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Determination of sulfur in coal and ash slurry by high-resolution continuum source electrothermal molecular absorption spectrometry
Spectrochimica Acta Part B 88 (2013) 80–84
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Analytical note
Determination of sulfur in coal and ash slurry by high-resolution continuum source electrothermal molecular absorption spectrometry Flávio V. Nakadi, Lilian R. Rosa, Márcia A.M.S. da Veiga ⁎ Departamento de Química, FFCLRP, Universidade de São Paulo, Av. Bandeirantes, 3900, 14040-900 Ribeirão Preto-SP, Brazil
a r t i c l e
i n f o
Article history: Received 16 February 2012 Accepted 25 April 2013 Available online 13 June 2013 Keywords: HR-CS MAS Molecular absorption Slurry sampling Coal Sulfur determination
a b s t r a c t We propose a procedure for the determination of sulfur in coal slurries by high resolution continuum source electrothermal molecular absorption spectrometry. The slurry, whose concentration is 1 mg mL−1, was prepared by mixing 50 mg of the sample with 5% v/v nitric acid and 0.04% m/v Triton X-100 and was homogenized manually. It sustained good stability. The determination was performed via CS molecular absorption at 257.592 nm, and the optimized vaporization temperature was 2500 °C. The accuracy of the method was ensured by analysis of certified reference materials SRM 1632b (trace elements in coal) and SRM 1633b (coal fly ash) from the National Institute of Standards and Technology, using external calibration with aqueous standards prepared in the same medium and used as slurry. We achieved good agreement with the certified reference materials within 95% confidence interval, LOD of 0.01% w/w, and RSD of 6%, which confirms the potential of the proposed method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The sulfur content in the earth's crust is about 0.05%, making it the 15th most abundant element in the planet. It is essential for plants and animals because it is part of various amino acids, co-enzymes, and vitamins. However, its gaseous compounds H2S and SO2 are highly toxic. The latter gas is emitted in large quantities during combustion of fossil fuel and mineral coal, and it has significantly contributed to acid rains. Reduction of the sulfur content in diesel fuel is currently one of the greatest international projects aiming at improved air quality, and this should diminish the environmental impact of SO2 [1]. The method D4239 proposed by the ASTM (American Society for Testing and Materials) details a standard procedure to determine sulfur in coal. This method subjects coal to high temperature (minimum operating temperature 1350 °C), generating sulfur oxides that can be determined by acid-base titration or infrared absorption spectroscopy [2]. Although the method D4239 can overcome problems such as the influence of different sulfur species, it is not precise for sulfur concentrations lower than 1% w/w. Another issue is to quantitatively transfer the SOx produced by coal combustion and determine their concentration. Therefore, some works have proposed different techniques for sulfur determination, such as X-ray fluorescence [3], electrothermal vaporization and inductively coupled plasma optical emission [4],
⁎ Corresponding author. Tel.: +55 16 3602 3177; fax: +55 16 3602 4838. E-mail address: [email protected] (M.A.M.S. da Veiga). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.04.011
and mass spectrometry (ICP-MS) [5–7]. However, the latter technique involves a critical, difficult, and time-consuming dissolution step. Miller-Ihli [8,9] established the analysis of coal slurries by electrothermal atomic absorption spectrometry (ET AAS) with satisfactory results. This method uses small amounts of sample (about 50 mg), and sample preparation is easy and fast. Maia and co-workers [10,11] developed a method involving slurry electrothermal vaporization and inductively coupled plasma mass spectrometry (ETV–ICP-MS) to quantify some elements, since this method consists of a faster and simpler way to achieve the same goal. Although this technique also allows to determine sulfur, it generates acid species, which can damage the equipment. Sulfur cannot be directly determined by atomic absorption spectroscopy (AAS) because its main resonance line at 180.671 nm and the two other lines at 181.974 nm and 182.565 nm lies in the range of vacuum-UV, which is not accessible with conventional instrumentation [12]. High-resolution continuum source molecular absorption spectrometry (HR-CS MAS) detects the rotational and vibrational bands of the diatomic molecules and allows one to visualize the entire spectral environment of several tenths of a nanometer in the vicinity, providing a new level of information [13]. Therefore, this method enables one to indirectly determine non-metals like sulfur, fluorine, bromine, and chlorine. Huang et al. determined sulfur by HR-CS MAS using flame (F AAS) [14]. This method generates carbon monosulfide absorption spectra in an air–acetylene flame using a high-resolution echelle spectrometer. Heitmann et al. [15] investigated electrothermal vaporization and CS molecular absorption, using Ca as modifier. Baumbach et al. [16]
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84
reported a new strategy to determine sulfur by HR-CS MAS using the molecule SnS which absorbs at 271.578 nm. Some authors combined HR-CS AAS and F AAS for direct analysis of wine and plants [17–19]. Baysal and Akman [20] also determined sulfur in coal using HR-CS flame absorption spectrometry. The coal samples were dissolved using microwave-assisted digestion and the calibration standards were prepared in sulfuric acid. Resano and Flórez [21] evaluated Pd, in the form of nanoparticles, as chemical modifier for the direct determination of sulfur in different types of solid samples using HR-CS MAS, circumventing the traditional drawbacks associated with sample digestion. Mior et al. [22] reported the determination of sulfur in coal by solid sampling HR-CS MAS using L-cysteine aqueous solution as standard. Jim et al. [23] determined sulfur in coal slurry using a low resolution absorption spectrometer with charge-coupled device detection and a continuum light source (deuterium lamp) coupled to platform or filter furnace vaporizers. The authors achieved a good limit of detection as compared with the ASTM D4239 method. The goal of this work was to develop a simple and reliable method to determine sulfur in coal and ash slurries using MAS and a high-resolution continuum source atomic absorption spectrometer with electrothermal atomization, to generate the target molecule. 2. Experimental 2.1. Instrumentation and furnace conditions All the measurements were performed with a high-resolution continuum source atomic absorption spectrometer ContrAA 700 (Analytik Jena AG, Jena, Germany) equipped with a transversely heated graphite tube atomizer. Sample injection was conducted with an MPE-60 furnace autosampler (Analytik Jena). The CS absorption at 257.592 nm was employed. The spectral bandwidth per pixel was approximately 1.5 pm, and assessment of all the 200 pixels of the detector corresponded to the evaluation of a spectral region of approximately 0.29 nm, with the analytical wavelength located in the center. The absorbance values were measured over three pixels (central pixel ±1). The experiment was carried out using pyrolytically coated graphite tubes with an integrated PIN platform (Analytik Jena Part No. 407-A81.025). Table 1 shows the temperature program used. Argon (99.999%, White Martins, São Paulo, Brazil) was used as purge and protective gas. 2.2. Standards, reagents, and samples
81
2.3. Procedures The slurries were prepared with a sample concentration of approximately 1 mg mL−1. The samples were weighed, and a 5% v/v nitric acid solution (previously prepared) was used to fill the slurry to a known volume. Triton X-100 was then added to a final concentration of 0.04% w/v in solution. The slurry was stable for about 4 min, which accounted for the diminished accuracy of the last replicate in relation to the former. This slurry was immediately submitted to the furnace temperature program (as shown in Table 1). Before each replicate, the sample was mixed by bubbling air with a pipette placed inside the cup to avoid precipitation of the coal. The calibration curve ranged from 5 to 30 mg L−1. 3. Results and discussion 3.1. CS molecular absorption spectra Sulfur exhibits a relatively strong molecular absorption spectrum, attributed to CS [14]. In Fig. 1, the most intense band of the spectrum appeared around 257.59 nm and therefore this wavelength was used to determine sulfur. The spectrometer software (Aspect CS) automatically fitted the base line during the absorption measurement, as represented by the dotted line in Fig. 1. Because the central peak did not fit the base line correctly, we applied a manual background correction; we used pixels 95 (257.577 nm) and 113 (257.602 nm) for this procedure, as shown by the straight line in Fig. 1. We employed this background correction in all the determinations, including the standard solution. Time-resolved absorbance signals for the CS molecule appear at 257.59 nm, and the highest absorbance peak occurs around 0.75 s. We decided to evaluate the signal by peak height and not by integrated absorbance, because the former gave lower standard deviation (2.5 vs 4.3%). 3.2. Optimization of the pyrolysis and vaporization temperature We conducted optimization of the pyrolysis and vaporization temperature with an ammonium sulfate standard solution (Appendix A, Fig. S1). As expected, a high pyrolysis temperature cannot be used in the absence of a chemical modifier, because sulfur loss may occur. Thus, we fixed pyrolysis at 300 °C, the lowest temperature
Two NIST (National Institute of Standards and Technology) certified reference materials, NIST 1632b (trace elements in coal) and NIST 1633b (coal fly ash), were used to assess the feasibility of this method. A stock standard sulfur solution with a concentration of 1000 mg L−1 was prepared using ammonium sulfate (Sigma-Aldrich, Munich, Germany). Water with a resistivity of 18.2 MΩ cm was purified in a Milli-Q system (Millipore, Bedford, MA, USA); nitric acid (Carlo Erba, Milan, Italy) was purified by double sub-boiling distillation in a quartz still (Marconi, Piracicaba, Brazil). Triton X-100 (Merck, Darmstadt, Germany) was employed to stabilize the slurry and to improve the CS molecule generation. Table 1 Temperature program used for the determination of sulfur in coal and ash slurries by HR-CS ET MAS without chemical modifier. Step
Temperature, °C
Ramp, °C s−1
Hold, s
Ar flow rate, L min−1
Drying Drying Pyrolysis Gas adaption Vaporization Cleaning
90 110 300 300 2500 2700
3 5 300 0 3000 500
20 10 10 5 10 4
2.0 2.0 2.0 0 0 2.0
Fig. 1. CS molecular spectra obtained with a 30 mg L−1 of S standard solution with automatic background correction of pixel (dotted) and manual background correction (solid). Wavelengths 257.577 and 257.602 nm are the pixels 95 and 113 respectively used for manual background correction.
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F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84 Table 3 Determination of sulfur in coal and ash CRMs using HR-CS ET MAS. The uncertainty is based on a 95% confidence level (n = 3). Certified material
NIST 1632b NIST 1633b
Sulfur concentration, % w/w Certified
Measured
1.89 ± 0.06 0.2075 ± 0.0011
1.84 ± 0.11 0.195 ± 0.029
3.4. Analysis of slurry samples We analyzed the different samples using the optimized conditions, which allowed us to compare the CS peaks obtained with the ammonium sulfate standard and those achieved for the slurry samples. Both spectra behave the same way, which suggests that the vaporization mechanism is the same and external calibration is allowed. Fig. 2 depicts the signal profiles for 5 mg L−1 sulfur solution, coal and coal fly ash. Fig. 2. Time resolved absorbance measured at 257.592 nm obtained after vaporization of 5 mg L−1 standard solution (blue), coal (NIST 1632b, black) and coal fly ash (NIST 1633b, red) slurries. Pyrolysis temperature = 300 °C and vaporization temperature = 2500 °C.
allowed by the equipment. We observed similar behavior for the coal and coal fly ash samples.
3.3. Optimization of the concentration of Triton X-100 During optimization of the concentration of Triton X-100, we observed that the signal obtained for the standard sulfur solution was approximately three times lower in the absence of Triton X-100. We investigated how increasing Triton X-100 concentrations in the coal slurry (NIST 1632b) affected sulfur determination using pre-established conditions: slurry sample concentration of approximately 1 mg mL−1 in 5% v/v nitric acid solution. Triton X-100 is a surfactant that can form bubbles in aqueous solutions during the drying step. This behavior can lead to sample loss and, thus, loss of the analyte, but we did not notice this event in the temperature program used in our experiments (Table 1). We varied the concentration of Triton X-100 from 0 to 0.10% w/v (Appendix A, Fig. S2). At a concentration higher than 0.02% w/v the absorbance increased slightly. Above 0.06% w/v, a sharp and narrow structured band appeared before the CS peak. The signals were not regular as in the case of molecular structured bands and they appeared randomly throughout the spectrum. Higher Triton concentrations intensified the band, which evidences the dependence of absorption on Triton concentration. Therefore, we decided to use a Triton X-100 concentration of 0.04% w/v in further experiments.
3.5. Analytical figures of merit Table 2 lists the figures of merit obtained by this method. The ET AAS technique generally yields limit of detection (LOD) values in the ng level, but in this case the values are in the μg range, because molecular absorption is not as sensitive as atomic absorption. We performed readings in the peak height mode, because it provided the best linear correlation coefficient (R), equal to 0.995. When we used the integrated absorbance mode, R was equal to 0.85. The LOD of this method was defined as being equal to three times the standard deviation of ten measurements of the blank (solution of nitric acid 5% v/v and Triton X-100 0.04% w/v) divided by the slope (sensitivity) of the calibration curve. The limit of quantification (LOQ) was defined as ten times the same standard deviation divided by the slope. The precision, shown as the relative standard deviation (RSD) obtained from five replicates of a sample, had an average value of 6%, which indicates good repeatability of the method. The LOD was 0.15 mg L−1, which allowed us to estimate LOD as 0.01% of sulfur weight per gram of coal/ash, considering that the slurry sample concentration is approximately 1 mg mL−1. This technique is at least 100 times more sensitive than ASTM D4239 (LOD ~1% w/w), and affords similar results to those reported by Baysal and Jim [20,23]. Mior et al. [22] obtained LOD of ca. 0.1 μg of sulfur in solid sampling determination, 70 times better than ours. This value is achieved due to proportionally higher masses of sample that can be analyzed by each method. However its precision is generally lower than other methods due to the requirement of high homogeneity of the sample and a micro balance is needed to weight small masses. Resano et al. [21] achieved better results for sulfur determination on other matrices (Polyethylene, steel, hay, oyster, petroleum coke) combining six CS lines available in the spectrum.
Table 2 Figures of merit for the determination of sulfur in coal and ash in this work compared to other sulfur determination methods using MAS. Parameter
This work
Ref [20]
Ref [21]
Ref [22]
Ref [23]
Matrix Sample preparation Sulfur amount range, μg Sample mass, mg LOD, % w/w LOQ, % w/w RSD, %
Coal and ash Slurry 0.1–0.6 ≈0.020b 0.01 0.05 4–8
Coal Acid digestion 0.005–20a 0.2 0.01 0.03 −
Polyethylene, steel, hay, oyster, petroleum coke Direct analysis 0.05–2.5 0.05–3.5 0.0001c (1 μg g−1) − 3–5
Coal Direct analysis 0.3–3.5 100–150 0.08 μg 0.3 μg ≈10
Coal Slurry 10–20 0.2–0.8 0.02 − 5–12
a b c
Amount range in % of sulfur, described by authors. Autosampler dispenses 20 μL of the 1 mg mL−1 slurry into the graphite furnace, which correspond to approximately 20 μg of sample. Converted in % w/w in order to compare with the others studies.
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84
83
Fig. 3. Time and wavelength resolved absorbance spectra obtained after the vaporization of sulfur in coal (NIST 1632b) according to time–temperature program in Table 1.
3.6. Sulfur determination
References
Table 3 summarizes the concentrations of sulfur obtained for the certified samples when we applied the described procedure. According to the Student's t-test for a confidence level of 95%, the results are in agreement with the certified values. We did not observe spectral interferences for this determination, despite the low temperature of pyrolysis. Determination of sulfur in both certified reference materials (CRM) samples evidenced two peaks in pixels 46 and 66, 257.509 and 257.540 nm, respectively, which correspond to two resonance lines of aluminum (Fig. 3). Nevertheless both peaks are approximately 0.05 nm far from the analytical line, so it does not interfere in the analysis.
[1] B. Welz, F.G. Lepri, R.G.O. Araujo, S.L.C. Ferreira, M.D. Huang, M. Okruss, H. Becker-Ross, Determination of phosphorus, sulfur and the halogens using high-temperature molecular absorption spectrometry in flames and furnaces — A review, Anal. Chim. Acta 647 (2009) 137–148. [2] ASTM D 4239-05 Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High-Temperature Tube Furnace Combustion Methods, ASTM International, W. Conshohocken, PA, USA, 2005. [3] M. Necemer, P. Kump, M. Rajcevic, R. Jacimovic, B. Budic, M. Ponikvar, Determination of sulfur and chlorine in fodder by X-ray fluorescence spectral analysis and comparison with other analytical methods, Spectrochim. Acta Part B 58 (2003) 1367–1373. [4] A. Mroczek, G. Werner, R.W.W. Schrön, Investigation of sulfur release in ETV-ICP-AES and its application for the determination of sulfates, Fresenius J. Anal. Chem. 361 (1998) 34–42. [5] H. Lachas, R. Richaud, K.E. Jarvis, A.A. Herod, D.R. Dugwell, R. Kandiyoti, Determination of 17 trace elements in coal and ash reference materials by ICP-MS applied to milligram sample sizes, Analyst 124 (1999) 177–184. [6] L.L. Yu, W.R. Kelly, J.D. Fassett, R.D. Vocke, Determination of sulphur in fossil fuels by isotope dilution electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 16 (2001) 140–145. [7] R. Clough, P. Evans, T. Catterick, H.E. Evans, δ34S Measurements of sulfur by multicollector inductively coupled plasma mass spectrometry, Anal. Chem. 78 (2006) 6126–6132. [8] N.J. Miller-Ihli, Slurry sample preparation for simultaneous multi-element graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 3 (1988) 73–81. [9] N.J. Miller-Ihli, Slurry sampling electrothermal atomic absorption spectrometry: results from the second phase of an international collaborative study, J. Anal. At. Spectrom. 12 (1997) 205–212. [10] S.M. Maia, J.B.B. da Silva, A.J. Curtius, B. Welz, Determination of As, Mn, Pb and Se in coal by slurry electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 15 (2000) 1081–1086. [11] S.M. Maia, D. Pozebon, A.J. Curtius, Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution, J. Anal. At. Spectrom. 18 (2003) 330–337. [12] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS, Wiley-VCH, Weinheim, 2005. [13] F. Vignola, D.L.G. Borges, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of Cd and Fe in sewage sludge by high-resolution continuum source electrothermal atomic absorption spectrometry with slurry sampling, Microchem. J. 95 (2010) 333–336. [14] M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, Determination of sulfur by molecular absorption of carbon monosulfide using a high-resolution continuum source absorption spectrometer and an air–acetylene flame, Spectrochim. Acta Part B 61 (2006) 181–188. [15] U. Heitmann, H. Becker-Ross, S. Florek, M.D. Huang, M. Okruss, Determination of non-metals via molecular absorption using high-resolution continuum source
4. Conclusions The results of the present paper agree with the values determined for the reference materials, demonstrating that the proposed technique is potentially applicable in routine analysis, since it allows to indirectly determine sulfur via CS bands. This technique enables a detailed visualization of the spectrum with enough resolution to detect the structural bands of CS, leading to better evaluation of the spectral region of analysis. The method employed to prepare the coal and ash slurry is effective and rapid, as compared with the traditional method of sulfur determination (D4239 ASTM). Acknowledgments The authors are grateful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the financial support. F.V. Nakadi and L.R. Rosa have scholarships from CNPq. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.sab.2013.04.011.
84
[16]
[17]
[18]
[19]
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84 absorption spectrometry and graphite furnace atomization, J. Anal. At. Spectrom. 21 (2006) 1314–1320. G. Baumbach, T. Limburg, J.W. Einax, Quantitative determination of sulfur by high-resolution graphite furnace molecular absorption spectrometry, Microchem. J. 106 (2013) 295–299. M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, Direct determination of total sulfur in wine using a continuum-source atomic-absorption spectrometer and an air–acetylene flame, Anal. Bioanal. Chem. 382 (2005) 1877–1881. M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, C.-D. Patz, Determination of sulfur forms in wine including free and total sulfur dioxide based on molecular absorption of carbon monosulfide in the air–acetylene flame, Anal. Bioanal. Chem. 390 (2008) 361–367. Y. Wang, J.-X. Li, Determination of sulfur in plant using a high-resolution continuum source atomic absorption spectrometer, Spectrosc. Spectr. Anal. 29 (2009) 1418–1421.
[20] A. Baysal, S. Akman, A pratical method for the method of sulphur in coal samples by high-resolution continuum source flame absorption spectrometry, Talanta 85 (2011) 2662–2665. [21] M. Resano, M. Flórez, Direct determination of sulfur in solid samples by means of high-resolution continuum source graphite furnace molecular absorption spectrometry using palladium nanoparticles as chemical modifier, J. Anal. At. Spectrom. 27 (2012) 401–412. [22] R. Mior, S. Morés, B. Welz, E. Carasek, J.B. de Andrade, Determination of sulfur in coal using direct solid sampling and high-resolution continuum source molecular absorption spectrometry of the CS molecule in a graphite furnace, 2013, http:// dx.doi.org/10.1016/j.talanta.2013.01.004 (in press). [23] G. Jim, D. Katskov, P. Tittarelli, Sulfur determination in coal using molecular absorption in graphite filter vaporizer, Talanta 83 (2011) 1687–1694.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Analytical note
Determination of sulfur in coal and ash slurry by high-resolution continuum source electrothermal molecular absorption spectrometry Flávio V. Nakadi, Lilian R. Rosa, Márcia A.M.S. da Veiga ⁎ Departamento de Química, FFCLRP, Universidade de São Paulo, Av. Bandeirantes, 3900, 14040-900 Ribeirão Preto-SP, Brazil
a r t i c l e
i n f o
Article history: Received 16 February 2012 Accepted 25 April 2013 Available online 13 June 2013 Keywords: HR-CS MAS Molecular absorption Slurry sampling Coal Sulfur determination
a b s t r a c t We propose a procedure for the determination of sulfur in coal slurries by high resolution continuum source electrothermal molecular absorption spectrometry. The slurry, whose concentration is 1 mg mL−1, was prepared by mixing 50 mg of the sample with 5% v/v nitric acid and 0.04% m/v Triton X-100 and was homogenized manually. It sustained good stability. The determination was performed via CS molecular absorption at 257.592 nm, and the optimized vaporization temperature was 2500 °C. The accuracy of the method was ensured by analysis of certified reference materials SRM 1632b (trace elements in coal) and SRM 1633b (coal fly ash) from the National Institute of Standards and Technology, using external calibration with aqueous standards prepared in the same medium and used as slurry. We achieved good agreement with the certified reference materials within 95% confidence interval, LOD of 0.01% w/w, and RSD of 6%, which confirms the potential of the proposed method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The sulfur content in the earth's crust is about 0.05%, making it the 15th most abundant element in the planet. It is essential for plants and animals because it is part of various amino acids, co-enzymes, and vitamins. However, its gaseous compounds H2S and SO2 are highly toxic. The latter gas is emitted in large quantities during combustion of fossil fuel and mineral coal, and it has significantly contributed to acid rains. Reduction of the sulfur content in diesel fuel is currently one of the greatest international projects aiming at improved air quality, and this should diminish the environmental impact of SO2 [1]. The method D4239 proposed by the ASTM (American Society for Testing and Materials) details a standard procedure to determine sulfur in coal. This method subjects coal to high temperature (minimum operating temperature 1350 °C), generating sulfur oxides that can be determined by acid-base titration or infrared absorption spectroscopy [2]. Although the method D4239 can overcome problems such as the influence of different sulfur species, it is not precise for sulfur concentrations lower than 1% w/w. Another issue is to quantitatively transfer the SOx produced by coal combustion and determine their concentration. Therefore, some works have proposed different techniques for sulfur determination, such as X-ray fluorescence [3], electrothermal vaporization and inductively coupled plasma optical emission [4],
⁎ Corresponding author. Tel.: +55 16 3602 3177; fax: +55 16 3602 4838. E-mail address: [email protected] (M.A.M.S. da Veiga). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.04.011
and mass spectrometry (ICP-MS) [5–7]. However, the latter technique involves a critical, difficult, and time-consuming dissolution step. Miller-Ihli [8,9] established the analysis of coal slurries by electrothermal atomic absorption spectrometry (ET AAS) with satisfactory results. This method uses small amounts of sample (about 50 mg), and sample preparation is easy and fast. Maia and co-workers [10,11] developed a method involving slurry electrothermal vaporization and inductively coupled plasma mass spectrometry (ETV–ICP-MS) to quantify some elements, since this method consists of a faster and simpler way to achieve the same goal. Although this technique also allows to determine sulfur, it generates acid species, which can damage the equipment. Sulfur cannot be directly determined by atomic absorption spectroscopy (AAS) because its main resonance line at 180.671 nm and the two other lines at 181.974 nm and 182.565 nm lies in the range of vacuum-UV, which is not accessible with conventional instrumentation [12]. High-resolution continuum source molecular absorption spectrometry (HR-CS MAS) detects the rotational and vibrational bands of the diatomic molecules and allows one to visualize the entire spectral environment of several tenths of a nanometer in the vicinity, providing a new level of information [13]. Therefore, this method enables one to indirectly determine non-metals like sulfur, fluorine, bromine, and chlorine. Huang et al. determined sulfur by HR-CS MAS using flame (F AAS) [14]. This method generates carbon monosulfide absorption spectra in an air–acetylene flame using a high-resolution echelle spectrometer. Heitmann et al. [15] investigated electrothermal vaporization and CS molecular absorption, using Ca as modifier. Baumbach et al. [16]
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84
reported a new strategy to determine sulfur by HR-CS MAS using the molecule SnS which absorbs at 271.578 nm. Some authors combined HR-CS AAS and F AAS for direct analysis of wine and plants [17–19]. Baysal and Akman [20] also determined sulfur in coal using HR-CS flame absorption spectrometry. The coal samples were dissolved using microwave-assisted digestion and the calibration standards were prepared in sulfuric acid. Resano and Flórez [21] evaluated Pd, in the form of nanoparticles, as chemical modifier for the direct determination of sulfur in different types of solid samples using HR-CS MAS, circumventing the traditional drawbacks associated with sample digestion. Mior et al. [22] reported the determination of sulfur in coal by solid sampling HR-CS MAS using L-cysteine aqueous solution as standard. Jim et al. [23] determined sulfur in coal slurry using a low resolution absorption spectrometer with charge-coupled device detection and a continuum light source (deuterium lamp) coupled to platform or filter furnace vaporizers. The authors achieved a good limit of detection as compared with the ASTM D4239 method. The goal of this work was to develop a simple and reliable method to determine sulfur in coal and ash slurries using MAS and a high-resolution continuum source atomic absorption spectrometer with electrothermal atomization, to generate the target molecule. 2. Experimental 2.1. Instrumentation and furnace conditions All the measurements were performed with a high-resolution continuum source atomic absorption spectrometer ContrAA 700 (Analytik Jena AG, Jena, Germany) equipped with a transversely heated graphite tube atomizer. Sample injection was conducted with an MPE-60 furnace autosampler (Analytik Jena). The CS absorption at 257.592 nm was employed. The spectral bandwidth per pixel was approximately 1.5 pm, and assessment of all the 200 pixels of the detector corresponded to the evaluation of a spectral region of approximately 0.29 nm, with the analytical wavelength located in the center. The absorbance values were measured over three pixels (central pixel ±1). The experiment was carried out using pyrolytically coated graphite tubes with an integrated PIN platform (Analytik Jena Part No. 407-A81.025). Table 1 shows the temperature program used. Argon (99.999%, White Martins, São Paulo, Brazil) was used as purge and protective gas. 2.2. Standards, reagents, and samples
81
2.3. Procedures The slurries were prepared with a sample concentration of approximately 1 mg mL−1. The samples were weighed, and a 5% v/v nitric acid solution (previously prepared) was used to fill the slurry to a known volume. Triton X-100 was then added to a final concentration of 0.04% w/v in solution. The slurry was stable for about 4 min, which accounted for the diminished accuracy of the last replicate in relation to the former. This slurry was immediately submitted to the furnace temperature program (as shown in Table 1). Before each replicate, the sample was mixed by bubbling air with a pipette placed inside the cup to avoid precipitation of the coal. The calibration curve ranged from 5 to 30 mg L−1. 3. Results and discussion 3.1. CS molecular absorption spectra Sulfur exhibits a relatively strong molecular absorption spectrum, attributed to CS [14]. In Fig. 1, the most intense band of the spectrum appeared around 257.59 nm and therefore this wavelength was used to determine sulfur. The spectrometer software (Aspect CS) automatically fitted the base line during the absorption measurement, as represented by the dotted line in Fig. 1. Because the central peak did not fit the base line correctly, we applied a manual background correction; we used pixels 95 (257.577 nm) and 113 (257.602 nm) for this procedure, as shown by the straight line in Fig. 1. We employed this background correction in all the determinations, including the standard solution. Time-resolved absorbance signals for the CS molecule appear at 257.59 nm, and the highest absorbance peak occurs around 0.75 s. We decided to evaluate the signal by peak height and not by integrated absorbance, because the former gave lower standard deviation (2.5 vs 4.3%). 3.2. Optimization of the pyrolysis and vaporization temperature We conducted optimization of the pyrolysis and vaporization temperature with an ammonium sulfate standard solution (Appendix A, Fig. S1). As expected, a high pyrolysis temperature cannot be used in the absence of a chemical modifier, because sulfur loss may occur. Thus, we fixed pyrolysis at 300 °C, the lowest temperature
Two NIST (National Institute of Standards and Technology) certified reference materials, NIST 1632b (trace elements in coal) and NIST 1633b (coal fly ash), were used to assess the feasibility of this method. A stock standard sulfur solution with a concentration of 1000 mg L−1 was prepared using ammonium sulfate (Sigma-Aldrich, Munich, Germany). Water with a resistivity of 18.2 MΩ cm was purified in a Milli-Q system (Millipore, Bedford, MA, USA); nitric acid (Carlo Erba, Milan, Italy) was purified by double sub-boiling distillation in a quartz still (Marconi, Piracicaba, Brazil). Triton X-100 (Merck, Darmstadt, Germany) was employed to stabilize the slurry and to improve the CS molecule generation. Table 1 Temperature program used for the determination of sulfur in coal and ash slurries by HR-CS ET MAS without chemical modifier. Step
Temperature, °C
Ramp, °C s−1
Hold, s
Ar flow rate, L min−1
Drying Drying Pyrolysis Gas adaption Vaporization Cleaning
90 110 300 300 2500 2700
3 5 300 0 3000 500
20 10 10 5 10 4
2.0 2.0 2.0 0 0 2.0
Fig. 1. CS molecular spectra obtained with a 30 mg L−1 of S standard solution with automatic background correction of pixel (dotted) and manual background correction (solid). Wavelengths 257.577 and 257.602 nm are the pixels 95 and 113 respectively used for manual background correction.
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F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84 Table 3 Determination of sulfur in coal and ash CRMs using HR-CS ET MAS. The uncertainty is based on a 95% confidence level (n = 3). Certified material
NIST 1632b NIST 1633b
Sulfur concentration, % w/w Certified
Measured
1.89 ± 0.06 0.2075 ± 0.0011
1.84 ± 0.11 0.195 ± 0.029
3.4. Analysis of slurry samples We analyzed the different samples using the optimized conditions, which allowed us to compare the CS peaks obtained with the ammonium sulfate standard and those achieved for the slurry samples. Both spectra behave the same way, which suggests that the vaporization mechanism is the same and external calibration is allowed. Fig. 2 depicts the signal profiles for 5 mg L−1 sulfur solution, coal and coal fly ash. Fig. 2. Time resolved absorbance measured at 257.592 nm obtained after vaporization of 5 mg L−1 standard solution (blue), coal (NIST 1632b, black) and coal fly ash (NIST 1633b, red) slurries. Pyrolysis temperature = 300 °C and vaporization temperature = 2500 °C.
allowed by the equipment. We observed similar behavior for the coal and coal fly ash samples.
3.3. Optimization of the concentration of Triton X-100 During optimization of the concentration of Triton X-100, we observed that the signal obtained for the standard sulfur solution was approximately three times lower in the absence of Triton X-100. We investigated how increasing Triton X-100 concentrations in the coal slurry (NIST 1632b) affected sulfur determination using pre-established conditions: slurry sample concentration of approximately 1 mg mL−1 in 5% v/v nitric acid solution. Triton X-100 is a surfactant that can form bubbles in aqueous solutions during the drying step. This behavior can lead to sample loss and, thus, loss of the analyte, but we did not notice this event in the temperature program used in our experiments (Table 1). We varied the concentration of Triton X-100 from 0 to 0.10% w/v (Appendix A, Fig. S2). At a concentration higher than 0.02% w/v the absorbance increased slightly. Above 0.06% w/v, a sharp and narrow structured band appeared before the CS peak. The signals were not regular as in the case of molecular structured bands and they appeared randomly throughout the spectrum. Higher Triton concentrations intensified the band, which evidences the dependence of absorption on Triton concentration. Therefore, we decided to use a Triton X-100 concentration of 0.04% w/v in further experiments.
3.5. Analytical figures of merit Table 2 lists the figures of merit obtained by this method. The ET AAS technique generally yields limit of detection (LOD) values in the ng level, but in this case the values are in the μg range, because molecular absorption is not as sensitive as atomic absorption. We performed readings in the peak height mode, because it provided the best linear correlation coefficient (R), equal to 0.995. When we used the integrated absorbance mode, R was equal to 0.85. The LOD of this method was defined as being equal to three times the standard deviation of ten measurements of the blank (solution of nitric acid 5% v/v and Triton X-100 0.04% w/v) divided by the slope (sensitivity) of the calibration curve. The limit of quantification (LOQ) was defined as ten times the same standard deviation divided by the slope. The precision, shown as the relative standard deviation (RSD) obtained from five replicates of a sample, had an average value of 6%, which indicates good repeatability of the method. The LOD was 0.15 mg L−1, which allowed us to estimate LOD as 0.01% of sulfur weight per gram of coal/ash, considering that the slurry sample concentration is approximately 1 mg mL−1. This technique is at least 100 times more sensitive than ASTM D4239 (LOD ~1% w/w), and affords similar results to those reported by Baysal and Jim [20,23]. Mior et al. [22] obtained LOD of ca. 0.1 μg of sulfur in solid sampling determination, 70 times better than ours. This value is achieved due to proportionally higher masses of sample that can be analyzed by each method. However its precision is generally lower than other methods due to the requirement of high homogeneity of the sample and a micro balance is needed to weight small masses. Resano et al. [21] achieved better results for sulfur determination on other matrices (Polyethylene, steel, hay, oyster, petroleum coke) combining six CS lines available in the spectrum.
Table 2 Figures of merit for the determination of sulfur in coal and ash in this work compared to other sulfur determination methods using MAS. Parameter
This work
Ref [20]
Ref [21]
Ref [22]
Ref [23]
Matrix Sample preparation Sulfur amount range, μg Sample mass, mg LOD, % w/w LOQ, % w/w RSD, %
Coal and ash Slurry 0.1–0.6 ≈0.020b 0.01 0.05 4–8
Coal Acid digestion 0.005–20a 0.2 0.01 0.03 −
Polyethylene, steel, hay, oyster, petroleum coke Direct analysis 0.05–2.5 0.05–3.5 0.0001c (1 μg g−1) − 3–5
Coal Direct analysis 0.3–3.5 100–150 0.08 μg 0.3 μg ≈10
Coal Slurry 10–20 0.2–0.8 0.02 − 5–12
a b c
Amount range in % of sulfur, described by authors. Autosampler dispenses 20 μL of the 1 mg mL−1 slurry into the graphite furnace, which correspond to approximately 20 μg of sample. Converted in % w/w in order to compare with the others studies.
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84
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Fig. 3. Time and wavelength resolved absorbance spectra obtained after the vaporization of sulfur in coal (NIST 1632b) according to time–temperature program in Table 1.
3.6. Sulfur determination
References
Table 3 summarizes the concentrations of sulfur obtained for the certified samples when we applied the described procedure. According to the Student's t-test for a confidence level of 95%, the results are in agreement with the certified values. We did not observe spectral interferences for this determination, despite the low temperature of pyrolysis. Determination of sulfur in both certified reference materials (CRM) samples evidenced two peaks in pixels 46 and 66, 257.509 and 257.540 nm, respectively, which correspond to two resonance lines of aluminum (Fig. 3). Nevertheless both peaks are approximately 0.05 nm far from the analytical line, so it does not interfere in the analysis.
[1] B. Welz, F.G. Lepri, R.G.O. Araujo, S.L.C. Ferreira, M.D. Huang, M. Okruss, H. Becker-Ross, Determination of phosphorus, sulfur and the halogens using high-temperature molecular absorption spectrometry in flames and furnaces — A review, Anal. Chim. Acta 647 (2009) 137–148. [2] ASTM D 4239-05 Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High-Temperature Tube Furnace Combustion Methods, ASTM International, W. Conshohocken, PA, USA, 2005. [3] M. Necemer, P. Kump, M. Rajcevic, R. Jacimovic, B. Budic, M. Ponikvar, Determination of sulfur and chlorine in fodder by X-ray fluorescence spectral analysis and comparison with other analytical methods, Spectrochim. Acta Part B 58 (2003) 1367–1373. [4] A. Mroczek, G. Werner, R.W.W. Schrön, Investigation of sulfur release in ETV-ICP-AES and its application for the determination of sulfates, Fresenius J. Anal. Chem. 361 (1998) 34–42. [5] H. Lachas, R. Richaud, K.E. Jarvis, A.A. Herod, D.R. Dugwell, R. Kandiyoti, Determination of 17 trace elements in coal and ash reference materials by ICP-MS applied to milligram sample sizes, Analyst 124 (1999) 177–184. [6] L.L. Yu, W.R. Kelly, J.D. Fassett, R.D. Vocke, Determination of sulphur in fossil fuels by isotope dilution electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 16 (2001) 140–145. [7] R. Clough, P. Evans, T. Catterick, H.E. Evans, δ34S Measurements of sulfur by multicollector inductively coupled plasma mass spectrometry, Anal. Chem. 78 (2006) 6126–6132. [8] N.J. Miller-Ihli, Slurry sample preparation for simultaneous multi-element graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 3 (1988) 73–81. [9] N.J. Miller-Ihli, Slurry sampling electrothermal atomic absorption spectrometry: results from the second phase of an international collaborative study, J. Anal. At. Spectrom. 12 (1997) 205–212. [10] S.M. Maia, J.B.B. da Silva, A.J. Curtius, B. Welz, Determination of As, Mn, Pb and Se in coal by slurry electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 15 (2000) 1081–1086. [11] S.M. Maia, D. Pozebon, A.J. Curtius, Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution, J. Anal. At. Spectrom. 18 (2003) 330–337. [12] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS, Wiley-VCH, Weinheim, 2005. [13] F. Vignola, D.L.G. Borges, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of Cd and Fe in sewage sludge by high-resolution continuum source electrothermal atomic absorption spectrometry with slurry sampling, Microchem. J. 95 (2010) 333–336. [14] M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, Determination of sulfur by molecular absorption of carbon monosulfide using a high-resolution continuum source absorption spectrometer and an air–acetylene flame, Spectrochim. Acta Part B 61 (2006) 181–188. [15] U. Heitmann, H. Becker-Ross, S. Florek, M.D. Huang, M. Okruss, Determination of non-metals via molecular absorption using high-resolution continuum source
4. Conclusions The results of the present paper agree with the values determined for the reference materials, demonstrating that the proposed technique is potentially applicable in routine analysis, since it allows to indirectly determine sulfur via CS bands. This technique enables a detailed visualization of the spectrum with enough resolution to detect the structural bands of CS, leading to better evaluation of the spectral region of analysis. The method employed to prepare the coal and ash slurry is effective and rapid, as compared with the traditional method of sulfur determination (D4239 ASTM). Acknowledgments The authors are grateful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the financial support. F.V. Nakadi and L.R. Rosa have scholarships from CNPq. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.sab.2013.04.011.
84
[16]
[17]
[18]
[19]
F.V. Nakadi et al. / Spectrochimica Acta Part B 88 (2013) 80–84 absorption spectrometry and graphite furnace atomization, J. Anal. At. Spectrom. 21 (2006) 1314–1320. G. Baumbach, T. Limburg, J.W. Einax, Quantitative determination of sulfur by high-resolution graphite furnace molecular absorption spectrometry, Microchem. J. 106 (2013) 295–299. M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, Direct determination of total sulfur in wine using a continuum-source atomic-absorption spectrometer and an air–acetylene flame, Anal. Bioanal. Chem. 382 (2005) 1877–1881. M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, C.-D. Patz, Determination of sulfur forms in wine including free and total sulfur dioxide based on molecular absorption of carbon monosulfide in the air–acetylene flame, Anal. Bioanal. Chem. 390 (2008) 361–367. Y. Wang, J.-X. Li, Determination of sulfur in plant using a high-resolution continuum source atomic absorption spectrometer, Spectrosc. Spectr. Anal. 29 (2009) 1418–1421.
[20] A. Baysal, S. Akman, A pratical method for the method of sulphur in coal samples by high-resolution continuum source flame absorption spectrometry, Talanta 85 (2011) 2662–2665. [21] M. Resano, M. Flórez, Direct determination of sulfur in solid samples by means of high-resolution continuum source graphite furnace molecular absorption spectrometry using palladium nanoparticles as chemical modifier, J. Anal. At. Spectrom. 27 (2012) 401–412. [22] R. Mior, S. Morés, B. Welz, E. Carasek, J.B. de Andrade, Determination of sulfur in coal using direct solid sampling and high-resolution continuum source molecular absorption spectrometry of the CS molecule in a graphite furnace, 2013, http:// dx.doi.org/10.1016/j.talanta.2013.01.004 (in press). [23] G. Jim, D. Katskov, P. Tittarelli, Sulfur determination in coal using molecular absorption in graphite filter vaporizer, Talanta 83 (2011) 1687–1694.