Multitechnique Determination of Halogens in Soil ... - ACS Publications

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Multitechnique Determination of Halogens in Soil after Selective Volatilization Using Microwave-Induced Combustion L. S. F. Pereira,† M. F. Pedrotti,† M. S. P. Enders,† C. N. Albers,‡,§ J. S. F. Pereira,∥ and E. M. M. Flores*,† †

Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, Rio Grande do Sul, Brazil Department of Geochemistry, Geological Survey of Denmark and Greenland, Ø. Voldgade 10, 1350 Copenhagen, Denmark § Center for Permafrost (CENPERM), Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350 Copenhagen, Denmark ∥ Departamento de Química Inorgânica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil ‡

S Supporting Information *

ABSTRACT: A method for digestion of soils with high inorganic matter content (ranging from 50 to 92%) by microwave-induced combustion (MIC) is proposed for the first time for further halogens (F, Cl, Br, and I) determination by ion chromatography (IC) and also by inductively coupled plasma mass spectrometry (ICP-MS). Microcrystalline cellulose (100−500 mg), used as a combustion aid, was mixed with sample and water or NH4OH solutions (10−100 mmol L−1) were investigated for analytes absorption. The use of cellulose (400 mg) was mandatory to volatilize the halogens from soils with high inorganic matter. It was possible to use diluted absorbing solutions (up to 100 mmol L−1 NH4OH) for halogens retention, providing limits of quantification in the range of 0.06 (I) to 60 (Cl) μg g−1. Accuracy was evaluated using certified reference materials (CRMs), spiked samples, and pyrohydrolysis method. Recoveries for halogens after spiked samples were in the range of 94 to 103% and the results after digestion of CRMs by MIC were in agreement better than 95% to certified values. Blanks were low, relative standard deviation was below 8% for all soils and no statistical difference was observed for results by pyrohydrolysis and MIC methods showing the feasibility of the proposed method for further halogens determination in soil samples.

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25:5:1.12−14 Recent studies showed that in the organic horizons of forests, all bromine may be covalently bound to organic structures,13,15 calling for more research in order to understand the terrestrial biogeochemical cycle of bromine, which is largely unexplored. In order to carry out future research on terrestrial halogen biogeochemistry, a high throughput analytical method capable of quantifying different halogen species in various soil types would be highly valued. Although the application of sample preparation methods in closed vessels has been extensively described for further multielement determination,16−22 some difficulties are generally related to the determination of halogens since the use of acid solutions for digestion may result in halogen losses during the decomposition step in addition to the high risk of contamination.23−25 Another difficulty is the suitability of final digests for analytical techniques. As an example, ion chromatography (IC) could not support high acidic solutions.

uring the last 25 years it has been recognized that chlorine, bromine, and iodine have biogeochemical cycles that significantly affect their behavior in the natural environment.1−3 For example, the chloride present in leaves of trees is quickly turned into organic chlorine after the leaves fall to the ground.4,5 Also within the soil, various halogenation and dehalogenation processes can occur, with enzymatic and abiotic processes turning halide from precipitation into organic halogen.6,7 The organic halogen may evaporate to the atmosphere if the compounds are volatile8 or be degraded through biotic and abiotic processes in soil and groundwater, which will eventually release chloride back to the soil.9−11 In this sense, halogens seem to be part of a very complex biogeochemical cycle, whose individual processes largely remain to be understood and quantified. Most previous studies have relied on nonspecific halogen determination of soil samples (total organic halogen, TOX) and have assumed that all TOX in soil is chlorine. Analyses of peat, forest soil, and soil humic acid samples suggest that also bromine and iodine may contribute to the total halogen content of unpolluted environmental samples with a ratio between organic chlorine, bromine, and iodine of around © XXXX American Chemical Society

Received: November 2, 2016 Accepted: November 29, 2016 Published: November 29, 2016 A

DOI: 10.1021/acs.analchem.6b04300 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

was used for the MIC method. The maximum operating temperature and pressure were 280 °C and 80 bar, respectively. A commercial quartz holder (Cat. No. 16427, Anton Paar) was used to insert the sample inside the quartz vessels. An ion chromatography system (850 professional IC, Metrohm, Switzerland) consisting of a pump (833 IC liquid handling unit) and a conductivity detector was used for F and Cl determination. It was equipped with an anion-exchange column (Metrosep A Supp 5, poly(vinyl alcohol) with quaternary ammonium groups, 250 × 4 mm i.d., Metrohm) and a guard column (Metrosep A Supp 4/5 Guard, Metrohm). The mobile phase consisted of a solution of 3.2 mmol L−1 Na2CO3 and 1.0 mmol L−1 NaHCO3. A sample loop of 100 μL was used, and the mobile phase flow rate was set at 0.7 mL min−1. An inductively coupled plasma mass spectrometer (Elan DRC II, PerkinElmer Sciex, Canada) equipped with a concentric nebulizer (Meinhard Associates, U.S.A.), a cyclonic spray chamber (Glass Expansion, Inc., Australia), and a quartz injection tube (2 mm i.d.) was used for determination of Br and I. Argon with 99.996% of purity (White Martins-Praxair, Brazil) was used for plasma generation, nebulization, and as auxiliary gas. Radio frequency power and plasma, auxiliary, and nebulizer gas flow rates were 1400 W, 15.0, 1.2, and 1.08 L min−1, respectively. The sample flow rate used was 1.4 mL min−1, and the isotopes measured were 79Br and 127I. Samples, Reagents, and Standards. Soil samples from five undisturbed areas in Greenland, Lapland, and the Alps were sampled with a steel core (Ø = 6 cm, depth 10 cm) after removing living mosses and lichens. The soil was frozen at the day of sampling and stored for subsequent freeze-drying and homogenization to fine powder by grinding after removal of larger roots (Supporting Information, Photo Material). Organic matter was determined as loss on ignition (2 h, 550 °C), and two groups, containing three samples each, were then evaluated: (i) group “A” with inorganic matter content of 92, 92, and 88% (samples “A1”, “A2”, and “A3”, respectively) and (ii) group “B” with inorganic matter content of 52, 51, and 50% (samples “B1”, “B2”, and “B3”, respectively). A more detailed description of samples location is shown in Supporting Information. Accuracy of the MIC method was evaluated using CRMs of coal (NIST 1632c - Coal bituminous, ash content 7.16 ± 0.05%, carbon content 77.45 ± 0.25%) and soil (NIST 2711 Montana soil, silicon content 30.44 ± 0.19%, carbon content 2%). Analytes recovery was also evaluated by the addition of reference solutions containing F, Cl, Br, and I on the pellet of soils “A1” and “B3” previously to soils digestion by MIC. Distilled and deionized water was further purified using a Milli-Q system (Millipore Corp., U.S.A.), and it was used to prepare all the standard solutions and reagents. Concentrated nitric acid (65%, Merck, Germany) was used for cleaning of quartz vessels after each digestion step by MIC. For fluoride and chloride determination by IC, a stock reference solution containing 10 mg L−1 of F and Cl (Fluka, Sigma-Aldrich, U.S.A.) was used. Reference solutions were prepared by sequential dilution of this solution in water and it was also used for spike recovery experiments (for addition of F, Cl, and Br). The mobile phase for IC was prepared by dissolution of Na2CO3 and NaHCO3 (Merck) in ultrapure water. The reference solutions used for the determination of Br and I by ICP-MS were prepared by dissolution of respective salts (Merck) in 10 mmol L−1 NH4OH. This solution was also used for addition of I in spike recovery experiments.

On the other hand, digests with high content of dissolved organic compounds are prone to interferences in inductively coupled plasma mass spectrometry (ICP-MS) determination, specially for iodine determination.23 From a practical point of view, if a digestion method could provide analytes in a suitable solution that could be analyzed by many techniques, it should by very attractive for routine analysis, saving time and reagents by avoiding specific digestion methods for each determination technique. In this way, microwave-induced combustion (MIC) has been applied for further halogens determination, since it is possible to ensure a complete sample digestion and, at same time, using nonconcentrated alkaline solutions or even water for absorption of analytes.26,27 Direct sample analysis can be employed to overcome these limitations and many analytical techniques have been used for halogens determination, such as neutron activation analysis (NAA),28,29 X-ray fluorescence spectrometry (XRF),30 X-ray absorption near-edge structure spectroscopy (XANES),4 and high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS).31 However, direct analysis of solid samples may be limited by the difficulties in the calibration step, in addition to the effects related to sample matrix and homogeneity.32 In this way, analytical techniques such as ICP-MS,33 ion-selective electrode (ISE),34,35 and IC36−38 have been used for halogens determination after a suitable sample pretreatment, such as pyrohydrolysis35,39 and combustion methods.33,40 The MIC method has been normally applied for the digestion of combustible samples (i.e., containing high amount of organic matter).41,42 However, in order to digest samples with a high inorganic content, it is necessary to mix the sample with a combustion aid such as microcrystalline cellulose to ensure a complete volatilization of analytes from matrix. Using this approach, it was possible the determination of As, Cd, and Pb by ICP-MS and inductively coupled plasma optical emission spectrometry (ICP-OES) and Hg by CVG-ICP-MS,43 allowing an agreement with certified values better than 95%. In another work,44 the use of high-purity microcrystalline cellulose was also investigated for the volatilization of Cl and F from Portland cement for further determination by IC. The agreement with certified values ranged from 98 to 103% for Cl, and the recovery was about 96% for F. The use of MIC for further determination of Br and other contaminants in polymeric waste of electrical and electronic equipment was also proposed.45 Considering the difficulties related to determination of halogens in soils, and also the few methods available in the literature for this challenging task, the aim of the present work was to demonstrate for the first time the feasibility of the MIC method to digest soil with high inorganic matter content for further halogens determination. Operational parameters of MIC such as sample mass of microcrystalline cellulose used as combustion aid and the type and concentration of absorbing solutions were investigated. After sample digestion, the determination of F and Cl was carried out by IC and Br and I by ICP-MS. The accuracy of the proposed method was evaluated by the analysis of certified reference materials (CRMs) of coal and soil by using a pyrohydrolysis method and also by using spikes with reference solutions.

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EXPERIMENTAL SECTION Instrumentation. A microwave sample preparation system (Multiwave 3000, Anton Paar, Austria), equipped with eight high pressure quartz vessels with an internal volume of 80 mL, B


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Figure 1. Influence of microcrystalline cellulose (100−500 mg) in concentration of halogens in soil “A1” after MIC. MIC was performed using 100 mg of sample and 50 mmol L−1 NH4OH as absorbing solution (mean ± standard deviation, n = 3).

digestion step, vessels were cleaned with 6 mL of 65% HNO3 by microwave heating at (i) 1400 W for 10 min and (ii) 0 W for 20 min as cooling step, followed by second cleaning step with 6 mL of Milli-Q water with the same heating program. After the end of each cleaning step, vessels and quartz holders were rinsed with water.

Ammonium nitrate (Merck) was dissolved in water (6 mol L−1), and it was used for the ignition step in the MIC method. Small discs of filter paper (15 mm of diameter, 15.3 ± 0.3 mg) with low ash content (Black Ribbon Ashless, Schleicher and Schuell, Germany) were used as a combustion aid for the MIC method, and they were previously cleaned with ethanol (95%, Merck) in an ultrasonic bath and further washed with water and dried in a laminar flow bench (CSLH-12, Veco, Brazil). Microcrystalline cellulose of pharmaceutical grade was used as a combustion aid by mixing with the sample prior pelleting in a hydraulic press (Specac, U.K.). Oxygen (99.6%, White MartinsPraxair) was used for vessel pressurization in the MIC method. Ammonium hydroxide (10−100 mmol L−1) was prepared by dilution of a commercial solution (25%, Merck), and it was used as absorbing solution for the MIC method. Microwave-Induced Combustion Method. Sample mass of about 100 mg was pressed as a pellet (13 mm) using a hydraulic press set at 3 ton. Soil sample (100 mg) was mixed with microcrystalline cellulose (100−500 mg), previously to the pelleting step, in order to aid the combustion process. Further, the sample pellet and filter paper were placed on the quartz holder and 50 μL of 6 mol L−1 NH4NO3 solution was added. The quartz holder was transferred to the quartz vessel previously charged with 6 mL of absorbing solution. The study of the more suitable absorbing solution was performed using H 2 O and a solution of NH 4 OH with variable concentrations (10−100 mmol L−1). After vessel closing and rotor capping, vessels were pressurized with O2 at 20 bar. The microwave heating program used for the MIC method was 5 min at 1400 W (ignition and reflux step) and 20 min at 0 W (cooling step). After the end of the heating program, the pressure of each vessel was carefully released and the resultant solution was diluted with water up to 25 mL. After each

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RESULTS AND DISCUSSION In spite of recent and successful applications, until now MIC method was not used to digest soil samples with high inorganic matter content for further halogens determination. In the case of inorganic matrices, which are not easily digested by combustion due to the low organic matter content, it has already been demonstrated the necessity of using a combustion aid, such as microcrystalline cellulose, resulting in a complete volatilization of metals and nonmetals.43−46 In the present work, some parameters were evaluated for the optimization of MIC method according to the inorganic matter content present in soils. Initially, one soil of each group (containing 92 and 50% of inorganic matter, named “A1” and “B3”, respectively) was arbitrarily chosen for optimization of the parameters of MIC. After sample digestion, analytes were determined by IC (F and Cl) and ICP-MS (Br and I). It is important to notice that Br and I were determined only by ICPMS and not by IC (as for F and Cl) due to the lower limit of quantification (LOQ) obtained by ICP-MS for these analytes. Optimization of MIC Method. In order to evaluate the suitability of MIC for samples containing high inorganic matter content, initial studies were carried out using soil sample “A1” (100 mg) and 50 mmol L−1 NH4OH solution for analytes absorption (without the use of microcrystalline cellulose). After combustion a solid residue remaining on the quartz holder was observed, indicating an incomplete combustion of the sample, C


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Figure 2. Influence of absorbing solution in concentration of halogens. MIC was performed using 100 mg of soil “A1” and 400 mg of microcrystalline cellulose (mean ± standard deviation, n = 3).

Figure 3. Influence of microcrystalline cellulose (100−500 mg) in concentration of halogens in soil “B3”. MIC was performed using 100 mg of sample and 50 mmol L−1 NH4OH as absorbing solution (mean ± standard deviation, n = 3).

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Figure 4. Influence of absorbing solution in concentration of halogens. MIC was performed using 100 mg of soil “B3” and 400 mg of microcrystalline cellulose (mean ± standard deviation, n = 3).

After optimization of cellulose mass, experiments were performed in order to evaluate the absorbing solution used for retention of analytes. For this, 100 mg of soil “A1” was digested using 400 mg of cellulose and water or NH4OH (10− 100 mmol L−1) were evaluated as absorbing solution. The obtained results are shown in Figure 2. According to the results presented in Figure 2, using water as absorbing solution, the concentration of F, Br and I was lower compared to values obtained when NH4OH solutions were used. Additionally, no statistical difference (ANOVA, confidence level of 95%) was observed for F concentration using 10−100 mmol L−1 NH4OH as absorbing solutions. For Br and I, higher concentrations were obtained using 100 mmol L−1 NH4OH as absorbing solution. Regarding Cl absorption, it was possible to observe that even water could be used, since no statistical difference was observed among the results obtained for water and for all the NH4OH solutions investigated. Thus, further experiments for samples with high inorganic matter content (“group A”) were carried out using 400 mg of microcrystalline cellulose mixed with 100 mg of sample and using 100 mmol L−1 NH4OH as absorbing solution. After the optimization of MIC conditions for soil samples of “group A”, MIC was also applied for sample with inorganic matter content of 50% (group “B”). In the same way, initial studies for halogens volatilization by MIC were performed using 100 mg of soil “B3” (without or using 100−500 mg of microcrystalline cellulose) and 6 mL of 50 mmol L−1 NH4OH as absorbing solution. The obtained results are shown in Figure 3. According to results, no statistical difference (ANOVA, confidence level of 95%) was observed among the results obtained for Br and I without or using 100−500 mg of

which could result in an ineffective volatilization of the analytes. In order to improve the digestion efficiency, the addition of microcrystalline cellulose was evaluated. A variable cellulose mass (100−500 mg) was mixed with 100 mg of sample previously to the pellet formation and combustion by MIC. In Figure 1 are shown the results obtained for halogens in soil “A1” after MIC without and using variable mass of microcrystalline cellulose. According to the results obtained for soil “A1”, it was possible to observe that without the use of microcrystalline cellulose, halogens concentration was lower in comparison to those obtained when cellulose was used. Moreover, a high relative standard deviation (RSD) was also observed for all halogens evaluated. This fact can be explained by the high inorganic matter content present in the soil sample (about 92%), releasing a low energy amount during the combustion that was not enough to allow the complete volatilization of analytes. Thus, a combustion aid is mandatory to obtain a quantitative volatilization of halogens when working with soils having low organic matter content. When microcrystalline cellulose was used, a progressive increase in the concentration of all analytes was observed when higher masses of cellulose were used. No statistical difference (t-test, confidence level of 95%) was observed for the results obtained for F, Br, and I when cellulose mass of 400 and 500 mg were used. Regarding Cl concentration, no statistical difference (ANOVA, confidence level of 95%) was observed when cellulose mass of at least 200 mg were used. Therefore, the use of 400 mg of cellulose microcrystalline was chosen as suitable condition to ensure an effective volatilization of all halogens in soil sample “A1”, with a high inorganic matter content. E


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Analytical Chemistry Table 1. Results for Halogens (μg g−1) after Digestion of Soils by MIC (Mean ± Standard Deviation, n = 3) concentration, μg g−1 F group A

B

Cl

Br

sample identification

without cellulose

optimized conditionsa

without cellulose

optimized conditionsa

A1 A2 A3 B1 B2 B3

10.9 ± 3.1 15.2 ± 1.1 16.2 ± 0.9 84.5 ± 4.3 21.1 ± 1.2 625 ± 36

534 ± 30 154 ± 4 504 ± 23 147 ± 9 84.2 ± 5.2 1107 ± 57

43.8 ± 6.3 42.0 ± 12.3 47.5 ± 5.5 196 ± 10 211 ± 11 164 ± 9

83.7 ± 4.7 126 ± 7 85.6 ± 5.9 230 ± 12 249 ± 13 190 ± 12

without cellulose 2.86 1.12 6.44 5.99 50.1 8.92

± ± ± ± ± ±

0.67 0.09 0.35 0.30 2.6 0.45

I optimized conditionsa 6.51 3.65 6.20 5.92 51.7 8.45

± ± ± ± ± ±

0.35 0.24 0.33 0.30 2.5 0.47

without cellulose 0.79 0.81 2.81 1.95 14.4 6.41

± ± ± ± ± ±

0.32 0.10 0.21 0.10 0.8 0.33

optimized conditionsa 2.27 1.47 2.76 2.06 15.3 6.55

± ± ± ± ± ±

0.12 0.11 0.16 0.11 0.9 0.34

a Optimized conditions. Group A: 100 mg of sample + 400 mg of cellulose and 100 mmol L−1 NH4OH as absorbing solution; Group B: 100 mg of sample + 400 mg of cellulose and 25 mmol L−1 NH4OH as absorbing solution.

recoveries for all the analytes, mainly for I. A reference solution containing F, Cl, and Br and a solution containing I, prepared by salt dissolution in water, were added on the pellet of samples “A1” and “B3”, and MIC method was performed using optimized conditions for each soil type. Recoveries were in the range of 94−103% for all analytes. Additionally, the pyrohydrolysis method was also applied for soil “A1” (containing 92% of inorganic matter) using conditions based on a previous work.39 Results obtained for sample “A1” after pyrohydrolysis method were 505 ± 46, 87.6 ± 5.2, 6.13 ± 1.07, and 2.06 ± 0.35 μg g−1 for F, Cl, Br, and I, respectively. No statistical difference (t-test, confidence level of 95%) was observed for results obtained by MIC and pyrohydrolysis method, showing the applicability of the proposed MIC method to digest soil samples with high inorganic matter content for further halogens determination by IC and ICP-MS. However, it is important to notice that for total volatilization of analytes of soil “A1” by pyrohydrolysis, the heating time used for each sample was 1 h, while using MIC method up to eight samples can be digested in 25 min (including the cooling step). Moreover, higher RSDs were observed for Br and I (about 17%) when pyrohydrolysis method was applied for soil “A1” in comparison with results obtained for this elements after MIC (about 5%). Halogens Determination in Soil Samples. After digestion of soil samples with 92 and 50% of inorganic matter content (soils “A1” and “B3”, respectively), MIC method was applied for other soil samples. In this sense, MIC was performed with the respective conditions optimized according to the inorganic matter content present in each soil sample. The conditions applied were as follows: (i) Group A: 400 mg of microcrystalline cellulose and 100 mmol L−1 NH4OH as absorbing solution; (ii) Group B: 400 mg of microcrystalline cellulose and 25 mmol L−1 NH4OH as absorbing solution. It is noteworthy that soil mass and volume of absorbing solution were kept in 100 mg and 6 mL, respectively. Results obtained for the determination of halogens by IC (F and Cl) and ICPMS (Br and I) after digestion of soils by MIC are shown in Table 1. It was possible to observe that for other soils evaluated for each group, in general, similar results were obtained. For example, it was necessary to use microcrystalline cellulose in order to improve volatilization of F, Cl, Br, and I for sample “A2”, in the same way as observed for sample “A1”, which was used for the method optimization. Additionally, for samples “B1” and “B2”, the use of cellulose was necessary for the total volatilization of F and Cl, but in absence of cellulose, quantitative results were obtained for Br and I. It was also

microcrystalline cellulose. However, for F and Cl, in the absence of cellulose, low concentration was obtained in comparison with results obtained by using cellulose. These results were similar to those obtained for soil “A1” (high inorganic matter content), since the use of cellulose was also necessary to improve analytes volatilization. In the experiments performed with the use of microcrystalline cellulose, it was possible to observe a higher volatilization for F and Cl with the increase of the cellulose mass. Using 200−500 mg of cellulose, no statistical difference (ANOVA, confidence level of 95%) was observed for F and Cl, but a higher RSD for Cl was obtained when up to 300 mg of cellulose was used. Thus, 400 mg of cellulose were chosen for subsequent experiments due to suitable volatilization and lower RSD for all halogens. In a similar way as performed for soil samples with high inorganic matter content, studies were also carried out to evaluate the absorbing solution for halogens retention. For this, sample “B3” was digested using 400 mg of microcrystalline cellulose and water or NH4OH were used as absorbing solution. The obtained results are shown in Figure 4. It was observed that even water could be used as absorbing solution for F, Cl, Br, and I for soil sample “B3”. However, a relatively high RSD, about 12%, was observed for Cl using water or 10 mmol L−1 NH4OH. Using higher concentrations of NH4OH (25−100 mmol L−1), no statistical difference (ANOVA, confidence level of 95%) was observed for F, Cl, Br, and I concentration. Thus, after the optimization of MIC method parameters, it was possible to conclude that the use of 400 mg of cellulose and 25 mmol L−1 NH4OH as absorbing solution, was suitable for the quantitative absorption of halogens of soil containing 50% of inorganic matter content. Accuracy Evaluation. The accuracy of the proposed MIC method was evaluated by the analysis of CRMs (NIST 1632c Coal bituminous and NIST 2711 - Montana soil), spiked samples and also by analysis of soil “A1” using pyrohydrolysis method. After CRMs digestion by MIC using optimized conditions for soil sample “B3”, halogens were determined by IC (F and Cl) and by ICP-MS (Br and I). Results obtained after NIST 1632c digestion by MIC were 69.1 ± 9.3, 1103 ± 39, and 18.2 ± 0.8 μg g−1 for F, Cl, and Br, respectively. These results were in agreement better than 95% to the certified values (72.7 ± 6.8, 1139 ± 41, and 18.7 ± 0.4 μg g−1 for F, Cl, and Br, respectively) showing the feasibility of MIC method. In the same way, results obtained after digestion of NIST 2711 by MIC (4.4 and 2.5 μg g−1 for Br and I, respectively) were similar to informed values in CRM (5 and 3 μg g−1 for Br and I, respectively). Since the CRMs used do not contain a certified value for I, spiked samples were used in order to evaluate the F


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Analytical Chemistry observed that the soils of groups “A” and “B” presented high concentration of F, probably due to the inorganic fraction present in these soils. Additionally, for all soils evaluated by MIC using optimized conditions the RSDs were lower than 8% (Table 1). Taking into account the optimizations performed in this work, if a soil sample with unknown inorganic matter content should be analyzed, digestion can be performed using 100 mg of sample mass, 400 mg of microcrystalline cellulose, and 6 mL of 100 mmol L−1 NH4OH as absorbing solution. Probably, using this condition, a complete volatilization of halogens could be assured for soils containing variable inorganic fraction. Regarding to the LOQs obtained by MIC using optimized conditions, it is possible to observe in Table 2 that, with the exception of Cl, the LOQs obtained by IC and ICP-MS were very low for F, Br, and I.

toward Br and I and would be well adequate for further detection of these elements in most soil types. In addition, the MIC method allowed the combustion of up to eight samples in less than 25 min, including the cooling step, providing a high sample throughput. Besides this, the use of diluted reagents such as 100 mmol L−1 NH4OH, makes it compatible with the commonly used determination techniques (such as IC and ICP-MS).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b04300. Table 1: Overview of soil samples used in MIC method; Figure A: Example of soil samples before and after homogenization by grinding; Figure B: Residues of soils obtained after MIC method (PDF).

Table 2. Limits of Quantification (μg g−1) Obtained by IC (F and Cl) and ICP-MS (Br and I) after Digestion of Soils by MIC (n = 3)

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concentration, μg g−1

AUTHOR INFORMATION

Corresponding Author

element

group A:a 88−92% of inorganic matter content

group B:b 50−52% of inorganic matter content

F Cl Br I

5 60 0.5 0.07

5 54 0.3 0.06

*Tel./Fax: +55-55-3220-9445. E-mail: [email protected]. ORCID

E. M. M. Flores: 0000-0001-9785-2477 Notes

The authors declare no competing financial interest.

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a 100 mg of soil + 400 mg of cellulose and 100 mmol L−1 NH4OH as absorbing solution. b100 mg of soil + 400 mg of cellulose and 25 mmol L−1 NH4OH as absorbing solution.

ACKNOWLEDGMENTS The authors are grateful to Conselho Nacional de Desenvolví mento Cientifico e Tecnológico (CNPq), Coordenaçaõ de ́ Superior (CAPES), and Aperfeiçoamento de Pessoal de Nivel Fundaçaõ de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for financial support. Co-funding (C.N.A.) was provided by Danish National Research Foundation (Grant No. CENPERMDNRF100) and the Villum Foundation (Grant No. 9934).

It is important to mention that for soils containing 92 and 50% of inorganic matter it was necessary to use 400 mg of microcrystalline cellulose as a combustion aid for volatilization of halogens (F, Cl, Br, and I), but it increased the blank values basically for Cl, consequently providing higher LOQs. However, if F would be not determined in these soils (“A1” and “B3”), cellulose masses of 200 and 100 mg, respectively, could be used for complete volatilization of Cl, Br, and I from soils. Therefore, the LOQ obtained for Cl in soils “A1” and “B3” would be lower (29 and 19 μg g−1, respectively). However, the MIC method can be considered suitable for further Cl determination in soils since the mean of total Cl content of worldwide soils is estimated as 300 μg g−1, although with a wide range of concentrations.47

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REFERENCES

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CONCLUSIONS The proposed MIC method was suitable for halogens volatilization from soils with high inorganic matter content and subsequent analytes determination by IC (F and Cl) and ICP-MS (Br and I). Operational conditions for the MIC method were optimized for each soil and the need of using a microcrystalline cellulose as a combustion aid for the complete volatilization of halogens for soils was observed. Results obtained by the proposed method were in agreement with those obtained after pyrohydrolysis and recoveries in the range of 94−103% were obtained using spiked samples. The accuracy was also checked by CRMs (NIST 1632c and NIST 2711) analysis and an agreement better than 95% to the certified values was achieved. The LOQs obtained using MIC method were 5, 54−60, 0.3−0.5, and 0.06−0.07 μg g−1, for F, Cl, Br, and I, respectively. Hence, MIC is very suitable especially G


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