Microwave-assisted digestion using diluted nitric acid for further trace elements determination in biodiesel by SF-ICP-MS

Fuel 204 (2017) 85–90

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Microwave-assisted digestion using diluted nitric acid for further trace elements determination in biodiesel by SF-ICP-MS P.S. Barela a, N.A. Silva a, J.S.F. Pereira a, J.C. Marques b, L.F. Rodrigues c, D.P. Moraes a,⇑ a

Institute of Chemistry, Federal University of Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil Laboratory for Isotope Geology, Federal University of Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil c Institute of Petroleum and Natural Resources, Pontifical Catholic University of Rio Grande do Sul, 90619-900 Porto Alegre, RS, Brazil b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 20 March 2017 Accepted 7 May 2017

Keywords: Microwave-assisted digestion Diluted nitric acid Biodiesel Trace elements determination SF-ICP-MS

a b s t r a c t A microwave-assisted digestion (MW-AD) using diluted nitric acid and hydrogen peroxide as auxiliary reagent was proposed for the first time for biodiesel for further trace elements determination by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). Biodiesel samples were digested under high-pressure closed vessels and the concentration of nitric acid (1, 2, 3.5, 7, 9.3 and 14 mol L 1) and volume of hydrogen peroxide (0, 1, 1.5, 2 and 3 mL) were evaluated in order to obtain high efficiency of digestion. The efficiency of biodiesel digestion was determined by residual carbon content and residual acidity in final solutions. Up to 700 mg of biodiesel were completely digested using 7 mol L 1 nitric acid and 2 mL of hydrogen peroxide in high-pressure quartz vessels assisted by microwave radiation at 900 W during 60 min (included 20 min for cooling step). Residual carbon content and residual acidity in digests obtained after MW-AD using optimized conditions were lower than 9 and 4%, respectively. After sample digestion, Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn were determined by SF-ICP-MS and analytes recoveries were in the range of 94 to 109%. Accuracy was evaluated using certified reference material (used oil) and agreement higher than 94% was achieved. Although biodiesel digestion could be performed using concentrated nitric acid, the use of diluted acid was preferable in view of the low reagent consumption and low blanks values, achieving better limits of detection (LODs) that is important aspect to green chemistry recommendations for trace elements analysis. Ó 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (D.P. Moraes). http://dx.doi.org/10.1016/j.fuel.2017.05.028 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.

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1. Introduction Nowadays, the renewable sources of energy has great interest worldwide due to the increase of energy demand, the shortage of light crude oil reserves and the reduction of carbon emissions to avoid global warming. In this way, the use of biodiesel has been recommended and it is considered an attractive alternative of diesel fuel because it is produced from renewable resources and involves lower emissions than petroleum diesel [1]. According to some official methods (ASTM, EN or NBR), trace elements determination in biodiesel plays a fundamental role to ensure the quality of the final product [2–4]. The raw material, usually vegetable oils or animal fats, presents low levels of inorganic contaminants. However, the final product needs to be analyzed to ensure catalyst traces are low or that no other inorganic contaminant has been introduced from reactant vessel, pipelines or stored vessel. In general, official methods recommend the determination of Na, K, Ca Mg, P and S in biodiesel [2–4]. Sodium, K, Ca, and Mg can promote insoluble compounds formation in the engine and can also cause corrosion of metal parts by abrasive solids. Phosphorus and S are catalysts poisoning and could affect the catalytic converts used in diesel engines [2]. Despite other elements such as Cd, Cu, Fe, Ni, Pb and Zn are not related in the official directives, their presence can affect the stability and quality requirements of biodiesel. The deleterious effects such as corrosion in metallic parts of engines, catalysts poisoning and incomplete combustion were also associated with the presence of trace elements [5–7]. For example, Cu and Cu-containing alloys should be avoided in storage tanks or pipelines for biodiesel due to the increase of sediments and deposit formation. Moreover, Pb, Ti and Zn can also induce the sediment formation that can plug filters [2]. The oxidation stability of biodiesel decreases in the presence of Cu, Mn and Ni as related by Jain and Sharma [8]. In addition, Ba, Cr, Mo, Sr and V could cause environmental pollution by emission of particulate matters during diesel/biodiesel combustion [9]. Typically, flame atomic absorption spectrometry (FAAS) and inductively coupled plasma optical emission spectrometry (ICPOES) are used for trace elements analysis of biodiesel after dilution step using organic solvent, as reported in official methods [2–4]. However, organic solvents as xylene or kerosene have a high toxicity, low stability related to analyte concentration and a more complex handling in comparison with aqueous solutions. In addition, this method is limited by severe interferences from the carbon matrix and in some cases the use of organometallic standard should be applied in the calibration step. The physical properties as density, viscosity and surface tension could be changed in the sample solution and then an intensification of matrix effects could be expected [10,11]. Moreover, high pure biodiesel may contain some elements in low concentration, that could not be determined by simple dilution step using an organic solvent due to poor sensitivity mainly related to plasma effects, aerosol generation/transport and spectral interferences [12]. In this aspect, the use of a sample preparation method for biodiesel digestion could be useful for further analysis of some elements. An interesting strategy to sample preparation of biodiesel is a microwave-assisted digestion (MW-AD). In the last years, the use of closed vessels combined with microwave radiation has been described as a potential sample preparation technique, which presents high efficiency of digestion, the loss of volatile elements is avoided and the risk of contamination may be reduced [13]. In addition, it is possible to use aqueous solution that is more convenient to handling during determination step when comparated to the use of organic solvents. However, some problems related to fuel digestion could be point out such as time consuming, eventual inefficiency of digestion, the use of high amounts of inorganic acids and production of high volumes of hazardous waste. Furthermore,

high pressure and temperature conditions must be used to improve the digestion efficiency [14]. It is important to mention that the use of concentrated acids could increase the blank values and sometimes the concentrated acid could not be supported by some analytical techniques, as ICP-MS and ICP-OES, and a subsequent step could be necessary to remove (or dilute) the excessive acidity [15,16]. Consequently, with the use of a dilution step, high limits of detection (LODs) will probably be obtained making difficult to perform the determination of some elements in low concentrations. Nowadays, the regeneration of nitric acid in the presence of oxygen was proposed to improve the digestion efficiency in a closed vessel using a diluted nitric acid [17,18]. The proposed mechanisms for nitric acid regeneration were investigated in more details by Bizzi et al. [19,20]. It was demonstrated that the digestion procedure is critically dependent on reactions occurring in liquid and gas phase induced by (i) presence of oxygen atmosphere inside of the closed vessel, and (ii) temperature gradient between gas and liquid phase specially during the first step of digestion [21]. This sample preparation method was successful applied for food [22–26], medicinal plants [27], biological [28,29] and botanical materials digestion [30,31]. It is important to notice that dilute acid solutions, as 2 mol L 1 nitric acid in the presence of hydrogen peroxide or oxygen were used in most of applications. In the present work, MW-AD using diluted nitric acid and hydrogen peroxide is proposed for the first time for biodiesel digestion for further Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn determination by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). Hydrogen peroxide was used as oxygen source to promote the nitric acid regeneration during MW-AD procedure. It is important to mention that is the first time that a diluted acid solutions is applied for biodiesel decomposition achieving high digestion efficiency even for high sample mass (700 mg) for subsequent trace elements determination.

2. Experimental 2.1. Instrumentation The microwave sample preparation system Multiwave PRO (Anton Paar, Austria) equipped with up to eight high-pressure quartz vessels (rotor type 8NXQ 80) was used for the MW-AD procedure, using the software version 2.42.7406.8. This system operates with the maximum pressure and temperature of 80 bar and 280 °C, respectively. The maximum pressure of all vessels and the temperature in each vessel were monitored in all runs. Total organic carbon (TOC) was determined by elemental analyzer LECO SC632 (LECO, USA) and residual carbon content (RCC) was determined by multi N/C 2100S (AnalytikJena, Germany) equipped with autosampler AS60. The system operation was based on CO2 formation after high-temperature combustion (up to 850 °C). An infrared detector equipped with a Focus Radiation System (NDIR, AnalytikJena) was used for precise detection of CO2. In order to remove the volatile carbon compounds before RCC determination in digests, aliquots were previously purged with oxygen for 3 min. For determination of residual acidity an automatic titration system 905 Titrando (Metrohm, Switzerland) coupled to a 20 mL burette (Dosino 800), magnetic stirrer (module 801 Stirrer) and pH electrode (LL Unitrode, model 6.0258.600) was used. Analytes (Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn) were measured in medium resolution mode on a sectorfield inductively coupled plasma mass spectrometer Element 2 (Thermo Fisher Scientific, Germany), equipped with a conikal concentric micronebulizer (Glass Expansion, Inc., Australia), coupled to a cinnabar

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cyclonic spray chamber (Glass Expansion, Inc., Australia) and quartz torch with a quartz injector tube (2 mm i.d.). Argon 99.996% (White Martins-Praxair, Brazil) was used for plasma generation, nebulization and as auxiliary gas. The operational conditions and the isotope measured are shown in Table 1. The SFICP-MS daily optimization was performed in agreement with manufacturer recommendation to evaluate the analytical response from 1 lg L 1 of 7Li+, 115In+ and 238U+ isotopes. In addition, in the medium resolution operation mode the separation of 56Fe+ from polyatomic interference (ArO+) was performed to optimization the high resolution lens. 2.2. Reagents and standards Distilled and deionized water further purified using a Milli-Q system (Millipore Corp., USA) with a minimum resistivity of 18.2 MX cm at 25 °C was used to prepare all the standard solutions, reagents and to perform dilutions. Concentrated nitric acid 65% (Merck, Germany) was purified using a sub-boiling system Distillacid, Model BSB-939-IR (Berghof, Germany). As was mentioned before, hydrogen peroxide EmsureÒ 30% (Merck, Germany) was used to generate oxygen, responsible for the nitric acid regeneration during biodiesel digestion by MW-AD. Carbon reference solutions used for RCC determination were prepared by dissolution of potassium hydrogen phthalate (Synth, Brazil) in water (0.5– 20 mg L 1 of C). For acidity determination, 0.1 mol L 1 KOH (Vetec, Brazil) was used for titration and this solution was previously standardized using potassium hydrogen phthalate. Multi-element stock solution containing 10 mg L 1 of Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn Plasma Cal SCP33MS (SCP Science, Canada) was used to prepare analytical standards by sequential dilution in 5% (v/v) HNO3 in the range of 0.5 to 20 lg L 1. A 1000 mg L 1 indium stock solution (Sigma-Aldrich, Switzerland) was used as internal standard with the final concentration of 1 mg L 1. All laboratory materials such as glasses, polypropylene vials and others flasks were cleaned before use by soaked in 10% (v/v) HNO3 bath for five days. 2.3. Samples and certified reference materials Biodiesel samples obtained by transesterification of vegetable oil and/or animal fats used in this work are in agreement with National Agency of Petroleum, Natural Gas and Biofuels (ANP) requirements. The commercial biodiesel samples (B100) are from different raw material and were obtained from different biodiesel producers: A (soy oil), B (60% soy oil, 20% bovine fat and 20% pork fat) and C (40% soy oil, 20% bovine fat and 40% pork fat). Certified reference material of used oil HU-1 (SCP Science, Canada) was used to evaluate the accuracy of the proposed digestion method for further Ba, Cr, Cu, Mn, Mo, Ni, Pb, V and Zn determination.

Table 1 Optimized conditions for trace elements determination by SF-ICP-MS. Parameter Radiofrequency Power (W) Plasma gas flow rate (L min 1) Auxiliar gas flow rate (L min 1) Sample flow rate (L min 1) Extraction lens (V) Focus lens (V) X-deflection (V) Y-deflection (V) Peristaltic pump (rpm) Resolution mode Isotopes measured (m/z) *

Internal standard.

1220 15 0.8 1.105 1980 1260 1.0 6.2 10 medium 51 + 52 V , Cr+, 55Mn+, 60Ni+, 63Cu+, 66Zn+, 98 Mo+, 138Ba+, 208Pb+ and 115In+*

88

Sr+,

87

Analyte recovery was performed by addition of multi-element reference solution (Plasma Cal SCP33MS) in the biodiesel sample B before MW-AD procedure using diluted nitric acid and hydrogen peroxide after optimization of the experimental parameters. 2.4. Microwave-assisted digestion (MW-AD) Initially, the efficiency of digestion was evaluated using variable concentration of HNO3 (1, 2, 3.5, 7 and 9.3 mol L 1) combined with 2 mL of H2O2 and concentrated nitric acid without hydrogen peroxide in high-pressure quartz vessels based on the parameters established in previous reports [31]. The proposed digestion procedure was initially evaluated using 500 mg of biodiesel (sample B) and, in agreement with manufacture recommendation, the total volume of solution was fixed in 6 mL. After that, the addition of H2O2 combined with nitric acid was evaluated in the following volumes: 0, 1, 1.5, 2, and 3 mL. Moreover, the sample mass was evaluated up to 700 mg. The heating program was carried out in agreement to conditions recommended by the microwave manufacturer for diesel digestion as follow: i) 900 W with a ramp of 20 min, ii) 900 W for 20 min and iii) 0 W for 20 min (cooling step). During the heating step the exhaust (air flow rate outside digestion vessel) was maintained in the first level (60 m3 h 1) and in the cooling step the exhaust was increased to the second level (125 m3 h 1). Cleaning of quartz digestion vessels was carried out using 6 mL of concentrated HNO3 by microwave radiation at 900 W during 20 min (5 min of ramp and 15 min of hold) and 20 min for cooling. After all digestions, resulting solutions were diluted with ultrapure water up to 30 mL for further determination by SF-ICP-MS. 2.5. Evaluation of digestion efficiency Efficiency of digestion was evaluated by determination of RCC and residual acidity in final digests. The RCC was expressed as organic carbon remained in the solution after digestion procedure related to TOC originally present in the sample. TOC was determined by elemental analysis and the samples presented 77.3– 77.8% of carbon. The final solutions were titrated with 0.1 mol L 1 KOH in order to determine the residual acidity which was expressed as percentage related to the initial acid concentration. 3. Results and discussion 3.1. Evaluation of nitric acid concentration The digestion procedure for biodiesel samples applied in this work were based on the use of H2O2 as auxiliary reagent to promote regeneration of diluted nitric acid in high-pressure quartz vessels. Initially, HNO3 concentration was varied from 1 to 14 mol L 1 for digestion of 500 mg of biodiesel (sample B) with addition of 2 mL of H2O2 or without H2O2 when 14 mol L 1 nitric acid was used. The results are shown in Fig. 1. It was observed that when digestion was carried out with 1 mol L 1 HNO3, black residues were observed in final solutions, showing that digestion was not complete. For this reason, the results for RCC and residual acidity were not show in the Fig. 1. However, using at least 2 mol L 1 HNO3 the final digests present a clear aspect without solid residues. Furthermore, when 2 and 3.5 mol L 1 HNO3 were used RCC values of 33.4 and 21.3% were achieved in digests, respectively (Fig. 1). Despite the low value obtained for residual acidity using 3.5 mol L 1 HNO3 (19.1%), the high value of RCC showed a poor efficiency of digestion. When 7 mol L 1 nitric acid was used the efficiency of digestion was significantly improved as could be observed by the RCC values. In this

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HNO3 concentration, mol L-1 Fig. 1. Influence of HNO3 concentration on the digestion efficiency of 500 mg of biodiesel using 2 mL of H2O2. Grey bars represent the procedure performed with H2O2 and white bar represent procedure performed without H2O2. Bars represent the RCC and the line represent the residual acidity (error bars are the standard deviation, n = 3).

case, RCC decreased to values lower than 4% and the residual acidity was 30.6%. This condition showed the better efficiency of digestion correlating RCC values with the residual acidity and therefore was used for further evaluations. When digestion was performed using at least 9.3 mol L 1 HNO3, the RCC values were below to 2%. However, as expected the residual acidity when 9.3 mol L 1 nitric acid was used increased considerably in comparison with 14 mol L 1 nitric acid due to presence of hydrogen peroxide that promote the nitric acid regeneration inside of closed vessel. Therefore, based on the results obtained and taking into account the RCC values in final solutions, the concentration of nitric acid was set at 7 mol L-1 for further studies to digest 500 mg of biodiesel.

3.2. Effect of H2O2 in the efficiency of biodiesel digestion using diluted nitric acid

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Residual Acidity, % (v/v)

Residual Carbon Content, % (m/m)

The H2O2 is known for its high oxidant power and therefore it is often used in wet digestion combined with inorganic acids. Furthermore, H2O2 can also act as O2 source contributing to the regeneration of HNO3 in closed systems.[31] In this sense, biodiesel digestion with diluted nitric acid (7 mol L 1) was evaluated using 1, 1.5, 2 and 3 mL of H2O2. Besides, the sample digestion was carried out without H2O2 and in the absence of oxygen the nitric acid regeneration is negligible. The results obtained are shown in Fig. 2.

0 0

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1.5

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H2O2, mL

As can be seen in Fig. 2, the digestion performed without addition of H2O2 showed the lower residual acidity (9.5%) and, on the other hand, presented the higher RCC value when compared to experiments carried out with addition of H2O2. When 1, 1.5 and 2 mL of H2O2 were added practically no differences were observed for residual acidity: 29.7, 26.7 and 30.6%, respectively. However, the RCC differs significantly. When digestions were performed with the addition of 1 and 2 mL of H2O2 the RCC in digests were 9.4% and 3.2%, respectively. Moreover, to improve the efficiency of digestion 3 mL of H2O2 were added as auxiliary reagent. However, despite the RCC value obtained was below 2%, it was observed an increase in the residual acidity of about 50%. This behavior was expected due to the increase of the H2O2 volume and these results may be explained by the high oxidant environment as reported in previous work to improve the regeneration of nitric acid in closed vessels [31]. Therefore, the compromise condition to achieve the best efficiency of digestion and suitability of final solution for further analytes determination by SF-ICP-MS was 7 mol L 1 HNO3 and 2 mL of H2O2 for digestion of biodiesel.

3.3. Evaluation of sample mass In order to use high sample mass in the digestion step for further determination of trace elements in biodiesel samples, the proposed method was investigated for digestion of up to 700 mg of sample using the previous optimized condition 7 mol L 1 HNO3 with 2 mL of H2O2. The results obtained are shown in Fig. 3. As shown in Fig. 3, the RCC values not differ significantly for digestion of 500 to 600 mg of sample and usually RCC values are below 5%. However, due to the increase of organic matter it was observed a high consumption of acid, thus the residual acidity is considerably lower for digestion of the 600 mg in comparison to digestion of 500 mg. For digestion of 650 and 700 mg of biodiesel the RCC were below 9% and residual acidity was lower than 11%. It is important to mention that the maximum pressure (80 bar) was achieved after 25 min of microwave heating for all mass range. Otherwise, the temperature increases for higher sample mass. For digestion of 500, 600 and 700 mg the maximum temperatures achieved were 210, 218 and 226 °C, respectively. However, the temperature gain was negligible to improve the digestion efficiency for higher sample mass. For digestion of 700 mg the microwave radiation was switch off for one minute during the heating step as can be showed in the Fig. 4. This could be explained due to the quickly increase of pressure (higher than 0.3 bar s 1). In this case, the exhaust (third level, 190 m3 h 1) was automatically turned on during heating for safety

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Residual Carbon Content, % (m/m)

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0 500

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Sample mass, mg Fig. 2. Influence of H2O2 combined with 7 mol L 1 HNO3 on digestion efficiency of biodiesel (500 mg). Bars represent the RCC and the line represent the residual acidity (error bars are the standard deviation, n = 3).

Fig. 3. Evaluation of sample mass. Bars represent the RCC and the line represent the residual acidity (error bars are the standard deviation, n = 3).

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Table 3 Results obtained by SF-ICP-MS of the certified reference material (used oil) after MWAD using diluted acid and recoveries obtained for the spiked sample (n = 3, mean ± standard deviation).

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V Cr 55 Mn 60 Ni 63 Cu 66 Zn 88 Sr 98 Mo 138 Ba 208 Pb 52

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Time, min Fig. 4. Profiles of pressure (——), temperature (



) and power ( ) obtained during digestion of 700 mg of biodiesel with 7 mol L 1 HNO3 and 2 mL of H2O2.

reasons. In view of this, sample masses higher than 700 mg were not evaluated. However, the pressure and temperature were not above the limits of the system. Although the RCC value (8.7%) was higher with the used of 700 mg of biodiesel, it is important to mention that interferences in the analytes determination were not observed. Additionally, the higher sample mass evaluated (700 mg) was adopted taking into account that, in general, analytes are present in low concentrations in biodiesel samples. 3.4. Trace elements determination in biodiesel by SF-ICP-MS after MW-AD method using diluted acid The determination of Ba, Cu, Cr, Mn, Mo, Ni, Pb, Sr, V and Zn was performed in digests by SF-ICP-MS after MW-AD using diluted nitric acid combined with hydrogen peroxide. The results obtained are shown in Table 2. It is important to observe that, in general, the analytes are present in low concentrations in the biodiesel samples evaluated. However, high concentration of Ba, Cr, Mn and Ni were observed only for biodiesel A and in this sample also Pb and V were presented. For other samples (biodiesel B and C), relatively low concentrations were observed for all analytes. The variable concentration of trace elements in the samples evaluated could be due to the fact that them are provided from different companies, and consequently the synthesis route could be different. Additionally, contamination of sample could be occurred during the biodiesel production. In general, several contaminants were presented in concentrations lower than the LODs. Taking into account the LODs achieved, it is important to mention that two hydrogen peroxide brands were evaluated for sample digestion and one of them presents

Table 2 Results, in mg g 1, obtained by SF-ICP-MS after MW-AD of biodiesel samples using diluted acid (n = 3, mean ± standard deviation). Isotope

Biodiesel samples A

B

C

51

0.110 ± 0.006 1.25 ± 0.04 0.727 ± 0.043 0.701 ± 0.033 0.091 ± 0.002 <0.239 0.056 ± 0.002 0.092 ± 0.005 0.411 ± 0.017 0.027 ± 0.001

<0.002 <0.007 <0.003 <0.009 0.067 ± 0.002 <0.239 <0.012 0.005 ± 0.0002 0.087 ± 0.003 <0.003

<0.002 <0.007 <0.003 <0.009 <0.014 <0.239 <0.012 0.005 ± 0.0003 0.139 ± 0.007 <0.003

V 52 Cr 55 Mn 60 Ni 63 Cu 66 Zn 88 Sr 98 Mo 138 Ba 208 Pb

HU-1 used oil Certified, mg g

*

7 ± 0.5 15 ± 2 18 ± 1 45 ± 3 3132 ± 226 16 ± 2 *

11 ± 1 9 ± 0.5 20 ± 1

Spiked sample 1

Found, mg g 7.4 ± 0.3 16.5 ± 0.5 18.3 ± 0.2 47.6 ± 2.3 3154 ± 77 15.1 ± 0.7 – 10.5 ± 0.3 8.7 ± 0.4 20.8 ± 0.9

1

Recovery, % 97 101 98 98 95 102 95 94 109 103

There is no certified or informed value for Sr.

some analytes in high concentration, increasing the LODs approximately 10 times in comparison with hydrogen peroxide of high purity (it is the reagent adopted in this study). This is an important point to trace elements determination. The LODs, in mg kg 1, obtained for the all the analytes by SF-ICPMS after MW-AD using diluted nitric acid and hydrogen peroxide are indicated between brackets: Ba (1), Cr (7), Cu (14), Mn (3), Mo (1), Ni (9), Pb (3), Sr (12), V (2) and Zn (239). The instrumental limits of detection were calculated as three times the standard deviation of 10 measurements of the blank signal plus mean of the blank (3r + Xb). Also, the LODs were calculated for the original sample considering 700 mg of biodiesel in 30 mL of final solution. Accuracy was evaluated by analysis of CRM of used oil (SCP Science HU-1) since there is no certified reference material for biodiesel containing all analytes studied in this work. The results obtained by SF-ICP-MS are shown in Table 3. Results obtained after digestion of the CRM by MW-AD with diluted nitric acid and hydrogen peroxide were in agreement better than 94% with certified values and no significant statistical difference was found with confidence level of 95% (t-test, p < 0.05). Furthermore, for all the analytes investigated the relative standard deviation (RSD) were lower than 5%. Spiked samples were also performed in order to verify the recoveries for all analytes. This study was performed using biodiesel sample B and the addition of a standard solution containing all elements above the sample. The digestion of spiked sample was carried out using 700 mg of biodiesel and a mixture of 7 mol L 1 HNO3 and 2 mL of H2O2. The determination of trace elements was performed in digests by SF-ICP-MS. Recoveries obtained for Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn were in the range of 94 to 109% and relative standard deviation values were lower than 6%. The low acid concentration present in the digests obtained by the proposed method of MW-AD using diluted nitric acid combined with hydrogen peroxide making unnecessary to perform a dilution step prior analysis in order to avoid interferences. Moreover, resultant solutions were suitable for determinations by SFICP-MS minimizing interferences due to low residual acidity and RCC even when high samples masses (up to 700 mg) were used. This is an important aspect concerning trace elements determination in biodiesel since better LODs could be achieved using the proposed procedure. In addition, it is important to notice that the sample mass that can be digested was significantly higher in comparison with other procedures reported in the literature [7,32]. 4. Conclusions The proposed method by MW-AD using diluted nitric acid combined with hydrogen peroxide was suitable for biodiesel digestion

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for further simultaneous determination of Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn by SF-ICP-MS. A solution composed by a mixture of 7 mol L 1 HNO3 and 2 mL H2O2 showed a high efficiency of digestion with low values of RCC and residual acidity. The high sample mass (up to 700 mg) can be digested without exceeding the maximum operating pressure (80 bar) of the microwave system, combining good performance for biodiesel digestion, safety and relatively high sample throughput (up to eight samples could be simultaneously digested). Therefore, possible interferences in the determination step caused by high acid concentration or high residual carbon content in resultant solutions could be avoided. Moreover, the use of diluted nitric acid instead of concentrated acid reduces the blank values significantly and it was possible to achieve better LODs. These facts are important aspects concerning trace elements determination in biodiesel. Acknowledgements The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq – Brazilian research funding agency) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS, Brazil) for supporting this study. References [1] Knothe G, Gerpen JV, Krahl J. The biodiesel handbook. Illinois: AOCS Press; 2010. [2] Annual Book of ASTM Standards, ASTM D6751-15c. Standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels. In: ASTM International; 2015. [3] BS EN 14538. Fat and oil derivatives – Fatty Acid Methyl Esters (FAME) – Determination of Ca, K, Mg and Na content by optical emission spectral analysis with inductively coupled plasma (ICP OES). In: European Standards; 2006. [4] ABNT NBR 15556. Produtos derivados de óleos e gorduras – Ésteres metílicos/ etílicos de ácidos graxos – Determinação do teor de sódio, potássio, magnésio e cálcio por espectrometria de absorção atômica. In: Associação Brasileira de Normas Técnicas; 2008. [5] Monteiro MR, Ambrozin ARP, Lião LM, Ferreira AG. Critical review on analytical methods for biodiesel characterization. Talanta 2008;77:593–605. [6] Lepri FG, Chaves ES, Vieira MA, Ribeiro AS, Curtius AJ, Oliveira LCC, et al. Determination of trace elements in vegetable oils and biodiesel by atomic spectrometric techniques—a review. Appl Spectrosc Rev 2011;46:175–206. [7] Maciel PB, Barros LLS, Duarte ECM, Harder MNC, Bortoleto GG, Abreu CH, et al. Determination of nutrients and potentially toxic elements in Jatropha curcas seeds, oil and biodiesel using inductively coupled plasma mass spectrometry. J Radioanal Nucl Chem 2013;297:209–13. [8] Jain S, Sharma MP. Effect of metal contents on oxidation stability of biodiesel/ diesel blends. Fuel 2014;116:14–8. [9] Wang Y-F, Huang K-L, Li C-T, Mi H-H, Luo J-H, Tsai P-J. Emissions of fuel metals content from a diesel vehicle engine. Atmos Environ 2003;37:4637–43. [10] Grindlay G, Gras L, Mora J, de Loos-Vollebregt MTC. Carbon-related matrix effects in inductively coupled plasma atomic emission spectrometry. Spectrochim Acta, Part B 2008;63:234–43. [11] Wiltsche H, Winkler M, Tirk P. Matrix effects of carbon and bromine in inductively coupled plasma optical emission spectrometry. J Anal At Spectrom 2015;30:2223–34. [12] Sanchez R, Sanchez C, Lienemann C-P, Todoli J-L. Metal and metalloid determination in biodiesel and bioethanol. J Anal At Spectrom 2015;30:64–101.

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