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Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption spectrometry
Accepted Manuscript Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption spectrometry
Clarice C. Leite, Alexandre de Jesus, Leandro Kolling, Marco F. Ferrão, Dimitrios Samios, Márcia M. Silva PII: DOI: Reference:
S0584-8547(17)30369-5 https://doi.org/10.1016/j.sab.2018.01.018 SAB 5369
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date: Revised date: Accepted date:
15 August 2017 26 January 2018 30 January 2018
Please cite this article as: Clarice C. Leite, Alexandre de Jesus, Leandro Kolling, Marco F. Ferrão, Dimitrios Samios, Márcia M. Silva , Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption spectrometry. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sab(2017), https://doi.org/10.1016/j.sab.2018.01.018
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ACCEPTED MANUSCRIPT
Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption
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spectrometry
Clarice C. Leitea, Alexandre de Jesusa, Leandro Kollinga, Marco F. Ferrãoa,b,
a
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Dimitrios Samiosa and Márcia M. Silva*a,c
Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto
Alegre, RS, Brazil
Instituto Nacional de Ciência e Tecnologia de Bioanalítica (INCT -
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b
Bioanalítica), C. Postal, 6154, Campinas, SP, Brazil. c
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Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energia e
Ambiente, Universidade Federal da Bahia, Salvador, BA, Brazil. E-mail: [email protected]
1
ACCEPTED MANUSCRIPT ABSTRACT This work reports a new method for extraction of Cu, Fe and Pb from Brazilian automotive gasoline and their determination by high-resolution continuous source flame atomic absorption spectrometry (HR-CS FAAS). The method was based on the formation of waterin-oil emulsion by mixing 2.0 mL of extraction solution constituted by 12% (w/v) Triton X100 and 5% (v/v) HNO3 with 10 mL of sample. After heating at 90 °C for 10 min, two welldefined phases were formed. The bottom phase (approximately 3.5 mL), composed of
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acidified water and part of the ethanol originally present in the gasoline sample, containing the extracted analytes was analyzed. The surfactant and HNO3 concentrations and the heating
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temperature employed in the process were optimized by Doehlert design, using a Brazilian
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gasoline sample spiked with Cu, Fe and Pb (organometallic compounds). The efficiency of extraction was investigated and it ranged from 80 to 89%. The calibration was accomplished by using
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matrix matching method. For this, the standards were obtained performing the same extraction procedure used for the sample, using emulsions obtained with a gasoline sample free of analytes and the addition of inorganic standards. Limits of detection obtained were 3.0,
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5.0 and 14.0 µg L-1 for Cu, Fe and Pb, respectively. These limits were estimated for the original sample taking into account the preconcentration factor obtained. The accuracy of the proposed method was assured by recovery tests spiking the samples with organometallic
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standards and the obtained values ranged from 98 to 105%. Ten gasoline samples were
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analyzed and Fe was found in four samples (0.04 - 0.35 mg L-1) while Cu (0.28 mg L-1) and Pb (0.60 mg L-1) was found in just one sample.
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Keywords: Gasoline; Trace metal determination; Extraction method; Extraction induced by
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emulsion breaking; HR-CS FAAS.
1. Introduction The presence of some metallic elements in gasoline is undesirable once they can cause several problems in different motor parts [1]. Pb, Cu and Fe can poison catalysts containing noble metals as Pt and Pd used in automobile engines and Fe and Cu can catalyze oxidation reactions of fuel favoring the formation of gum [2, 3, 4]. Although metal species are 2
ACCEPTED MANUSCRIPT present in trace concentrations, the burning of the fuel can lead to emission of significant quantities of metals into the atmosphere, becoming a serious environmental concern. Lead can be considered one of the most important element because Pb organic compounds were added in automotive gasoline for decades, as additives. Although the use of Pb is nowadays extinguished in almost all countries around the world, its content is still controlled. The Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP), establishes the maximum of 5.0 mg kg-1 of Pb in gasoline [5]. However, despite their importance in the
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quality control of the fuel and as potential environmental pollutants, other metals content is not controlled, either in Brazil or elsewhere.
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A variety of techniques have been used for the determination of metals and metalloids in gasoline. The most common techniques used for the determination of metals in fuels are
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inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP OES), flame atomic absorption spectrometry (FAAS) and
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graphite furnace atomic absorption spectrometry (GFAAS) [6,7]. High-resolution continuum source flame atomic absorption spectrometry, which was introduced commercially in 2003,
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was used for the determination of Pb in gasoline samples [8], but this technique probably could be a good alternative for determination of other elements. The xenon short-arc lamp, used as a continuum source, provides the possibility to determine many elements in a fast
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sequential mode when FAAS is used. The possibility of including part of the line wings in the
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measurement to enhance sensitivity and the improvement in the signal to noise ratio (S/N) and thus in the limit of detection (LOD) are among the important advantages brought about by this technique [9]. Application to other fuels, petroleum derivatives and biofuels analysis have
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been reported in literature [10,11,12,13]. The sample pre-treatment is probably the most critical step in the determination of
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metals in fuels and biofuels. It involves exposing the samples to procedures that make it more compatible with instrumentation available for analysis. However, excessive manipulation can lead to contamination of the samples or even losses of analytes, affecting the accuracy of measurements [1,7].
The direct analysis of fuel and biofuel samples for the determination of metals by FAAS is critical. General problems caused by the physicochemical proprieties of the gasoline such as volatility, flammability, and immiscibility with water and some specific technical problems have been reported [7,14,15]. In order to overcome the problems, different approaches may be considered as sample preparation for gasoline samples, such as: acid digestion [7], dilution with suitable solvents [7,8,16], conversion of samples in 3
ACCEPTED MANUSCRIPT emulsion/microemulsion [6,7,17], extraction/preconcentration of the analyte [1,7,18,19] and others. The use of emulsified systems (emulsion / microemulsion) as sample preparation allows homogeneous dispersion and stabilization of the samples in the aqueous phase, decreasing the sample viscosity and organic load. This procedure also make possible the use of inorganic standards for calibration [6,7,17]. On the other hand, extraction methods allow the separation of metals from fuel samples, avoiding possible interferences, and improve
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limits of detection (LOD) because of the preconcentration when the final extracted volume is lower than the initial sample volume [7,18]. The solid phase extraction (SPE) has been
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proposed in the literature for gasoline samples [1,18,19].
An extraction method induced by emulsion breaking (EIEB) has been proposed by
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Cassella et al. as sample preparation method for diesel oil samples [20, 21]. The process is based on the formation of a water-in-oil emulsion with surfactant and acid and subsequent
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breaking by heating or centrifugation, with formation of two or three phases: (i) organic phase containing the oil sample, (ii) aqueous phase containing acid and metals and/or trace elements
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extracted and (iii) surfactant-rich phase (depending on the type of oil used). Due to the greater affinity of the metal ions to the acidic aqueous phase, these are concentrated in this phase, resulting in a better detection limits and a solution that is compatible with various analytical
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techniques. This approach has been used for determination of metals in diesel oil [20, 21],
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lubricating oil [22], biodiesel [23], crude oil [24], edible oils [25, 26] and fish oil [27]. Bakircioglu et al. [25] have evaluated different procedures for the determination of metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in edible oils, such as: extraction induced by emulsion
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breaking, ultrasonic extraction and wet digestion. Considering the procedures evaluated, the EIEB showed the best results, being fast, reliable and simple, when compared with the other
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procedures. The EIEB method has not been applied for metals extraction from gasoline yet. Recently, Vicentino and Cassella [28] proposed a similar procedure based on microemulsion breaking (EIMB) for determination of Hg in gasoline. The phase separation was accomplished by water addition, which brought the destabilization of the microemulsion. The goal of this work was to develop a simple and fast extraction method based on emulsion breaking (EIEB) for the sequential determination of Cu, Fe and Pb in automotive gasoline samples by HR-CS FAAS. This approach was based on the formation and breaking of gasoline emulsions with a consequent extraction of the analytes from gasoline to the extracting phase. In order to obtain the best condition for sample preparation and extraction the multivariate optimization based on Doehlert design was used. 4
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2. Experimental section 2.1. Instrumentation A high resolution continuous source flame atomic absorption spectrometer, ContrAA model 300 (Analytik Jena AG, Germany) was used for all measurements and determinations.
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The equipment has a xenon short-arc lamp (which emits a continuum spectrum between 190 and 900 nm), operating at a hot-spot mode, and a double monochromator with high resolution
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linear array detector (charge-coupled device — CCD) with spectral resolution of 588 pixels
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and 1.2 pm per pixel. The main analytical lines (highest sensitivity) at 324.754 nm, 248.327 nm and 217.000 nm, were used for Cu, Fe and Pb, respectively. For all elements, atomic absorption was measured using the center pixel (CP) and the two adjacent pixels (CP ± 1). The
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determinations were performed in a sequential mode using an air−acetylene flame with a 50 mm burner and an optimized reading height of 5.0 mm for all analytes. The optimized
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acetylene flow rates were 50, 40 and 50 L h-1, for Cu, Fe and Pb, respectively. The air flow rate was 400 L h-1 for all elements. Sealing rings resistant to organic solvents, supplied by Analytik Jena AG, were used in the nebulizer chamber. For all elements, the aspiration rate
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was 3.4 mL min−1. All measurements were carried out in triplicates.
2.2. Reagents, Solutions, and Samples
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All reagents were of analytical grade. Distilled–deionized water with a specific resistivity of 18.2 MΩ cm at 25 °C from a Milli-Q water purification system (Millipore,
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Bedford, MA, USA), coupled to a water distiller (Fisatom, São Paulo, Brazil). All containers and glassware were soaked in 1.4 mol L-1 HNO3 for at least 24 hours and rinsed three times with distilled–deionized water before use. The HNO3 (Merck, Darmstadt, Germany) used for sample preparation was further purified by sub-boiling distillation in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim, Germany). Triton X-100 (octylphenol ethylene oxide condensate) from Acros Organics (St. Louis, USA) was used as surfactant. The multi-elementary inorganic working standards were prepared from serial dilutions of the inorganic stock solution of 1000 mg L-1 Specsol (Quimlab, Jacareí-SP, Brazil) to give concentrations in the range of 0.5 - 2.0 mg L-1 for Cu, 1.0 - 5.0 mg L-1 for Fe and 3.0 - 12 mg 5
ACCEPTED MANUSCRIPT L-1 for Pb in 0.014 mol L-1 HNO3. Organometallic standards of 1000 mg kg-1 Cu, Fe and Pb (Specsol) diluted with mineral base oil (Specsol) were used to prepare the intermediate solutions of 50 mg kg-1 Cu, 100 mg kg-1 Fe and 300 mg kg-1 of Pb used to spike the samples Automotive gasoline samples from Porto Alegre city - Brazil (GB1, GB2, GB3, GB4, GB5, GB6 and GB7) and from Ciudad del Leste - Paraguay (GP1, GP2 and GP3), were analyzed The samples were stored in high-density polyethylene bottles, previously decontaminated with 3.0 mol L−1 HNO3, and kept under refrigeration. The anhydrous ethanol content,
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determined in gasoline samples from Brazil and Paraguay, ranged from 24.8% (v/v) to 25%
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(v/v) for all samples.
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2.3. Analytical Procedure
The optimization of the experimental conditions of the spectrometer for sequential
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analysis was performed with the extracted phase of a gasoline sample free of metals (GB2), spiked with organometallic standards of each analyte to obtain the final concentration of 1.0, 1.8 and 3.5 mg L-1 of Cu, Fe and Pb, respectively. The aspiration rate adjustment was
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performed manually, observing the intensity of the analytical signal of one element at time. The best signal was obtained at 3.5 mL min-1 for Cu and s 3.4 mL min-1 for Fe and Pb. Thus, aspiration rate was adjusted to approximately 3.4 mL min-1. The sequential measurements of
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the three elements in blank, standards, and samples were carried out according to the
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following sequence: Fe, Pb and Cu. This sequence was optimized 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.
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Recovery experiments were performed to evaluate the matrix effect and accuracy of the proposed method. In these experiments, the samples GB6 and GB7 were spiked with each
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analyte (organometallic standards) to obtain the final concentration of 1.0, 2.0 and 6.0 mg L-1 of Cu, Fe and Pb, respectively. All samples and spiked samples were prepared in triplicate and all measurements were conducted under optimized conditions.
2.3.1 General procedure of extraction based on emulsion breaking The extraction method investigated in this work was based on the EIEB method proposed by Cassella et al. [20,21]. The emulsion was obtained with vigorous manual mixing of 10 mL of gasoline with 2.0 mL of aqueous solution containing HNO3 and Triton X-100 6
ACCEPTED MANUSCRIPT (emulsifying agent) in glass tubes of 15 mL. After the emulsion formation, the tube was transferred to a temperature-controlled bath and maintained 90 ± 2 °C for a time varying from 10 to 15 min. After the heating time, the emulsion was completely broken forming two separate phases: (i) top organic phase, (ii) lower acid aqueous phase containing the extracted metals (extract) and part of the ethanol originally present in the gasoline (approximately 3.5 mL). The organic phase was removed with Pasteur pipette and only the extract was analyzed. The multivariate optimization of the extraction procedure was carried out considering
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three variables: the concentration of the HNO3, concentration of surfactant Triton X-100 and the temperature of heating. All experiments were performed with the sample GB2 spiked with
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organometallic standards. A model for evaluation of the system was constructed, assessing the signal obtained (absorbance), used as response. The data obtained in the experiment were
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processed with Doehlert 1.0 program developed by Teófilo and Ferreira [29] at the Chemistry Institute of Universidade Estadual de Campinas (Campinas, SP, Brazil). This program can be
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used online [30]. Three factors were evaluated for optimization of the method of response surface based on Doehlert design. This was applied directly on optimizing the level of the
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variables. The experimental variance was estimated by the central point of the repetition calculation in the model as individual value.
The matrix matching calibration curves were prepared using a gasoline sample (GB2)
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free of analytes (concentration < LOD) to which the mentioned concentrations of inorganic
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standards were added as described in the item 2.2 and the extraction procedure described above was performed. For comparison, external aqueous calibration curves were constructed (with inorganic standard) for all three analytes at the same concentrations mentioned in the
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item 2.2.
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3. Results and discussion The main problem related to the application of liquid-liquid extraction methods for the analysis of oily samples is the low interaction between the two phases (oily sample and aqueous extracting solution), which can decrease the extraction efficiency. The EIEB procedure can improve the performance of the extraction as a surfactant is added to the extracting solution in order to promote sufficient mixing between the two phases through the formation of an emulsion [20, 28]. The use of EIEB permitted a convenient preconcentration of the analytes in the aqueous solution and avoids the use of large amounts of solvents or acids. 7
ACCEPTED MANUSCRIPT The extraction method investigated in this work for determination of Cu, Fe and Pb in automotive gasoline samples was based on the EIEB method proposed by Cassella et al. [20,21] for diesel oil samples. For the formation of the gasoline emulsions, an aqueous phase containing HNO3 and an emulsifying agent (Triton X-100) was used. The presence of HNO3 in the emulsion has an important role in the proposed method since it is responsible for the extraction of metals, as the H+ ion displaces the metallic cations from the organic structures where they are in the oily samples [22]. The emulsifying agent Triton X-100 has been used in
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the most of emulsion/microemulsion systems reported in the literature for fuel/biofuel analysis [6, 17]. Cassella and coworkers [20-23] have investigated the use of Triton X-114
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and Triton X-100 in the EIEB systems. These authors choose the former because in this case the emulsions were broken in a reduced time in comparison with those prepared with Triton
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X-100, indicating that the emulsions formed with Triton X-114 are less stable and more suitable for the procedure. The emulsion obtained in this work for automotive gasoline with
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Triton X-100, unlike that obtained by Cassella et al. [20,21], was not stable, generating phase separation in less than one minute, possibly due to the high amount of ethanol present in the
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Brazilian gasoline. Even though, this composition (HNO3 and Triton X-100) provided a satisfactory extraction efficiency when the system was heat for a short period of time. After phase separation, approximately 3.5 mL of the extract was collected and used for analysis by
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HR-CS FAAS. The increase in the volume of the aqueous phase from 2.0 to 3.5 mL was due
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to the transference of the part of ethanol content from gasoline to the aqueous phase. As discussed, the temperature was not necessary to break the emulsion. However, without heating, the extraction in a short period of time (up to 15 min) was not efficient. In order to
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develop a fast method, the time used for heating was 10 min. Thus, three variables (HNO3 concentration, Triton X-100 concentration and temperature) were evaluated for multivariate
breaking.
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optimization in order to find out the best conditions for the extraction based on emulsion
3.1 Multivariate optimization of extraction method for determination of Cu, Fe and Pb in gasoline The Doehlert design is easily applicable to the optimization variables requiring few experiments. Basically, this optimization process involves three main steps: performing statistically designed experiments, by estimating the coefficients of a mathematical model; predicting the response and check the suitability of the model. The groups tested were: 5 to 15% (v/v) HNO3 solution; 3.0 to 13% (w/v) of Triton X-100 and temperatures from 70 to 90 8
ACCEPTED MANUSCRIPT °C. The response was the absorbance signal obtained in each experiment. The numbered experiments and the response obtained for each experiment are shown in Table S1 (Appendix). Fifteen experiments were carried out for planning and performed in a random order to avoid any bias in the measurements. Data were modeled using the spreadsheet Doehlert developed by Teófilo and Ferreira [29] to determine the effect of each variable in the response. The values of the variables employed were coded to eliminate the effect of different magnitudes on the evaluation.
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The normal probability graphics for Cu, Fe and Pb are shown in Fig. S1 (Appendix). The first order effects are: (1) temperature, (2) HNO3 and (3) Triton X-100. All effects that
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deviate from zero are the most significant. The normal probability graphics for Cu (Fig. S1.A) and Fe (Fig. S1.B) were similar, where the positive effects of second order for HNO3 (22) and
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interactions between HNO3 and Triton X-100 (23) were highlighted. A negative effect of first order for HNO3 (2) was also observed for both elements, suggesting that this reagent should
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be used in lower concentrations. For Pb (Fig. S1.C), besides the positive effect of second order for HNO3 (22), a positive effect of first order was observed for Triton X-100 (3),
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suggesting the use of this reagent in higher concentrations in order to improve the signal for this element. Moreover, a significant negative effect of second order for temperature and Triton X-100 (13) was highlighted, indicating an antagonistic effect between these reagents,
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suggesting that an adequate condition can be achieved with opposite levels of them.
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The response surfaces for the experiments performed for multivariate optimization based on Doehlert design for extraction of Cu, Fe and Pb from gasoline are shown in Fig. S2 (Appendix). A similar behavior for Cu and Fe was observed, showing that the HNO3
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concentration in the investigated range (3% to 15% v/v) do not affect the response, while the increase in the temperature affects significantly. For Pb a slight effect can be observed for
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lower HNO3 concentrations. Cassella et al. [20] investigated the concentration of HNO3 in the extracting solution in the range of 2% to 20% (v/v) and they reached to the conclusion that the acid concentration of 2% (v/v) was already enough to promote metals migration from the oil to water phase. They also showed that higher concentrations reduced the time for emulsion breaking and suggested that this behavior can be credited to the effect of the ionic concentration on the stability of the emulsion. These authors adopted the concentration of 10% (v/v) of HNO3 in order to decrease the time for emulsion breaking. Nevertheless, in our work even using the lower concentration of HNO3 investigated, the emulsion breaking was very fast, not requiring higher concentrations, that could destabilize even more the emulsion, impairing the extraction performance for some elements. Regarding the concentration of 9
ACCEPTED MANUSCRIPT Triton X-100, higher concentrations should improve the emulsion formation and the interaction between the phases, enhancing the extraction of more stable organic molecules. The results obtained in this work for Pb suggested that this element required a little more stable emulsion for appropriate extraction. Thus, the lower concentration of HNO3 and high concentration of Triton X-100 was adopted. Based on the results obtained in both normal probability (Fig. S1) and response surface (Fig. S2) graphics, the following compromise conditions were adopted: 5% (v/v)
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HNO3 and 12% (w/v) Triton X-100 in 2.0 mL of extracting solution; the heating temperature
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of 90 °C for 10 minutes.
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3.2 Evaluation of extraction efficiency
After establishing the optimized conditions for extraction, the efficiency of the proposed
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procedure for metals extraction from gasoline was investigated. In this experiment, a gasoline sample (GB2) spiked with organometallic standards was subjected to EIEB procedure and the analytical signal was measured for each analyte in the extracted phase. The oily phase
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obtained after the emulsion breaking was separated and again subjected to the same procedure. The new extracted phase obtained after emulsion breaking was also analyzed. Three successive extractions were carried out with the same spiked sample in triplicate.
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The obtained results showed that the percentage of transfer of metals from the gasoline
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to the extract in the first extraction were 80% for Cu, 83% for Fe and 89% for Pb. Even considering that there was a small residual amount in the second extraction, the reproducibility of the first extraction (RSD between 5 - 7%) as well as the efficiency of
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extraction were satisfactory. As investigated by Cassella and coworkers, quantitative extraction was obtained only with 60 min of extraction time for Cu, Mn and Ni from biodiesel
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[23] and 30 min for Cu, Fe and Mn from used lubricating oils [22]. However, quantitative extraction for Cu, Fe, Ni and Pb from diesel oil was obtained in 15 min [20]. In this work, the increase of time from 10 to 15 min did not enhance the extraction efficiency. Higher times were not investigated as the intention was to propose a simple and fast procedure.
3.3 Figures of merit
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ACCEPTED MANUSCRIPT One of the main problems in the analysis of fuels is the correct choice of calibration strategy. When nebulization is used for sample introduction, the composition of the extract and the calibration solutions must be the same. Thus, in order to prevent any physical interference caused by the content of alcohol and/or surfactant in the extract, the matrixmatching calibration using the analytical curve prepared with the extract obtained from a gasoline sample free of analytes was performed. This calibration method also prevents errors due to the fact that the efficiency of the extraction of the analytes from the gasoline samples
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was not 100%. For this, the standards were obtained performing the same extraction procedure used for the sample, using emulsions obtained with a gasoline sample free of
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analytes and the addition of inorganic standards of the analytes. The same calibration strategy was used by Vicentino and Cassella [28] for Hg determination in Brazilan gasoline samples
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using EIMB.
The figures of merit obtained using calibration curves prepared with inorganic
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standards in water (external calibration) and by the emulsion breaking extraction (matrixmatching) are shown in Table 1. The limit of detection (LOD) and limit of quantification
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(LOQ) were calculated according to International Union of Pure and Applied Chemistry (IUPAC) recommendations, as 3 times and 10 times, respectively, the standard deviation of 10 measurements of a blank, divided by the slope of the calibration curve. The LOD and LOQ
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of the proposed method were estimated for the original sample (volume of 10 mL), thus
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taking into account the preconcentration factor. The characteristic concentration (C0) defined as the analyte concentration corresponding to an integrated absorbance of 0.0044 (1% of absorption) [31].
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As can be seen, all curves presented determination coefficients > 0.99 demonstrating adequate linearity. As expected, the sensitivity obtained with calibration curves without
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extraction and after extraction by emulsion breaking are different, due to the preconcentration factor (PF) achieved with the extraction. The PF is the most widely used criterion for evaluation of preconcentration systems and were calculated as the ratio between the angular coefficient of the linear part of the calibration curve without preconcentration and the calibration curve obtained using the preconcentration procedure [32]. The obtained PF shown in Table 1 were close to the expected PF of 2.86, considering the initial sample volume (10 mL) and the final volume of extract (3.5 mL). The PF values obtained in this work were lower than those found by Cassella et al. for the determination of Cu, Fe, Ni and Pb in diesel oil by GFAAS [20] and Zn in diesel oil by FAAS [21] using the same method used in this study, but 11
ACCEPTED MANUSCRIPT it should be take into account that these authors obtained a final volume of 2.0 mL of extracted solution for 10 mL of sample.
Table 1. Figures of merit obtained for the determination of Cu, Fe and Pb by HR-CS FAAS Linear regression
R2
equation
Externalb
y = 0.1429x – 0.0040
Matrix-matchingc
-1
-1
PFa
(µg L )
(µg L )
0.9965
32
8.2
27
-
y = 0.3470x -0.0020
0.9972
12
3.2
11
2.4
External
y = 0.0549x – 0.0020
0.9987
75
12
42
-
Matrix-matching
y = 0.1359x + 0.0020
0.9934
37
5.2
17
2.5
External
y = 0.0303x – 0.0054
0.9971
148
30
99
-
Matrix-matching
y = 0.0610x – 0.0070
74
14
48
2.0
Pb b
performed with inorganic standards in 0.014 mol L-1 ;
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PF = preconcentration factor;
0.9980
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Fe
c
-1
LOQ
(µg L )
Cu
a
LOD
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calibration
Co
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Method of
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Analyte
performed with the extract obtained from a gasoline sample free of analytes spiked with
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inorganic standards.
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As there are no studies in the literature that relate extraction based on emulsion breaking for Cu, Fe and Pb from gasoline the LOD obtained in this work was compared with those obtained by alternative methods from the literature using FAAS technique. Similar
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results were found in the literature for Cu [19] and Fe [18,19] using preconcentration by SPE methods. However, lower results for Pb [19] and Cu [1,18] were also reported for the same
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technique. Kowalewska et al. [8] found a similar LOD for Pb for direct analysis of gasoline by HR-CS FAAS. Nevertheless, even if the LODs and LOQS found in this work are slightly higher than some methods described in the literature, the values are in the µg L-1 level for all elements and are smaller than the maximum levels stipulated for Pb in the legislation of both countries (Brazil: 5.0 mg L-1; Paraguay: 13 mg L-1). The precision of the method was estimated from the measurement of metals, applying the procedure for three separate aliquots of the same sample, containing the amount of 1.0, 1.8 and 3.5 mg L-1 of Cu, Fe and Pb, respectively. The relative standard deviation for Cu, Fe and Pb were 4.7; 3.5 and 5.0%, respectively, indicating that the developed method has good 12
ACCEPTED MANUSCRIPT precision. Finally, it should be pointed out that this method was optimized for the Brazilian automotive gasoline containing 25% of ethanol. Considering that the content of ethanol influences the final volume of the extract, some adjustments / optimizations should be necessary when applying this method to other gasoline samples with different compositions.
3.4 Determination of Cu, Fe and Pb in gasoline samples by HR-CS FAAS using extraction
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based on emulsion breaking as sample preparation and recovery tests
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The proposed method has been applied for the multi-element sequential determination of Cu, Fe and Pb in ten gasoline samples (from Brazil and Paraguay). The samples GB2, GB3,
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GB4, GB7, GP1 and GP3 did not present concentration above the limit of quantification for any investigated elements. The concentrations of Cu, Fe and/or Pb found in four gasoline
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samples are showed in Table 2. As can be seen, concentrations of Pb were below the limit established by the legislation of both countries. It should be pointed out that this method could
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be applied for the Paraguayan automotive gasoline, since these samples presented the same composition of the Brazilian automotive gasoline (25% v/v of ethanol). In order to attest the accuracy of the method two samples (GB6 and GB7) were spiked
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with different concentrations of the organometallic standards (1.0, 2.0 and 6.0 mg L-1 of Cu,
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Fe and Pb, respectively)). The recovery values (Table S2 – Appendix) obtained through the matrix matching calibration ranged from 98 to 105%, for all analytes and confirmed the absence of the matrix effects. These results were satisfactory, confirming the accuracy of the
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proposed method and showing that it can be successfully used in routine analysis.
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Table 2. Determination of Cu, Fe and Pb in gasoline samples by HR-CS FAAS using emulsion breaking as sample preparation. The concentrations are expressed as mean ± standard deviation (n=3). Sample
Cu (mg L-1)
Fe (mg L-1)
Pb (mg L-1)
GB1
LOD
0.044 ± 0.001
LOD
GB5
0.283 ± 0.001
0.056 ± 0.005
LOD
GB6
LOD
0.350 ± 0.001
LOD
GP2
LOD
0.208 ± 0.001
0.60 ± 0.01 13
ACCEPTED MANUSCRIPT 4. Conclusion
The application of extraction based on emulsion breaking allowed preconcentration of analytes in the extract, leading to an appropriate improvement in detectability, allowing the determination of the analytes in mg L-1 level by HR-CS FAAS. Multivariate optimization based on Doehlert design was efficient for establishing the optimal conditions for the procedure. The extraction method proposed in this work allied with sequential determination
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by HR-CS FAAS showed to be simple, fast and thus suitable for the routine determination of
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Cu, Fe and Pb in Brazilian automotive gasolines and others with the same composition.
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Acknowledgments
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This work was supported by the Conselho Nacional de Desenvolvimento Cientifíco e Tecnológico (CNPq; Edital 40/2013 grant no. 405011/2013-0 and Universal 2012 grant no. 478998/2012-0). M.M.S., D.S. and L.K. have research scholarships from CNPq (grant nos.
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307815/2016-1 (M.M.S.), 305331/2013-2 (D.S.)) and A.J. has a scholarship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).
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'Appendix A. Supplementary data'
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determination of copper in gasoline by flame atomic absorption spectrometry after sorption and preconcentration on Moringa oleifera husks, Microchem. J. 110 (2013) 320 - 325.
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ACCEPTED MANUSCRIPT [10] C.C. Leite, A. Jesus, M.L Potes, M.A. Vieira, D. Samios, M.M. Silva, 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, Energy Fuels 29 (2015) 7358 7363. [11] J.A.G. Neto, V.R.A. Filho, Evaluation of lubricating oil preparation procedures for the determination of Al, Ba, Mo, Si and V by high-resolution continuum source FAAS -Anal. Sci.
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ACCEPTED MANUSCRIPT [20] R.J. Cassella, D.M. Brum, C.E.R. De Paula, C.F. Lima, Extraction induced by emulsion breaking: a novel strategy for the trace metals determination in diesel oil samples by electrothermal atomic absorption spectrometry. J. Anal. At. Spectrom., 25 (2010) 1704–1711. [21] R.J. Cassella, D.M. Brum, C.F. Lima, L.F.S. Caldas, C.E.R. De Paula, Multivariate optimization of the determination of zinc in diesel oil employing a novel extraction strategy based on emulsion breaking, Anal. Chim. Acta 690 (2011) 79-85.
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biodiesel by electrothermal atomic absorption spectrometry, Talanta 117 (2013) 32-38. [24] A.M. Trevelin, R.E.S. Marotto, E.V.R. Castro, G.P. Brandão, R.J. Cassella, M.T.W.D.
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emulsion breaking, ultrasonic extraction and wet digestion procedures for determination of metals in edible oil samples in Turkey using ICP-OES, Food Chem. 138 (2013) 770 – 775. [26] N.F. Robaina, D.M. Brum, R.J. Cassella, Application of the extraction induced by
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electrothermal atomic absorption spectrometry, Talanta 99 (2012) 104-112. [27] M.Z. Corazza, C.R.T. Tarley, Development and feasibility of emulsion breaking method for the extraction of cadmium from omega-3 dietary supplements and determination by flow injection TS-FF-AAS, Microchem. J 127 (2016) 145-151. [28] P.O. Vicentino, R.J. Cassella, Novel extraction induced by microemulsion breaking: a model study for Hg extraction from Brazilian gasoline, Talanta 162 (2017) 249-255. [29] R.F. Teófilo, M.M.C. Ferreira, Quimiometria II: planilhas eletrônicas para cálculos de planejamentos experimentais, um tutorial, Quim. Nova 29 (2006) 338-350. 17
ACCEPTED MANUSCRIPT [30] Laboratório de Quimiometria Teórica e Aplicada, Instituto de Química, Universidade Estadual de Campinas. Planilhas Eletrônicas Cálculos de Planejamentos Experimentais; http://lqta.iqm.unicamp.br/portugues/Downloads.html#teofilo (accessed 02.06.2017). [31] B. Welz, M. Sperling, Atomic Absorption Spectrometry, 3rd ed. Wiley-VCH, Weinhein,
Germany, 1999. [32] J. Ruzicka, G.D. Cristia, Flow injection analysis and chromatography: twins or siblings?,
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Highlights
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- A new extraction method is proposed for metal determination in Brazilian gasoline.
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- The extraction is based on the formation and breaking of gasoline emulsions.
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- The proposed method allowed preconcentration of analytes in the aqueous phase
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- The emulsion breaking extraction method was optimized using Doehlert design.
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- The method was simple, fast, accurate and required a minimum sample manipulation.
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Clarice C. Leite, Alexandre de Jesus, Leandro Kolling, Marco F. Ferrão, Dimitrios Samios, Márcia M. Silva PII: DOI: Reference:
S0584-8547(17)30369-5 https://doi.org/10.1016/j.sab.2018.01.018 SAB 5369
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date: Revised date: Accepted date:
15 August 2017 26 January 2018 30 January 2018
Please cite this article as: Clarice C. Leite, Alexandre de Jesus, Leandro Kolling, Marco F. Ferrão, Dimitrios Samios, Márcia M. Silva , Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption spectrometry. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sab(2017), https://doi.org/10.1016/j.sab.2018.01.018
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Extraction method based on emulsion breaking for the determination of Cu, Fe and Pb in Brazilian automotive gasoline samples by high-resolution continuum source flame atomic absorption
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spectrometry
Clarice C. Leitea, Alexandre de Jesusa, Leandro Kollinga, Marco F. Ferrãoa,b,
a
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Dimitrios Samiosa and Márcia M. Silva*a,c
Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto
Alegre, RS, Brazil
Instituto Nacional de Ciência e Tecnologia de Bioanalítica (INCT -
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b
Bioanalítica), C. Postal, 6154, Campinas, SP, Brazil. c
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Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energia e
Ambiente, Universidade Federal da Bahia, Salvador, BA, Brazil. E-mail: [email protected]
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ACCEPTED MANUSCRIPT ABSTRACT This work reports a new method for extraction of Cu, Fe and Pb from Brazilian automotive gasoline and their determination by high-resolution continuous source flame atomic absorption spectrometry (HR-CS FAAS). The method was based on the formation of waterin-oil emulsion by mixing 2.0 mL of extraction solution constituted by 12% (w/v) Triton X100 and 5% (v/v) HNO3 with 10 mL of sample. After heating at 90 °C for 10 min, two welldefined phases were formed. The bottom phase (approximately 3.5 mL), composed of
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acidified water and part of the ethanol originally present in the gasoline sample, containing the extracted analytes was analyzed. The surfactant and HNO3 concentrations and the heating
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temperature employed in the process were optimized by Doehlert design, using a Brazilian
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gasoline sample spiked with Cu, Fe and Pb (organometallic compounds). The efficiency of extraction was investigated and it ranged from 80 to 89%. The calibration was accomplished by using
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matrix matching method. For this, the standards were obtained performing the same extraction procedure used for the sample, using emulsions obtained with a gasoline sample free of analytes and the addition of inorganic standards. Limits of detection obtained were 3.0,
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5.0 and 14.0 µg L-1 for Cu, Fe and Pb, respectively. These limits were estimated for the original sample taking into account the preconcentration factor obtained. The accuracy of the proposed method was assured by recovery tests spiking the samples with organometallic
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standards and the obtained values ranged from 98 to 105%. Ten gasoline samples were
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analyzed and Fe was found in four samples (0.04 - 0.35 mg L-1) while Cu (0.28 mg L-1) and Pb (0.60 mg L-1) was found in just one sample.
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Keywords: Gasoline; Trace metal determination; Extraction method; Extraction induced by
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emulsion breaking; HR-CS FAAS.
1. Introduction The presence of some metallic elements in gasoline is undesirable once they can cause several problems in different motor parts [1]. Pb, Cu and Fe can poison catalysts containing noble metals as Pt and Pd used in automobile engines and Fe and Cu can catalyze oxidation reactions of fuel favoring the formation of gum [2, 3, 4]. Although metal species are 2
ACCEPTED MANUSCRIPT present in trace concentrations, the burning of the fuel can lead to emission of significant quantities of metals into the atmosphere, becoming a serious environmental concern. Lead can be considered one of the most important element because Pb organic compounds were added in automotive gasoline for decades, as additives. Although the use of Pb is nowadays extinguished in almost all countries around the world, its content is still controlled. The Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP), establishes the maximum of 5.0 mg kg-1 of Pb in gasoline [5]. However, despite their importance in the
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quality control of the fuel and as potential environmental pollutants, other metals content is not controlled, either in Brazil or elsewhere.
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A variety of techniques have been used for the determination of metals and metalloids in gasoline. The most common techniques used for the determination of metals in fuels are
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inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP OES), flame atomic absorption spectrometry (FAAS) and
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graphite furnace atomic absorption spectrometry (GFAAS) [6,7]. High-resolution continuum source flame atomic absorption spectrometry, which was introduced commercially in 2003,
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was used for the determination of Pb in gasoline samples [8], but this technique probably could be a good alternative for determination of other elements. The xenon short-arc lamp, used as a continuum source, provides the possibility to determine many elements in a fast
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sequential mode when FAAS is used. The possibility of including part of the line wings in the
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measurement to enhance sensitivity and the improvement in the signal to noise ratio (S/N) and thus in the limit of detection (LOD) are among the important advantages brought about by this technique [9]. Application to other fuels, petroleum derivatives and biofuels analysis have
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been reported in literature [10,11,12,13]. The sample pre-treatment is probably the most critical step in the determination of
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metals in fuels and biofuels. It involves exposing the samples to procedures that make it more compatible with instrumentation available for analysis. However, excessive manipulation can lead to contamination of the samples or even losses of analytes, affecting the accuracy of measurements [1,7].
The direct analysis of fuel and biofuel samples for the determination of metals by FAAS is critical. General problems caused by the physicochemical proprieties of the gasoline such as volatility, flammability, and immiscibility with water and some specific technical problems have been reported [7,14,15]. In order to overcome the problems, different approaches may be considered as sample preparation for gasoline samples, such as: acid digestion [7], dilution with suitable solvents [7,8,16], conversion of samples in 3
ACCEPTED MANUSCRIPT emulsion/microemulsion [6,7,17], extraction/preconcentration of the analyte [1,7,18,19] and others. The use of emulsified systems (emulsion / microemulsion) as sample preparation allows homogeneous dispersion and stabilization of the samples in the aqueous phase, decreasing the sample viscosity and organic load. This procedure also make possible the use of inorganic standards for calibration [6,7,17]. On the other hand, extraction methods allow the separation of metals from fuel samples, avoiding possible interferences, and improve
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limits of detection (LOD) because of the preconcentration when the final extracted volume is lower than the initial sample volume [7,18]. The solid phase extraction (SPE) has been
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proposed in the literature for gasoline samples [1,18,19].
An extraction method induced by emulsion breaking (EIEB) has been proposed by
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Cassella et al. as sample preparation method for diesel oil samples [20, 21]. The process is based on the formation of a water-in-oil emulsion with surfactant and acid and subsequent
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breaking by heating or centrifugation, with formation of two or three phases: (i) organic phase containing the oil sample, (ii) aqueous phase containing acid and metals and/or trace elements
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extracted and (iii) surfactant-rich phase (depending on the type of oil used). Due to the greater affinity of the metal ions to the acidic aqueous phase, these are concentrated in this phase, resulting in a better detection limits and a solution that is compatible with various analytical
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techniques. This approach has been used for determination of metals in diesel oil [20, 21],
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lubricating oil [22], biodiesel [23], crude oil [24], edible oils [25, 26] and fish oil [27]. Bakircioglu et al. [25] have evaluated different procedures for the determination of metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in edible oils, such as: extraction induced by emulsion
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breaking, ultrasonic extraction and wet digestion. Considering the procedures evaluated, the EIEB showed the best results, being fast, reliable and simple, when compared with the other
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procedures. The EIEB method has not been applied for metals extraction from gasoline yet. Recently, Vicentino and Cassella [28] proposed a similar procedure based on microemulsion breaking (EIMB) for determination of Hg in gasoline. The phase separation was accomplished by water addition, which brought the destabilization of the microemulsion. The goal of this work was to develop a simple and fast extraction method based on emulsion breaking (EIEB) for the sequential determination of Cu, Fe and Pb in automotive gasoline samples by HR-CS FAAS. This approach was based on the formation and breaking of gasoline emulsions with a consequent extraction of the analytes from gasoline to the extracting phase. In order to obtain the best condition for sample preparation and extraction the multivariate optimization based on Doehlert design was used. 4
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2. Experimental section 2.1. Instrumentation A high resolution continuous source flame atomic absorption spectrometer, ContrAA model 300 (Analytik Jena AG, Germany) was used for all measurements and determinations.
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The equipment has a xenon short-arc lamp (which emits a continuum spectrum between 190 and 900 nm), operating at a hot-spot mode, and a double monochromator with high resolution
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linear array detector (charge-coupled device — CCD) with spectral resolution of 588 pixels
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and 1.2 pm per pixel. The main analytical lines (highest sensitivity) at 324.754 nm, 248.327 nm and 217.000 nm, were used for Cu, Fe and Pb, respectively. For all elements, atomic absorption was measured using the center pixel (CP) and the two adjacent pixels (CP ± 1). The
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determinations were performed in a sequential mode using an air−acetylene flame with a 50 mm burner and an optimized reading height of 5.0 mm for all analytes. The optimized
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acetylene flow rates were 50, 40 and 50 L h-1, for Cu, Fe and Pb, respectively. The air flow rate was 400 L h-1 for all elements. Sealing rings resistant to organic solvents, supplied by Analytik Jena AG, were used in the nebulizer chamber. For all elements, the aspiration rate
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was 3.4 mL min−1. All measurements were carried out in triplicates.
2.2. Reagents, Solutions, and Samples
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All reagents were of analytical grade. Distilled–deionized water with a specific resistivity of 18.2 MΩ cm at 25 °C from a Milli-Q water purification system (Millipore,
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Bedford, MA, USA), coupled to a water distiller (Fisatom, São Paulo, Brazil). All containers and glassware were soaked in 1.4 mol L-1 HNO3 for at least 24 hours and rinsed three times with distilled–deionized water before use. The HNO3 (Merck, Darmstadt, Germany) used for sample preparation was further purified by sub-boiling distillation in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim, Germany). Triton X-100 (octylphenol ethylene oxide condensate) from Acros Organics (St. Louis, USA) was used as surfactant. The multi-elementary inorganic working standards were prepared from serial dilutions of the inorganic stock solution of 1000 mg L-1 Specsol (Quimlab, Jacareí-SP, Brazil) to give concentrations in the range of 0.5 - 2.0 mg L-1 for Cu, 1.0 - 5.0 mg L-1 for Fe and 3.0 - 12 mg 5
ACCEPTED MANUSCRIPT L-1 for Pb in 0.014 mol L-1 HNO3. Organometallic standards of 1000 mg kg-1 Cu, Fe and Pb (Specsol) diluted with mineral base oil (Specsol) were used to prepare the intermediate solutions of 50 mg kg-1 Cu, 100 mg kg-1 Fe and 300 mg kg-1 of Pb used to spike the samples Automotive gasoline samples from Porto Alegre city - Brazil (GB1, GB2, GB3, GB4, GB5, GB6 and GB7) and from Ciudad del Leste - Paraguay (GP1, GP2 and GP3), were analyzed The samples were stored in high-density polyethylene bottles, previously decontaminated with 3.0 mol L−1 HNO3, and kept under refrigeration. The anhydrous ethanol content,
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determined in gasoline samples from Brazil and Paraguay, ranged from 24.8% (v/v) to 25%
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(v/v) for all samples.
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2.3. Analytical Procedure
The optimization of the experimental conditions of the spectrometer for sequential
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analysis was performed with the extracted phase of a gasoline sample free of metals (GB2), spiked with organometallic standards of each analyte to obtain the final concentration of 1.0, 1.8 and 3.5 mg L-1 of Cu, Fe and Pb, respectively. The aspiration rate adjustment was
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performed manually, observing the intensity of the analytical signal of one element at time. The best signal was obtained at 3.5 mL min-1 for Cu and s 3.4 mL min-1 for Fe and Pb. Thus, aspiration rate was adjusted to approximately 3.4 mL min-1. The sequential measurements of
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the three elements in blank, standards, and samples were carried out according to the
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following sequence: Fe, Pb and Cu. This sequence was optimized 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.
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Recovery experiments were performed to evaluate the matrix effect and accuracy of the proposed method. In these experiments, the samples GB6 and GB7 were spiked with each
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analyte (organometallic standards) to obtain the final concentration of 1.0, 2.0 and 6.0 mg L-1 of Cu, Fe and Pb, respectively. All samples and spiked samples were prepared in triplicate and all measurements were conducted under optimized conditions.
2.3.1 General procedure of extraction based on emulsion breaking The extraction method investigated in this work was based on the EIEB method proposed by Cassella et al. [20,21]. The emulsion was obtained with vigorous manual mixing of 10 mL of gasoline with 2.0 mL of aqueous solution containing HNO3 and Triton X-100 6
ACCEPTED MANUSCRIPT (emulsifying agent) in glass tubes of 15 mL. After the emulsion formation, the tube was transferred to a temperature-controlled bath and maintained 90 ± 2 °C for a time varying from 10 to 15 min. After the heating time, the emulsion was completely broken forming two separate phases: (i) top organic phase, (ii) lower acid aqueous phase containing the extracted metals (extract) and part of the ethanol originally present in the gasoline (approximately 3.5 mL). The organic phase was removed with Pasteur pipette and only the extract was analyzed. The multivariate optimization of the extraction procedure was carried out considering
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three variables: the concentration of the HNO3, concentration of surfactant Triton X-100 and the temperature of heating. All experiments were performed with the sample GB2 spiked with
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organometallic standards. A model for evaluation of the system was constructed, assessing the signal obtained (absorbance), used as response. The data obtained in the experiment were
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processed with Doehlert 1.0 program developed by Teófilo and Ferreira [29] at the Chemistry Institute of Universidade Estadual de Campinas (Campinas, SP, Brazil). This program can be
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used online [30]. Three factors were evaluated for optimization of the method of response surface based on Doehlert design. This was applied directly on optimizing the level of the
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variables. The experimental variance was estimated by the central point of the repetition calculation in the model as individual value.
The matrix matching calibration curves were prepared using a gasoline sample (GB2)
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free of analytes (concentration < LOD) to which the mentioned concentrations of inorganic
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standards were added as described in the item 2.2 and the extraction procedure described above was performed. For comparison, external aqueous calibration curves were constructed (with inorganic standard) for all three analytes at the same concentrations mentioned in the
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item 2.2.
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3. Results and discussion The main problem related to the application of liquid-liquid extraction methods for the analysis of oily samples is the low interaction between the two phases (oily sample and aqueous extracting solution), which can decrease the extraction efficiency. The EIEB procedure can improve the performance of the extraction as a surfactant is added to the extracting solution in order to promote sufficient mixing between the two phases through the formation of an emulsion [20, 28]. The use of EIEB permitted a convenient preconcentration of the analytes in the aqueous solution and avoids the use of large amounts of solvents or acids. 7
ACCEPTED MANUSCRIPT The extraction method investigated in this work for determination of Cu, Fe and Pb in automotive gasoline samples was based on the EIEB method proposed by Cassella et al. [20,21] for diesel oil samples. For the formation of the gasoline emulsions, an aqueous phase containing HNO3 and an emulsifying agent (Triton X-100) was used. The presence of HNO3 in the emulsion has an important role in the proposed method since it is responsible for the extraction of metals, as the H+ ion displaces the metallic cations from the organic structures where they are in the oily samples [22]. The emulsifying agent Triton X-100 has been used in
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the most of emulsion/microemulsion systems reported in the literature for fuel/biofuel analysis [6, 17]. Cassella and coworkers [20-23] have investigated the use of Triton X-114
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and Triton X-100 in the EIEB systems. These authors choose the former because in this case the emulsions were broken in a reduced time in comparison with those prepared with Triton
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X-100, indicating that the emulsions formed with Triton X-114 are less stable and more suitable for the procedure. The emulsion obtained in this work for automotive gasoline with
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Triton X-100, unlike that obtained by Cassella et al. [20,21], was not stable, generating phase separation in less than one minute, possibly due to the high amount of ethanol present in the
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Brazilian gasoline. Even though, this composition (HNO3 and Triton X-100) provided a satisfactory extraction efficiency when the system was heat for a short period of time. After phase separation, approximately 3.5 mL of the extract was collected and used for analysis by
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HR-CS FAAS. The increase in the volume of the aqueous phase from 2.0 to 3.5 mL was due
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to the transference of the part of ethanol content from gasoline to the aqueous phase. As discussed, the temperature was not necessary to break the emulsion. However, without heating, the extraction in a short period of time (up to 15 min) was not efficient. In order to
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develop a fast method, the time used for heating was 10 min. Thus, three variables (HNO3 concentration, Triton X-100 concentration and temperature) were evaluated for multivariate
breaking.
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optimization in order to find out the best conditions for the extraction based on emulsion
3.1 Multivariate optimization of extraction method for determination of Cu, Fe and Pb in gasoline The Doehlert design is easily applicable to the optimization variables requiring few experiments. Basically, this optimization process involves three main steps: performing statistically designed experiments, by estimating the coefficients of a mathematical model; predicting the response and check the suitability of the model. The groups tested were: 5 to 15% (v/v) HNO3 solution; 3.0 to 13% (w/v) of Triton X-100 and temperatures from 70 to 90 8
ACCEPTED MANUSCRIPT °C. The response was the absorbance signal obtained in each experiment. The numbered experiments and the response obtained for each experiment are shown in Table S1 (Appendix). Fifteen experiments were carried out for planning and performed in a random order to avoid any bias in the measurements. Data were modeled using the spreadsheet Doehlert developed by Teófilo and Ferreira [29] to determine the effect of each variable in the response. The values of the variables employed were coded to eliminate the effect of different magnitudes on the evaluation.
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The normal probability graphics for Cu, Fe and Pb are shown in Fig. S1 (Appendix). The first order effects are: (1) temperature, (2) HNO3 and (3) Triton X-100. All effects that
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deviate from zero are the most significant. The normal probability graphics for Cu (Fig. S1.A) and Fe (Fig. S1.B) were similar, where the positive effects of second order for HNO3 (22) and
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interactions between HNO3 and Triton X-100 (23) were highlighted. A negative effect of first order for HNO3 (2) was also observed for both elements, suggesting that this reagent should
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be used in lower concentrations. For Pb (Fig. S1.C), besides the positive effect of second order for HNO3 (22), a positive effect of first order was observed for Triton X-100 (3),
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suggesting the use of this reagent in higher concentrations in order to improve the signal for this element. Moreover, a significant negative effect of second order for temperature and Triton X-100 (13) was highlighted, indicating an antagonistic effect between these reagents,
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suggesting that an adequate condition can be achieved with opposite levels of them.
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The response surfaces for the experiments performed for multivariate optimization based on Doehlert design for extraction of Cu, Fe and Pb from gasoline are shown in Fig. S2 (Appendix). A similar behavior for Cu and Fe was observed, showing that the HNO3
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concentration in the investigated range (3% to 15% v/v) do not affect the response, while the increase in the temperature affects significantly. For Pb a slight effect can be observed for
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lower HNO3 concentrations. Cassella et al. [20] investigated the concentration of HNO3 in the extracting solution in the range of 2% to 20% (v/v) and they reached to the conclusion that the acid concentration of 2% (v/v) was already enough to promote metals migration from the oil to water phase. They also showed that higher concentrations reduced the time for emulsion breaking and suggested that this behavior can be credited to the effect of the ionic concentration on the stability of the emulsion. These authors adopted the concentration of 10% (v/v) of HNO3 in order to decrease the time for emulsion breaking. Nevertheless, in our work even using the lower concentration of HNO3 investigated, the emulsion breaking was very fast, not requiring higher concentrations, that could destabilize even more the emulsion, impairing the extraction performance for some elements. Regarding the concentration of 9
ACCEPTED MANUSCRIPT Triton X-100, higher concentrations should improve the emulsion formation and the interaction between the phases, enhancing the extraction of more stable organic molecules. The results obtained in this work for Pb suggested that this element required a little more stable emulsion for appropriate extraction. Thus, the lower concentration of HNO3 and high concentration of Triton X-100 was adopted. Based on the results obtained in both normal probability (Fig. S1) and response surface (Fig. S2) graphics, the following compromise conditions were adopted: 5% (v/v)
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HNO3 and 12% (w/v) Triton X-100 in 2.0 mL of extracting solution; the heating temperature
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of 90 °C for 10 minutes.
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3.2 Evaluation of extraction efficiency
After establishing the optimized conditions for extraction, the efficiency of the proposed
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procedure for metals extraction from gasoline was investigated. In this experiment, a gasoline sample (GB2) spiked with organometallic standards was subjected to EIEB procedure and the analytical signal was measured for each analyte in the extracted phase. The oily phase
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obtained after the emulsion breaking was separated and again subjected to the same procedure. The new extracted phase obtained after emulsion breaking was also analyzed. Three successive extractions were carried out with the same spiked sample in triplicate.
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The obtained results showed that the percentage of transfer of metals from the gasoline
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to the extract in the first extraction were 80% for Cu, 83% for Fe and 89% for Pb. Even considering that there was a small residual amount in the second extraction, the reproducibility of the first extraction (RSD between 5 - 7%) as well as the efficiency of
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extraction were satisfactory. As investigated by Cassella and coworkers, quantitative extraction was obtained only with 60 min of extraction time for Cu, Mn and Ni from biodiesel
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[23] and 30 min for Cu, Fe and Mn from used lubricating oils [22]. However, quantitative extraction for Cu, Fe, Ni and Pb from diesel oil was obtained in 15 min [20]. In this work, the increase of time from 10 to 15 min did not enhance the extraction efficiency. Higher times were not investigated as the intention was to propose a simple and fast procedure.
3.3 Figures of merit
10
ACCEPTED MANUSCRIPT One of the main problems in the analysis of fuels is the correct choice of calibration strategy. When nebulization is used for sample introduction, the composition of the extract and the calibration solutions must be the same. Thus, in order to prevent any physical interference caused by the content of alcohol and/or surfactant in the extract, the matrixmatching calibration using the analytical curve prepared with the extract obtained from a gasoline sample free of analytes was performed. This calibration method also prevents errors due to the fact that the efficiency of the extraction of the analytes from the gasoline samples
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was not 100%. For this, the standards were obtained performing the same extraction procedure used for the sample, using emulsions obtained with a gasoline sample free of
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analytes and the addition of inorganic standards of the analytes. The same calibration strategy was used by Vicentino and Cassella [28] for Hg determination in Brazilan gasoline samples
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using EIMB.
The figures of merit obtained using calibration curves prepared with inorganic
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standards in water (external calibration) and by the emulsion breaking extraction (matrixmatching) are shown in Table 1. The limit of detection (LOD) and limit of quantification
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(LOQ) were calculated according to International Union of Pure and Applied Chemistry (IUPAC) recommendations, as 3 times and 10 times, respectively, the standard deviation of 10 measurements of a blank, divided by the slope of the calibration curve. The LOD and LOQ
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of the proposed method were estimated for the original sample (volume of 10 mL), thus
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taking into account the preconcentration factor. The characteristic concentration (C0) defined as the analyte concentration corresponding to an integrated absorbance of 0.0044 (1% of absorption) [31].
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As can be seen, all curves presented determination coefficients > 0.99 demonstrating adequate linearity. As expected, the sensitivity obtained with calibration curves without
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extraction and after extraction by emulsion breaking are different, due to the preconcentration factor (PF) achieved with the extraction. The PF is the most widely used criterion for evaluation of preconcentration systems and were calculated as the ratio between the angular coefficient of the linear part of the calibration curve without preconcentration and the calibration curve obtained using the preconcentration procedure [32]. The obtained PF shown in Table 1 were close to the expected PF of 2.86, considering the initial sample volume (10 mL) and the final volume of extract (3.5 mL). The PF values obtained in this work were lower than those found by Cassella et al. for the determination of Cu, Fe, Ni and Pb in diesel oil by GFAAS [20] and Zn in diesel oil by FAAS [21] using the same method used in this study, but 11
ACCEPTED MANUSCRIPT it should be take into account that these authors obtained a final volume of 2.0 mL of extracted solution for 10 mL of sample.
Table 1. Figures of merit obtained for the determination of Cu, Fe and Pb by HR-CS FAAS Linear regression
R2
equation
Externalb
y = 0.1429x – 0.0040
Matrix-matchingc
-1
-1
PFa
(µg L )
(µg L )
0.9965
32
8.2
27
-
y = 0.3470x -0.0020
0.9972
12
3.2
11
2.4
External
y = 0.0549x – 0.0020
0.9987
75
12
42
-
Matrix-matching
y = 0.1359x + 0.0020
0.9934
37
5.2
17
2.5
External
y = 0.0303x – 0.0054
0.9971
148
30
99
-
Matrix-matching
y = 0.0610x – 0.0070
74
14
48
2.0
Pb b
performed with inorganic standards in 0.014 mol L-1 ;
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PF = preconcentration factor;
0.9980
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Fe
c
-1
LOQ
(µg L )
Cu
a
LOD
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calibration
Co
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Method of
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Analyte
performed with the extract obtained from a gasoline sample free of analytes spiked with
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inorganic standards.
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As there are no studies in the literature that relate extraction based on emulsion breaking for Cu, Fe and Pb from gasoline the LOD obtained in this work was compared with those obtained by alternative methods from the literature using FAAS technique. Similar
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results were found in the literature for Cu [19] and Fe [18,19] using preconcentration by SPE methods. However, lower results for Pb [19] and Cu [1,18] were also reported for the same
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technique. Kowalewska et al. [8] found a similar LOD for Pb for direct analysis of gasoline by HR-CS FAAS. Nevertheless, even if the LODs and LOQS found in this work are slightly higher than some methods described in the literature, the values are in the µg L-1 level for all elements and are smaller than the maximum levels stipulated for Pb in the legislation of both countries (Brazil: 5.0 mg L-1; Paraguay: 13 mg L-1). The precision of the method was estimated from the measurement of metals, applying the procedure for three separate aliquots of the same sample, containing the amount of 1.0, 1.8 and 3.5 mg L-1 of Cu, Fe and Pb, respectively. The relative standard deviation for Cu, Fe and Pb were 4.7; 3.5 and 5.0%, respectively, indicating that the developed method has good 12
ACCEPTED MANUSCRIPT precision. Finally, it should be pointed out that this method was optimized for the Brazilian automotive gasoline containing 25% of ethanol. Considering that the content of ethanol influences the final volume of the extract, some adjustments / optimizations should be necessary when applying this method to other gasoline samples with different compositions.
3.4 Determination of Cu, Fe and Pb in gasoline samples by HR-CS FAAS using extraction
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based on emulsion breaking as sample preparation and recovery tests
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The proposed method has been applied for the multi-element sequential determination of Cu, Fe and Pb in ten gasoline samples (from Brazil and Paraguay). The samples GB2, GB3,
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GB4, GB7, GP1 and GP3 did not present concentration above the limit of quantification for any investigated elements. The concentrations of Cu, Fe and/or Pb found in four gasoline
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samples are showed in Table 2. As can be seen, concentrations of Pb were below the limit established by the legislation of both countries. It should be pointed out that this method could
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be applied for the Paraguayan automotive gasoline, since these samples presented the same composition of the Brazilian automotive gasoline (25% v/v of ethanol). In order to attest the accuracy of the method two samples (GB6 and GB7) were spiked
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with different concentrations of the organometallic standards (1.0, 2.0 and 6.0 mg L-1 of Cu,
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Fe and Pb, respectively)). The recovery values (Table S2 – Appendix) obtained through the matrix matching calibration ranged from 98 to 105%, for all analytes and confirmed the absence of the matrix effects. These results were satisfactory, confirming the accuracy of the
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proposed method and showing that it can be successfully used in routine analysis.
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Table 2. Determination of Cu, Fe and Pb in gasoline samples by HR-CS FAAS using emulsion breaking as sample preparation. The concentrations are expressed as mean ± standard deviation (n=3). Sample
Cu (mg L-1)
Fe (mg L-1)
Pb (mg L-1)
GB1
LOD
0.044 ± 0.001
LOD
GB5
0.283 ± 0.001
0.056 ± 0.005
LOD
GB6
LOD
0.350 ± 0.001
LOD
GP2
LOD
0.208 ± 0.001
0.60 ± 0.01 13
ACCEPTED MANUSCRIPT 4. Conclusion
The application of extraction based on emulsion breaking allowed preconcentration of analytes in the extract, leading to an appropriate improvement in detectability, allowing the determination of the analytes in mg L-1 level by HR-CS FAAS. Multivariate optimization based on Doehlert design was efficient for establishing the optimal conditions for the procedure. The extraction method proposed in this work allied with sequential determination
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by HR-CS FAAS showed to be simple, fast and thus suitable for the routine determination of
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Cu, Fe and Pb in Brazilian automotive gasolines and others with the same composition.
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Acknowledgments
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This work was supported by the Conselho Nacional de Desenvolvimento Cientifíco e Tecnológico (CNPq; Edital 40/2013 grant no. 405011/2013-0 and Universal 2012 grant no. 478998/2012-0). M.M.S., D.S. and L.K. have research scholarships from CNPq (grant nos.
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307815/2016-1 (M.M.S.), 305331/2013-2 (D.S.)) and A.J. has a scholarship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).
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'Appendix A. Supplementary data'
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'Supplementary data to this article can be found online at doi:...'
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Analyst 115 (1990) 475-486.
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Highlights
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- A new extraction method is proposed for metal determination in Brazilian gasoline.
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- The extraction is based on the formation and breaking of gasoline emulsions.
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- The proposed method allowed preconcentration of analytes in the aqueous phase
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- The emulsion breaking extraction method was optimized using Doehlert design.
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- The method was simple, fast, accurate and required a minimum sample manipulation.
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