Analytical Methods Employed at Quality Control of Fuel Ethanol

Publication Date (Web): September 29, 2009 ... processes, lead to storage instability, and others) implies a rigorous quality control of these fuels, ...
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Energy Fuels 2009, 23, 4852–4859 Published on Web 09/29/2009

: DOI:10.1021/ef900493k

Analytical Methods Employed at Quality Control of Fuel Ethanol )

Marcelo Firmino de Oliveira,† Adelir Aparecida Saczk,‡ Leonardo Luiz Okumura,§ and Nelson Ramos Stradiotto*, Departamento de Quı´mica, Faculdade de Filosofia, Ci^ encias e Letras de Ribeir~ ao Preto, Universidade de S~ ao Paulo, 14040-901, Ribeir~ ao Preto, SP, Brasil, ‡Departamento de Quı´mica, Universidade Federal de Lavras, 37200-000, Lavras, MG, Brasil, § Departamento de Quı´mica, Centro de Ci^ encias Exatas, Universidade Federal de Vic-osa, 36570-000, Vic- osa, MG, Brasil, and Departamento de Quı´mica Analı´tica, Instituto de Quı´mica, UNESP - 14800-900, Araraquara, SP, Brasil )

†

Received May 20, 2009. Revised Manuscript Received September 9, 2009

The existence of organic and inorganic contaminants present in both fossil and biomass fuels and the fact that they can provide undesirable effects (environmental problems, corrosion processes, lead to storage instability, and others) implies a rigorous quality control of these fuels, although these contaminants make up a small part of the final fuel composition. Considering the rising importance of fuel ethanol in the worldwide panorama, this review aims at reporting the use of successful alternative analytical methods in the monitoring of organic and inorganic contaminants at trace levels, used to determine and to quantify these substances in fuel ethanol and also presenting all official norms for quality control of fuel ethanol employed by ABNT (Brazilian Association of Technical Norms), ASTM (American Society for Testing and Materials), and ECS (European Committee for Standardization).

1.1. Historical Insights. The history of humankind can be promptly associated with the history of energy, where the first form of energy used by man was his own force, employed in hunting, transportation, and building. One of the main characteristics of modern society is its intrinsic demand for energy supply, which results in an ever-increasing fossil fuel consumption to keep the “life quality” standards. A direct consequence of this is the emission of large amounts of gases from combustion processes, which increases the carbon level in the environment and leads to several disruptions in many of the planet0 s natural cycles, thus contributing to the increasing greenhouse effect that is being responsible for the increasing in the Earth’s temperature. Most of the conventional transportation vehicles use petroleum-derived fuels such as gasoline and diesel. However, in many countries (USA, Brazil, European community, China, Thailand, Australia, and Caribbean Basin countries) ethyl alcohol (CH3CH2OH) is employed both as direct fuel (hydrated alcohol) or as additive to fossil fuels (anhydrous alcohol). This historic panorama has occurred with the objective of minimizing the commercial dependence in relation to petroleum exporter countries.1 Although this biofuel can be obtained from several industrial plants, such as corn2 and lignocelulosis,3 the use of sugar cane has been preferentially employed in Brazil due to inherent operational advantages1,4 such as its easy adaptation to the tropical weather and consolidated knowledge about sugar cane plantation, acquired throughout the past five centuries.

1.2. Fuel Ethanol. The initial purpose of fuel ethanol was as an additive in gasoline. Later, it was also applied in its hydrated form as the only fuel source for explosion engines, which turned into a new renewable energetic source. Although the initial use of ethanol as an alternative fuel had economic purposes, it was clear that it was also environmentally advantageous because: (1) it eliminated the need for lead-based additives and toxic oxygen compounds, such as methyl tert-butyl ether (MTBE); (2) it reduces SO2, CO, and hydrocarbon emission; (3) it is renewable,2,5 so the global process of plantation, alcohol production, and consumption results in a closed carbon emission cycle, which does not contribute to the atmospheric carbon level; and (4) besides being an excellent additive in gasoline,6 ethanol has also been studied as a diesel additive,7 thus leading to a considerable decrease in the emission of pollutants, and providing higher engine lifetime and performance. However, there are some problems associated to these applications, such as the environmental risk of a leakage to the groundwater, once ethanol can, for example, make water-soluble some insoluble diesel compounds. Nowadays, environmental aspects have been of high concern in developed countries, so the use of ethanol and other renewable fuels as the main energy’s sources in the planet must be stimulated, especially if the Kyoto protocol is adopted by the industrialized countries. Because of the widespread use of ethanol as a fuel or industrial material (fuel cells, perfumes, paints, cosmetics, medicine, etc.), rigorous quality control of this substance is required. Even though the broad spectrum of chemical species present in ethanol play a secondary role in the final

*To whom correspondence should be addressed. E-mail: nrstradi@ iq.unesp.br. Fax: 55-16-3301-6692. (1) Calle, F. R.; Cortez, L. A. B. Biomass Bioenergy 1998, 14, 115–124. (2) Hammerschlag, R. Environ. Sci. Technol. 2006, 40, 1744–1750. (3) Gnansounou, E.; Dauriat, A. J. Sci. Ind. Res. 2005, 64, 809–821. (4) Macedo, I. D. Biomass Bioenergy 1998, 14, 77–81.

(5) Hahn-Hagerdal, B.; Galbe, M.; Gorwa-Grauslund, M. F.; Liden, G.; Zacchi, G. Trends Biotechnol. 2006, 24, 549–556. (6) Niven, R. K. Renew Sust. Energy Rev. 2005, 9, 535–555. (7) Hansen, A. C.; Zhang, Q.; Lyne, P. W. L. Biores. Tech. 2005, 96, 277–285.

1. Introduction

r 2009 American Chemical Society

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product, knowledge and control of these species is crucial when it comes to high-technology engines industrial processes. Since the 1980s, the appearance of flexible fuel vehicles (FFVs);which can run on gasoline or a blend of up to 85% ethanol;has resulted in an increase in the consumption of ethanol as a fuel. The possibility of supplying a vehicle with gasoline or ethanol brings direct advantages to users, once a more economical option can be chosen. Considering the importance of ethanol in the international economic panorama, as well as the need for new strategies for its quality control, this review aims at citing and comparing the scientific works reported in the literature (1980 to present) that lend themselves to the research and development of alternative methodologies of organic and inorganic trace analysis in fuel ethanol.

Table 1. Summarized Comparison between the Brazilian Official Analytical Methodologies Used for the Trace Analysis of Species in Fuel Ethanola work range (ppm) species acetal acetaldehyde benzene chloride copper dimethylcetone ethyl acetate formaldehyde high alcohols iron methanol sodium sulfate

2. Literature Review 2.1. Official Analytical Methodologies. 2.1.1. Official Norms for Trace Analysis. Due to its pioneering exploration of fuel ethanol, the Brazilian official norms of quality control deserve special attention. In Brazil, the agency responsible for the inspection and commercialization rules of all fuels is ANP ; Ag^encia Nacional do Petr oleo, G as Natural e Biocombustı´ veis (National Agency of Petroleum, Natural Gas and Biofuels). Their resolutions establish the criteria of quality control, following the specifications of the Brazilian Norms (NBR), as set by Associac-~ ao Brasileira de Normas Tecnicas (ABNT, Brazilian Association of Technical Norms) and other norms of the American Society for Testing and Materials (ASTM) and European Committee for Standardization (ECS) concerning the characteristics of the final product (national or imported) for use as fuel. However, Brazilian legislation contemplates quality standards for only a few number of chemical species in fuel ethanol. In this context, official methodology exists for testing organic substances by gas chromatography technique for acetal, acetaldehyde, ethyl acetate, acetone, methanol, higher alcohols,8 benzene8,9 and formaldehyde;10 official methodoloby also exists for testing inorganic compounds such as sodium11 (analyzed by flame photometry), copper12 (by flame atomic absorption spectrometry), iron13,14 (by flame atomic absorption spectrometry and UV-vis spectrophotometry), chloride15,16 (by ionic chromatography and potentiometry) and sulfate15,17 (by ionic chromatography and volumetry). For instance, the detection limits for chloride and sulfate in hydrated ethanol are 1 and 4 mg L-1,

methodology GC GC UV/vis IC potentiometry FAAS GC GC GC GC FAAS UV/vis GC FAAS IC volumetry

lower N/S N/S 0.3 0.1 0.05 0.01 N/S N/S 10 N/S N/S 0.2 N/S 0.5 0.1 1

upper 20 20 N/S 100 N/S N/S 10 20 60 50 8 N/S 30 2.5 100 4

ref. 8 8 9 15 16 12 8 8 10 8 13 14 8 11 15 17

a N/S = not specified; UV/Vis = molecular spectrophotometry; FAAS = flame atomic absorption spectrometry; GC = gas chromatography; IC = ionic chromatography.

respectively. For higher aldehydes, there are no set norms to date. There are still no rules in Brazilian legislation for other substances with toxic effects for the environment and life, such as heavy metals. According to the literature,19,20 the transport, handling, and use of fuels are the main cause of volatile organic emission into the atmosphere in urban centers. Therefore, it is of utmost importance to know about fuel ethanol composition, in order to stimulate the creation of new norms for the storage and handling of this fuel. The classical methods for analysis of inorganic anions are based on colorimetric and chromatographic tests. Formaldehyde10 and acetaldehyde8 determination in fuel ethanol is restricted to colorimetric and chromatographic methods, and an intensive pretreatment of the samples is required before the final analysis. Among the parameters employed for fuel ethanol evaluation in the present legislation, it is possible to cite analysis of metallic ions, which can take place during the production, storage, and transport steps. However, only iron and copper are actually investigated. The Brazilian ANP, for example, through its Resolution No. 36 from 17/12/2005, establishes a maximum level of iron in hydrated fuel ethanol not higher than 5 mg L-1, while the copper level must be lower than 0.07 mg L-1. The same Resolution predicts that the determination of these species must be carried out using flame atomic absorption spectrometry, through the norms NBR 1133113 and NBR 10893.12 The analytical characteristics of the Brazilian official techniques for the trace analysis of organic and inorganic compounds in fuel ethanol reported in this review are summarized in Table 1. The insertion of ethanol as an interesting alternative biofuel produced a demand for official norms of quality

(8) ABNT. NBR10260. Ethylic Alcohol - Determination of Levels of Aketal, Aketaldehyde, Ethyl Acetate, Ketone, Methylic Alcohol, Higher Alcohols and Benzene by Gaseous Chromatography. 01/04/1988. (9) ABNT. NBR10649. Ethylic Alcohol - Determination of Benzene by Ultraviolet Spectrophotometry. 30/04/1989. (10) ABNT. NBR9868. Ethylic Alcohol - Determination of Level of Formaldehyde. 30/05/1987. (11) ABNT. NBR10422. Ethylic Alcohol - Determination of Level of Sodium by Flame Photometry. 23/04/2007. (12) ABNT. NBR10893. Ethylic Alcohol - Determination of Level of Copper by Atomic Absorption Spectrophotometry. 01/01/1990. (13) ABNT. NBR11331. Ethylic Alcohol - Determination of Level of Iron by Atomic Absorption Spectrophotometry. 01/05/1990. (14) ABNT. NBR11486. Ethylic Alcohol - Determination of Level of Iron in Nonaffected Alcohol by Spectrophotometry. 01/12/1989. (15) ABNT. NBR10894. Ethylic Alcohol - Determination of Chloride and Sulphate Ions by Ionic Chromatography. 01/01/1990. (16) ABNT. NBR10895. Ethylic Alcohol - Determination of Content of Chloride Ions by Potentiometric Technique. 01/01/1990. (17) ABNT. NBR12120. Ethylic Alcohol - Determination of Level of Sulphate by Volumetry. 01/11/1991.

(18) Pereira, E. A.; Tavares, M. F. F.; Stevanato, A.; Cardoso, A. A. Anal. Bioanal. Chem. 2004, 380, 178–182. (19) Pereira, E. A.; Tavares, M. F. F.; Stevanato, A.; Cardoso, A. A. Quim. Nova 2006, 29, 66–71. (20) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; Wiley: New York, 1998. (21) Bruning, I. M. R. A.; Malm, E. B. Bol. Tec. Petrobr as 1982, 25, 217–228.

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Table 2. Summarized Comparison between the American and European Official Analytical Standards Used for the Trace Analysis of Species in Fuel Ethanola species chloride

copper high alcohols methanol sulfur phosphorus

agency

methodology

ASTM ASTM ECS ECS ASTM ECS ASTM ECS AST M ECS ASTM ECS ECS

titration/ISE microcoulometry potentiometry IC GF AAS GF AAS GC GC GC GC UV-FS XRFS volumetry

maximum allowed 40 ppm 10 ppm 25 ppm 25 ppm 0.1 ppm 0.1 ppm 2.0% m/m 2.0% m/m 0.5% v/v 1.0% v/v 50 ppm 10 ppm 0.5 mg L-1

standard

ref.

D512/04 D4929/04 PrEn15484 PrEn15492 D1668/87 PrEn15488 D5501/04 En13132/00 D5501/04 En13132/00 D5453/06 PrEn15485 PrEn15487

55 55 56 56 55 56 55 56 55 56 55 56 56

a

N/S = not specified; GF AAS = graphite furnace atomic absorption spectrometry; GC = gas chromatography; IC = ionic chromatography; UV-FS: ultraviolet fluorescence spectroscopy; XRFS = X-ray fluorescence spectroscopy.

control for this fuel. In this context, two important sources of standard methods have been used as support for fuel ethanol analysis by several countries: The norms of American Society for Testing and Materials (ASTM)55 and European Committee for Standardization (ECS).56 Actually, the chemical species investigated in fuel ethanol by importer/new producer countries by these norms are restricted to few species, as Cu, Cl, S, P, methanol, and higher alcohols.54 Additionally, a good part of these norms are adapted from other kinds of matrixes, such as crude oil or water, that is, they were not developed specifically for fuel ethanol analysis. These norms can be better compared in Table 2. 2.2. Alternative Methodologies for Analysis of Organic Species. In this topic it is possible to review analytical methodologies that still do not make part of those used as the official norms by ABNT, ASTM, or ECS. The refinement of instrumental methods of analysis in the last decades, associated with the development of new methodologies, such as spectrometry (atomic absorption and emission), chromatography (high performance liquid chromatography, gas chromatography), and electrochemistry (adsorptive and anodic stripping methods, chemically modified electrodes), has made the detection and quantification of several organic and inorganic compounds in fuel ethanol possible. Among these compounds, several species such as lead, aluminum, cadmium, esters, and aromatic aldehydes are devoid of specific legislation. The high reliability of instrumental methods associated with low detection limits has enabled a series of technological innovation in terms of fuel ethanol quality control, thus allowing the introduction of new official norms and the improvement of the existing ones, leading to a more detailed profile of the species contained in this fuel. The analytical characteristics of the alternative techniques of trace analysis of organic compounds in fuel ethanol reported in this review are summarized in Table 3 to allow easy comparison between the methods, with all concentration units standardized to ppm and ppb according to the reported unit (ppm corresponds to mg L-1 or μg g-1, and ppb corresponds to μg L-1 or ng g-1). According to literature survey, the first independent studies on the determination of chemical species in fuel ethanol were carried out in the 1980s by Bruning and Malm.21 In these studies, methanol, acetic aldehyde, and ethyl acetate were determined by gaseous chromatography, constituting a pioneering work in this research field.

Table 3. Summarized Comparison between the Alternative Analytical Methodologies Used for the Trace Analysis of Organic Species in Fuel Ethanola work range species acetaldehyde

methodology

GC CE GC HPLC-UV/vis HPLC-ED voltammetry voltammetry propanaldehyde GC 2-furfuraldehyde voltammetry HPLC-ED HPLC-UV-vis 5-hydroxymethyl furfural HPLC-ED HPLC-UV-vis butyraldehyde HPLC-ED HPLC-UV-vis total aldehydes HPLC-UV/vis ethyl formiate GC ethyl propionate GC methyl acetate GC isopropyl acetate GC dimethylketone CE HPLC-ED HPLC-UV-vis GC methyl-ethylketone HPLC-ED HPLC-UV-vis methanol GC FTIR dimethyl ether GC diethyl ether GC high alcohols GC formaldehyde GC FIA-UV/vis solvent Orange 7 voltammetry

lower

upper

ref.

1 ppm 1.3 ppm 1 ppm 3.8 ppm 3.8 ppm 0.05 ppm 0.05 ppm 1 ppm 115 ppm 6 ppb 6 ppb 6 ppb 6 ppb 6 ppb 6 ppb 0.4 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1.3 ppm 6 ppb 6 ppb 1 ppm 6 ppb 6 ppb 1 ppm N/S 1 ppm 1 ppm 1 ppm 1 ppm 0.063 ppm 1.4 ppm

N/S 25 ppm N/S 379 ppm 379 ppm 5 ppm 0.5 ppm N/S 704 ppm 500 ppb 500 ppb 500 ppb 500 ppb 500 ppb 500 ppb 7.6 ppm N/S N/S N/S N/S 25 ppm 500 ppb 500 ppb N/S 500 ppb 500 ppb N/S N/S N/S N/S N/S N/S 1.9 ppm 6.3 ppm

21 19 21 24 24 27 28 21 22 29 30 29 30 29 30 26 21 21 21 21 19 29 30 21 29 30 21 65 21 21 21 21 23 64

a N/S = not specified; CE = capillary electrophoresis; UV/Vis = molecular spectrophotometry; GC = gas chromatography; HPLC-UV/ Vis = high performance liquid chromatography with molecular spectrophotometric detection; HPLC-ED = high performance liquid chromatography with electrochemical detection; IC = ionic chromatography; FIA-UV/Vis = flow injection analysis with molecular spectrophotometric detection; FTIR = Fourier transform Infra-red spectroscopy.

A direct consequence of the actual increasing interest in fuel ethanol is the publication of 27 scientific articles on fuel ethanol quality control in scientific journals, in the last 5 (22) Saczk, A. A.; de Oliveira, M. F.; Okumura, L. L.; Stradiotto, N. R. Eclet. Quim. 2002, 27, 141–51.

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: DOI:10.1021/ef900493k LOD values ranging between 1.7 and 2.0 ng mL-1. The obtainment of recovery values up to 95% and the satisfactory simultaneous analysis time (15 min per analysis) indicated that this is an excellent method for the simultaneous analysis of organic species in fuel ethanol. The identification of several organic components present in fuel ethanol has been also carried out by mass spectrometry technique. Vilar et al.58 have provided the characterization of about 65 organic species in this matrix by means of initial separation using a gaseous chromatograph, followed by acquisition of mass spectrum and subsequent comparison with spectral data from the library of the equipment. In this study, saturated linear hydrocarbons and aromatic hydrocarbons eluted in the first fraction and oxygenated compounds such as aldehydes, ketones, and alcohols, eluted in the second one, when n-hexane, n-hexane-benzene (1:1, v/v), and dichloromethane were used has mobile phases. 2.3. Alternative Methodologies for Analysis of Inorganic Species. 2.3.1. Cationic Species. Besides the significant number of organic substances present in fuel ethanol, it is also crucial to bear in mind that inorganic compounds are also present in this matrix, for example, anions and metallic cations. Therefore, the detection and dosage of these species is necessary since there are problems related to their emission into the environment, as well as the accentuated corrosion phenomenon in engines and contamination of industrial stages in plants where ethanol is employed as raw material. The analytical characteristics of the alternative techniques in relation to official norms (ABNT, ASTM, and ECS) for the trace analysis of cationic compounds in fuel ethanol reported in this review are summarized in Table 4, with all concentration units also standardized to ppm and ppb in order to allow easy comparison between the methods. Regarding the analysis of cationic species in fuel ethanol, it is noteworthy that actual legislation (ABNT, ASTM, or ECS) still does not present official norms for quality control of other species such as lead and nickel, among others. As reported in Section 2.1, the first work on cationic species present in fuel ethanol was carried out by Bruning and Malm21 in 1982. They analyzed iron, nickel, copper, potassium, sodium, and chromium by flame atomic absorption spectrometry. After 1998 there was a real boom in the analysis of cationic species by different techniques, and several spectrometric methodologies such as flame atomic emission spectrometry (FAES),31 flame atomic absorption spectrometry (FAAS),32-36,59,60,70 graphite furnace atomic absorption

years. Among the typical instrumental techniques used for analysis of organic species, it is possible to cite electrophoretic,19 voltammetric,22,27,28,64 spectrophotometric,23,65 and chromatografic24-26,29,58 techniques. Some of these works can be more detailed in the sequence. Pereira and colleagues19 employed the technique of capillary electrophoresis for determination of dimethylketone, acetaldehyde, and formaldehyde in fuel ethanol using the procedure of derivatization of aldehydes and ketones with 3-methyl-2benzothiazoline hydrazone adducts. The species formed were spectrophotometrically detected at 216 nm. This methodology allowed the determination of these organic species in a linear calibration range from 32.0 to 91.3 mg L-1, with obtained detection limits between 0.45 and 0.58 mg L-1. Teixeira and colleagues23 proposed a spectrophotometric method for the determination of formaldehyde in fuel ethanol, with the direct measurement of light in a solid support of a C18 composite. In this work, formaldehyde specifically reacted with the species Fluoral P, which had been previously added to medium, thus producing the species 3,5-diacetyl1,4-dihydrolutidine, promptly determined by UV-vis spectrophotometry at 412 nm. The insertion of a flow injection analysis (FIA) system in the determination step allowed determination of formaldehyde in the range 0.050-1.5 mg L-1, with a limit of detection (LOD) of 30 μg L-1 and an analytical frequency of 20 determinations per hour. The importance of determining aldehydes in fuel ethanol allowed the development of a sequential injection analysis (SIA) system proposed for the determination of total aldehydes in fuel ethanol samples by Oliveira and colleagues.26 The possibility of reacting aliphatic aldehydes with the derivatizing agent 3-methyl-2-benzotiazolone hydrazone (MBTH), allowed analysis of total aldehydes in this fuel in the range 0.3-6 mg L-1, the obtainment of an LOD value of 60 μg L-1, and an analytical frequency of up to 10 samples per hour. The voltammetric analysis of acetaldehyde in glassy carbon electrodes was the objective of the studies carried out by Rodgher et al.27 when a previous step of acetaldehyde derivatization with DNPH was provided. Well-defined voltammograms concerning the reduction of the derivatized product at -1.77 V in the glassy carbon electrodes were observed. The transient signal of icp enabled the obtainment of LOD at 8.1  10-7 mol L-1 for the indirect determination of acetaldehyde, dismissing the need for other steps of sample pretreatment and thus demonstrating a good robustness of the proposed methodology. A simultaneous analysis of 5-hydroxymethylfurfural, 2-furfuraldehyde, butyraldehyde, acetone, and methylethyl-ketone in fuel ethanol was done by Saczk et al.29 by the HPLC technique coupled with electrochemical detector. The derivation step of above-mentioned analytes allowed their determination in the range 5.0-400.0 ng mL-1, with

(30) Saczk, A. A.; Okumura, L. L.; de Oliveira, M. F. Zanoni, M. V. B.; Stradiotto, N. R. Anal. Lett. 2009; in press. (31) de Oliveira, A. P.; Okumura, L. L.; Neto, J. A. G.; de Moraes, M. Eclet. Quim. 2002, 27, 285–91. (32) Padilha, P. M.; Padilha, C. C. F.; Rocha, J. C. Quim. Anal. 1999, 18, 299–303. (33) Gomes, L. A. M.; Padilha, P. M.; Moreira, J. C.; Filho, N. L. D.; Gushikem, Y. J. Braz. Chem. Soc. 1998, 9, 494–498. (34) Roldan, P. S.; Alcantara, I. L.; Castro, G. R.; Rocha, J. C.; Padilha, C. C. F.; Padilha, P. M. Anal. Bioanal. Chem. 2003, 375, 574– 577. (35) da Silva, E. L.; Budziak, D.; Carasek, E. Anal. Lett. 2004, 37, 1909–1924. (36) Teixeira, L. S. G.; Bezerra, M. A.; Lemos, V. A.; Santos, H. C.; Jesus, D. S.; Costa, A. C. S. Sep. Sci. Technol. 2005, 40, 2555–2565. (37) de Oliveira, A. P.; de Moraes, M.; Neto, J. A. G.; Lima, E. C. Atom. Spectrosc. 2002, 23, 39–43. (38) de Oliveira, A. P.; de Moraes, M.; Neto, J. A. G.; Lima, E. C. Atom. Spectrosc. 2002, 23, 190–195. (39) Curtius, A. J.; Giacomelli, M. B. O.; Silva, J. B. B.; SaintPierre, T. D. Microchem. J. 2004, 77, 151–156.

(23) Teixeira, L. S. G.; Le~ao, E. S.; Dantas, A. F.; Pinheiro, H. C. L.; Costa, A. C. S.; Andrade, J. B. Talanta 2004, 64, 711–715. (24) Okumura, L. L.; Saczk, A. A.; de Oliveira, M. F.; Zanoni, M. V. B.; Stradiotto, N. R. Anal. Sci. 2005, 21, 441–444. (25) Okumura, L. L.; Saczk, A. A.; de Oliveira, M. F.; Zanoni, M. V. B.; Stradiotto, N. R. Anal. Bioanal. Chem. 2005, 381, 1619–1624. (26) Oliveira, F. S.; Leite, B. C. O.; Andrade, M. V. A. S.; Korn, M. J. Braz. Chem. Soc. 2005, 16, 87–92. (27) Rodgher, V. S.; Okumura, L. L.; Saczk, A. A.; Stradiotto, N. R.; Zanoni, M. V. B. J. Anal. Chem. 2006, 61, 889–895. (28) Rodgher, V. S.; Stradiotto, N. R.; Zanoni, M. V. B. Quim. Nova 2006, 29, 662–665. (29) Saczk, A. A.; Okumura, L. L.; de Oliveira, M. F.; Zanoni, M. V. B.; Stradiotto, N. R. Chromatographia 2006, 63, 45–51.

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Table 4. Summarized Comparison between the Analytical Methodologies Used for the Trace Analysis of Cationic Species in Fuel Ethanola work range species

methodology

amonium arsenium

CE GF AAS GF AAS ET AAS ICP-MS GF AAS GF AAS ICP-MS GF AAS ICP-MS FAAS FAAS FAAS ET AAS ETV ICP-MS ICP-MS ASV GF AAS ICP-MS CE FAAS FAAS GF AAS FAAS FAAS ICP-MS ICP-MS FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS GF AAS GF AAS ICP-MS ETV ICP-MS DP ASV UV/vis DP ASV SW ASV potentiometry LS-ASV GF AAS ICP-MS FAAS ICP-MS FAAS FAAS FAAS FAAS FAAS FAAS GF AAS GF AAS ICP-MS UV/vis SW ASV GF AAS ICP-MS FAAS FAAS ET AAS ETV ICP-MS ICP-MS DP ASV SW ASV DP ASV GF AAS GF AAS

aluminum antimony barium bismuth cadmium

calcium chromium

cobalt

copper

gallium iron

lead

lower 0.15 ppm 3 ppb 0.004 ppb 3 ppb 6.3 ppb 3 ppb 3 ppb 0.63 ppb 1 ppb 0.63 ppb 35 ppm 28 ppm 0.3 ppb 0.06 ppb 0.10 ppb 0.15 ppb 0.14 ppb 0.12 ppb 0.63 ppb 0.17 ppm 1.3 ppb 16.4 ppm 0.12 ppb 14.8 ppm 1.8 ppb 0.05 ppb 0.63 ppb 1.3 ppb 20 ppm 16 ppm 19 ppb 1.8 ppb 2.9 ppb 11 ppb 8.9 ppb 2.5 ppb 0.72 ppb 0.5 ppb 0.12 ppb 0.08 ppb 40 ppb 6 ppb 0.15 ppb 12 ppb 4 ppb 0.76 ppb 0.63 ppb 0.12 ppb 0.63 ppb 1.3 ppb 18 ppm 14 ppm 6.3 ppb 11 ppb 5 ppb 2.5 ppb 1.6 ppb 34 ppb 14 ppb 0.42 ppm 2.3 ppb 6.3 ppb 126 ppb 14 ppm 1.4 ppb 0.06 ppb 0.25 ppb 0.26 ppb 0.297 ppb 2,5 ppb 0.4 ppb 0.8 ppb

4856

upper N/S 63 ppb N/S N/S 126 ppb 63 ppb N/S 12.6 ppb N/S 12.6 ppb 141 ppm 359 ppm N/S N/S N/S N/S 70 ppb N/S 12.6 ppb N/S N/S 65.8 ppm N/S 185 ppm N/S N/S 12.6 ppb N/S 80 ppm 200 ppm 252 ppb N/S N/S N/S N/S 63 ppb N/S N/S N/S 40 ppb N/S 200 ppb N/S N/S 80 ppb N/S 12.6 ppb 1.26 ppb 12.6 ppb N/S 70 ppm 140 ppm N/S N/S N/S 63 ppb N/S N/S N/S 1.4 ppm N/S 126 ppb 1260 ppb 176 ppm N/S N/S N/S 130 ppb N/S 312 ppb N/S N/S

ref. 52 37 38 39 68 37 38 68 67 68 32 33 35 39 41 40 44 66 68 52 21 32 67 33 35 40 68 21 32 33 34 35 36 59 60 37 38 40 41 44 42 48 45 57 63 67 68 70 68 21 32 33 36 59 60 37 38 40 42 61 67 68 70 33 39 41 40 44 45 49 66 67

Energy Fuels 2009, 23, 4852–4859

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species magnesium manganese

molibdenium nickel

potassium rubidium selenium silver sodium strontium talium tin vanadium zinc

methodology

lower

upper

ref.

ICP-MS CE GF AAS GF AAS FAAS ICP-MS ICP-MS GF AAS ICP-MS FAAS FAAS FAAS FAAS FAAS FAAS FAAS GF AAS GF AAS ICP-MS DPV DPV GF AAS ICP-MS FAAS FAAS CE ICP-MS ICP-MS ICP-MS ETV ICP-MS FAAS FAAS CE ICP-MS ETV ICP-MS ICP-MS ICP-MS ICP-MS FAAS FAAS FAAS FAAS FAAS FAAS FAAS DP ASV DP ASV ICP-MS FAAS

0.63 ppb 0.18 ppm 2.5 ppb 0.5 ppb 2.5 ppb 0.9 ppb 0.63 ppb 0.3 ppb 0.63 ppb 1.3 ppb 18 ppm 15 ppm 19 ppb 10 ppb 4.0 ppb 5.0 ppb 3 ppb 1.6 ppb 0.5 ppb 5.0  10-9 mol L-1 7.5  10-9 mol L-1 0.5 ppb 0.63 ppb 126 ppb 1.3 ppb 0.22 ppm 0.63 ppb 6.3 ppb 0.10 ppb 0.03 ppb 1.3 ppb N/S 0.16 ppm 0.63 ppb 0.001 ppb 0.63 ppb 0.63 ppb 0.63 ppb 20 ppm 16 ppm 13 ppb 1.3 ppb 0.1 ppb 2.5 ppb 2.5 ppb 40 ppb 0.08 ppb 6.3 ppb 126 ppb

12.6 ppb N/S 63 ppb N/S N/S N/S 12.6 ppb N/S 12.6 ppb N/S 74 ppm 182 ppm 253 ppb N/S N/S N/S 63 ppb N/S N/S 5.0  10-7 mol L-1 1.0  10-6 mol L-1 N/S 12.6 ppb 1260 bbp N/S N/S 12.6 ppb 126 ppb N/S N/S N/S 0.375 ppm N/S 12.6 ppb N/S 12.6 ppb 12.6 ppb 12.6 ppb 82 ppm 205 ppm 126 ppb N/S N/S N/S N/S 411 ppb 41 ppb 126 ppb 1260 ppb

68 52 37 38 60 40 68 67 68 21 32 33 34 36 59 60 37 38 40 46 47 67 68 70 21 52 68 68 40 41 21 31 52 68 41 68 68 68 32 33 34 35 36 59 60 43 44 68 70

a N/S = not specified; CE = capillary electrophoresis; UV/Vis = molecular spectrophotometry; GF AAS = graphite furnace atomic absorption spectrometry; ET AAS = electrothermal atomic absorption spectrometry; FAAS = flame atomic absorption spectrometry; ICP-MS = inductively coupled plasma mass spectrometry; ETV = electrothermal vaporization; ASV = anodic stripping voltammetry; LS = linear sweep; SW = square wave; DPV = differential pulse voltammetry.

spectrometry (GF AAS),37,38,66,67 and electrothermal (ET AAS)39 can be cited. UV Vis spectrophotometry,40 inductively coupled plasma mass spectrometry (ICP-MS)41,42,68

and electroanalytical methodologies43-49,57,61,63 can also be used to analyze cationic species. Atomization techniques can also be enhanced by chemical preconcentration steps coupled to the conventional apparatus. As an example, Padilha et al.34 applied the FAAS technique for determination of several metallic species in fuel ethanol, such as Cu, Ni, and Zn, after enrichment in columns packed with 2-aminothiazole-modified silica gel. In these cases, the concentration step allowed precise determinations of metallic

(40) Saint’Pierre, T. D.; Tormen, L.; Frescura, V. L. A.; Curtius, A. J. J. Anal. Atom. Spectrom. 2006, 21, 1340–1344. (41) Saint’Pierre, T. D.; Frescura, V. L. A.; Curtius, A. J. Talanta 2006, 68, 957–962. (42) Teixeira, L. S. G.; Brasileiro, J. F.; Borges Junior, M. M.; Cordeiro, P. W. L. Quim. Nova 2006, 29, 741–745. (43) de Oliveira, M. F.; Saczk, A. A.; Okumura, L. L.; Stradiotto, N. R. Eclet. Quim. 2002, 27, 153–60. (44) de Oliveira, M. F.; Saczk, A. A.; Okumura, L. L.; Fernandes, A. P.; de Moraes, M.; Stradiotto, N. R. Anal. Bioanal. Chem. 2004, 380, 135–140. (45) Munoz, R. A. A.; Angnes, L. Microchem. J. 2004, 77, 157–62. (46) Tartarotti, F. O.; de Oliveira, M. F.; Balbo, V. R.; Stradiotto, N. R. Microchim. Acta 2006, 155, 397–401.

(47) Bergamini, M. F.; Santos, A. L.; Stradiotto, N. R. Eclet. Quı´m. 2006, 31, 45–52. (48) Takeuchi, R. M.; Santos, A. L.; Padilha, P. M.; Stradiotto, N. R. Anal. Chim. Acta 2007, 584, 295–301. (49) Takeuchi, R. M.; Santos, A. L.; Padilha, P. M.; Stradiotto, N. R. Talanta 2007, 71, 771–777.

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: DOI:10.1021/ef900493k electrophoresys,18,19,52 potentiometry,21 UV-vis spectrophotometry,50 conductometry,51 and voltammetry53,62,69 have been used for research of anionic species in this matrix. As reported in Section 2.2, the first work on anionic species present in fuel ethanol was carried out by Bruning and Malm21 in 1982. They analyzed chloride and sulfate by potentiometry. The analytical characteristics of the alternative techniques in relation to official norms (NBR, ASTM, and ECS) for the trace analysis of anionic compounds in fuel ethanol reported in this review are summarized in Table 5, wherea all concentration units have been standardized to ppm and ppb in order to allow easy comparison between the methods. De Oliveira and Korn50 applied the reagent barium dimethylsulfonazo(III) for subsequent reaction with sulfate, and they monitored the decrease in the absorbance of the initial reagent. This allowed the indirect spectrophotometric determination of sulfate in fuel ethanol at 665 nm. When an SIA system was coupled to the spectrophotometer, it was possible to obtain LOD values around 10 mg L-1 for sulfate and an analytical frequency of 27 analyses per hour. The determination of chloride in fuel ethanol has been also investigated by the technique of conductometry by Avelar and Barbeira.51 In this work, a simple, fast, and sensitive procedure was carried out by titration of 100 mL of fuel samples with about 0.5 mL of titrant solution (AgNO3), which presented recovery values from 98.3% to 102.4% as well as a limit of determination of 0.88 mg L-1. The use of capillary electrophoresis (CE) for the determination of sulfate, chloride, and nitrate in fuel ethanol52 led to

species (recovery values between 98 and 99%) present in fuel samples in levels ranging from 4.4 to 78 ppm. Still, in terms of trace analysis of metallic species by atomization methods, an important contribution to the development of analytical methodologies for quality control of fuel ethanol was provided by de Oliveira et al.37 by using the GF AAS technique. This research group investigated the determination of Al, As, Cu, Fe, Mn, and Ni in fuel ethanol samples using additions of Pd(NO3)2 and Mg(NO3)2 as chemical modifier for the analytes. In this case, the selected pyrolysis temperature was enough to eliminate the fuel ethanol matrix. This technique provided detection limits ranging from 0.2 to 1.5 μg L-1 for the metallic species. Saint’Pierre et al.40 applied the ICP-MS technique in the analysis of a broad spectrum of cationic species in fuel ethanol. In this work, it was possible to obtain detection limits (in μg L-1) at 0.08 (Ag), 0.12 (Cd), 0.04 (Co), 0.4 (Cu), 27 (Fe), 0.7 (Mn), and 0.4 (Ni) by isotope dilution method, which demonstrates the high robustness of this methodology. The UV-Vis spectrophotometric method is among the spectroscopic methodologies available for the determination of metallic ions. As an example, it is possible to perform the determination of copper and iron using ligands of the ferroine class,42 due to formation of considerable stable complexes between these ligands and the ionic metallic species. Other metallic ions also form complexes with this class of ligands, but the spectral characteristics are different enough to avoid interference with the spectrophotometric analysis of copper and iron. As for electroanalytical methodologies, which commonly are of low operational costs and are compatible with conventional techniques in terms of robustness and precision, there can be cited seven works about determination of inorganic species in fuel ethanol. As an example, glassy carbon electrodes modified with mercury film were employed at analysis of cationic species by de Oliveira et al.44 in 2004, using a mixed medium of ethanol/water (20/80 w/w) and two additional voltammetric modalities: differential pulse and square wave voltammetry. This methodology, developed for the simultaneous determination of Zn, Cu, Pb, and Cd in fuel ethanol, allowed direct determination without previous pretreatment of samples. The application of voltammetric modalities of linear scan, square wave, and differential pulse after a preconcentration step by the anodic stripping method allowed the obtainment of detection limit values ranging from 10-9 to 10-8 mol L-1, and an analytical frequency of 6 analyses per hour. Among the different metallic species studied by electrochemical methods, Cu species was also determined in fuel ethanol by chemically modified carbon paste electrodes in fuel ethanol. Takeuchi et al.49 also used 2-aminothiazole organofunctionalized silica (SiAt-SPCPE), as chemical modifier at carbon paste for detection of Cu in ethanol fuel samples without any sample treatment. Differential pulse anodic stripping voltammetry was employed as voltammetric modality. It was possible to obtain a linear work range from 7.5  10-8 to 2.5  10-6 mol L-1 with detection limit of 3.1  10-8 mol L-1. 2.3.2. Anionic Species. There are only a small number of scientific works on the determination of inorganic anions in fuel ethanol. Instrumental methodologies such as capillary

(51) Avelar, H. M.; Barbeira, P. J. S. Fuel 2007, 86, 299–302. (52) Munoz, R. A. A.; Richter, E. M.; Jesus, D. P.; Lago, C. L.; Angnes, L. J. Braz. Chem. Soc. 2004, 15, 523–526. (53) Aleixo, L. M.; Rath, S.; Godinho, O. E. S. Analyst 1988, 113, 1427–9. (54) Junior, J. F. S. Market specification and methods for ethanol analysis. www.nist.gov/oiaa/felix.pdf (Accessed May 10th 2009). (55) American Society for Testing and Materials; ASTM homepage. www.astm.org (Accessed on October 28th 2007). (56) European Committee for Standardization. www.cen.eu/cenorm/ homepage.htm (Accessed on November 21th 2007). (57) Neves, E. F. A.; Neto, G. O.; Serrano, S. H. P. Anal. Lett. 1987, 20, 1363–1377. (58) Vilar, R. B. C.; da Silva, R.; Schossler, P.; Veses, R. C.; Piatnicki, C. M. S.; Samios, D. J. Chrom. A 2003, 985, 367–373. (59) Moreira, J. C.; Gushikem, Y. Anal. Chim. Acta 1985, 176, 263– 267. (60) Filho, N. L. D.; Gushikem, Y.; Polito, W. L.; Moreira, J. C.; Ehirim, E. O. Talanta 1995, 42, 1625–1630. (61) Mattos, C. S.; Carmo, D. R.; de Oliveira, M. F.; Stradiotto, N. R. Int. J. Electrochem. Sci. 2008, 3, 338–345. (62) Ferreira, H. B. P.; Lima, T. A. F.; Barbeira, P. J. S. Electroanalysis 2008, 20, 390–395. (63) Takeuchi, R. M.; Santos, A. L.; Medeiros, M. J.; Stradiotto, N. R. Microchim. Acta 2009, 164, 101–106. (64) Romanini, D. C.; Trindade, M. A. G.; Zanoni, M. V. B. Fuel 2009, 88, 105–109. (64) Romanini, D. C.; Trindade, M. A. G.; Zanoni, M. V. B. Fuel 2009, 88, 105–109. (65) Carneiro, H. S. P.; Medeiros, A. R. B.; Oliveira, F. C. C.; Aguiar, G. H. M.; Rubim, J. C.; Suarez, P. A. Z. Energy Fuels 2008, 22, 2767– 2770. (66) Saint’Pierre, T. D.; Maranh~ao, T. A.; Frescura, V. L. A.; Curtius, A. J.; Aucelio, R. Q. Quim. Nova 2008, 31, 1626–30. (67) Amorim, V. R.; Caldas, N. M.; Raposo, J. L.; Flumignan, D. L.; de Oliveira, J. E.; Neto, J. A. G. Atom. Spectrosc. 2008, 29, 230–235. (68) Tormen, L.; Chaves, E. S.; Saint’Pierre, T. D.; Frescura, V. L. A.; Curtius, A. J. J. Anal. Atom. Spectrom. 2008, 23, 1300–1304. (69) de Paula, D. T.; Yamanaka, H.; de Oliveira, M. F.; Stradiotto, N. R. Chem. Tech. Fuels Oils 2008, 44, 435–440. (70) de Oliveira, M. F.; Balbo, V. R.; de Andrade, J. F.; Saczk, A. A.; Okumura, L. L.; Stradiotto, N. R. Chem. Tech. Fuels Oils 2008, 44, 430– 434.

(50) de Oliveira, F. S.; Korn, M. Talanta 2006, 68, 992–999.

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: DOI:10.1021/ef900493k 103% and LOD for sulfate and chloride of 0.033 and 0.041 mg kg-1, respectively. Aleixo et al.53 developed a voltammetric methodology for determination of total sulfur in ethanol, using a previous step of reduction of sulfur compounds to sulphide using Raney nickel, with a subsequent conversion to H2S and detection by differential pulse-cathodic stripping voltammetry. The calibration graph covered the concentration range of 140 ppb, and the method was employed for determination of sulfur in fuel ethanol samples containing from 15 ng g-1 to 20 μg g-1. 2.3.3. Perspectives. Despite the broad number of alternative methodologies developed for the quality control of fuel ethanol, there is still need for the research and development of detection methods for chemical species, such as nitrogen and phosphorus compounds and nonvolatile organic substances.54 These new complementary methodologies are necessary to relieve the environmental impact of the evaluations and to enable the rapid adequation to the quality control norms of importer countries.

Table 5. Summarized Comparison between the Analytical Methodologies Used for the Trace Analysis of Anionic Species in Fuel Ethanola work range species

methodology

lower

upper

ref.

chloride

CE CE potentiometry conductometry CE AdSV FIA AD colorimetry CE CE CE FIA UV/vis potentiometry CE DP CSV

0.065 ppm 0.02 ppm 1 ppm 1.1 ppm 0.08 ppm 0.079 ppm 22 ppm 100 ppb 0.10 ppm 0.25 ppm 0.2 ppm 12 ppm 1 ppm 0.10 ppm 1 ppb

0.65 ppm 0.1 ppm N/S N/S N/S 0.236 ppm 223 ppm N/S N/S 4 ppm 4 ppm N/S N/S N/S 40 ppb

18 19 21 51 52 62 69 21 52 18 19 50 21 52 54

cyanide nitrate sulfate

sulfur

a N/S = not specified; CE = capillary electrophoresis; UV/Vis = molecular spectrophotometry; DP-CSV = differential pulse-cathodic stripping voltammetry; IC = ionic chromatography; AdSV = adsorptive stripping voltammetry; FIA AD = flow injection analysis with amperometric detection.

3. Conclusions

LOD values between 0.06 and 0.18 mg L-1, which is in good agreement with the LOD values of conventional and official methodologies. In this case, a previous evaporation of ethanol with subsequent resuspension of residue was carried out. The CE technique was also applied by Pereira et al.18 for indirect determination of sulfate and chloride in ethanol by previous evaporation of alcoholic sample and further resuspension in water (preconcentration factor = 10). This methodology allowed recovery values ranged from 85 to

After observing the several scientific articles on the analysis of organic and inorganic species present in fuel ethanol developed by research projects mainly from Brazilian institutions and published in a relatively short time period, it is possible to realize the importance of this research in the context of the production and commercialization of this biofuel, which is seen in the global market as a concrete alternative for nonrenewable fossil fuels and for the decrease in the greenhouse effect.

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