (details in Table 8.1). After the establishment of Tessier’s method, the interest in sequential extraction has grown considerably for element extraction in lake and river sediment, sewage sludge, and fly ash, among other matrices. Thus, in order to ensure the harmonization of different methodologies for sequential extraction, the Community Bureau of Reference (BCR) developed a general method (Table 8.1) that has been widely

TABLE 8.1 Tessier and BCR Sequential Extraction Protocols

Steps Tessier [16] BCR [17]

1 Exchangeable: 1 g of sample + 8 ml of 1 mol/l MgCl2 at pH 7 and agitation by 1 h.

Exchangeable, water and acid soluble: 1 g of sample + 40 ml of 0.11 mol/l CH3COOH and agitation by 16 h.

2 Bound to carbonates: Residue of step 1 + 8 ml of 1 mol/l CH3COONa at pH 5 and agitation by 1 h.

Reducible: Residue of step 1 + 40 ml of 0.1 mol/l NH2OH·HCl at pH 2 and agitation by 16 h.

3 Bound to Fe and Mn oxides: Residue of step 2 + 20 ml of 0.04 mol/l NH2OH·HCl in 25% (v/v) CH3COOH with occasional agitation up to complete dissolution of the free iron oxides (96 °C).

Oxidizable: Residue of step 2 + 10 ml of 8.8 mol/l H2O2 with heating at 85 °C for 1 h, followed by a second aliquot of 10 ml of 30% H2O2 with heating at 85 °C for 1 h. After cooling, the addition of 50 ml of 1 mol/l CH3COONH4 at pH 2.

4 Bound to organic matter and sulfides: Residue of step 3 + 3 ml of 0.02 mol/l HNO3 and 5 ml of 30% H2O2 at pH 2 with heating at 85 °C for 2 h, followed by a second aliquot of 3 ml of 30% H2O2 at pH 2 with heating at 85 °C for 3 h. After cooling, the addition of 5 ml of 3.2 mol/l CH3COONH4 in 20% (v/v) HNO3.

Residual: Residue of step 3 + 8 ml of aqua regia

5 Residual: Residue of step 4 + 2 ml of HClO4 digestion in a platinum crucible and 10 ml of HF to near dryness. Addition of 1 ml of HClO4 + 10 ml of HF and evaporation to near dryness. Addition of 1 ml of HClO4 until the appearance of white fumes. Dissolution with 12 mol/l HCl and dilution up to 25 ml.

–


accepted [16,18–20]. The main difference and advantage of BCR in comparison to Tessier’s method is that the first two steps were replaced by a single step. In addition, Tessier’s method recommends the use of relatively high amounts of HClO4 (step 5). However, the use of HClO4 is not recommended due to the risk of explosion, mainly during the evaporation step [16]. Both methods (BCR and Tessier) have been adapted in order to improve the extraction as well as reduce the extraction time by the insertion of microwave radiation in one or more steps.

Time is one of the main limitations of sequential extraction, which could be improved by the replacement of conventional agitation by ultrasonic shaking or microwave heating [21,22]. Microwave radiation can accelerate chemical pro-cesses for single or multistep sequential extraction [23,24]. Another important advantage of the MAE is the possibility to replace the sequential extraction by a single step extraction using the same reagents and conditions, but using dif-ferent aliquots of sample for each extraction [21,22]. When a single microwave-assisted step is performed, all extractions can be performed simultaneously except for the oxidizable fraction, which is obtained with the residue of the reducible fraction.

It is important to mention that when the BCR method is modified for its use with microwave radiation, the temperature needs to be controlled and should be always lower than the boiling temperature of reagents. In addition, some parameters such as power and irradiation time need to be carefully evaluated. As an example, the use of microwave irradiation with a power of 500–600 W dur-ing 60–90 s provided quantitative recoveries for Cd, Cu, Cr, Ni, Pb, and Zn for sediment applying the conventional BCR method and single MAE [21]. Some applications for environmental analysis were chosen to cover the last 20 years, and they are summarized in Table 8.2.

One of the relevant works was performed by Bettinelli et al. [39] where the particular importance of the determination of total content of toxic metals in soils and sediments was demonstrated. The authors evaluated an MAE method by using aqua regia to extract Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn from the soil. It was demonstrated that the MAE is a viable alternative to the traditional reflux systems, since contamination was minimized, sample preparation time was shorter, and suitable extraction efficiency (from 82 to 110%) was obtained [39].

Popescu et al. [40] determined the concentration and the lability of Pb, Cd, Cu, and Zn in soils affected by smelting activities. A BCR sequential extrac-tion method was used, and the residual fraction was submitted to a microwave-assisted digestion method with aqua regia. Potentially toxic elements appeared to be much above the intervention limits, while their lability was extremely high, revealing a serious risk for the resident population. Barra et al. [41] reminded the necessity to develop sensitive methods for the determination of inorganic arsenic in different matrices since these species are more toxic than the organic form. They reported the development of an inexpensive microwave-assisted distillation procedure for the quantitative determination of inorganic arsenic in soils by AFS.

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TABLE 8.2 Selected Applications of the MAE for Environmental Samples

Elements Matrix Typical Recoveries Comments Reference

Cr, Cu, Ni, Pb, and Zn

Sewage sludge

Recoveries (considering the comparison of the sum of MAE steps and Tessier’s method) ranged from 93% to 100%, except for Cr and Pb. The relative standard deviation (RSD) was <4%.

Sequential extraction was performed in four steps and the analytes were determined by FAAS.Step 1: 2 g of sample + 8 ml of 1 mol/l MgCl2 (pH 7). Microwave irradiation: 90 W for 30 s (polyethylene vessels).Step 2: Residue step 1 + 8 ml of 1 mol/l CH3COONa (pH 5). Microwave irradiation: 90 W for 30 s (polyethylene vessels).Step 3: Residue step 2 + 20 ml of 0.04 mol/l NH2OH·HCl in 25% CH3COOH. Microwave irradiation: 90 W for 30 s (polyethylene vessels).Step 4: Residue step 2 + 3 ml of 0.02 mol/l HNO3 + 5 ml of 30% H2O2 with microwave irradiation at 270 W for 30 s. Addition of 3 ml of 30% H2O2 with microwave irradiation at 270 W for 30 s. Addition of 5 ml of 3.2 mol/l CH3COONH4 with microwave irradiation at 270 W for 10 s (polyethylene vessels).

[24]

Hg Sediment Recovery was about 98%, with an RSD <3%.

Extractions were performed using 0.7 g of the sample and 4.5 ml of water + 10 ml of concentrated HNO3. Microwave irradiation at 570 W for 8 min (PTFE closed vessels).Mercury was determined by flow injection cold vapor generation atomic absorption spectrometry (FI–CVG–AAS).

[25]

Al, As, Cd, Co, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Si, and Zn

Sediment Recoveries ranged from 72% to 123%, except for Al and Cr, with an RSD <6%.

Extraction was performed using 0.25 g of sample and 8.5 ml of 0.11 mol/l CH3COOH. Microwave irradiation at 70 °C for 10 min (PTFE closed vessels).Analytes were determined by ICP OES.

[26]

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TABLE 8.2 Selected Applications of the MAE for Environmental Samples—cont’d

Cr, Ni, Zn, Cu, Cd, and Pb

Soil Recoveries ranged from 94% to 113%, 94% to 122%, 83% to 114%, and 81% to 157%, for CaCl2, ethylenediaminetetraacetic acid (EDTA) CH3COOH, and HCl, respectively.

Extractions were performed using 1 g of sample and 20 ml of 0.05 mol/l CaCl2 or 0.05 mol/l EDTA or 0.10 mol/l CH3COOH or 0.1 mol/l HCl. Microwave irradiation at 378 W for 30 min.Analytes were determined by ICP–MS.

[27]

Cr Soil Recoveries using ET AAS ranged from 95% to 102%, with an RSD <4%.

Extraction was performed using 0.3 g of sample and 3 ml of concentrated HNO3 + 0.5 ml of H2O. Microwave irradiation at 650 W for 15 min (PTFE closed vessels).Chromium was determined by F AAS and ET AAS.

[28]

Li, Na, K, Ca, Mg, Co, Mn, Ni, and Pb

Soil Recoveries ranged from 91% to 100%.

Extractions were performed using 2 g of sample and 20 ml of ultrapure water. Microwave irradiation at 150 °C for 15 min.Analytes were determined by ICP OES.

[29]

Cr, Cu, Ni, Pb, and Zn

Sewage sludge and sediment

Recoveries considering the comparison of the sum of MAE steps and Tessier’s method values ranged from 93% to 109% for sewage sludge and 92% to 106% for sediment, except for Cr.

Extraction with EDTA was performed using 2.5 g of sample and 25 ml of 0.05 mol/l EDTA. Microwave irradiation at 75 W for 20 and 40 s for sediment and sewage sludge, respectively (polyethylene vessels).Analytes were determined by F AAS.

[30]

Hg Coal fly ash and sediment

Recoveries ranged from 98% to 102%, with and an RSD <6%.

Extraction was performed using 0.3 g of the sample and 8 ml of 30% HNO3 + 0.02% thiourea. Microwave irradiation at 640 W for 30 s (polypropylene closed vessels).Mercury was determined by FI–CVG-ICP–MS.

[31]


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Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Tb, Tm, Y, and Yb

Soil Recoveries ranged from 89% to 108%, 96% to 109%, 92% to 109%, and 91% to 112%, for CaCl2, EDTA, CH3COOH, and HCl, respectively.

Extractions were performed using 1 g of sample and 20 ml of 0.05 mol/l CaCl2 or 0.05 mol/l EDTA or 0.1 mol/l CH3COOH or 0.1 mol/l HCl. Microwave irradiation at 378 W for 30 min.Analytes were determined by ICP–MS.

[32]

Cd, Cr, Cu, Ni, Pb, and Zn

Sewage sludge

Recoveries considering the comparison of the sum of the MAE steps and BCR method values ranged from 97% to 104%, with an RSD <7%.

Sequential extraction was performed in four steps, and analytes were determined by F AAS and ET AAS.Step 1: 0.25 g of sample + 20 ml of 0.11 mol/l CH3COOH. Microwave irradiation at 540 W for 60–90 s (PTFE open vessels).Step 2: Residue step 1 + 20 ml of 0.5 mol/l NH2OH·HCl (pH 1.5). Microwave irradiation at 540 W for 60–90 s (PTFE open vessels).Step 3: Residue step 2 + 5 ml of 8.8 mol/l H2O2. Microwave irradiation at 540 W for 70–90 s (PTFE open vessels). Addition of 20 ml of 1 mol/l CH3COONH4 (pH 2).Step 4: Residue step 3 + 4 ml of aqua regia.

[33]

Cr Sediment Recovery was about 102%, with an RSD <4%.

Sequential extraction was performed in three steps and Cr was determined by ET AAS.Step 1: 0.25 g of sample + 10 ml of 0.11 mol/l CH3COOH. Microwave irradiation at 66 W for 30 s.Step 2: Residue step 1 + 10 ml of 0.1 mol/l NH2OH·HCl (pH 2). Microwave irradiation at 66 W for 30 s.Step 3: Residue step 2 + 2.5 ml of 33% H2O2 and conventional digestion at 85 °C for 1 h (2 times). Addition of 12.5 ml of 1 mol/l CH3COONH4 (pH 2) and microwave irradiation at 198 W for 60 s.

[34]

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Cu, Pb, and Zn

Soil Recoveries considering the comparison of the sum of MAE steps and certified values ranged from 102% to 115%, 104% to 120% and 98% to 102% for Cu, Pb, and Zn, respectively.

Sequential extraction was performed in four steps (glass open vessels) and analytes were determined by F AAS.Step 1: 1 g of sample + 20 ml of 1 mol/l CH3COONa (pH 5.0). Microwave irradiation at 300 W for 5 min.Step 2: Residue step 1 + 20 ml of 0.072 mol/l NH2OH·HCl in 4.25 mol/l CH3COOH. Microwave irradiation at 150 W for 30 min.Step 3: Residue step 2 + 3.5 ml of 0.02 mol/l HNO3 + 5 ml of 30% H2O2 (2 times). Microwave irradiation at 300 W (first extraction) and 450 W (second extraction) near dryness.Step 4: Digestion with HClO4 of residue step 3 without microwave.

[35]

Cu, Pb, and Zn

Soil and sediment

Recovery ranged from 73% to 102%, with an RSD < 4%.

Sequential extraction was performed in three steps and the analytes were determined by ICP OES.Step 1: 0.5 g of sample + 20 ml of 0.11 mol/l CH3COOH. Microwave irradiation: Ramp to 70 °C in 7 min and hold for 1.6 min.Step 2: Residue step 1 + 20 ml of 0.5 mol/l NH2OH·HCl. Microwave irradiation: Ramp to 70 °C in 7 min and hold for 1.6 min.Step 3: Residue step 2 + 10 ml of 8.8 mol/l H2O2 and microwave irradiation with ramp to 75 °C in 7 min and hold for 0.17 min. Addition of 20 ml of 1 mol/l CH3COONH4.

[36]

TABLE 8.2 Selected Applications of the MAE for Environmental Samples—cont’d


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Cr, Cu, Fe, Mn, Ni, and Zn

Sediment Recoveries considering the comparison of the sum of the MAE steps and certified values ranged from 72% to 103%, with an RSD <14%.

Sequential extraction was performed in four steps (polyethylene vessels) and analytes were determined by F AAS and ET AAS.Step 1: 0.5 g of sample + 20 ml of 1 mol/l CH3COONa (pH 5). Microwave irradiation at 750 W for 3 min.Step 2: Residue step 1 + 25 ml of 0.04 mol/l NH2OH·HCl in 25% CH3COOH. Microwave irradiation at 750 W for 3 min.Step 3: Residue step 2 + 15 ml of 30% H2O2 + 10 ml of 0.02 mol/l HNO3. Microwave irradiation at 750 W for 3 min. Addition of 5 ml of 3.2 mol/l CH3COONH4.Step 4: Residue step 3 + 10 ml of aqua regia + 5 ml of HF.

[37]

Cd, Cr, Cu, Ni, and Pb

Sediment Recoveries ranged from 96% to 103%, 85% to 103% and 82% to 103% for steps 1, 2, and 3, respectively.

Sequential extraction was performed in three steps (TFM closed vessels) and analytes were determined by ICP–MS.Step 1: 0.25 g of sample + 10 ml of 0.11 mol/l CH3COOH. Microwave irradiation: Ramp to 300 W in 4.6 min and hold for 2 min.Step 2: Residue step 1 + 10 ml of 0.5 mol/l NH2OH·HCl (pH 1.5). Microwave irradiation: Ramp to 400 W in 6.5 min and hold for 1.75 min.Step 3: Residue step 2 + 2 ml of 30% H2O2 + 5 ml of 5 mol/l CH3COOH. Microwave irradiation: Ramp to 300 W in 4.6 min and hold for 2 min.

[38]


Concerning the use of BCR or Tessier methods, element determination by atomic absorption spectrometry (AAS) in extracts is less susceptible to matrix effects in comparison to plasma-based techniques. In addition, for extracts with a high ionic content, matrix-matching calibration for ICP–MS instruments is commonly required.

Even though the majority of applications have been executed in batch sys-tems, the MAE can also be carried out in flow systems. In this case, a plug containing soil slurry was injected into an acid carrier stream and transported to a focused microwave reactor. After a few minutes, some analytes such as As, Cr, Cu, Mn, Mo, Ni, Pb, Sb, and Zn could be completely leached and introduced into different detection systems such as AFS, ICP–MS, among others [42,43].

8.3.2. Biological and Food Analysis

Element extraction in biological and food samples is less frequently performed once the organic matrix can be oxidized using conventional wet digestion, microwave-assisted digestion, or combustion methods. Another alternative for these matrices is the dry ashing, but this procedure is rather time consuming and can often result in the loss of volatile elements [44,45].

The main limitation using MAE with diluted acids (e.g., HNO3 or HCl) is the unsuitable recovery for some elements. For example, recoveries of As, Cd, and Cu in mussel tissues are <72%, using an extraction solution composed of 15 ml of 10% (v/v) HNO3. On the other hand, quantitative recoveries (from 82 to 114%) were observed for Co, Cr, Hg, Mn, Ni, Pb, V, and Zn [45]. In order to avoid the use of concentrated acids and H2O2, MAE can be carried out using chelating reagents (e.g., EDTA) or just ultrapure water. Even with a suitable recovery for elements such as B, Cd, Ni, Pb, Sr, and Zn using 0.02 mol/l EDTA, Borkowska-Burnecka [44] has been reported that with the use of 1 mol/l of HCl, quantitative recoveries were also obtained for these elements as well as for Ba, Cu, and Mn from plant samples (cabbage and tobacco leaves). It is important to mention that in some cases, a careful optimization of extraction time and reagent concentration needs to be performed to improve analyte recovery.

Some applications for biological and food analysis were chosen to cover the last 20 years and are summarized in Table 8.3.

In general, the major advantage of the MAE is the reduced time in compari-son to traditional extraction methods such as Soxhlet and liquid (solid–liquid or liquid–liquid) extractions. For marine organisms, after 2 min (75 °C), MAE recoveries were improved in comparison to traditional methods for As and Cd using 2 mol/l HNO3 as the extraction solution. Quantitative recoveries can be observed for other elements, such as Co, Mo, and Se, when some characteristics of solvent (e.g., type, polarity, and concentration) are modified [56].

Halogens determination in biological and food samples is a difficult task owing to their volatility when acidic solutions are employed. Moreover, memory effects in plasma-based techniques have been reported, and the use of sample

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TABLE 8.3 Selected Applications of the MAE for Biological and Food Samples


Al, Ca, Mg, and Mn

Tea Recoveries ranged from 37% to 82%, 72% to 95%, 73% to 103%, and 72% to 106%, for Al, Ca, Mg, and Mn, respectively.

Extractions were performed using 0.1–0.5 g of sample and 20 ml of 1.2 mol/l HCl. Microwave irradiation with ramp to 95 °C in 2 min and hold for 3 min (borosilicate open vessels). Analytes were determined by F AAS and ICP OES.

[46]

Cd and Pb Plant leaves Recoveries were about 102%, with an RSD <4%.

Extractions were performed using 0.5 g of sample and 2 ml of 1% HNO3. Microwave irradiation at 300 W for 10 min (Cd) or 15 min (Pb).Analytes were determined by ET AAS.

[47]

Cu and Zn Wheat and corn flour

Recoveries ranged from 100% to 105% for Cu and 98% to 99% for Zn and an RSD <13%.

Extraction was performed using 1 g of sample and 10 ml of 1 mol/l HNO3. Microwave irradiation at 90 W for 9 min.Analytes were determined by F AAS.

[48]

Cu, Fe, Mn, Ni, Pb, and Zn

Fish muscle – Extraction was performed using 1 g of sample and 10 ml of 2.5 mol/l HNO3. Microwave irradiation at 685 W for 38 min.Analytes were determined by F AAS.

[49]

Se Medicinal plants Recovery ranged from 98% to 99%, with an RSD <13%.

Extraction was performed using 1.0 g of the sample and 25 ml of water. Microwave irradiation at 720 W for 2 min (PTFE closed vessels).Selenium was determined by ET AAS.

[50]

Br and I Seaweed Recovery for Br ranged from 90% to 118%.

Extraction was performed using 0.1–0.2 g of sample and 5 ml of ultrapure water + 5 ml of 25% tetramethylammonium hydroxide (TMAH). Microwave irradiation with ramp to 200 °C in 10 min and hold for 5 min (PTFE closed vessels).Analytes were determined by ICP–MS.

[51]

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TABLE 8.3 Selected Applications of the MAE for Biological and Food Samples—cont’d


Ag, As, Cd, Cu, Ni, Pb, V, and Zn

Mussel tissue Recoveries ranged from 70% to 109%, except for Cd.

Extraction was performed using 5 ml of 25% TMAH. Microwave irradiation at 40 W for 3 min (borosilicate open vessels).Analytes were determined by ICP–MS.

[52]

Br and I Seaweed Recoveries were about 101% for both analytes, with an RSD <11%.

Extraction was performed using 0.25 g of sample and 10 ml of ultrapure water + 10 ml of 25% TMAH. Microwave irradiation with ramp to 200 °C in 10 min and hold for 5 min (PTFE closed vessels).Analytes were determined by ICP–MS.

[53]

Al, Ba, Cd, Cr, Cu, Fe, Mn, Pb, Sn, V, and Zn

Mussel tissue Recoveries ranged from 73% to 105%, with an RSD <12%.

Extraction was performed using 0.5 g of the sample and 15 ml of a mixture containing 2.5 mol/l HNO3, 3 mol/l HCl, and 0.5% H2O2. Microwave irradiation at 65 °C for 3 min (PTFE closed vessels).Analytes were determined by ICP OES.

[54]

Br and Cl Cigarette tobacco Recoveries of 70% and 90% were obtained for Br and Cl, respectively, using water or alkaline solution.

Extraction was performed using 0.5 g of sample and 6 ml of water or 100 mmol/l (NH4)2CO3 solution. Microwave radiation was applied for 30 min at 1400 W (ramp of 10 min) and further 20 min for cooling. The maximum temperature and pressure were set at 280 °C and 80 bars, respectively. After cooling, resultant solutions from extraction procedures were diluted with water to 20 ml in a polypropylene vessel and then centrifuged at 3000 rpm for 10 min prior to the determination by ICP OES.

[55]


preparation in alkaline solutions is recommended [57,58]. Muller et al. [55] investigated the use of water and 100 mmol/l (NH4)2CO3 solution for Br and Cl extraction from cigarette tobacco samples. Extraction was performed under microwave radiation at a high pressure and temperature using closed vessels, and 1400 W was applied for 30 min (ramp of 10 min). According to the authors, recoveries in the same order were obtained using a water or alkaline solution (70–90%).

Nevertheless, the MAE usually requires less solvent, in addition to it being mild, relatively simple, and highly reproducible. Regardless of the extraction method chosen, some parameters such as extraction conditions, analyte, sol-vent, and sample matrix are essential for method selectivity.

Element determination after MAE in biological tissues is critical, espe-cially using detection techniques such as ICP OES and ICP–MS. In these cases, the use of internal standards can reduce the matrix effects caused by residual organic matter. Moreover, in order to avoid nebulizers clogging, filtration (0.22 or 0.45 μm), and/or centrifugation steps are recommended [45]. An alternative to high solid dissolved content in the extracts is the use of nebulizers with a high solid tolerance, such as GemCone™, V-groove, and Babington. These nebuliz-ers are considered suitable for extracts with high solid contents, even though some of them have a poor limit of detection in comparison to concentric or crossflow nebulizers.

8.3.3. Miscellaneous

The MAE has been typically applied for environmental (sediment, soil, sew-age sludge, and fly ash) and biological (animal and plants tissues) samples. Additionally, microwaves may be used for element extraction from refractory samples, such as some types of polymers [59,60], crude oil [61–63], and air-borne particulate matter [64,65].

In general, polymers (especially fluoropolymers) are highly inert, difficult to solubilize, and generally require complex and time-consuming sample prepara-tion methods. For this type of matrix [e.g., PTFE, PTFE–TFM, and fluorinated ethylene propylene], combustion methods are frequently recommended for total element determination [59,60]. However, using concentrated HNO3, some ele-ments such as Cr, Fe, Mn, and Zn can be quantitatively leached with microwave heating (e.g., 1400 W for 40 min), and it is important to point out that high pres-sure (∼80 bar) is required for complete analyte recovery [59]. Concerning halo-gens, their determination in elastomers is necessary because they are frequently added as flame retardants to prevent ignition and accidental fires [60]. A method based on the MAE was applied using closed vessels and using 6 ml of water or 50 mmol/l of (NH4)2CO3 solution. Extraction was performed according to the following heating program: 1400 W (ramp of 10 min) and hold for 20 min, and 0 W for 20 min for cooling. However, a better agreement for Br was only 33% and 40% and for Cl it was 38% and 42% using water and 50 mmol/l (NH4)2CO3


as extractants, respectively [60]. Therefore, quantitative recoveries were not obtained for halogens extraction from elastomer samples using the MAE.

The MAE has been also used to extract some elements from crude oil sam-ples [61–63] or petrochemical samples, as refining catalysts [66]. In general, MAE procedures applied for crude oils are related to Cl extraction because this element is responsible for corrosion during crude oil processing in refineries. In addition, the presence of salts could change the quality of final products such as petroleum coke and residues of petroleum distillation [61–63]. Heavy crude oil emulsions (10 g of sample, °API lower than 14) were submitted to microwave radiation using high-pressure quartz vessels, and water (20 ml) was also added. The microwave radiation program, such as heating time, microwave power, and the number of extractions, was evaluated. The extraction efficiency of Cl from heavy crude oil emulsion was >95% using subsequent extraction steps [61]. In order to evaluate the extraction efficiency, Cl was determined by IC in the aque-ous phase obtained after the MAE and after sample digestion by the microwave-induced combustion method [67]. For crude oil samples and for residues from crude oil distillation, recoveries for Cl were not quantitative when the MAE was applied using one step [62,63]. The microwave irradiation program was performed at 1000 W for 10 min (ramp for 10 min) and at 0 W for 20 min (cool-ing step) using 0.4 g of vacuum residue [62] or 0.5 g of crude oil [63] and 6 ml of water. Recoveries <30% were obtained for Cl extraction from vacuum distil-lation residues [62] and about 70% for extra heavy crude oil samples (°API 11) [63]. Sulfur was also determined, and the recovery was about only 1% using MAE for extra heavy crude oil samples [63].

The element monitoring of airborne particulate matter can be performed using filter papers or filters composed of inert materials, such as PTFE, glass, and quartz. Using microwave-assisted digestion methods, the pretreatment of filter paper is relatively easy in comparison to that of inert materials. On the other hand, due to the high stability of glass (or quartz) or polymers filters, the MAE has been successfully employed for the extraction of acid [64] or water-soluble [65,68] analytes. An advantage of the MAE for this type of matrix is the high sample mass that can be used (up to 4 g). In this case, recoveries ranging from 83 to 105% for Mo, La, Ce, Nd, and Sr were observed after irradiation (650 W) for 5 min using 0.24 mol/l of HCl as the extraction solution [64]. In addition, anionic (F−, Cl−, NO3

−, PO43−, and SO4

2−) or cationic (Na+, NH4+,

K+, Ca2+, Mg2+, and others) analytes were also extracted from filters using only water [65,68].

Among the nonrestrictive pharmaceuticals, the multivitamin/multimineral preparations containing several elements and many vitamins and/or provitamins are the most consumed [69,70]. Once these formulations have different compo-sitions, some of the most common elements are Cu, Fe, Mn, and Zn, which are essential for humans, because of their physiological importance and biologi-cal roles [71,72]. Extraction from commercial samples of ground multivitamin/multimineral tablets may be easily performed using 10 ml of 0.7 mol/l of HNO3


and irradiation power of 360 W for 15 min in PTFE–TFM vessels. For these elements, the MAE efficiency is in agreement with the results obtained after microwave-assisted digestion [68].

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