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A simple dilute-and-shoot procedure for Si determination in diesel and biodiesel by microwave-induced plasma optical emission spectrometry
Microchemical Journal 106 (2013) 318–322
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A simple dilute-and-shoot procedure for Si determination in diesel and biodiesel by microwave-induced plasma optical emission spectrometry Renata S. Amais a, George L. Donati a,⁎, Daniela Schiavo b, Joaquim A. Nóbrega a a b
Group of Applied Instrumental Analysis, Department of Chemistry, Federal University of São Carlos, São Carlos, SP, Brazil Agilent Technologies, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 5 July 2012 Received in revised form 14 August 2012 Accepted 5 September 2012 Available online 12 September 2012 Keywords: Silicon determination Liquid fuel MIP OES Ethanol dilution
a b s t r a c t In the present work, microwave-induced plasma optical emission spectrometry (MIP OES) and the flow blurring nebulizer (FBN) technology are used to determine silicon in diesel and biodiesel samples. A simple diluteand-shoot procedure with ethanol is presented. Two additional sample preparation procedures are also evaluated for comparison: closed-vessel microwave-assisted acid digestion and microemulsion preparation in n-propanol. Limits of detection (LOD) vary from 5 to 20 μg L−1 and relative standard deviations (RSD) are lower than 2% in all cases. Accuracy is evaluated by spike experiments, with recoveries between 80 and 103%. The influence of Ca, K, Mg and Na as concomitant ions on silicon determination by MIP OES is also investigated. Significant effects on Si emission signals are observed neither in single nor multiple concomitant experiments. Although good results are obtained with all sample preparation procedures evaluated, sample dilution in ethanol represents a simpler and faster strategy, which allows external calibration with inorganic standards and aqueous solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The performance of engines is directly affected by the presence of metals and metalloids in fuels. Depending on their concentrations, these species can cause several problems such as corrosion of engine parts and shortening of the machinery lifetime. From an environmental perspective, some elements act as catalyst poison, which contributes to increasing air pollution from toxic gases and particulate matter emitted by vehicles. Silicon compounds, such as siloxanes, are added to diesel fuel as anti-foaming agents. Species produced during the combustion of these compounds can form a coating layer on the catalyst element of the exhaust system, significantly increasing air pollution [1]. Thus, the Brazilian National Agency of Petroleum, Natural Gas and Biofuels has established the maximum concentration of Si plus Al in diesel as 80 mg kg −1 [2]. Different procedures have been proposed to determine metallic species in fuels. However, due to the complexity of this matrix, time consuming sample preparation procedures are usually required [3]. Korn et al. proposed two different digestion procedures for biodiesel based either on closed-vessel microwave-assisted or on open-vessel conductive heating [4]. In both cases low residual carbon contents were obtained, but considering better accuracies and lower consumption of acids, the former approach was recommended by the authors.
⁎ Corresponding author. Tel.: +55 16 3351 8058; fax: +55 16 3351 8350. E-mail address: [email protected] (G.L. Donati). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.09.001
Emulsions and microemulsions have also been used as alternative sample preparation strategies, especially because they require less toxic reagents and present relatively high sample throughput [5]. An additional advantage is that emulsions and microemulsions are compatible with external calibration using aqueous standard solutions [6]. Cassella et al., for example, recently proposed an approach to extract analytes from diesel fuel into an aqueous solution by employing the emulsion breaking extraction-induced procedure [7]. In this case, the samples are emulsified in an acidic solution containing a surfactant (Triton X-114) and the inorganic analytes are subsequently extracted into an aqueous solution by centrifugation and emulsion breaking. The official method for biodiesel analysis according to the Brazilian Legislation (ABNT NBR 15553), for example, is based on the dilution of samples in xylene before elemental determination [8]. This procedure presents a high sample throughput, but some important limitations are the toxicity of xylene and the necessity of using organometallic standards [5]. According to Barros et al., sample dilution with short chain alcohols instead of toxic hydrocarbons can reduce costs, improve nebulization and allow calibration with aqueous standards [9]. For the determination of Al and Si in fuels, the standard method ASTM D 5184 recommends the organic matrix removal by combustion in a muffle furnace and elemental determination by atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP OES) [10]. Inductively coupled plasma OES has been widely used for elemental determination due to its multi-element capacity, high dynamic linear range and versatility [11]. On the other hand, the introduction of organic samples may require oxygen addition into the plasma to improve its stability and reduce high background signals caused by
R.S. Amais et al. / Microchemical Journal 106 (2013) 318–322 Table 1 N2-MIP OES operating conditions for Si determination in diesel and biodiesel samples. Instrument parameter
Operating condition
Nebulizer Spray chamber Read time (s) Number of replicates Stabilization time (s) Background correction
Flow blurring (OneNeb) Cyclonic, double-pass 10 3 15 Auto
carbon emission (Swan bands) [12]. According to Young et al., the addition of 165 mL min −1 of oxygen gas as a combustion aid drastically reduces the Swan bands in the plasma while analyzing n-propanol microemulsions [13]. Another alternative to determine trace levels of Si in fuels is inductively coupled plasma mass spectrometry (ICP-MS). This, however, is not a trivial task. In addition to difficulties associated with plasma instability, accuracy in 28Si+, 29Si+ and 30Si+ determinations is severely compromised by spectral interferences caused by molecular ions abundantly present in the plasma, e.g. 14N2+, 12C 16O+, 14N2H+ and 14N16O+ [14]. High resolution ICP-MS, isotopic dilution and collision/reaction cells have been the most efficient strategies to overcome these difficulties [15]. Pohl et al., for example, determined Si in organic solutions and petrochemical samples using a double focusing magnetic sector high resolution ICP-MS [16]. On the other hand, microwave-induced plasma optical emission spectrometry (MIP OES) has also been used in several applications [17]. Determination of hydride forming elements (As, Bi, Ge, Sb, Se, and Sn) and Hg in dogfish liver, sediment, soil and water was performed by Ar/He mixed gas MIP OES [18]. In another work, a slurry sampling procedure was used to determine macro and trace elements in a reference sample of biological material by an Ar-based MIP operated at 300 W [19]. Recently, an improved magneticallycoupled, torch-based MIP OES instrument has been made commercially available. One of its main advantages is that it runs on nitrogen gas, so an air compressor and a N2 generator are sufficient to maintain a stable plasma, which reflects in significantly lower costs. The preliminary evaluation of the analytical capabilities of this instrument has demonstrated its applicability for the determination of Al, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, and Zn in plant materials [20], and Ca, K, Mg, and Na in diesel samples previously diluted in organic solvent (synthetic isoparaffinic hydrocarbon) [21]. Microwave-induced plasmas (MIPs) operate at lower applied powers (up to 1 kW), which may lead to more severe matrix effects. On the other hand, different gases can be used in MIP instruments and contribute to decomposing matrix components more efficiently than Ar-ICPs. A strategy to overcome the negative effects of MIP's lower temperatures and improve its analytical performance is to use an efficient nebulization system, which is capable of introducing smaller, more homogeneously distributed particles. According to Gañán-Calvo et al. [22]
Table 2 Viewing positions, nebulizer pressures and EGCM settings for Si determination using the 4100 MP-AES. Sample medium
Wavelength (nm)
Microemulsion 251.611 288.158 Aqueous 251.611 288.158
Viewing position (mm) 0 10 −20 −30
a
319
Table 3 Heating program for microwave-assisted acid digestion of diesel and biodiesel samples. Step
Applied power (W)
Time (min)
Temperature (°C)
1 2 3 4 5
250 0 550 650 750
2 3 4 5 5
80 80 120 200 200
and Aguirre et al. [23], the flow blurring nebulizer (FBN) technology can improve sensitivity because it provides a more homogeneous aerosol, with smaller and narrow-distributed particles. Matusiewicz et al. [24] compared the performances of seven different nebulizers in Ar–He MIP OES determinations: FBN, high efficiency nebulizer (HEN), demountable direct injection high efficiency nebulizer (D-DIHEN), AriMist (AM), MiraMist CE (MMCE), ultrasonic nebulizer (NOVA‐1), and a conventional Meinhard pneumatic concentric nebulizer (PN). According to these authors, the use of FBN and D-DIHEN presented better sensitivity than the other nebulizers evaluated. The present work describes the determination of Si in diesel and biodiesel samples by combining a simple dilute-and-shoot procedure with ethanol, the flow blurring nebulization and MIP OES. Two additional sample preparation procedures are also evaluated for comparison. The instrument robustness is demonstrated by analyzing samples diluted in 90% v v −1 ethanol without any temperature control of the nebulization chamber. Part of the results here presented was used in an application note for the Agilent's 4100 MP-AES instrument [25]. 2. Experimental 2.1. Instrumentation A N2-MIP OES (4100 MP-AES, Agilent Technologies, Melbourne, Australia) was used in all determinations. This instrument is equipped with an external gas control module (EGCM), which injects air into the plasma to prevent carbon deposition on both the torch and the optical components. The EGCM also contributes for plasma stability and minimization of background emission in organic sample analyses. The sample introduction system is composed of solvent-resistant tubing, a double-pass cyclonic chamber and the FBN-based OneNeb nebulizer. The instrument software (MP Expert) allows automatic background correction (Auto) by recording, storing and automatically subtracting a background spectrum from each standard and sample solution analyzed. The MP Expert also performs the optimization of the nebulizer pressure and the viewing position for each wavelength monitored. In this case, each analytical line is measured sequentially in its optimal condition. A standard reference solution (1 mg L −1) was used to optimize such parameters. The instrumental operating conditions and settings used to determine Si are presented in Tables 1 and 2. A closed-vessel microwave oven (Ethos 1600, Milestone, Sorisole, Italy) equipped with 45 mL PFA vessels was employed for the acid digestion of diesel and biodiesel samples. 2.2. Reagents and standard reference solutions
b
Nebulizer Pressure (kPa)
EGCM
100 120 120 160
Medium Medium Medium Medium
a Position of one optical mirror related to the torch. It is the position in the plasma where the analytical signal is monitored. b External gas control module.
Analytical grade ethanol (J.T. Baker, Hexis, São Paulo, SP, Brazil) was used for direct sample dilution. A 1000 mg L−1 Si stock solution in organic medium (Conostan, Quimlab, Jacareí, SP, Brazil) was used in spike studies of ethanol-diluted diesel and biodiesel samples. For the comparative sample preparation procedures, nitric acid (Merck, Darmstadt, Germany) previously purified by a sub-boiling distillation system (Milestone), and hydrogen peroxide 30% m m−1 (Synth, São Paulo, SP, Brazil) were employed to digest the samples. Polyoxyethylene(10)isooctylphenyl
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Table 4 Figures of merit for silicon determination by N2-MIP OES. HNO3 1% v v−1
Si (251.611 nm) Si (288.158 nm) a b
Microemulsion
LOD (μg L−1)
LDRa (decades)
RSDb (%)
LOD (μg L−1)
LDRa (decades)
RSDb (%)
20 240
2.3 0.9
1.6 1.3
5 5
2.6 2.5
1.6 0.4
Linear dynamic range starting at the limit of detection. Repeatability presented as the relative standard deviation for a 2 mg L−1 Si solution (n = 10).
ether (Triton X-100, Acros Organics, Geel, Belgium), n-propanol and light mineral oil (Tedia, Rio de Janeiro, RJ, Brazil), were used for the preparation of microemulsions. A 1000 mg L−1 Si stock solution (Tec-Lab, Hexis, São Paulo, SP, Brazil) was adequately diluted to prepare aqueous and microemulsion standard reference solutions and to carry out spike studies in digested samples and microemulsions. The external calibration method was used in all determinations. For digested samples and the ones simply diluted in 90% v v−1 ethanol, aqueous standards prepared in HNO3 1% v v−1 were used to build the analytical calibration curves. For the microemulsion procedure, the analytical calibration standards and the samples were prepared similarly. In this case, 0.2 mL of mineral oil was used to simulate the sample matrix viscosity while preparing the calibration curve standards [13].
2.3. Samples and sample preparation Biodiesel samples were provided by the Center of Characterization and Development of Materials (CCDM, Federal University of São Carlos, São Carlos, SP, Brazil). Diesel fuel samples containing 5% v v −1 of biodiesel (B5), in accordance to the Brazilian legislation [2], were obtained in local gas stations of São Carlos, SP, Brazil. The direct dilution of samples in ethanol was carried out by adding 9 mL of the solvent to 1 mL of sample. For comparison, microwaveassisted acid digestion and microemulsion preparation in n-propanol were also evaluated. Sample digestions were performed by adding 5 mL of 50% v v−1 HNO3 (7 mol L−1) and 3.0 mL of H2O2 30% m m−1 to approximately 0.2 g of sample. Table 3 presents the heating program adopted in this case. It must be mentioned that closed-vessel microwave-assisted digestions, usually performed at high temperatures and pressures, are potentially dangerous procedures and all safety recommendations must be strictly followed. Microemulsions were prepared by adding 0.5 mL of Triton X-100 and 0.5 mL of a 20% v v−1 HNO3 aqueous solution to 1.0 mL of diesel or biodiesel samples. Then, the volume was made up to 10 mL with n-propanol and the mixture was homogenized for 2 min with a vortex mixer [5].
3. Results and discussion 3.1. Instrumental limits of detection in different media Limit of detection (LOD), relative standard deviation (RSD, n = 10) and linear dynamic range (LDR) values for the different procedures evaluated are presented in Table 4. The LOD for both 1% v v −1 HNO3 and microemulsion media were calculated by using background equivalent concentrations (BEC), signal-to-background ratios (SBR) obtained with a 2.0 mg L −1 Si standard reference solution, and RSD for 10 consecutive measurements of the blank in each case [26]. From Table 4, it can be observed that Si can be determined with adequate sensitivity and according to the Brazilian legislation requirements by adopting any of the three sample preparation procedures evaluated [2]. It is important to note that aqueous standard reference solutions prepared in HNO3 1% v v −1 were used to determine Si in ethanol-diluted samples. Therefore, it may be observed that this element could be detected at concentrations as low as 200 μg L −1 (considering a 10-fold sample dilution) using this simple dilute-and-shoot procedure. It was observed that the N2-MIP remained stable even while introducing high concentrations of n-propanol or ethanol. In addition, carbon deposition was observed neither on the torch nor on the pre-optical window after 8 h of operation. These facts may be related to carbon conversion to gaseous species in its reaction with air introduced into the plasma by the EGCM. Additional plasma stability and lower background emission is related to both the FBN sample introduction efficiency and the automatic background spectrum subtraction.
3.2. Accuracy Accuracy was evaluated by spike studies with both sample matrices, i.e. diesel and biodiesel. Two different spike concentrations were evaluated for the ethanol dilution procedure: 0.5 and 1.0 mg L −1. In this case, different volumes of a Si reference solution prepared in
Table 5 Spike experiments for Si determination by N2-MIP OES in diesel and biodiesel after dilution in 90% v v−1 ethanol, sample digestion, or microemulsion preparation. Concentrations reported in mg L−1 (mean ± standard deviation, n = 3). Sample
Si emission line (nm)
Biodiesel
251.611 288.158
Diesel
251.611 288.158
a b
Spike solution in organic medium. Spike solution in aqueous medium.
Ethanola
Digestionb
Microemulsionb
Added
Found
Added
Found
Added
Found
0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0
0.45 ± 0.03 0.99 ± 0.09 0.40 ± 0.04 1.02 ± 0.17 0.47 ± 0.01 0.91 ± 0.01 0.46 ± 0.01 0.95 ± 0.01
3.0
3.05 ± 0.07
1.0
0.89 ± 0.05
3.0
3.05 ± 0.01
1.0
0.89 ± 0.06
3.0
3.09 ± 0.10
1.0
0.96 ± 0.03
3.0
3.07 ± 0.15
1.0
0.96 ± 0.04
R.S. Amais et al. / Microchemical Journal 106 (2013) 318–322
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Fig. 1. Recoveries for Si (0.5 mg L−1) in HNO3 1% v v−1 at 251.611 nm without and with Ca, K, Mg or Na in separated solutions at different concentrations (1, 10 or 50 mg L−1). Mix contains 0.5 mg L−1 of Si and all concomitants (Ca, K, Mg and Na) present at 10 mg L−1 each.
organic medium were added directly to the samples. For digested samples and microemulsions, different volumes of Si aqueous standards were added either after the digestion or directly to the samples during the microemulsion preparation. In each case, Si final concentrations were 3.0 and 1.0 mg L −1, respectively. The results for these studies are shown in Table 5. As it can be seen, all recoveries were between 80 and 103%. For the ethanol dilution procedure, recoveries were in the 91–102% range, except for one sample: 80% recovery for biodiesel with Si determination at 288.158 nm. Considering that a 90% recovery was obtained for the same sample using an alternative wavelength, and that no signal correction method such as internal standardization was employed, these results are acceptable. From Table 5, it can also be observed that the simple sample dilution in ethanol combined with a non matrix-matching external calibration method using aqueous solutions presents similar results to other conventional, more laborious procedures. The combination of efficient nebulization, resulting in homogeneous aerosols composed of small, narrow-distributed particles; and background minimization achieved by automatic signal correction and air introduction into the plasma are important aspects contributing to such performance.
3.3. Interference study The influence of Ca, K, Mg, and Na as concomitants on silicon analytical signals was evaluated. These elements were chosen based on their potential high concentrations in the samples analyzed. The interference experiments were carried out by monitoring Si signal intensity in a binary solution containing the analyte at 0.5 mg L−1 and the concomitant at 1, 10 or 50 mg L −1 in 1% HNO3 v v −1. A multi-element solution containing all concomitants at 10 mg L −1 was also analyzed. Interferences were considered significant for signal enhancements or suppressions equal or larger than 10%. From Fig. 1, it is possible to observe that no concomitant interfered significantly on Si emission signal either individually or as a group even for concentrations 100-fold higher than the analyte.
4. Conclusions Accurate Si determinations in diesel and biodiesel were carried out simply by diluting samples in ethanol. For comparison, adequate recoveries were also obtained by microwave-assisted digestion or microemulsion preparation. Diesel or biodiesel dilution in ethanol is preferred when compared to the other sample preparation strategies evaluated due to the higher sample throughput and the possibility of using external calibration with inorganic standards prepared in aqueous solutions. Another important advantage of this procedure is its simplicity, which may result in easy implementation in routine analyses. Despite the N2-MIP lower temperatures, neither carbon deposits nor loss of instrument performance was observed while introducing n-propanol microemulsions or 90% v v−1 ethanol solutions into the plasma. Considering that the N2 gas used is obtained from an air compressor and a nitrogen generator, an important advantage of the instrument used (4100 MP-AES) is its low cost of operation. Acknowledgments The authors would like to thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the scholarship and fellowship provided to R.S.A. and G.L.D. (grants 2010/17387-7 and 2010/50238-5, respectively). Research grants provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) to J.A.N. is also especially acknowledged. The technical support provided by Agilent Technologies is also greatly appreciated. References [1] R. Sánchez, J.L. Todolí, C.P. Lienemann, J.M. Mermet, Effect of the silicon chemical form on the emission intensity in inductively coupled plasma atomic emission spectrometry for xylene matrices, J. Anal. At. Spectrom. 24 (2009) 391–401. [2] Brazilian National Agency of Petroleum, Natural Gas and Biofuels. Resolution ANP 52 of December, 2010, DOU 30/12/2010. [3] A.V. Soin, T.A. Maryutina, T.V. Arbuzov, B.Y. Spivakov, Sample preparation in the determination of metals in oil and petroleum products by ICP MS, J. Anal. Chem. 65 (2010) 571–576.
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[4] M.G.A. Korn, D.C.M.B. Santos, M.A.B. Guida, I.S. Barbosa, M.L.C. Passos, M.L.M.F.S. Saraiva, J.L.F.C. Lima, Evaluation of digestion procedures for simultaneous determination of Ca, P, Mg, K and Na in biodiesel by inductively coupled plasma optical emission spectrometry, J. Braz. Chem. Soc. 21 (2010) 2278–2284. [5] J.L. Burguera, M. Burguera, Pretreatment of oily samples for analysis by flow injection-spectrometric methods, Talanta 83 (2011) 691–699. [6] F.H. Lyra, M.T.W.D. Carneiro, G.P. Brandão, H.M. Pessoa, E.V. Castro, Determination of Na, K, Ca and Mg in biodiesel samples by flame atomic absorption spectrometry (F AAS) using microemulsion as sample preparation, Microchem. J. 96 (2010) 180–185. [7] R.J. Cassella, D.M. Brum, N.F. Robaina, A.A. Rocha, C.F. Lima, Extraction induced by emulsion breaking for metals determination in diesel oil by ICP-MS, J. Anal. At. Spectrom. 27 (2012) 364–370. [8] ABNT NBR 15553 — Produtos derivados de gorduras e óleos — Ésteres metílicos/etílicos de ácidos graxos — Determinação dos teores de cálcio, magnésio, sódio, fósforo e potássio por espectrometria de emissão ótica com plasma indutivamente acoplado (ICP OES). [9] A.I. Barros, A.P. Oliveira, M.R.L. Magalhães, R.D. Villa, Determination of sodium and potassium in biodiesel by flame atomic emission spectrometry, with dissolution in ethanol as a single sample preparation step, Fuel 93 (2012) 381–384. [10] ASTM D 5184 — Standard test methods for determination of aluminum and silicon in fuel oils by ashing, fusion, inductively coupled plasma atomic emission spectrometry, and atomic absorption spectrometry. [11] J. Nölte, ICP Emission Spectrometry: A Practical Guide, first ed. Wiley-VCH, Weinheim, 2003. [12] M. Edlund, H. Visser, P. Heitland, Analysis of biodiesel by argon–oxygen mixed-gas inductively coupled plasma optical emission spectrometry, J. Anal. At. Spectrom. 17 (2002) 232–235. [13] C.G. Young, R.S. Amais, D. Schiavo, E.E. Garcia, J.A. Nóbrega, B.T. Jones, Determination of sulfur in biodiesel microemulsions using the summation of the intensities of multiple emission lines, Talanta 84 (2011) 995–999. [14] R.B. Georg, B.C. Reynolds, M. Frank, A.N. Halliday, New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS, Chem. Geol. 235 (2006) 95–104. [15] J.S. Becker, State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS — plenary lecture, J. Anal. At. Spectrom. 17 (2002) 1172–1185.
[16] P. Pohl, N. Vorapalawut, B. Bouyssiere, R. Lobinski, Trace-level determination and insight in speciation of silicon in petrochemical samples by flow-injection high resolution ICP MS and HPLC-high resolution ICP MS, J. Anal. At. Spectrom. 25 (2010) 1461–1466. [17] K.J. Jankowski, E. Reszke, Microwave Induced Plasma Analytical Spectrometry, first ed. RSC Publishing, Cambridge, 2011. [18] H. Matusiewicz, M. Ślachciśki, Method development for simultaneous multielement determination of hydride forming elements (As, Bi, Ge, Sb, Se, Sn) and Hg by microwave induced plasma optical emission spectrometry using integrated continuous-microflow ultrasonic nebulizer-hydride generator sample introduction system, Microchem. J. 95 (2010) 213–221. [19] H. Matusiewicz, B. Golik, Simultaneous determination of macro and trace elements in biological reference materials by microwave induced plasma optical emission spectrometry with slurry sample introduction, Spectrochim. Acta B 59 (2004) 749–754. [20] C. Wu, Y. Chen, K. Ouyang, Z. Zhang, C. Taylor, Analysis of Chinese herbal medicines by microwave plasma-atomic emission spectrometry (MP-AES). (Application Note 5990-9791EN) Agilent Technol. (2012). [21] P. Lowenstern, E. Reisman, Analysis of diesel using the 4100 MP-AES. (Application Note 5990-9005EN) Agilent Technol. (2011). [22] A.M. Gañán-Calvo, Enhanced liquid atomization: from flow-focusing to flowblurring, Appl. Phys. Lett. 86 (2005) 214101. [23] M.A. Aguirre, N. Kovachev, B. Almagro, M. Hidalgo, A. Canals, Compensation for matrix effects on ICP-OES by on-line calibration methods using a new multinebulizer based on Flow Blurring® technology, J. Anal. At. Spectrom. 25 (2010) 1724–1732. [24] H. Matusiewicz, M. Ślachciśki, B. Almagro, A. Canals, Evaluation of various types of micronebulizers and spray chamber configurations for microsamples analysis by microwave induced plasma optical emission spectrometry, Chem. Anal. 54 (2009) 1219–1244. [25] R.S. Amais, G.L. Donati, D. Schiavo, J.A. Nóbrega, Determination of silicon in diesel and biodiesel by microwave plasma-atomic emission spectrometry. (Application Note 5991-0490EN) Agilent Technol. (2012). [26] V. Thomsen, G. Roberts, K. Burgess, The concept of background equivalent concentration in spectrochemistry, Spectrosc. 15 (2000) 33–36.
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A simple dilute-and-shoot procedure for Si determination in diesel and biodiesel by microwave-induced plasma optical emission spectrometry Renata S. Amais a, George L. Donati a,⁎, Daniela Schiavo b, Joaquim A. Nóbrega a a b
Group of Applied Instrumental Analysis, Department of Chemistry, Federal University of São Carlos, São Carlos, SP, Brazil Agilent Technologies, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 5 July 2012 Received in revised form 14 August 2012 Accepted 5 September 2012 Available online 12 September 2012 Keywords: Silicon determination Liquid fuel MIP OES Ethanol dilution
a b s t r a c t In the present work, microwave-induced plasma optical emission spectrometry (MIP OES) and the flow blurring nebulizer (FBN) technology are used to determine silicon in diesel and biodiesel samples. A simple diluteand-shoot procedure with ethanol is presented. Two additional sample preparation procedures are also evaluated for comparison: closed-vessel microwave-assisted acid digestion and microemulsion preparation in n-propanol. Limits of detection (LOD) vary from 5 to 20 μg L−1 and relative standard deviations (RSD) are lower than 2% in all cases. Accuracy is evaluated by spike experiments, with recoveries between 80 and 103%. The influence of Ca, K, Mg and Na as concomitant ions on silicon determination by MIP OES is also investigated. Significant effects on Si emission signals are observed neither in single nor multiple concomitant experiments. Although good results are obtained with all sample preparation procedures evaluated, sample dilution in ethanol represents a simpler and faster strategy, which allows external calibration with inorganic standards and aqueous solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The performance of engines is directly affected by the presence of metals and metalloids in fuels. Depending on their concentrations, these species can cause several problems such as corrosion of engine parts and shortening of the machinery lifetime. From an environmental perspective, some elements act as catalyst poison, which contributes to increasing air pollution from toxic gases and particulate matter emitted by vehicles. Silicon compounds, such as siloxanes, are added to diesel fuel as anti-foaming agents. Species produced during the combustion of these compounds can form a coating layer on the catalyst element of the exhaust system, significantly increasing air pollution [1]. Thus, the Brazilian National Agency of Petroleum, Natural Gas and Biofuels has established the maximum concentration of Si plus Al in diesel as 80 mg kg −1 [2]. Different procedures have been proposed to determine metallic species in fuels. However, due to the complexity of this matrix, time consuming sample preparation procedures are usually required [3]. Korn et al. proposed two different digestion procedures for biodiesel based either on closed-vessel microwave-assisted or on open-vessel conductive heating [4]. In both cases low residual carbon contents were obtained, but considering better accuracies and lower consumption of acids, the former approach was recommended by the authors.
⁎ Corresponding author. Tel.: +55 16 3351 8058; fax: +55 16 3351 8350. E-mail address: [email protected] (G.L. Donati). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.09.001
Emulsions and microemulsions have also been used as alternative sample preparation strategies, especially because they require less toxic reagents and present relatively high sample throughput [5]. An additional advantage is that emulsions and microemulsions are compatible with external calibration using aqueous standard solutions [6]. Cassella et al., for example, recently proposed an approach to extract analytes from diesel fuel into an aqueous solution by employing the emulsion breaking extraction-induced procedure [7]. In this case, the samples are emulsified in an acidic solution containing a surfactant (Triton X-114) and the inorganic analytes are subsequently extracted into an aqueous solution by centrifugation and emulsion breaking. The official method for biodiesel analysis according to the Brazilian Legislation (ABNT NBR 15553), for example, is based on the dilution of samples in xylene before elemental determination [8]. This procedure presents a high sample throughput, but some important limitations are the toxicity of xylene and the necessity of using organometallic standards [5]. According to Barros et al., sample dilution with short chain alcohols instead of toxic hydrocarbons can reduce costs, improve nebulization and allow calibration with aqueous standards [9]. For the determination of Al and Si in fuels, the standard method ASTM D 5184 recommends the organic matrix removal by combustion in a muffle furnace and elemental determination by atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP OES) [10]. Inductively coupled plasma OES has been widely used for elemental determination due to its multi-element capacity, high dynamic linear range and versatility [11]. On the other hand, the introduction of organic samples may require oxygen addition into the plasma to improve its stability and reduce high background signals caused by
R.S. Amais et al. / Microchemical Journal 106 (2013) 318–322 Table 1 N2-MIP OES operating conditions for Si determination in diesel and biodiesel samples. Instrument parameter
Operating condition
Nebulizer Spray chamber Read time (s) Number of replicates Stabilization time (s) Background correction
Flow blurring (OneNeb) Cyclonic, double-pass 10 3 15 Auto
carbon emission (Swan bands) [12]. According to Young et al., the addition of 165 mL min −1 of oxygen gas as a combustion aid drastically reduces the Swan bands in the plasma while analyzing n-propanol microemulsions [13]. Another alternative to determine trace levels of Si in fuels is inductively coupled plasma mass spectrometry (ICP-MS). This, however, is not a trivial task. In addition to difficulties associated with plasma instability, accuracy in 28Si+, 29Si+ and 30Si+ determinations is severely compromised by spectral interferences caused by molecular ions abundantly present in the plasma, e.g. 14N2+, 12C 16O+, 14N2H+ and 14N16O+ [14]. High resolution ICP-MS, isotopic dilution and collision/reaction cells have been the most efficient strategies to overcome these difficulties [15]. Pohl et al., for example, determined Si in organic solutions and petrochemical samples using a double focusing magnetic sector high resolution ICP-MS [16]. On the other hand, microwave-induced plasma optical emission spectrometry (MIP OES) has also been used in several applications [17]. Determination of hydride forming elements (As, Bi, Ge, Sb, Se, and Sn) and Hg in dogfish liver, sediment, soil and water was performed by Ar/He mixed gas MIP OES [18]. In another work, a slurry sampling procedure was used to determine macro and trace elements in a reference sample of biological material by an Ar-based MIP operated at 300 W [19]. Recently, an improved magneticallycoupled, torch-based MIP OES instrument has been made commercially available. One of its main advantages is that it runs on nitrogen gas, so an air compressor and a N2 generator are sufficient to maintain a stable plasma, which reflects in significantly lower costs. The preliminary evaluation of the analytical capabilities of this instrument has demonstrated its applicability for the determination of Al, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, and Zn in plant materials [20], and Ca, K, Mg, and Na in diesel samples previously diluted in organic solvent (synthetic isoparaffinic hydrocarbon) [21]. Microwave-induced plasmas (MIPs) operate at lower applied powers (up to 1 kW), which may lead to more severe matrix effects. On the other hand, different gases can be used in MIP instruments and contribute to decomposing matrix components more efficiently than Ar-ICPs. A strategy to overcome the negative effects of MIP's lower temperatures and improve its analytical performance is to use an efficient nebulization system, which is capable of introducing smaller, more homogeneously distributed particles. According to Gañán-Calvo et al. [22]
Table 2 Viewing positions, nebulizer pressures and EGCM settings for Si determination using the 4100 MP-AES. Sample medium
Wavelength (nm)
Microemulsion 251.611 288.158 Aqueous 251.611 288.158
Viewing position (mm) 0 10 −20 −30
a
319
Table 3 Heating program for microwave-assisted acid digestion of diesel and biodiesel samples. Step
Applied power (W)
Time (min)
Temperature (°C)
1 2 3 4 5
250 0 550 650 750
2 3 4 5 5
80 80 120 200 200
and Aguirre et al. [23], the flow blurring nebulizer (FBN) technology can improve sensitivity because it provides a more homogeneous aerosol, with smaller and narrow-distributed particles. Matusiewicz et al. [24] compared the performances of seven different nebulizers in Ar–He MIP OES determinations: FBN, high efficiency nebulizer (HEN), demountable direct injection high efficiency nebulizer (D-DIHEN), AriMist (AM), MiraMist CE (MMCE), ultrasonic nebulizer (NOVA‐1), and a conventional Meinhard pneumatic concentric nebulizer (PN). According to these authors, the use of FBN and D-DIHEN presented better sensitivity than the other nebulizers evaluated. The present work describes the determination of Si in diesel and biodiesel samples by combining a simple dilute-and-shoot procedure with ethanol, the flow blurring nebulization and MIP OES. Two additional sample preparation procedures are also evaluated for comparison. The instrument robustness is demonstrated by analyzing samples diluted in 90% v v −1 ethanol without any temperature control of the nebulization chamber. Part of the results here presented was used in an application note for the Agilent's 4100 MP-AES instrument [25]. 2. Experimental 2.1. Instrumentation A N2-MIP OES (4100 MP-AES, Agilent Technologies, Melbourne, Australia) was used in all determinations. This instrument is equipped with an external gas control module (EGCM), which injects air into the plasma to prevent carbon deposition on both the torch and the optical components. The EGCM also contributes for plasma stability and minimization of background emission in organic sample analyses. The sample introduction system is composed of solvent-resistant tubing, a double-pass cyclonic chamber and the FBN-based OneNeb nebulizer. The instrument software (MP Expert) allows automatic background correction (Auto) by recording, storing and automatically subtracting a background spectrum from each standard and sample solution analyzed. The MP Expert also performs the optimization of the nebulizer pressure and the viewing position for each wavelength monitored. In this case, each analytical line is measured sequentially in its optimal condition. A standard reference solution (1 mg L −1) was used to optimize such parameters. The instrumental operating conditions and settings used to determine Si are presented in Tables 1 and 2. A closed-vessel microwave oven (Ethos 1600, Milestone, Sorisole, Italy) equipped with 45 mL PFA vessels was employed for the acid digestion of diesel and biodiesel samples. 2.2. Reagents and standard reference solutions
b
Nebulizer Pressure (kPa)
EGCM
100 120 120 160
Medium Medium Medium Medium
a Position of one optical mirror related to the torch. It is the position in the plasma where the analytical signal is monitored. b External gas control module.
Analytical grade ethanol (J.T. Baker, Hexis, São Paulo, SP, Brazil) was used for direct sample dilution. A 1000 mg L−1 Si stock solution in organic medium (Conostan, Quimlab, Jacareí, SP, Brazil) was used in spike studies of ethanol-diluted diesel and biodiesel samples. For the comparative sample preparation procedures, nitric acid (Merck, Darmstadt, Germany) previously purified by a sub-boiling distillation system (Milestone), and hydrogen peroxide 30% m m−1 (Synth, São Paulo, SP, Brazil) were employed to digest the samples. Polyoxyethylene(10)isooctylphenyl
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Table 4 Figures of merit for silicon determination by N2-MIP OES. HNO3 1% v v−1
Si (251.611 nm) Si (288.158 nm) a b
Microemulsion
LOD (μg L−1)
LDRa (decades)
RSDb (%)
LOD (μg L−1)
LDRa (decades)
RSDb (%)
20 240
2.3 0.9
1.6 1.3
5 5
2.6 2.5
1.6 0.4
Linear dynamic range starting at the limit of detection. Repeatability presented as the relative standard deviation for a 2 mg L−1 Si solution (n = 10).
ether (Triton X-100, Acros Organics, Geel, Belgium), n-propanol and light mineral oil (Tedia, Rio de Janeiro, RJ, Brazil), were used for the preparation of microemulsions. A 1000 mg L−1 Si stock solution (Tec-Lab, Hexis, São Paulo, SP, Brazil) was adequately diluted to prepare aqueous and microemulsion standard reference solutions and to carry out spike studies in digested samples and microemulsions. The external calibration method was used in all determinations. For digested samples and the ones simply diluted in 90% v v−1 ethanol, aqueous standards prepared in HNO3 1% v v−1 were used to build the analytical calibration curves. For the microemulsion procedure, the analytical calibration standards and the samples were prepared similarly. In this case, 0.2 mL of mineral oil was used to simulate the sample matrix viscosity while preparing the calibration curve standards [13].
2.3. Samples and sample preparation Biodiesel samples were provided by the Center of Characterization and Development of Materials (CCDM, Federal University of São Carlos, São Carlos, SP, Brazil). Diesel fuel samples containing 5% v v −1 of biodiesel (B5), in accordance to the Brazilian legislation [2], were obtained in local gas stations of São Carlos, SP, Brazil. The direct dilution of samples in ethanol was carried out by adding 9 mL of the solvent to 1 mL of sample. For comparison, microwaveassisted acid digestion and microemulsion preparation in n-propanol were also evaluated. Sample digestions were performed by adding 5 mL of 50% v v−1 HNO3 (7 mol L−1) and 3.0 mL of H2O2 30% m m−1 to approximately 0.2 g of sample. Table 3 presents the heating program adopted in this case. It must be mentioned that closed-vessel microwave-assisted digestions, usually performed at high temperatures and pressures, are potentially dangerous procedures and all safety recommendations must be strictly followed. Microemulsions were prepared by adding 0.5 mL of Triton X-100 and 0.5 mL of a 20% v v−1 HNO3 aqueous solution to 1.0 mL of diesel or biodiesel samples. Then, the volume was made up to 10 mL with n-propanol and the mixture was homogenized for 2 min with a vortex mixer [5].
3. Results and discussion 3.1. Instrumental limits of detection in different media Limit of detection (LOD), relative standard deviation (RSD, n = 10) and linear dynamic range (LDR) values for the different procedures evaluated are presented in Table 4. The LOD for both 1% v v −1 HNO3 and microemulsion media were calculated by using background equivalent concentrations (BEC), signal-to-background ratios (SBR) obtained with a 2.0 mg L −1 Si standard reference solution, and RSD for 10 consecutive measurements of the blank in each case [26]. From Table 4, it can be observed that Si can be determined with adequate sensitivity and according to the Brazilian legislation requirements by adopting any of the three sample preparation procedures evaluated [2]. It is important to note that aqueous standard reference solutions prepared in HNO3 1% v v −1 were used to determine Si in ethanol-diluted samples. Therefore, it may be observed that this element could be detected at concentrations as low as 200 μg L −1 (considering a 10-fold sample dilution) using this simple dilute-and-shoot procedure. It was observed that the N2-MIP remained stable even while introducing high concentrations of n-propanol or ethanol. In addition, carbon deposition was observed neither on the torch nor on the pre-optical window after 8 h of operation. These facts may be related to carbon conversion to gaseous species in its reaction with air introduced into the plasma by the EGCM. Additional plasma stability and lower background emission is related to both the FBN sample introduction efficiency and the automatic background spectrum subtraction.
3.2. Accuracy Accuracy was evaluated by spike studies with both sample matrices, i.e. diesel and biodiesel. Two different spike concentrations were evaluated for the ethanol dilution procedure: 0.5 and 1.0 mg L −1. In this case, different volumes of a Si reference solution prepared in
Table 5 Spike experiments for Si determination by N2-MIP OES in diesel and biodiesel after dilution in 90% v v−1 ethanol, sample digestion, or microemulsion preparation. Concentrations reported in mg L−1 (mean ± standard deviation, n = 3). Sample
Si emission line (nm)
Biodiesel
251.611 288.158
Diesel
251.611 288.158
a b
Spike solution in organic medium. Spike solution in aqueous medium.
Ethanola
Digestionb
Microemulsionb
Added
Found
Added
Found
Added
Found
0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0
0.45 ± 0.03 0.99 ± 0.09 0.40 ± 0.04 1.02 ± 0.17 0.47 ± 0.01 0.91 ± 0.01 0.46 ± 0.01 0.95 ± 0.01
3.0
3.05 ± 0.07
1.0
0.89 ± 0.05
3.0
3.05 ± 0.01
1.0
0.89 ± 0.06
3.0
3.09 ± 0.10
1.0
0.96 ± 0.03
3.0
3.07 ± 0.15
1.0
0.96 ± 0.04
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Fig. 1. Recoveries for Si (0.5 mg L−1) in HNO3 1% v v−1 at 251.611 nm without and with Ca, K, Mg or Na in separated solutions at different concentrations (1, 10 or 50 mg L−1). Mix contains 0.5 mg L−1 of Si and all concomitants (Ca, K, Mg and Na) present at 10 mg L−1 each.
organic medium were added directly to the samples. For digested samples and microemulsions, different volumes of Si aqueous standards were added either after the digestion or directly to the samples during the microemulsion preparation. In each case, Si final concentrations were 3.0 and 1.0 mg L −1, respectively. The results for these studies are shown in Table 5. As it can be seen, all recoveries were between 80 and 103%. For the ethanol dilution procedure, recoveries were in the 91–102% range, except for one sample: 80% recovery for biodiesel with Si determination at 288.158 nm. Considering that a 90% recovery was obtained for the same sample using an alternative wavelength, and that no signal correction method such as internal standardization was employed, these results are acceptable. From Table 5, it can also be observed that the simple sample dilution in ethanol combined with a non matrix-matching external calibration method using aqueous solutions presents similar results to other conventional, more laborious procedures. The combination of efficient nebulization, resulting in homogeneous aerosols composed of small, narrow-distributed particles; and background minimization achieved by automatic signal correction and air introduction into the plasma are important aspects contributing to such performance.
3.3. Interference study The influence of Ca, K, Mg, and Na as concomitants on silicon analytical signals was evaluated. These elements were chosen based on their potential high concentrations in the samples analyzed. The interference experiments were carried out by monitoring Si signal intensity in a binary solution containing the analyte at 0.5 mg L−1 and the concomitant at 1, 10 or 50 mg L −1 in 1% HNO3 v v −1. A multi-element solution containing all concomitants at 10 mg L −1 was also analyzed. Interferences were considered significant for signal enhancements or suppressions equal or larger than 10%. From Fig. 1, it is possible to observe that no concomitant interfered significantly on Si emission signal either individually or as a group even for concentrations 100-fold higher than the analyte.
4. Conclusions Accurate Si determinations in diesel and biodiesel were carried out simply by diluting samples in ethanol. For comparison, adequate recoveries were also obtained by microwave-assisted digestion or microemulsion preparation. Diesel or biodiesel dilution in ethanol is preferred when compared to the other sample preparation strategies evaluated due to the higher sample throughput and the possibility of using external calibration with inorganic standards prepared in aqueous solutions. Another important advantage of this procedure is its simplicity, which may result in easy implementation in routine analyses. Despite the N2-MIP lower temperatures, neither carbon deposits nor loss of instrument performance was observed while introducing n-propanol microemulsions or 90% v v−1 ethanol solutions into the plasma. Considering that the N2 gas used is obtained from an air compressor and a nitrogen generator, an important advantage of the instrument used (4100 MP-AES) is its low cost of operation. Acknowledgments The authors would like to thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the scholarship and fellowship provided to R.S.A. and G.L.D. (grants 2010/17387-7 and 2010/50238-5, respectively). Research grants provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) to J.A.N. is also especially acknowledged. The technical support provided by Agilent Technologies is also greatly appreciated. References [1] R. Sánchez, J.L. Todolí, C.P. Lienemann, J.M. Mermet, Effect of the silicon chemical form on the emission intensity in inductively coupled plasma atomic emission spectrometry for xylene matrices, J. Anal. At. Spectrom. 24 (2009) 391–401. [2] Brazilian National Agency of Petroleum, Natural Gas and Biofuels. Resolution ANP 52 of December, 2010, DOU 30/12/2010. [3] A.V. Soin, T.A. Maryutina, T.V. Arbuzov, B.Y. Spivakov, Sample preparation in the determination of metals in oil and petroleum products by ICP MS, J. Anal. Chem. 65 (2010) 571–576.
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