Direct Determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb, and Zn in

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Direct determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn in ethanol fuel by high resolution continuum source flame atomic absorption spectrometry Clarice Caldeira Leite, Alexandre de Jesus, Mariana Luz Potes, Mariana Antunes Vieira, Dimitrios Samios, and Marcia Messias da Silva Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01796 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Direct determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn in ethanol fuel by high resolution continuum source flame atomic absorption spectrometry Clarice C. Leite1, Alexandre de Jesus1, Mariana L. Potes1, Mariana A. Vieira2, Dimitrios Samios1 and Márcia M. Silva1,3*

1

Centro de Combustíveis, Biocombustíveis, Lubrificantes e Óleos (CECOM) - Instituto

de Química, Universidade Federal do Rio Grande do Sul (UFRGS). Av. Bento Gonçalves 9500, CP 15003, CEP 91501-970, Porto Alegre – RS, Brazil 2

Programa de Pós-Graduação em Química, Universidade Federal de Pelotas. Campus

Capão do Leão. CEP 96160-000, Capão do Leão – RS, Brazil. 3

Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energia e Ambiente,

Universidade Federal da Bahia, 40170-115 Salvador, BA, Brazil

*Corresponding author; Phone: (55) 51 3308-6278; Fax: (55) 51 3308-7304 e-mail: [email protected]

Keywords: Metal determination, ethanol fuel, high-resolution continuum source flame atomic absorption spectrometry, multi-element analysis.

1

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Abstract

A method to determine nine elements (Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn) in ethanol fuel using high resolution continuum source flame atomic absorption spectrometry (HR- CS FAAS) was developed. The proposed method consists on the direct analysis of samples, making use of inorganic standards for calibration and multielement sequential determination. To evaluate the accuracy of the proposed method, the obtained results were compared with the results obtained by the following methods: i) the standard method NBR 11331, for the determination of Cu and Fe in ethanol fuel by line source atomic absorption spectrometry (LS - FAAS); ii) the standard method NBR 10422 for the determination of Na in ethanol fuel by flame photometry; iii) recovery tests for the other elements. Good accuracy was attained since the results obtained for Cu, Fe and Na by the proposed method and the standard methods were not significantly different within 95% level of confidence (Student’s t-test) and the recovery tests for the other analytes ranged from 97% to 105%. Limits of detection of 0.006; 0.02; 0.001; 0.02; 0.003; 0.1; 0.04; 0.06 and 0.024 mg Kg-1, were obtained for Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn, respectively. The relative standard deviations were in general lower than 3.0% for all analytes. The proposed method showed to be an excellent alternative for the determination of metals in ethanol fuel, emphasizing multi-element sequential analysis reducing the costs and analysis time, thus can be recommended for routine analysis.

1. INTRODUCTION

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The use of ethanol (or so-called bioethanol) as a fuel shows several advantages over fossil fuels, such as: it is a renewable energy; ethanol improves the octane rating of fuels; a reduction of greenhouse emissions; and burning is clean, therefore the toxicity of the generated compounds is low. For these reasons the production of ethanol is growing with the simultaneous increase in the research related to its production and characterization.1 The Brazil intensified its investments in the production of ethanol fuel from sugarcane in 1975 with the implementation of the National Alcohol Program (Proálcool).2 The main objective was to reduce the dependence on oil imports.3,4,5,6 After the end of Proálcool, at the end of the 80s, the ethanol fuel production gained new momentum from 2003 with the development of flex-fuel vehicles.7 Currently, Brazil is the second largest producer of ethanol fuel (23.5 billion liters in 2012/2013) being United States, the first producer.2 Ethanol is used as a fuel in two ways in Brazil: hydrated ethanol is used directly as fuel; and anhydrous ethanol is added to gasoline either as an oxygenate additive, (27 %, according to current Brazilian legislation). The quality control of ethanol fuel is monitored by the National Agency of Petroleum, Natural Gas and Biofuels from Brazil (ANP). Besides the characterization analysis, some contaminants are controlled, such as Cu, Fe and Na. According to the Resolution ANP Nº 19, DE 15.4.20158, the maximum permitted levels of Cu, Fe and Na are 0.07, 5.0 and 2.0 mg kg-1, respectively. In Europe and the United States, a limit of 0.1 mg kg-1 is only established the Cu.1,9 The source of metals in ethanol could be the raw material, which depends on the soil where the raw material has grown, as well as the atmospheric pollution and the products used to obtain ethanol. It can also appear during the fuel distillation process, storage and transport in metallic containers. Finally, some metallic species can be used as additives to promote

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the combustion process. Although the concentrations are frequently low, their determination is important for several reasons: Fe and Cu cause damage of the vehicle engine, prevent the stability of the ethanol and in presence of gasoline can catalyze oxidation reactions which favor the formation of gum.10,11 Sodium, if not controlled, increases the electrical conductivity of ethanol, resulting in corrosion processes in the vehicles. In addition, the fuel combustion can release to the environment several potentially toxic elements, representing a significant source of pollution.12,13 The concentration of metals in ethanol fuel is normally low and therefore highly sensitive analytical techniques are required for analysis.1,14 For this reason inductively coupled plasma mass spectrometry (ICP-MS) is the most widely employed technique for the determination 19 elements in ethanol fuel, as stated by Sanches et al.1 in a recent review on biofuel analysis. Inductively coupled plasma optical emission spectrometry (ICP-OES) is the second technique used for the determination of 7 elements at concentrations of around a few mg L-1. However, ICP-based techniques are very susceptible to organic solvents. In this review several plasma effects and spectral interferences caused by biofuel analysis by ICP-based techniques are summarized. Besides these techniques are comparatively expensive considering the implementation and operation costs. This review also shows that atomic absorption spectrometry (AAS) techniques have also been fairly used for ethanol fuel analysis. Graphite furnace atomic absorption spectrometry (GFAAS) has been used as a good alternative due to its good sensitivity and high tolerance to complex matrices. On the other hand, all applications using

flame

atomic

absorption

spectrometry

(FAAS)

involves

a

previous

preconcentration stage using appropriate sorbents or by evaporation, due to the higher limits of detection than techniques cited previously (ICP-OES, ICP-MS and GFAAS). A

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few applications using electrochemical techniques1 and energy dispersive X-ray fluorescence (EDXRF)15, have also been reported. High-resolution continuum source atomic absorption spectrometry (HR-CS AAS), which was introduced commercially in 2003, has not been used for the determination of trace elements in ethanol fuel samples yet, but it appears to be an alternative technique for this application. The xenon short-arc lamp, used as a continuum source, provides the possibility to determine almost all known elements in a fast sequential mode when FAAS is used. The possibility of using the principal and secondary lines without loss in signal-to-noise ratio, the choice of the line wings to enhance sensitivity and/or to extend the linear working range are among the most important advantages brought about by this technique16. Moreover, the high tolerance of the flame to organic solvents allows the direct analysis of ethanol fuel samples. Besides, an additional increase in the sensitivity should be expected due to the decrease the mean droplet size distribution caused by the physics characteristics of ethanol, thus enhancing and the aerosol loading compared to aqueous solution. Some authors have proposed a method the analysis of alcohol beverages using this technique. Boschetti et al.

17

developed a method for rapid sequential determination of metals (Be, Ca, Co, Cu, K, Li, Mn, Na, Rb and Sr) in wine samples produced in different regions of Brazil and Raposo et al.18 used the Bi and Sn as an internal standard in the direct determination of Cu in distilled spirits. HR-CS FAAS allows a rapid sequential multi-element routine determination, there by increasing the speed of the analytical procedures and reducing the number of steps. In this context, the main objective of this work was to develop a rapid, precise and accurate method for direct multi-element determination of trace elements in ethanol fuel using HR-CS FAAS. With this method, a fast and low cost

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routine analysis of ethanol fuel for quality control of contaminants or for environmental purposes can be accomplished.

2. EXPERIMENTAL SECTION

2.1. Instrumentation The measurements for Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn in ethanol fuel were carried out with a high resolution continuous source atomic absorption spectrometer, ContrAA model 300 (Analytik Jena AG, Germany) equipped with a xenon short-arc lamp (which emits a continuum spectrum between 190 and 900 nm), operating at hot-spot mode, a prism pre-monochromator, an echelle grating monochromator for high resolution, and a charge-coupled device (CCD) array detector; the resolution is about 1.5 pm per pixel at 200nm. The determinations were performed in air - acetylene flame with a burner of 50 mm. Sealing rings resistant to organic solvents, supplied by Analytik Jena AG, was used in the nebulizer chamber. For all elements, the aspiration rate was 3.6 mL min-1 and the number of evaluated pixels was CP ± 3. All measurements were carried out in triplicates. The optimized instrumental parameters for each analyte are shown in table 1.

Insert Table 1

For comparison purposes, the obtained results for Fe and Cu were compared to the results obtained by the application of NBR 11331, which is a Brazilian standard method for the determination of Fe and Cu in ethanol fuel, by FAAS. A flame atomic absorption spectrometer, AAS 6 Vario (Analytik Jena AG, Germany), was employed.

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Hollow cathode lamps of Fe and Cu (Photon, Australia) were used as primary radiation source for the measurements of the signals, operating in specific current for each element. The spectral bandpass were used according to the manufacturer's instructions. The instrumental parameters for each element are shown in Table 2. For the all measurements with AAS high-purity acethylene (99.0% (v/v), from White Martins, Brazil, were used as fuel. The compressed air obtained by an air compressor, model FIAC CDS 8/50 (Araraquara, São Paulo, Brazil), were used as oxidant.

Insert Table 2

The NBR 10422, which is a Brazilian standard method for the determination of Na in ethanol fuel by flame photometry, was applied for validation. A flame photometer Model B462 (Micronal, São Paulo, SP, Brazil), was used operating under the following conditions: compressed air of 9 L min-1, pressure of 1 kgf cm-2, and butane gas flame (liquefied petroleum gas).

2.2 Reagents, solutions and samples All samples and solutions were prepared with analytical grade reagents and distilled water (Fisatom, São Paulo, Brazil) additionally purified by a Milli-Q system (Millipore, Bedford, USA), that results in a ultra-pure water with a specific resistivity of 18.2 MΩ cm at 25° C. Aqueous stock solutions with 1000 mg L-1 of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn (Specsol, São Paulo, Brazil) were used. The working standard solutions were

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prepared by serial dilutions from stock solutions and acidified with 0.014 mol L-1 HNO3 (Merck, Darmstadt, Germany). High purity ethanol with 99.9% (v/v) of purity (Merck, Darmstad, Germany) was used to prepare the multi-element calibration curves with a blank and 4 standards containing each element in the following range: Cd (0.15 - 0.75 mg L-1), Co (0.25 - 1.0 mg L-1), Cu (0.25 - 0.75 mg L-1), Fe (0.25 - 0.75 mg L-1), Mn (0.25 - 1.00 mg L-1), Na using main wavelength (0.25 - 1.0 mg L-1) and using wavelength with 50% sensitivity (0.5 - 3.0 mg L-1), Ni (0.5 - 2.0 mg L-1), Pb (0.50 - 2.0 mg L-1) and Zn (0.2 - 0.8 mg L-1); all standards with 0.014 mol L-1 HNO3. The final water content in the blank and in the working standards after addition of the aqueous standards and acid varied between 1.5 to 4.5% (v/v). In this work only hydrated ethanol fuel (94.5 – 96.3 % v/v of purity) samples, that are used directly as fuel, obtained from local gas stations of two Brazilian cities, Porto Alegre (EF-01, EF-02, EF-04 and EF-05) and Pelotas (EF-03), were analyzed. The samples were stored in high density polyethylene bottles, previously decontaminated with HNO3 (3 mol L-1), and kept under refrigeration.

2.3 Analytical Procedure The measurements of the 9 elements in blank, standards and ethanol fuel samples by HR-CS FAAS was carried out according to the following sequence: Mn, Fe, Co, Cd, Pb, Ni, Na, Cu and Zn. This sequence was optimized with standard and a spiked sample (enriched of each element) according to the acetylene flow-rate; the flame conditions and the burner height were optimized automatically by the software, taking into account the maximum absorbance as criterion. The sequence was repeated three times for each sample to obtain an average and standard deviation for each element and sample. For all elements, the most sensitive lines was used for the

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measurements, except for Na, that two lines were used in order to perform the measurements of the samples with low and high concentration without need of dilution. The optimized conditions are summarized in Table 1. In order to follow the variations of the equipment during the time of working, a blank and a control solution containing intermediate concentrations of the calibration curve in (0.4 mg L-1 for Cd and Zn, 0.50 mg L-1 for Co, Cu, Fe and Mn, and 1.0 mg L-1 for Na, Ni and Pb) in 0.014 mol L-1 HNO3, was analyzed periodically. To evaluate the matrix effect and the accuracy of the proposed method for the elements Cd, Co, Mn, Ni, Pb, and Zn, recovery experiments were performed. In these experiments, the samples were spiked with each analyte in order to obtain the final concentration, in mg kg-1: 0.62 for Fe, Cu and Na; 0.49 for Mn; 0.92 for Co; 1.85 for Ni; 0.31 for Cd; 1.24 for Pb and 0.50 for Zn. Triplicate (n=3) measurements of all standards, samples and spiked samples were conducted under optimized conditions.

3. RESULTS AND DISCUSSION

3.1 Optimization of the experimental conditions of the HR-CS FAAS The optimization of the experimental conditions of the equipment was performed using a sample spiked with aqueous standard of each analyte. The adjustment of the aspiration rate was performed manually, observing the intensity of the analytical signal of each element, taking into account the maximum value of analytical signal. The maximum values were obtained in the range of 3.4 mL/min (Ni) up to 3.8 mL/min (Pb). With the aspiration rate near to 3.6 mL min-1 the higher values of analytical signals were

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obtained for most of the analytes. Thus the aspiration rate of 3.6 mL min-1 was adopted. The software performs the optimization of equipment operating conditions automatically, varying the burner height and the acetylene flow rate in order to obtain the highest values of absorbance for each element. Therefore, a compromise condition was only used for aspiration rate, while the optimum condition of atomization and measurement were used for each element. The optimized conditions used for the following experiments are showed in Table 1. For the elements Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn, the most sensitive lines were used because their concentrations are normally low in ethanol fuel.19 For Na, two lines of resonance were used: 588.995 nm (100% sensitivity) for samples with low Na concentration and 589.592 nm (50% sensitivity) for samples with high Na concentration. In this way dilutions of samples were avoided.

3.2 Tests with ionization suppressor The flame temperature directly affects the balance of ionization of elements that have low ionization energy, as in the case of Na, thus influencing their sensitivity. Moreover, the presence of ethanol can further increase the flame temperature. The presence of a suitable ionization suppressor could reduce this effect. In this work, the Cs was investigated as ionization suppressor because it presents low ionization energy (3.9 eV), resulting in an increase of electrons in the flame, thus preventing the ionization of other elements. The addition of different amount of Cs in the samples and in the blank solution was evaluated for all analytes because it was intended to carry out a multi-element analysis. The absorbance signal of each analyte was evaluated in the presence of 50 -

10


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2000 mg L-1 Cs. The obtained results showed that the elements Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn were not influenced by the variation in the concentration of Cs, maintaining the variation of the analytical signal with RSD less than 5%, as expected. In the case of Na, even with the use of the maximum amount of ionization suppressor (2000 mg L-1 Cs) there was an increase of only about 7% in the absorbance signal. This enhancement in is in agreement with the literature20 which states that the air-acetylene flame may ionized Na around of 9% in aqueous solution. This result shows that the presence of ethanol as solvent did not influence on the ionization of Na. The lower ionization than expected could be owing to the increase of loading of sample due to solvent effect on the nebulization that lead to an increasing the amount of Na in the flame, thus reducing the degree of ionization, compared to aqueous solution. 21 As there was no significant influence on the analytical signals, the ionization suppressor was not used in the future determinations.

3.3 Figures of merit After finishing the optimization steps and establish the instrumental and operational conditions of the equipment for each analyte, calibration curves were constructed for each element using aqueous inorganic standards diluted in pure ethanol (without the presence of metals). The linear regression equations and correlation coefficient square were established for the proposed method and for the comparative methods22, 23 (NBR 11331 and NBR 10422). Calibration curves were established using a blank and four standard solutions in the concentration ranges already mentioned. It should be mentioned that the final water content was 1.5% (v/v) in the blank and varied between 1.5 to 4.5% (v/v) in the working standards, after addition of the aqueous standards and acid. The results were expressed in mg kg-1, according to NBR 11331 and 11


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NBR 10422. The figures of merit for the calibration curves are shown in the tables 3, 4 and 5. The characteristic concentration (C0) defined as the analyte concentration corresponding to an integrated absorbance of 0.0044 (1% of absorption)21 and the limits of detection (LOD) and quantification (LOQ) calculated according to IUPAC recommendations, as three times and ten times, respectively, the standard deviation of ten measurements of a blank, divided by the slope of the calibration curve, was obtained for all analytes. These parameters are shown in Tables 3, 4 and 5. As can be seen in these tables, for all analytes investigated the calibration curves presented a good linearity (R2  0.99). The sensitivities were generally 20% higher than those obtained for aqueous solution. This increase in the sensitivity can be explained by the enhancing and the aerosol loading caused by the physics characteristics of ethanol and the higher vapor pressure compared to aqueous solution.21 The LOD and LOQ values obtained for the proposed method are higher than those obtained by other more sensitive techniques such as GFAAS19, electrothermal vaporization inductively coupled plasma-mass spectrometry (ETV-ICP-MS)24 and preconcentration techniques25. Nevertheless, for Fe, Cu and Na, they are lower than those obtained by the standards methods NBR 11331 (LS - FAAS)21 and NBR 10422 (flame photometry).22 This is expected since the intensity of the radiation source is one to three orders of magnitude higher than in line source for conventional AAS, leading to an improvement in the signal / noise ratio (S / N) and consequently in the standard deviation of the blank.16 Additionally the LOQ are lower than the maximum limits established by the Brazilian legislation. Thus the method is suitable for routine application on the Brazilian monitoring program of quality control of fuel and biofuels, according to Resolution ANP Nº 7/2011.8

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For the other elements, the C0 obtained are in agreement with the data from the literature 16, and the LOQ for Co and Mn are in agreement with the values obtained by Boschetti et al.,17 for the same technique (HR-CS FAAS). Furthermore, the LOD values are in the g L-1 levels, being appropriate for a screening analysis of the sample for these contaminants. The relative standard deviations were in general lower than 3.0% for all analytes. The sample throughput was 35 samples by hour, considering nine analytes and measurements in triplicates for each analyte.

Insert Table 3

Insert Table 4

Insert Table 5

3.4 Recovery Tests In order to evaluate the matrix effect and the accuracy of the proposed method, in addition to the comparison of results obtained by the proposed method and by the comparative methods NBR 11331 (LS FAAS)22 and NBR 10422 (flame photometry)23, recovery assays were performed by spiking some samples of ethanol fuel randomly chosen, obtained from different gas stations of Porto Alegre, with aqueous standard of all analytes, as described in the experimental section. The recovery values obtained

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ranged from 97% to 105% for all analytes were very satisfactory, confirming the accuracy of the proposed method, and showing that the new method can be successfully used in routine analysis. The obtained results from the recovery tests also confirmed the absence of the matrix effects for all analytes investigated and also evidenced that the small difference in the water content did not influence on the analytical result.

3.5 Direct determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb, and Zn in ethanol fuel The found concentrations of the Cu, Fe and Na in five samples of hydrated ethanol fuel from different gas stations from the Brazilian cities of Porto Alegre and Pelotas, obtained by the standard methods and the proposed method are shown in Table 6. In relation to the determination of Na, the samples EF-01, EF-02, EF-04 and EF-05 were analyzed using the wavelength of 588.995 nm (100% sensitivity) while the sample EF-03 was analyzed using the wavelength of 589.592 nm (50% sensitivity) due to the high concentration of analyte present in this sample.

Insert Table 6

The Student’s t-test was applied to the data of Table 6 and the calculated t values obtained was 0.6 for Na and ranged from 0.3 to 3.5 for Cu and from 0.2 – 1.7 for Fe, which were lower than t-critical obtained for all elements (t= 4.303), indicating that the results for both methods were not significantly different at the 95% confidence level. The data of Table 6 showed that Cu was found in all samples, and for two samples the concentration were above the limit established by the legislation, which is 0.07 mg kg-1 Cu. Regarding the values found in the literature, as in the work of Saint' Pierre et al.24,

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the concentrations of Cu in ethanol fuel samples were found ranging from 0.002 to 0.014 mg L-1, being similar to those found in this work. Fe was found in two samples (EF-04 and EF-05) but the concentrations were lower than the limit established by the legislation, which is 5.0 mg kg-1. Sodium was found only in one sample and the concentration was above the limit established by Brazilian legislation, which is 2.0 mg kg-1. For the other elements (Cd, Co, Mn, Ni, Pb and Zn) the only elements found in concentrations above the LOD in the samples analyzed in this work were: Co in the samples EF-01, EF-03 and EF-05 (0.02, 0.03 and 0.02 mg kg-1 respectively), and Zn in the sample EF-02 (0.09 ± 0.001 mg kg-1). The overall results were satisfactory and showed that the proposed method is suitable for this application and provided accurate results. The analyses were simple and fast and calibration with aqueous standards could be used.

4. CONCLUSIONS

The method developed in this work was efficient for the determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb and Zn in ethanol fuel samples by HR-CS FAAS. The proposed method was fast, simple and accurate. The LOD, LOQ and sensitivity obtained were similar to those found with the standard methods adopted by the ANP. The obtained LOQs were lower than the maximum limits proposed by the Brazilian legislation for Fe, Cu and Na in ethanol fuel. In relation to the metals Ni, Co, Cd, Pb and Zn, the accuracy of the proposed method was checked by recovery tests and the values found were excellent. The feasibility of sequential determination of the elements was the main advantage observed. Other important advantage of the method is the direct

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analysis of samples without any pre-treatment, i.e., it was not necessary the use of chemical reagents to prepare the samples, contributing to “Green chemistry”.

Acknowledgements The authors are grateful to FAPERGS (Edital PRONEX 08/2009 Process no. 10/0012-7) and CNPq (Edital 40/2013 Process no. 405011/2013-0 and Universal 2012 Process no. 478998/2012-0) for the financial support. C.C.L. and M.L.P. has a scholarship from CAPES/REUNI. M.M.S., M.A.V. and D.S. have research scholarships from

CNPq

(Processes

no.

308775/2013-9,

310917/2013-1,

305331/2013-2,

respectively).

References (1) Sánchez, R.; Sánchez, C.; Liememann, C. P.; Todoli , J. L. J. Anal. At. Spectrom. 2015, 30, 64-101. (2) Programa Brasileiro de Álcool (http://www.biodieselbr.com/proalcool/proalcool/programa-etanol.htm), 2015 (accessed in 22/06/2015). (3) Furtado, A. T.; Scandiffio, M. I. G.; Cortez, L. A. B. Energy Policy 2011, 39, 156166. (4) Goldemberg, J.; Coelho, S. T.; Guardabassi, P. Energy Policy 2008, 36, 2086– 2097. (5) Rosillo-Calle, F.; Cortez, L .A. B. Biomass Bioenergy 1998, 14, 115-124. (6) Zapata, C.; Nieuwenhuis, P. D. Bus. Strat. Env. 2009, 18, 528–541. (7) Anuário da Indústria Automobilistica Brasileira 2014. (http://www.anfavea.com.br/ anuario.html), 2014 (accessed 22.06/2015). (8) RESOLUÇÃO ANP Nº 19, DE 15.4.2015-Especificações para comercialização do Álcool Etílico Anidro Combustível (AEAC) e do Álcool Etílico Hidratado Combustível (AEHC) em todo o território nacional e define obrigações dos agentes econômicos sobre o controle de qualidade do produto. 16


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(9) Instituto

Nacional de Metrologia, Normalização e Qualidade Industrial (INMETRO); Orientações sobre Validação de Métodos de Ensaios Químicos, DOQ-CGCRE-008, 2003. (10) Teixeira, L. S. G.; Souza, J. C.; Santos, H. C.; Pontes, L. A. M.; Guimarães, P. R. B.; Sobrinho, E. V.; Vianna, R. F. Fuel Process. Technol. 2007, 88, 73–76. (11) Pereira, R. C. C.; Pasa, V. M. D. Energy Fuels 2005, 19, 426-432. (12) Turunen, M.; Peraniemi, S.; Ahlgren, M.; Westerholm, H. Anal. Chim. Acta 1995, 311, 85–91. (13) Elik, A. Int. J. Environ. Anal. Chem. 2002, 82, 37–45. (14) Korn, M. G. A.; dos Santos, D. S. S.; Welz, B.; Vale, M. G. R.; Teixeira, A. P.; Lima, D. C.; Ferreira, S. L. C. Talanta 2007, 73, 1–11. (15) Teixeira, L. S. G.; Santos, E. S.; Nunes, L. S. Anal. Chim. Acta 2012, 722, 29– 33. (16) Welz, B.; Becker-Ross, H.; Florek, S.; Heitmann, U.; High-Resolution Continuum Source AAS; Wiley-VCH: Weinheim, 2005. (17) Boschetti, W.; Rampazzo, R. T.; Dessuy, M. B.; Vale, M. G. R.; Rios, A. O.; Hertz, P.; Manfroi, V.; Celso, P. G.; Ferrão, M. F. Talanta 2013, 111, 147–155. (18) Raposo Jr, J. L.; de Oliveira, A. P.; Jones, B. T.; Gomes Neto, J. A. Talanta 2012, 92, 53– 57. (19) Saint’Pierre, T.; Aucélio, R. Q.; Curtius, A. J. Microchem. J. 2003, 75 59–67. (20) Pinta. M., ed. Spectrometrie d´absorption atomique. Volumes I e II, Masson, Paris, 1980. (21) Welz, M. Sperling, Atomic Absorption Spectrometry, Wiley VCH, Weinhein, 3rd edn, 1999. (22) NBR 11331: Ethanol - Determination of iron and copper content - Flame atomic absorption spectrometric method (Standard Method). (23) NBR 10422: Fuel ethanol — Determination of sodium content — Flame photometry method (Standard Method).

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(24) Saint’Pierre, T. D.; Tormen, L.; Frescura, V. L. A.; Curtius, A. J. Talanta 2006, 68, 957–962. (25) Teixeira, L. S. G.; Brasileiro, J. F.; Borges Jr., M. M.; Cordeiro, P. W. L.; Rocha, S. A. N.; Costa, A. C. S. Quim. Nova 2006, 29, 741-745.

Table 1. Instrumental parameters for direct determination of Mn, Fe, Co, Pb, Cd, Ni, Na, Cu and Zn by HR-CS FAAS. Wavelength (nm)

Analyte

Relative sensitivity (%)

Reading height (mm)

C2H2 / air (L h-1)

Mn

279.481

100

7

0.086

Fe

248.327

100

5

0.100

Co

240.725

100

6

0.108

Pb

217.000

100

6

0.119

Cd

228.801

100

6

0.119

Ni

232.003

100

6

0.119

Na

588.995

100

5

0.122

Na

589.592

50

5

0.122

Cu

324.754

100

5

0.125

Zn

213.857

100

5

0.130

Table 2. Instrumental parameters for determination of Cu and Fe by FAAS according to NBR 11331.

Analyte

Wavelength (nm)

Lamp Current (mA)

Spectral bandpass (nm)

Reading height (mm)

C2H2 / air (L h -1)

Cu

324.8

4.0

1.2

5

0.106

Fe

248.3

6.0

0.2

5

0.128

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Energy & Fuels

Table 3. Figures of merit obtained for the determination of Cu and Fe by LS - FAAS ( NBR 11331) and HR-CS FAAS (proposed method). Analyte Cu

Fe

Method

Linear regression equation

R2

LS - FAAS

y = 0.2174 x + 0.0035

HR-CS FAAS LS - FAAS HR-CS FAAS

Co

LOD

LOQ

(mg L )

(mg kg )

(mg kg-1 )

0.9889

0.02

0.01

0.02

y = 0.2370 x + 0.0008

0.9985

0.02

0.001

0.003

y = 0.0835 x + 0.0009

0.9987

0.05

0.06

0.2

0.9948

0.05

0.02

0.07

y = 0.0818 x + 0.0012

-1

-1

Table 4. Figure of merit obtained for the determination of Na by flame photometry (NBR 1042) and HR-CS F AAS (proposed method).

Method

Linear regression equation

R2

Flame photometry

y = 0.2207x + 0.0003

0.9908

HR- CS F AAS*

y = 0.1230x + 0.0178

HR- CS F AAS**

y = 0.4277x + 0.004

Co (mg L-1)

LOD (mg kg-1 )

LOQ (mg kg-1 )

-

0.3

0.9

0.9823

0.03

0.2

0.8

0.9909

0.008

0.1

0.3

 * = 589.592 nm ** = 588.995 nm

Table 5. Figures of merit obtained for the determination of Mn, Co, Pb, Cd, Ni and Zn by HR-CS FAAS. Linear regression Analyte equation

R2

Co

LOD

LOD

LOQ

LOQ

(mg L-1)

(mg L-1)

(mg kg-1)

(mg L-1 )

(mg kg-1)

Mn

y = 0.2444x + 0.0089

0.9922

0.01

0.002

0.003

0.007

0.009

Co

y = 0.07902x – 0.0012

0.9933

0.06

0.017

0.020

0.055

0.068

Pb

y = 0.04632x + 0.0034

0.9895

0.10

0.05

0.06

0.17

0.21

Cd

y = 0.3244x + 0.0086

0.9806

0.01

0.005

0.006

0.017

0.021

Ni

y = 0.04096x + 0.0048

0.9883

0.08

0.03

0.04

0.11

0.14

Zn

y = 0.4475x + 0.01454

0.9897

0.01

0.007

0.024

0.009

0.01

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Table 6 . Determination of Cu, Fe and Na in ethanol fuel by LS - FAAS, HR-CS FAAS and flame photometry.

Sample

Concentration Cu

Concentration Fe

Concentration Na

(mg kg-1 )( mean ± Sd, n=3)

(mg kg-1 ) ( mean ± Sd, n=3)

(mg kg-1 ) ( mean ± Sd, n=3)

LS - FAAS

HR-CS FAAS

LS - FAAS

HR-CS FAAS

Flame photometry

HR-CS FAAS

EF-01

0.03 ± 0.001

0.03 ± 0.001

< LOD

< LOD

< LOD

< LOD**

EF-02

0.09 ± 0.004

0.09 ± 0.001

< LOD

< LOD

< LOD

< LOD* *

EF-03

0.09 ± 0.003

0.09 ± 0.001

< LOD

< LOD

2.2 ± 0.2

2.2 ± 0.3*

EF-04

0.01 ± 0.004

0.01 ± 0.003

0.2 ± 0.003

0.2 ± 0.001

< LOD

< LOD**

EF-05

< LOD

0.001

3.2 ± 0.12

3.1 ± 0.1

< LOD

< LOD**

 * = 589.5 nm ** = 589.0 nm

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Direct Determination of Cd, Co, Cu, Fe, Mn, Na, Ni, Pb, and Zn in

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