Article pubs.acs.org/EF
Determination of Iron, Copper, Zinc, Aluminum, and Chromium in Biodiesel by Flame Atomic Absorption Spectrometry Using a Microemulsion Preparation Method Greici A. Antunes,† Heldiane S. dos Santos,† Yara P. da Silva,† Márcia M. Silva,*,†,‡ Clarisse M. S. Piatnicki,† and Dimitrios Samios† Instituto de Química, Centro de Combustíveis, Biocombustíveis, Lubrificantes e Ó leos, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 9500, CP 15003, CEP 91501-970, Porto Alegre, RS, Brazil ‡ Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energia e Ambiente Universidade Federal da Bahia, 40170-115 Salvador, BA, Brazil †
ABSTRACT: This work proposes a flame atomic absorption spectrometry (FAAS) method for Fe, Cu, Zn, Al, and Cr determination in biodiesel using water in oil (w/o) microemulsion (ME) as a sample preparation method. A less toxic solvent (npropanol), instead of those used in the standard method (ABNT NBR 15556), the absence of surfactant, and calibration using inorganic reference standards were investigated. A ternary phase diagram of biodiesel as oil, water, nitric acid, and n-propanol showed a single-phase ME region. The composition of ME adopted for analysis was 1.7 g of biodiesel and 1.1 mL of 1.4 mol L−1 HNO3, completed with n-propanol to 10 mL volume. Linear calibration graphs with R > 0.99 were obtained using ME prepared with oleic acid and inorganic standards. Analytes were stable for at least 3 days both in sample and in metallic standards MEs. The limits of detection obtained were 0.3, 0.1, 0.07, 1.7, and 1.0 mg kg−1 for Fe, Cu, Zn, Al, and Cr, respectively. These limits of detection were lower than those obtained by the reference method of dilution with organic solvent, due to lower fluctuation in the measurements. Recovery test values (93−105%) indicated absence of matrix effects. In order to assess the method accuracy, several real samples were analyzed by the proposed method, by the standard method, and also by acid digestion, without significant differences between results when applying the student’s t test. Relative standard deviations lower than 5% were obtained for standard and samples MEs. Therefore, combination of w/o ME with the FAAS technique and calibration with inorganic standards proved to be an accurate, simple, fast, and practical strategy, with greater analyte stability and appropriate sensitivity, so that the proposed method is suitable for biodiesel routine analysis.
1. INTRODUCTION Ecological concerns such as sustainability and the need for alternative and renewable energy have helped in increasing biodiesel inclusion as an energy source less aggressive to the environment. Use of biodiesel as a fuel reduces emissions of carbon monoxide, particulate matter, total hydrocarbons, and sulfur compounds.1 For these reasons, biodiesel production is growing simultaneously with increasing research on its production and characterization.2 In Brazil, investments in biodiesel production from vegetal oil started in 1970 with implementation of the Vegetable Oil Production Plan for Energy Purposes (Pro-oleo). However, high production cost and taxes relative to diesel prevented its use on a commercial scale. Only from 2005, the National Production and Use of Biodiesel Program (ProBiodiesel) fixed the amount of biodiesel to be added to diesel, so that Brazil became one of the largest biodiesel producers in the world (3.9 billion liters in 2015).3 According to Brazilian legislation, the amount of biodiesel mixed with diesel is currently 7% (B7) and should reach 10% (B10) in 2019. One of the challenges in the biodiesel production and distribution chain is guaranteeing fuel quality. Biodiesel’s low oxidative stability with respect to temperature and exposure to air and its higher hygroscopic character as compared to diesel are the critical factors for its storage and transport.4,5 These © XXXX American Chemical Society
characteristics can lead to biodiesel chemical degradation and corrosion of fuel lines, pumps, engine components, alloys, and metallurgy used in storage tank and barrel manufacturing. The corrosion process may start in the water−metal interface due to the free water (droplets) present in biodiesel.4,6,7 On the other hand, the presence of metals in biodiesel from corrosion processes or from other sources such as the raw material, production, or contamination from additives can cause several problems. Metals such as Co, Cu, Fe, Mn, and Ni in powdered form can catalyze biodiesel oxidation8 while Na+, K+, Cu2+, Pb2+, and Zn2+ promote biodiesel oxidative degradation through gum formation reactions.2,9,10 Among the metals investigated, Cu appears to have the strongest detrimental effect.5,10 Searching for ideal materials to be used in storage tanks in order to prevent corrosion and biodiesel degradation, several authors investigated corrosion products generated by different materials and temperatures at various storage times.4 Therefore, monitoring of metals concentration in biodiesel and its variation with time is of great importance in terms of costs to industry, trade, services, and consumers and to evaluate new material performance in transport and storage tanks. Received: December 16, 2016 Revised: January 31, 2017 Published: February 7, 2017 A
DOI: 10.1021/acs.energyfuels.6b03360 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 1. Instrumental Parameters Used for Determination of Fe, Cu, Zn, Al, and Cr by FAAS Using Different Methods of Sample Preparation analyte
wavelenth (nm)
Fe 248.3 Cu 324.8 Zn 213.9 Al 309.3 Cr 357.9 flame composition C2H2/air (Fe, Cu, Zn) C2H2/N2O (Al, Cr) a
DX Fe Cu Zn Al Cr a
0.066 0.082 0.086 0.390 0.342
a
ME
b
0.081 0.082 0.070 0.430 0.354
c
lamp curent (mA)
slit (nm)
6.0 3.0 4.0 6.0 6.0
0.2 0.8 0.2 0.8 0.2
aspiration rate (mL min−1) a
AD
DX
0.096 0.085 0.078 0.480 0.339
2.7 2.8 2.6 3.2 3.2
ME
b
3.0 3.0 3.0 2.8 2.8
burner height (mm) c
a
AD
DX
MEb
ADc
3.1 3.2 3.2 5.2 4.0
6 7 7 5 5
6 7 7 7 5
6 7 7 7 6
Dilution with xylene. bMicroemulsion. cAcid digestion.
vaporization (ETV) coupled to ICP-MS and methods using a solid sampling device for GFAAS have been reported for direct analysis of biodiesel.2 For techniques based on nebulization as sample introduction, a suitable sample preparation is need. Although several drawbacks of using dilution with organic solvents in spectrometric methods have been reported, this procedure is adopted in the standard methods and is quite used in the published works due to its simplicity and speed.2,9 Emulsion/microemulsion sample preparation has been successfully used for fuel analysis12−15 due to the homogeneous dispersion and stabilization of the oil microdroplets in the aqueous phase, which brings the viscosity close to that of an aqueous solution and reduces the organic load of the system. In addition, it allows using acids and inorganic standards for calibration thus increasing analyte stability.9 In this paper, a method for Fe, Cu, Zn, Al, and Cr determination in biodiesel samples by FAAS using water in oil (w/o) surfactant-free ME as a sample preparation method is proposed. Due to the fact that biodiesel’s molecular structure is similar to that of amphiphilic compounds, it probably plays the role of surfactant in this type of ME,16 so that sample preparation becomes simpler and faster. ME formation without surfactant using a less toxic organic solvent instead of that used in the standard method11 was investigated through a three phase diagram. Combining this technique with ME and the use of inorganic standards for calibration, a low cost routine analysis of biodiesel for quality control or screening analysis of contaminants can be achieved. To evaluate the accuracy of the developed method, the obtained results were compared with the standard (ABNT NBR 15556)11 and acid digestion methods results.
In Brazil, biodiesel monitoring and characterization is carried out by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). Besides physicochemical parameters and oxidative stability, Na, K, Ca, and Mg content is controlled; the maximum amount of (Na + K) and (Ca + Mg) is 5.0 mg kg−1 each, the same as that allowed in Europe and in United States of America. However, despite their importance on the quality control of the biodiesel and as potential environmental pollutants, the content of other metals is not controlled, either in Brazil or elsewhere. According to recent reviews on this subject,2,9 several methods have been proposed for the determination of 30 elements in biodiesel. Spectrometric techniques are most frequently reported, although some electrochemical and chromatographic methods have also been proposed.2,9 Based on data recently published by Sánchez et al.,2 the largest number of publications refers to inductively coupled plasma optical emission spectrometry (ICP-OES) while about the same number refers to graphite furnace atomic absorption spectrometry (GFAAS), flame atomic absorption spectrometry (FAAS), and mass spectrometry (ICP-MS). Regarding the number of elements, ICP-based techniques encompass a greater number of applications due to features such as multielement analysis and high sensitivity. However, since ICP-based techniques are very sensitive to organic solvents, biofuel analyses cause several plasma effects and spectral interferences.2 Besides, considering implementation and operation costs, these are comparatively expensive techniques. In this sense GFAAS has been used as a good alternative due to its good sensitivity and higher tolerance to organic solvents and lower susceptibility to spectral interferences. FAAS has been used mainly for Na, K, Ca, and Mg, including the standard methods of quality control.2 When few elements need to be determined, the relation between sample throughput and costs favors the application of FAAS in spite of it being a monoelementary technique. Moreover, the flame presents higher tolerance to organic solvents and lower susceptibility to interferences. Following this trend toward suitable methods, a standard method to determine Na, K, Ca, and Mg in biodiesel using dilution with organic solvents and determination by FAAS (ABNT NBR 15556)11 was published in Brazil. Several problems related to direct analysis of biodiesel by spectrometric techniques have been discussed by Lepri et al.9 However, methods based on the use of electrothermal
2. EXPERIMENTAL SECTION 2.1. Instrumentation. An atomic absorption spectrometer Vario AAS 6 (Analytic Jena AG, Jena, Germany) was used for absorbance measurements. Hollow cathode lamps of Fe, Cu, Zn, Al, and Cr (Photon, Australia) were employed as radiation sources operating in specific current for each element, according to the manufacturer’s instruction. Compressed air (FIAC CDS 8/50, Brazil) was used as the oxidant for Fe, Cu, and Zn, and nitrous oxide (White Martins, Brazil) was used for Al and Cr; meanwhile, acetylene (White Martins, Brazil) was used as fuel for all elements. Instrumental parameters, sample aspiration rates, and acetylene flow rate were optimized for each method in order to obtain the maximum and stable absorbance signals. Table 1 shows the optimized instrumental parameters. B
DOI: 10.1021/acs.energyfuels.6b03360 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Weighing of samples and reagents was carried out at a Europe balance (Gibertini, Italy), and the Ostwald viscometer was used for measurements of viscosity of both standard and sample MEs. 2.2. Reagents and Samples. Analytical grade reagents were used throughout. Solutions and MEs were prepared with distilled and deionized water (DDW) from a Milli-Q water purification system (Millipore, Bedford, MA, USA) with 18.2 MΩ cm resistivity. All containers and glassware were soaked in 3 mol L−1 nitric acid for at least 24 h and rinsed three times with DDW before use. The nitric acid (Merck, Germany) used either for ME preparation and digestion was further purified by sub-boiling distillation in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim, Germany). Other reagents used are oleic acid (Nuclear, São Paulo, Brazil), n-propanol (Merck, Darmstadt, Germany), base mineral oil (Specsol, Quimlab, São Paulo, Brazil), xylene (Nuclear, Brazil), Fe, Cu, Zn, Al, and Cr inorganic (1000 mg L−1), and organic (1.000 mg g−1) standards from Specsol (São Paulo, Brazil). According to the suppliers, all biodiesel samples analyzed in this work were of B100 (100% biodiesel) type obtained by the transesterification process using basic catalysts, except samples 69T− 73T, that were obtained by using heterogeneous catalysis. The samples characteristics are shown in Table 2. Most of the samples were
investigated through the daily monitoring of the analytical signal of standards and samples. 2.4. Determination of Metals According to ABNT NBR 15556. As there are no standard methods for determination of the Fe, Cu, Zn, Al, and Cr in biodiesel, the Brazilian standard method ABNT NBR 15556,11 applied for the determination of Na, K, Ca, and Mg in products derived from oils and fatsmethyl/ethyl esters of fatty acidswas used. This method consists in sample dilution with organic solvents (xylene or cyclohexane) and determination by FAAS. In this work xylene was used as organic solvent for dilution of samples and organometallic standards for calibration. The standards were prepared from a stock solution of 100 mg kg−1, in 10 mL volumetric flasks, and with addition of base mineral oil for viscosity adjustment. Samples were prepared weighing 1.0 g of biodiesel in a 10 mL volumetric flask and then diluted with xylene. The standards and samples were analyzed immediately after the dilution, as recommended by the standard method. Aspiration rate and fuel/oxidant ratio were optimized before analysis. The analytes stability was investigated through the daily monitoring of the analytical signal of standards and diluted samples. A matrix effect was also investigated for this method. Biodiesel samples were spiked with organic standards and then diluted with xylene. 2.5. Acid Digestion Procedure. Five selected samples were submitted to acid digestion for validation purpose. A mass of 1.0 g of each sample were weighed directly into an open borosilicate glass tubes and 10 mL of concentrated nitric acid was added. The mixture was gradually heated up to 120 °C and then kept at this temperature for 16 h in a digestion block under reflux using a “cold finger”.17 The tubes were then left to cool at room temperature: 2.5 mL of hydrogen peroxide (30% v/v) was added to the mixture that was gradually heated up to 40 °C for 10 min. Finally, tubes were left to rest during 6 h to complete the digestion and the products were transferred to volumetric flasks completed up to 20 mL with ultrapure water. All samples were digested in triplicate. Standards for analysis of the digested samples were prepared by serial dilutions from aqueous stock solutions (1000 mg L−1) of Fe, Cu, Zn, Al, and Cr in 0.014 mol L−1 nitric acid.
Table 2. Characteristics of Biodiesel Samples Analyzed sample
origin
BD1, B04, B28T, B98T, B133T, soybean B262T, S01 ASE1, ASE2, ASE3, BD, BSA, BS1, BS2, soybean P1, P2 FR01, FR02, FR03, FR04 fry oil/ soybean BSC Canola GZE not informed 69T−73T soybean
route methylic ethylic ethylic methylic methylic heterogeneous catalysis
provided by the Fuel Center (CECOM) and from research groups that are working in development of biodiesel production methods at Federal University of Rio Grande do Sul (UFRGS). Some samples were provided by research groups from Technological and Research Center of the Pontifical Catholic University of Rio Grande do Sul (TECNOPUC) and National Institute of Technology (INT)−Rio de Janeiro that have been performing corrosion assays. 2.3. Microemulsion Preparation and Analysis. ME were obtained by mixing biodiesel, aqueous phase (1.4 mol L−1 HNO3) and n-propanol. After the addition of all components, the system was shaken manually for a few seconds. A ternary diagram phase was built by varying the proportions of biodiesel, diluted nitric acid and npropanol; the formation of ME being evidenced through visual transparency. This procedure was performed at 25 °C, maintained by air conditioning. The composition adopted in the preparation of the ME for analysis was 1.7 g of biodiesel, 1100 μL of water and the final volume of 10 mL filled with the n-propanol (about 5.4 g). Nitric acid (1.4 mol L−1) was added within the water component in order to increase the stability of the analytes. All samples were prepared in triplicate. The ME standards for calibration were prepared similarly to the sample, using 1.7 g of oleic acid replacing the biodiesel and metals inorganic standards added into the aqueous phase. A blank solution was prepared with oleic acid, aqueous phase (0.14 mol L−1 HNO3) and n-propanol. Standards were prepared from aqueous stock solutions described above, pipetting the volume into the aqueous phase to give concentrations in the ranges of 1.0−7.0 mg L−1 for Fe, 1.0−6.0 mg L−1 for Cu, Al, and Cr, and 0.2−0.7 mg L−1 for Zn, in 10 mL volumetric flasks. The aspiration rate and fuel/oxidant ratio were optimized before analysis. In order to investigate the matrix effect, recovery tests were carried out. The samples MEs were spiked with inorganic standards added into the water component. The stability of the analytes was
3. RESULTS AND DISCUSSION 3.1. Microemulsion Formation. Solvents such as npropanol, 2-propanol, and n-butanol, as well as the surfactant Triton X-100 have been investigated in different proportions in order to obtain a homogeneous and stable system (ME) to be used as a sample preparation method for the determination of Fe, Cu, Zn, Al, and Cr in biodiesel. The immediate preparation was aimed to make the proposed procedure fast, handy, and competitive with the standard method of dilution with organic solvent. In the presence of Triton X-100, the ME was obtained with all investigated solvents. Nevertheless, using n-propanol a homogeneous system was achieved even in absence of the surfactant. In the proposed system biodiesel, a mixture of long chain carboxylic acid esters among other components, probably plays the role of nonionic surfactant with significant emulsifying properties compared to other nonionic types.16 Fatty acid alkyl esters present hydrophobic alkyl groups in one end and hydrophilic ester groups in the other. Therefore, they exhibit the characteristics of surfactants when biodiesel is spiked into water. This behavior has been evidenced recently in a study18 on photolysis of biodiesel and its blends with crude oil in simulated freshwater. The presence of FAMEs (fatty acid methyl esters) could stabilize oil droplets in the water phase therefore reducing oil droplet reaggregation. Propanol has been used with success by several authors for fuel and biofuel analysis.2,9,12 Based on our previous experience with the use of ME for analysis of biodiesel,19−21 in this work emphasis was given in using less reagents and higher amount of water with C
DOI: 10.1021/acs.energyfuels.6b03360 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels 1.4 mol L−1 nitric acid (10−15% v/v), in order to obtain a simpler system, more similar to aqueous solutions, thus facilitating the optimization of instrument for analysis without drastic changes after aqueous solutions analysis. Thus, the chosen composition was biodiesel, n-propanol, and water/nitric acid. In order to obtain the region where a homogeneous and stable system could be achieved a ternary phase diagram was built (Figure 1) as described hereafter. The diagram has three
aspiration rate and nebulization. In order to avoid transport interferences in the FAAS standards must have very similar physical characteristics as the sample. Thus, after definition of the ME composition to be used as sample preparation, the composition of standards for calibration was investigated. In the standard method of dilution with xylene, mineral oil is used for viscosity adjustment. Mineral oil has also been used in the standards ME for calibration.19,20,22 In this work the use of mineral oil in the same amount of biodiesel (1.7 g) did not bring to ME formation with n-propanol and water. Based on the higher physical and chemical similarity of oleic acid with biodiesel,23 the use of this compound was investigated as oil phase in the ME to simulate the matrix. Biodiesel viscosity can vary between 3.5−5.0 mm2 s−1 depending of the nature of the raw material (4.4 mm2 s−1 for soybean oil biodiesel) whereas for oleic acid it is approximately 4 mm2 s−1,23 and for the mineral oil used in this work, it is around 14 mm2 s−1, as specified by the supplier. Moreover, MEs with oleic acid, npropanol, and water was previously investigated by Xu et al.24 Thus, the standards for calibration were prepared in the same way, replacing the sample by 1.7 g of oleic acid to simulate the biodiesel and adding the inorganic standards into the aqueous phase. After the addition of all components, the system was shaken manually for a few seconds and the formation of a clear and stable solution was achieved. The viscosity of both biodiesel and oleic acid MEs measured using the Ostwald viscometer were 1.54 and 1.48 mm2 s−1, respectively, evidencing the similarity of the systems. Both standard and samples MEs remained stable, without phase separation, for at least 24 months. 3.3. Optimization of Instrumental Parameters. Although flames are quite tolerant for most organic solvents, the load of organic solvents in the flame requires a careful optimization of instrumental parameters, in order to guarantee a good analysis performance. For Fe, Cu, Zn, Al, and Cr determination through the standard method11 of dilution with organic solvent (DX) and the proposed method using microemulsion (ME), optimization studies were performed for the following instrumental parameters: gas fuel/oxidant (C2H2/air), burner height, and aspiration rate. The optimized and selected values are shown in Table 1. As can be seen all parameters were similar for the three methods. Although better sensitivity could be achieved increasing the aspiration rate for DX method, this procedure had to be avoided in order to prevent low stability of the signal due to the high flame fluctuation caused by the higher loading of organic solvent. For the proposed method instead, due to the higher viscosity of this system, the aspiration rate was set to larger loading in order to increase the analytical response. 3.4. Stability Studies. The method of biodiesel dilution in organic solvent has been frequently used due to its simplicity and speed.2,9 However, as pointed out by Lepri et al.,9 few works that investigated signal stability of the analyte in the diluted solution of organic solvents have observed a short-term stability of these solutions. This fact must be considered in the analysis strategy, especially in relation to the waiting time of these solutions in large autosampler trays.9 In order to investigate the stability of analytes in xylene solution and in ME, the monitoring of analytical signal was carried either for samples and standards for a period of 5 h. Considering that in ME the analytical signal remained stable for all elements for this period, the monitoring was extended for 5 days for this system. Results are shown in Figure 2.
Figure 1. Ternary phase diagram of biodiesel, 0.14 mol L−1 HNO3, and n-propanol at 25 °C: region I microemulsion; region II emulsion.
components: biodiesel, water (containing 0.14 mol L −1 HNO3), and n-propanol. The points inside the triangle were plotted fixing the quantity of one component and varying the others (the variations were 5% for each). The procedure ended when the whole interior of the diagram was plotted. Each point represents a mixture and, therefore, has its proper physical characteristics (emulsified or transparent mixture) visually identified. Figure 1 shows two distinct regions: region I, where a homogeneous and transparent solution is found (ME), and region II, where mixtures form emulsions. A high amount of biodiesel could be stabilized considering the percentage of water not exceeding 12% in mass. The use of a maximum amount of sample in the ME is desirable to increase the analysis sensitivity, but it also increases the viscosity which reduces the nebulization efficiency. Therefore, the amount of 20% (w/w) of biodiesel was adopted. High amount of water is also desirable but more than 12% (w/w) destabilizes the ME and leads the emulsion formation. Thus, the adopted composition was 1.7 g of biodiesel and 1.1 mL of H2O/HNO3, completed to 10 mL with n-propanol. It is important to emphasize that this composition is not in the border of the ME phase, shown in Figure 1. This condition was selected because, even with a small variation in the composition of the system, e.g. the mass of sample or using samples with different characteristics (biodiesel from animal fat or vegetable oils), the formation of the ME is not affected. The ME developed in this work is of the water in oil (w/o) type, where the aqueous phase is dispersed in a continuous oil phase. 3.2. Microemulsion for Calibration Curve. The organic solvents can affect the nebulization process in many ways as emphasized by Sánchez et al.2 The viscosity, e.g., affects both D
DOI: 10.1021/acs.energyfuels.6b03360 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Stability study of the analytes (∗ Al, ● Cr, Δ Cu, □ Fe, and ▶ Zn) in xylene solution and in microemulsion. (A) Standards and (B) biodiesel sample.
Table 3. Analytical Figures of Merit for Iron, Copper, Zinc, Aluminum, and Chromium Determination by ME, DX, and AD Methods analyte
method
linear regression
R
Co (mg L−1)
LODa (mg kg−1)
LOQa (mg kg−1)
Fe
ME DX AD ME DX AD ME DX AD ME DX AD ME DX AD
A = 0.0428c + 0.0011 A = 0.0380c + 0.0035 A = 0.0435c + 0.0018 y = 0.0603x + 0.0159 y = 0.0561x − 0.0020 y = 0.0657c + 0.0020 y = 0.1946x + 0.0097 y = 0.1939x + 0.0064 y = 0.1419c + 0.0055 y = 0.0041x + 0.0004 y = 0.0064x + 0.0011 y = 0.0062c − 0.0001 y = 0.0159x − 0.0002 y = 0.0163x + 0.0024 y = 0.0127x − 0.0011
0.9998 0.9985 0.9996 0.9934 0.9948 0.9988 0.9971 0.9983 0.9925 0.9999 0.9997 0.9996 0.9998 0.9985 0.9987
0.10 0.12 0.10 0.06 0.08 0.07 0.02 0.03 0.03 1.06 0.60 0.70 0.27 0.10 0.34
0.3 1.1 0.4 0.1 0.5 0.1 0.07 0.4 0.1 1.7 1.5 1.2 0.9 1.4 1.2
1.0 3.7 1.3 0.3 1.7 0.3 0.2 1.3 0.3 5.0 5.0 3.9 3.3 4.0 3.9
Cu
Zn
Al
Cr
The LOD and LOQ were calculated considering the mass of sample and final volume used in each method (1.7 g and 10 mL for ME; 1.0 g and 10 mL for DX; 1.0 g and 20 mL for AD). a
In the xylene dilution method, it was observed that the absorbance signals of Fe and Zn have been decreased significantly while for Cu the decay was around 20% in 5 h and for Al and Cr it remained constant over the investigated period both for standard and samples (Figure 2). The measurement of the signal of Al and Cr in the next day showed that the absorbance signal remained constant for at least 24 h for these elements. These results showed that the
behavior of the Fe and Zn in this medium was unstable, possibly due to the adsorption of analytes on the walls of the bottles. On the other hand, the absorbance signal of analytes in MEs of standards and biodiesel samples remained almost stable for a period of 4 days (Figure 2). The increased stability of the analytes in ME method might be associated with the presence of nitric acid that avoids the adsorption phenomenon, allowing standard and samples E
DOI: 10.1021/acs.energyfuels.6b03360 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels preparation in a day before the measurements, as is generally done on acid digestion methods. Moreover, the use of aqueous inorganic standards in the calibration curve makes the proposed method appropriate and advantageous in relation to the standard method of dilution with xylene. 3.5. Figures of Merit. After finishing the optimization steps and establishing the instrumental and operational conditions for each analyte, calibration curves were constructed for each element using inorganic standards in ME. Linear regression equations and correlation coefficients were established for the proposed method (ME) and for the standard method of dilution with xylene (DX).11 For comparison purposes, the data obtained with aqueous solution for analysis of the digested samples are also shown in this table. Figures of merit obtained for all methods are presented in Table 3. The limit of detection (LOD) and limit of quantitation (LOQ) were calculated considering the mass of sample and final volume used in each method. Comparing the sensitivity obtained by all methods it was found that the ME and the standard method showed similar sensitivities for all analytes, as demonstrated by characteristic concentrations, enabling the use of the standard method for validation of the proposed method. Correlation coefficients greater than 0.99 were obtained for all investigated analytes. The Co values obtained for different methods were similar, and the LOD and LOQ values obtained with the ME were better than those found by the standard method (DX) for the analytes investigated, except for the Al. The better LOD and LOQ for the ME method are due to the higher amount of sample that can be used and the lower standard deviation of the measurements obtained with this method. Sánchez et al.2 reported recently a summary of LOD obtained in biodiesel samples by several authors. Comparing the LODs obtained in this work for ME/FAAS with the values reported,2 it can be seen that the former values were higher than those obtained by other more sensitive techniques, such as ICP-OES, ICP-MS, and ETAAS, but similar to data reported for Zn by FAAS. Nevertheless, except for Al and Cr, the LOD values are at microgram per kilogram levels, being appropriate for a screening analysis of the sample for these contaminants. 3.6. Analytical Results. In order to evaluate the matrix effect and the trueness of the proposed method and of the comparative method of dilution with xylene (ABNT NBR 15556),11 recovery assays were performed by spiking some selected samples of biodiesel with aqueous standard or organic standards of all analytes, as described in the Experimental Section. The recovery values obtained for the proposed method were very satisfactory ranging from 95% to 102% for all analytes, as shown in Table 4, confirming the absence of the matrix effect and showing that the new method can be successfully used for routine analysis. The results obtained for the reference method were also satisfactory confirming that it is suitable for comparison of results. Concentrations found for Cu, Fe, and Zn in several biodiesel samples (B100) from different suppliers, obtained by the standard method ABNT NBR 1555611 (DX) and the proposed method (ME) are presented in Table 5. It should be emphasized that most of the samples were not commercial but provided by research groups working with different methods of production of biodiesel such as ethylic route, heterogeneous catalysis, use of fry oil, etc., and also from research groups developing corrosion studies. This explains the
Table 4. Recovery Tests for Iron, Copper, Zinc, Chromium, and Aluminum Carried out by Dilution with Xylene and Microemulsions (n = 3) recovery (%) −1
a
analyte
sample
spike (mg L )
Fe
BS1 BS2 FR03 FR04 BSC FR02 BD1 BSC FR01 GZE B004 B262 B004 B262
4.0 4.0 5.0 2.0 4.0 3.0 5.0 0.4 0.4 0.3 2.0 4.0 2.0 3.0
Cu
Zn
Cr Al
ME 97 98 98 97 102 95 98 98 96 97
± ± ± ± ± ± ± ± ± ±
DX 1 1 1 1 1 1 1 2 1 1
104 103 96 95 96 92 94 92 91 94 105
± ± ± ± ± ± ± ± ± ± ±
2 1 2 2 1 1 1 3 2 2 3
93 ± 2 103 ± 3 100 ± 2
a
Samples were spiked with inorganic standard for the ME and with organic standard for the DX.
Table 5. Determination of Fe, Cu, Zn, Cr, and Al in Biodiesel by ME, DX, and AD Methods (n = 3) concentration (mg kg−1) analyte
sample
DX
ME
Fe
P01 P02 S01 ASE1 ASE2 ASE3 BSA FR01 FR02 FR03 FR04 GZE BS1 BS2 FR02 FR03 BS2 ASE1 ASE2 ASE3 BSC BD1 BD1 BSC ASE1 ASE2 ASE3 FR01 GZE
28.0 ± 1.1 30.4 ± 1.4 35.0 ± 0.9 6.5 ± 0.8 10.2 ± 0.7 8.0 ± 0.9 23.5 ± 1.0 27.3 ± 0.6 19.7 ± 0.5
