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Shake, shut, and go – A fast screening of sulfur in heavy crude oils by high-resolution continuum source graphite furnace molecular absorption spectrometry via GeS molecule detection
Spectrochimica Acta Part B 160 (2019) 105671
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Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Analytical note
Shake, shut, and go – A fast screening of sulfur in heavy crude oils by highresolution continuum source graphite furnace molecular absorption spectrometry via GeS molecule detection
T
⁎
Carlos Abada,b, , Stefan Florekc, Helmut Becker-Rossc, Mao-Dong Huangc, Francisco Lopez-Linaresd, Laura Poirierd, Norbert Jakubowskib,e, Sebastian Recknagela, Ulrich Pannea,b Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany Humboldt-Universität zu Berlin, School of Analytical Sciences Adlershof (SALSA), Unter den Linden 6, 10099 Berlin, Germany Leibniz-Institut für Analytische Wissenschaften - ISAS - e. V., Department Berlin, Schwarzschildstr. 8, 12489 Berlin, Germany d Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94801, United States e Spetec GmbH, Berghamer Str. 2, 85435 Erding, Germany a
b c
A B S T R A C T
A fast and simple method for sulfur quantification in crude oils was developed by using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS-GFMAS). For this, heavy crude oil samples were prepared as microemulsion (shake) and injected into a graphite furnace (shut). Finally, the concentration of sulfur was determined by monitoring in situ the transient molecular spectrum of GeS at wavelength 295.205 nm after adding a germanium solution as molecular forming agent (and go). Zirconium dioxide in the form of nanoparticles (45–55 nm) was employed as a permanent modifier of the graphite furnace. Calibration was done with an aqueous solution standard of ammonium sulfate, and a characteristic mass (m0) of 7.5 ng was achieved. The effectiveness of the proposed method was evaluated analizing, ten heavy crude oil samples with sulfur amounts ranging between 0.3 and 4.5% as well as two NIST standard reference materials, 1620c and 1622e. Results were compared with those obtained by routine ICP-OES analysis, and no statistical relevant differences were found.
1. Introduction Sulfur is a natural but undesired element present in fossil crude oils [1], and combustion of their derivatives fuels releases sulfur dioxide into the atmosphere. In the presence of water, this produces sulfuric acid, one of the principal causes of acid rain [2]. Sulfur dioxide also pollutes the air, with proven consequences to human health [3]. Therefore, US and European governments have implemented strict regulations to control the amount of sulfur in fuels which shall not exceed 95 mg Kg−1 in the United States of America, and 10 mg Kg−1 in the European Union [4,5]. Additionally, in different crude oil refining steps, the catalytic processes needed to achieve desired products, and the presence of refractory sulfur in the feedstocks could act as a catalyst poison, and consequently, sulfur present needs to be removed via hydrotreatment to achieve product quality [6,7]. Therefore, there is a need for developing analytical methods for a fast and reliable sulfur determination in such feedstocks. Many methods have been proposed for sulfur determination in crude oils, such as high temperature combustion with infrared (IR) [8] and ultraviolet fuorescence (UVF) detection [9], X-ray fluorescence ⁎
spectrometry (XRF) [10], gas chromatography (GC, mainly for speciation) [11,12], inductively coupled plasma mass spectrometry (ICP-MS, as GC detector for speciation), and optical emission spectroscopy (ICPOES) [13]. Besides XRF, ICP-OES is the work-horse for sulfur determination for petroleum feedstocks and derivatives. ICP-OES has been widely used for routine analysis due to the possibility of multielement analysis and the high sample throughput [13]. The bottleneck of ICPOES methods lies in the need for sample preparation due to the high complexity of the matrix. For ICP-based analysis, sample digestion (hot plate, microwave assisted acid), or direct dilutions in organic solvents are essential. Even though the above methods are routinely used in many laboratories, electrothermal vaporization with graphite furnaces has demonstrated to be a robust and fast alternative for complex crude oil matrices [14]. However, the typical atomic absorption spectrometry (AAS) is not suitable for sulfur determination, because the main absorption lines of sulfur atoms lie in the vacuum ultraviolet spectral range [15,16]. Nevertheless, in the last decade many improvements were introduced to the AAS instrumentation, mainly in the form of a single xenon lamp with a high radiance continuum radiation from UV
Corresponding author at: Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany. E-mail address: [email protected] (C. Abad).
https://doi.org/10.1016/j.sab.2019.105671 Received 15 May 2019; Received in revised form 2 August 2019; Accepted 2 August 2019 Available online 05 August 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.
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acid 0.4 M (Merck, Germany) was used. Other chemical modifiers investigated were Ni, Pd, Ru, Rh, and Au (all metal reagents ICP grade, 1 g L−1, Merck, Germany). For analyis of samples by HR-CS-GFMAS, a total of 12 heavy crude oil samples were analyzed (among them 2 standard reference materials from NIST, 1620c and 1622e). To reduce the viscosity of samples and for a practical introduction of them in graphite furnaces, a microemulsion of these samples was prepared by a modified procedure previously described by Cadorim et al. [30]. Approximately 0.05 g of sample was dissolved in 2 mL of o-xylene (semiconductor grade, Alfa Aesar, Germany), and 0.5 mL of concentrated HNO3 was added and sonicated for 5 min in an ultrasonic bath. Next, an aliquot of 0.5 mL of water and 2 mL methyl isobutyl ketone (for extraction analysis grade, Merck, Germany) was added and n-propanol (Rotisolv® HPLC grade, Carl Roth, Germany) was used to reach a final volume of 14 mL. Finally, this preparation was vigorously shaken and rapidly used for measurements. Triton® X-100 (the GC grade, Merck, Germany) was used to improve the distribution of the micro-emulsion droplets in the graphite furnace. Samples with a high sulfur content were diluted again by adding a microemulsion mix in ranges between 1:10 to 1:100. For analysis of samples by ICP-OES, the procedure reported by Poirier et al. was followed [34] but with the addition of a calibration for Sulfur. SRM NIST 1622e was used to calibrate for sulfur on the ICP-OES. Heavy crude oil samples and SRM NIST 1620c were prepared by taking an aliquot of aproximatly 1 g which was dissolved in aproximatly 9 g of an o-xylene diluent containing a matrix modifier and internal standard. Sample dilutions ranged from 1:10 to 1:30 depending on sulfur concentration. All samples were agitated in a mechanical shaker to assist the dissolution of the heavy crude oil samples. After 30 min shaking, the heavy crude oils were well dissolved in the o-xylene diluent. The samples were inspected after shaking to make sure that all oil was off the walls of the vial, and no particles were visible in the liquid. If any oil had been found on the vial walls, a vortex mixer would be used to ensure a better homogenization of the sample. While the samples were running, o-xylene was used in between samples to rinse the sample introduction system. The samples were prepared less than 24 h before ICP-OES analysis.
to near IR, coupled with a high-resolution Double Echelle Monochromator (DEMON) [17], allowing the extention of the application range of AAS from atomic to molecular absorption spectrometry and now allows for the analysis of non-metals and also isotopes [18,19]. By using high-resolution continuum source molecular graphite furnace absorption spectrometry (HR-CS-GFMAS), non-metals like sulfur can be quantified by adding a molecule forming agent and monitoring the transient spectrum of a formed diatomic molecule at high temperature. This is an advantage because it is possible to fine-tune the temperature program of vaporization in a graphite furnace and to eliminate matrix interferences and optimize the molecule formation at the same time. To Tittarelli and Lavorato in 1987 belong the first determination of sulfur in crude oils via CS molecule in graphite furnaces. However, they were limited by the technology at that time and this method did not gain popularity due to the lack of sensitivity [20]. Huang et al. re-introduced the use of CS molecule for analysis of sulfur content in wine by using a flame HR-CS-MAS [21], and other work has been reported [22–24]. Later, Kowalewska successfully applied HR-CS-MAS with flame and graphite furnace for sulfur determination in crude oils and petroleum products [25]. Resano et al. and Ozbek et al. reviewed the latest advances in sulfur determination by diatomic molecules using HR-CS-MAS [26–28]. In a recent work on sulfur determination, the diatomic sulfides of group 14 elements were evaluated in graphite furnace (CS, SiS, GeS, SnS, and PbS) [29]. From this work, it was found the most suitable molecule for quantitative analysis of sulfur is the GeS, which is more sensitive and less prone to interferences in comparison to SnS, previously used for the sulfur determination in petroleum crude oil [30]. The GeS molecule was first proposed by Dittrich et al. for sulfur determination more than three decades ago using a H2 lamp as light source [31]. In this work, the molecule GeS is evaluated for the sulfur quantification in heavy crude oils which so far has not been analyzed by HR-CSMAS. Further, a micro-emulsion has been applied for sample introduction, which is a fast and well-known method for graphite furnaces [32]. 2. Materials and methods
2.2. Instrumentation and measurements
2.1. Reagents and sample preparation
A high-resolution continuum source graphite furnace absorption spectrometer (HR-CS-GFAA) ContrAA 800D (Analytik Jena, Germany) was employed for monitoring the molecular absorption spectra of GeS between wavelengths 295.0117 and 295.3969 nm with a resolution Δλ = 1.93 pm. Pyrolytically coated standard graphite tubes with PINplatform (Part no. 407-A81.025, Analytik Jena, Germany) and an autosampler (Analytik Jena, Germany) was used throughout this work. The optimized temperature program in Table 1 was employed for all measurements. All parameters were optimized. After homogenization by vigorously shaking, an automatic sampler immediately took 10 μL of a microemulsion and added it to 2 μL of a 1% Triton X-100 solution and 5 μL of the germanium molecule forming agent in the graphite furnace. For analysis of samples by ICP-OES, a Thermo Radial ICAP 6000 Series was employed (Thermo Fisher Scientific) as described in a previous work [34]. The emission line of sulfur at wavelength 182.034 nm was used.
Ultrapure deionized water (18.2 MΩ cm−1) was used for dilutions (Milli-Q Gradient, Merck Millipore, Germany). HNO3 was purified by sub-boiling in PFA devices. A sulfur standard solution was prepared by dilution of 2.0720 g of (NH4)2SO4 (Suprapur, Merck, Germany) in 50 mL of water for 1% of S. For the modification of the graphite furnace, zirconium oxide was used in the form of the dispersion nanoparticles (ZrO2 nanoparticles aqueous dispersion 20 wt%, 45–55 nm, US Research Nanomaterials, Inc., USA) [33]. This Zr modification was prepared before analysis by applying 40 μL of a 1% dispersion of zirconium dioxide nanoparticles in 50% methanol, and a similar program displayed in Table 1 was implemented. This procedure was repeated once. As a molecular forming agent, a solution of Ge as GeO2 in oxalic Table 1 Optimized temperature program used for monitoring the GeS molecular absorption in HR-CS-GFMAS. Step
Temperature (°C)
Heating rate (°C s−1)
Holding time (s)
Drying 1 Drying 2 Drying 3 Drying 4 Pyrolysis GeS vaporization Clean out
60 80 90 130 400 1000 2500
6 6 3 5 100 2000 1000
10 20 10 20 20 10 3
3. Results and discussion 3.1. Sample preparation Due to the high viscosity of heavy crude oil samples, it is not recommendable to introduce them in graphite furnaces as a raw material. The step of weighing is not reproducible with these samples, as it is hard to homogenize and handle them. Therefore, solid sampling devices were not considered. Instead, a microemulsion was prepared, as liquids 2
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C. Abad, et al.
Fig. 1. (A) Time-resolved spectra of GeS molecule around wavelength 295.205 nm observed during the analysis SRM NIST 1620c and Ge as a molecular forming reagent. (B) Integrated absorption spectrum of (A); arrows indicate the nine strongest absorption lines for sulfur quantification.
improve considerably the sensitivity of sulfur determination by molecular absorption spectroscopy of GeS. The best conditions for sulfur determination were found to be a zirconium coating prepared from a nanoparticle ZrO2 precursor, with the temperature of pyrolysis and vaporization at 400 °C and 1000 °C, respectively, sample preparation done as microemulsions, and a calibration with an aqueous ammonium sulfate standard.
are easy to handle. This method has been successfully explored for the determination of metals and sulfur in crude oils [30,35]. 3.2. Optimization of parameter and modifiers
Intergrated Absorbance (s)
The GeS molecule has been proposed as the better choice for analytical determination of sulfur. First, germanium is not a natural or anthropogenic component of fossil crude oils, and therefore its concentration as a molecular forming agent can be quantitatively controlled. Second, the spectral region for the electronic transition X → A exhibits no spectral interferences from the matrix or even the molecule forming agent (Fig. 1(A)), as for example SnO interferes in flames, if SnS is the target molecule [36,37]. Third and finally, the limit of detection and characteristic mass are far better than for the known proposed molecules for sulfur determination. The low vaporization temperature is an additional advantage avoiding the loss of volatile sulfur species. The use of zirconium modifier was proven to decrease the vaporization temperature and, consequently, the limits of detection were improved. Recently, it was found that ZrO2 is the chemical form of zirconium modifiers at the top layer of modified graphite tubes [33]. Therefore, the surface of this permanent graphite modifier is increased by using ZrO2 nanoparticles precursor for graphite modification. Thus, an improvement in sensitivity of around 20% compared to a traditional Zr coating was obtained. Sulfide-forming elements were investigated as chemical modifiers in combination with Zr (Fe, Ni, Ru, Rh, Pd, and Au) and their influence in the GeS absorbance is shown in Fig. 2. None of these chemical modifiers
3.3. Determination of sulfur in heavy crude oils The concentration of sulfur in heavy crude oils was determined in a prepared micro-emulsion by adding a germanium solution as a chemical forming agent in a graphite furnace. Thus, by heating the graphite furnace, the transient molecule of GeS is formed, and its molecular spectra were monitored during the vaporization, and then used as an analytical signal. This method was calibrated by standard addition of an aqueous solution of (NH4)2SO4 on Sample 1. This sample was used as the standard for calibration due to its middle sulfur concentration compared with all other samples (known from ICP-OES results, Table 3). As shown in Fig. 1(B), the molecular spectrum of GeS is present as nine well-resolved absorption lines around the highest peak. These absorption lines are found at wavelengths 295.1414 nm, 295.1569 nm, 295.1724, 295.1891, 295.2057 nm (highest peak), 295.2227 nm, 295.2401 nm, 295.2582 nm, and 295.2764 nm. By integration of these absorption lines, it is possible to improve the sensitivity of the analytical method. However, due to the high sulfur concentration in the studied samples, measurements were done by monitoring only the highest absorption line. The analytical signal was monitored for the central pixel of the CCD camera with a width of 5 pixels. The figures of merit are summarized in Table 2. The obtained results for sulfur determination by our proposed method were compared to a routine analysis using ICP-OES to analyze the 10 heavy oil samples, and 2 standard reference materials. These results are summarized in Table 3. A relatively large standard deviation in HR-CS-GFMAS may be explained by the formation and loss of volatile sulfur compounds [25]. Also, a selected representative sample
0.10
0.05 Table 2 Figures of merit for an ammonium sulfate aqueous standard solution used in calibration for sulfur determination. GeS molecule λ = 295.2053 nm, Triton-X 2 μL 0.1% and 5 μL Ge 0.1%). Parameter Slope Coefficient R2 Linear range Characteristic mass, m0 Limit of detection LOD (absolute/relative)a Limit of quantification LOQ (absolute/relative)a
0.00
Zr
e Zr-F
i Zr-N
u Zr-R
h Zr-R
d Zr-P
u Zr-A
Modifiers Fig. 2. Evaluation of chemical modifiers. Error bars are reported as standard deviation of n = 3.
a
3
For 0.05 g sample.
0.380 s μg−1 0.9963 0.1–1.5 μg 7.5 ng 8.05 ng/0.16 mg kg −1 24.16 ng/0.48 mg kg −1
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Table 3 Results for sulfur determination in, heavy oil samples and standard reference materials (SRM NIST 1620c & 1622e) by HR-CS-GFMAS compared to an ICPOES method. Sample
ICP-OES
(%)a
HR-CSGFMAS
(%)b
CoAc
1 2 3 4 5 6 7 8 9 10 SRM NIST 1620c SRM NIST 1622e
3.13 5.17 2.66 1.66 4.15 1.53 3.35 0.61 1.39 0.27 4.66
± 0.09 ± 0.21 ± 0.13 ± 0.09 ± 0.12 ± 0.11 ± 0.11 ± 0.05 ± 0.06 ± 0.08 ± 0.14
3.26 4.24 2.44 1.19 5.05 1.75 3.72 0.67 1.27 0.22 4.30
± 0.16 ± 0.23 ± 0.29 ± 0.18 ± 0.09 ± 0.35 ± 0.12 ± 0.26 ± 0.06 ± 0.09 ± 0.19
– – – – – – – – – – 4.561
± 0.015
2.15
± 0.10
2.11
± 0.30
2.1468
± 0.0041
[2] [3] [4] [5]
[6] [7] [8]
[9]
[10] [11]
a
Uncertainties are reported as standard deviation of n = 3. b Uncertainties are reported as the relative standard deviation of n = 3 with a coverage factor of k = 2, corresponding to a level of confidence of 95%. c From certificates of analysis.
[12]
[13]
calibrated the method although heavy crude oil samples differ in their chemical composition. Nevertheless, results were accurate and reliable for fast screening of sulfur in complex heavy crude oils. Despite a higher standard deviation observed for the HR-CS-GFMAS compared to the ICP-OES measurements, no statistical difference was found for t critical α = 0.05 when performing a t-test for almost all samples. Samples # 2 and # 4 differ statistically from the results reported by the ICP-OES method. However, they are still in the same order of magnitude.
[14]
[15] [16]
[17]
4. Conclusion [18]
A method for a fast and accurate quantification of sulfur in heavy oils was developed by monitoring the transient molecular spectra of GeS using HR-CS-GFMAS. The preparation of a microemulsion was a convenient way for a fast and simple sample introduction into the graphite furnace. The application of a ZrO2 coating with a nanoparticulated precursor improves previous analytical methods by lowering the vaporization temperature. This probably avoids the release of gaseous sulfur species before the vaporization step. The avoidance of chemical modifiers in favor of the molecular forming agent is an advantage in comparison to other reported methods. Results presented herein are compatible with those obtained by routine analytical methods. The proposed method also provides a possible control mechanism of sulfur content to ensure compliance with US and European regulations [4,5].
[19]
[20]
[21]
[22]
[23]
Acknowledgments [24]
Financial support by the Bundesministerium für Wirtschaft und Energie and the Deutsche Forschungsgemeinschaft (DFG) within the School of Analytical Sciences Adlershof (SALSA) is gratefully acknowledged. This research was also financially supported by the Senatsverwaltung für Wirtschaft, Technologie und Forschung des Landes Berlin, and the Bundesministerium für Bildung und Forschung. We thank Analytik Jena AG for the support of this research with a HRCS-AS instrument. Chevron Energy Technology Company is acknowledged for permission to publish this work.
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Analytical note
Shake, shut, and go – A fast screening of sulfur in heavy crude oils by highresolution continuum source graphite furnace molecular absorption spectrometry via GeS molecule detection
T
⁎
Carlos Abada,b, , Stefan Florekc, Helmut Becker-Rossc, Mao-Dong Huangc, Francisco Lopez-Linaresd, Laura Poirierd, Norbert Jakubowskib,e, Sebastian Recknagela, Ulrich Pannea,b Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany Humboldt-Universität zu Berlin, School of Analytical Sciences Adlershof (SALSA), Unter den Linden 6, 10099 Berlin, Germany Leibniz-Institut für Analytische Wissenschaften - ISAS - e. V., Department Berlin, Schwarzschildstr. 8, 12489 Berlin, Germany d Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94801, United States e Spetec GmbH, Berghamer Str. 2, 85435 Erding, Germany a
b c
A B S T R A C T
A fast and simple method for sulfur quantification in crude oils was developed by using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS-GFMAS). For this, heavy crude oil samples were prepared as microemulsion (shake) and injected into a graphite furnace (shut). Finally, the concentration of sulfur was determined by monitoring in situ the transient molecular spectrum of GeS at wavelength 295.205 nm after adding a germanium solution as molecular forming agent (and go). Zirconium dioxide in the form of nanoparticles (45–55 nm) was employed as a permanent modifier of the graphite furnace. Calibration was done with an aqueous solution standard of ammonium sulfate, and a characteristic mass (m0) of 7.5 ng was achieved. The effectiveness of the proposed method was evaluated analizing, ten heavy crude oil samples with sulfur amounts ranging between 0.3 and 4.5% as well as two NIST standard reference materials, 1620c and 1622e. Results were compared with those obtained by routine ICP-OES analysis, and no statistical relevant differences were found.
1. Introduction Sulfur is a natural but undesired element present in fossil crude oils [1], and combustion of their derivatives fuels releases sulfur dioxide into the atmosphere. In the presence of water, this produces sulfuric acid, one of the principal causes of acid rain [2]. Sulfur dioxide also pollutes the air, with proven consequences to human health [3]. Therefore, US and European governments have implemented strict regulations to control the amount of sulfur in fuels which shall not exceed 95 mg Kg−1 in the United States of America, and 10 mg Kg−1 in the European Union [4,5]. Additionally, in different crude oil refining steps, the catalytic processes needed to achieve desired products, and the presence of refractory sulfur in the feedstocks could act as a catalyst poison, and consequently, sulfur present needs to be removed via hydrotreatment to achieve product quality [6,7]. Therefore, there is a need for developing analytical methods for a fast and reliable sulfur determination in such feedstocks. Many methods have been proposed for sulfur determination in crude oils, such as high temperature combustion with infrared (IR) [8] and ultraviolet fuorescence (UVF) detection [9], X-ray fluorescence ⁎
spectrometry (XRF) [10], gas chromatography (GC, mainly for speciation) [11,12], inductively coupled plasma mass spectrometry (ICP-MS, as GC detector for speciation), and optical emission spectroscopy (ICPOES) [13]. Besides XRF, ICP-OES is the work-horse for sulfur determination for petroleum feedstocks and derivatives. ICP-OES has been widely used for routine analysis due to the possibility of multielement analysis and the high sample throughput [13]. The bottleneck of ICPOES methods lies in the need for sample preparation due to the high complexity of the matrix. For ICP-based analysis, sample digestion (hot plate, microwave assisted acid), or direct dilutions in organic solvents are essential. Even though the above methods are routinely used in many laboratories, electrothermal vaporization with graphite furnaces has demonstrated to be a robust and fast alternative for complex crude oil matrices [14]. However, the typical atomic absorption spectrometry (AAS) is not suitable for sulfur determination, because the main absorption lines of sulfur atoms lie in the vacuum ultraviolet spectral range [15,16]. Nevertheless, in the last decade many improvements were introduced to the AAS instrumentation, mainly in the form of a single xenon lamp with a high radiance continuum radiation from UV
Corresponding author at: Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany. E-mail address: [email protected] (C. Abad).
https://doi.org/10.1016/j.sab.2019.105671 Received 15 May 2019; Received in revised form 2 August 2019; Accepted 2 August 2019 Available online 05 August 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.
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acid 0.4 M (Merck, Germany) was used. Other chemical modifiers investigated were Ni, Pd, Ru, Rh, and Au (all metal reagents ICP grade, 1 g L−1, Merck, Germany). For analyis of samples by HR-CS-GFMAS, a total of 12 heavy crude oil samples were analyzed (among them 2 standard reference materials from NIST, 1620c and 1622e). To reduce the viscosity of samples and for a practical introduction of them in graphite furnaces, a microemulsion of these samples was prepared by a modified procedure previously described by Cadorim et al. [30]. Approximately 0.05 g of sample was dissolved in 2 mL of o-xylene (semiconductor grade, Alfa Aesar, Germany), and 0.5 mL of concentrated HNO3 was added and sonicated for 5 min in an ultrasonic bath. Next, an aliquot of 0.5 mL of water and 2 mL methyl isobutyl ketone (for extraction analysis grade, Merck, Germany) was added and n-propanol (Rotisolv® HPLC grade, Carl Roth, Germany) was used to reach a final volume of 14 mL. Finally, this preparation was vigorously shaken and rapidly used for measurements. Triton® X-100 (the GC grade, Merck, Germany) was used to improve the distribution of the micro-emulsion droplets in the graphite furnace. Samples with a high sulfur content were diluted again by adding a microemulsion mix in ranges between 1:10 to 1:100. For analysis of samples by ICP-OES, the procedure reported by Poirier et al. was followed [34] but with the addition of a calibration for Sulfur. SRM NIST 1622e was used to calibrate for sulfur on the ICP-OES. Heavy crude oil samples and SRM NIST 1620c were prepared by taking an aliquot of aproximatly 1 g which was dissolved in aproximatly 9 g of an o-xylene diluent containing a matrix modifier and internal standard. Sample dilutions ranged from 1:10 to 1:30 depending on sulfur concentration. All samples were agitated in a mechanical shaker to assist the dissolution of the heavy crude oil samples. After 30 min shaking, the heavy crude oils were well dissolved in the o-xylene diluent. The samples were inspected after shaking to make sure that all oil was off the walls of the vial, and no particles were visible in the liquid. If any oil had been found on the vial walls, a vortex mixer would be used to ensure a better homogenization of the sample. While the samples were running, o-xylene was used in between samples to rinse the sample introduction system. The samples were prepared less than 24 h before ICP-OES analysis.
to near IR, coupled with a high-resolution Double Echelle Monochromator (DEMON) [17], allowing the extention of the application range of AAS from atomic to molecular absorption spectrometry and now allows for the analysis of non-metals and also isotopes [18,19]. By using high-resolution continuum source molecular graphite furnace absorption spectrometry (HR-CS-GFMAS), non-metals like sulfur can be quantified by adding a molecule forming agent and monitoring the transient spectrum of a formed diatomic molecule at high temperature. This is an advantage because it is possible to fine-tune the temperature program of vaporization in a graphite furnace and to eliminate matrix interferences and optimize the molecule formation at the same time. To Tittarelli and Lavorato in 1987 belong the first determination of sulfur in crude oils via CS molecule in graphite furnaces. However, they were limited by the technology at that time and this method did not gain popularity due to the lack of sensitivity [20]. Huang et al. re-introduced the use of CS molecule for analysis of sulfur content in wine by using a flame HR-CS-MAS [21], and other work has been reported [22–24]. Later, Kowalewska successfully applied HR-CS-MAS with flame and graphite furnace for sulfur determination in crude oils and petroleum products [25]. Resano et al. and Ozbek et al. reviewed the latest advances in sulfur determination by diatomic molecules using HR-CS-MAS [26–28]. In a recent work on sulfur determination, the diatomic sulfides of group 14 elements were evaluated in graphite furnace (CS, SiS, GeS, SnS, and PbS) [29]. From this work, it was found the most suitable molecule for quantitative analysis of sulfur is the GeS, which is more sensitive and less prone to interferences in comparison to SnS, previously used for the sulfur determination in petroleum crude oil [30]. The GeS molecule was first proposed by Dittrich et al. for sulfur determination more than three decades ago using a H2 lamp as light source [31]. In this work, the molecule GeS is evaluated for the sulfur quantification in heavy crude oils which so far has not been analyzed by HR-CSMAS. Further, a micro-emulsion has been applied for sample introduction, which is a fast and well-known method for graphite furnaces [32]. 2. Materials and methods
2.2. Instrumentation and measurements
2.1. Reagents and sample preparation
A high-resolution continuum source graphite furnace absorption spectrometer (HR-CS-GFAA) ContrAA 800D (Analytik Jena, Germany) was employed for monitoring the molecular absorption spectra of GeS between wavelengths 295.0117 and 295.3969 nm with a resolution Δλ = 1.93 pm. Pyrolytically coated standard graphite tubes with PINplatform (Part no. 407-A81.025, Analytik Jena, Germany) and an autosampler (Analytik Jena, Germany) was used throughout this work. The optimized temperature program in Table 1 was employed for all measurements. All parameters were optimized. After homogenization by vigorously shaking, an automatic sampler immediately took 10 μL of a microemulsion and added it to 2 μL of a 1% Triton X-100 solution and 5 μL of the germanium molecule forming agent in the graphite furnace. For analysis of samples by ICP-OES, a Thermo Radial ICAP 6000 Series was employed (Thermo Fisher Scientific) as described in a previous work [34]. The emission line of sulfur at wavelength 182.034 nm was used.
Ultrapure deionized water (18.2 MΩ cm−1) was used for dilutions (Milli-Q Gradient, Merck Millipore, Germany). HNO3 was purified by sub-boiling in PFA devices. A sulfur standard solution was prepared by dilution of 2.0720 g of (NH4)2SO4 (Suprapur, Merck, Germany) in 50 mL of water for 1% of S. For the modification of the graphite furnace, zirconium oxide was used in the form of the dispersion nanoparticles (ZrO2 nanoparticles aqueous dispersion 20 wt%, 45–55 nm, US Research Nanomaterials, Inc., USA) [33]. This Zr modification was prepared before analysis by applying 40 μL of a 1% dispersion of zirconium dioxide nanoparticles in 50% methanol, and a similar program displayed in Table 1 was implemented. This procedure was repeated once. As a molecular forming agent, a solution of Ge as GeO2 in oxalic Table 1 Optimized temperature program used for monitoring the GeS molecular absorption in HR-CS-GFMAS. Step
Temperature (°C)
Heating rate (°C s−1)
Holding time (s)
Drying 1 Drying 2 Drying 3 Drying 4 Pyrolysis GeS vaporization Clean out
60 80 90 130 400 1000 2500
6 6 3 5 100 2000 1000
10 20 10 20 20 10 3
3. Results and discussion 3.1. Sample preparation Due to the high viscosity of heavy crude oil samples, it is not recommendable to introduce them in graphite furnaces as a raw material. The step of weighing is not reproducible with these samples, as it is hard to homogenize and handle them. Therefore, solid sampling devices were not considered. Instead, a microemulsion was prepared, as liquids 2
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Fig. 1. (A) Time-resolved spectra of GeS molecule around wavelength 295.205 nm observed during the analysis SRM NIST 1620c and Ge as a molecular forming reagent. (B) Integrated absorption spectrum of (A); arrows indicate the nine strongest absorption lines for sulfur quantification.
improve considerably the sensitivity of sulfur determination by molecular absorption spectroscopy of GeS. The best conditions for sulfur determination were found to be a zirconium coating prepared from a nanoparticle ZrO2 precursor, with the temperature of pyrolysis and vaporization at 400 °C and 1000 °C, respectively, sample preparation done as microemulsions, and a calibration with an aqueous ammonium sulfate standard.
are easy to handle. This method has been successfully explored for the determination of metals and sulfur in crude oils [30,35]. 3.2. Optimization of parameter and modifiers
Intergrated Absorbance (s)
The GeS molecule has been proposed as the better choice for analytical determination of sulfur. First, germanium is not a natural or anthropogenic component of fossil crude oils, and therefore its concentration as a molecular forming agent can be quantitatively controlled. Second, the spectral region for the electronic transition X → A exhibits no spectral interferences from the matrix or even the molecule forming agent (Fig. 1(A)), as for example SnO interferes in flames, if SnS is the target molecule [36,37]. Third and finally, the limit of detection and characteristic mass are far better than for the known proposed molecules for sulfur determination. The low vaporization temperature is an additional advantage avoiding the loss of volatile sulfur species. The use of zirconium modifier was proven to decrease the vaporization temperature and, consequently, the limits of detection were improved. Recently, it was found that ZrO2 is the chemical form of zirconium modifiers at the top layer of modified graphite tubes [33]. Therefore, the surface of this permanent graphite modifier is increased by using ZrO2 nanoparticles precursor for graphite modification. Thus, an improvement in sensitivity of around 20% compared to a traditional Zr coating was obtained. Sulfide-forming elements were investigated as chemical modifiers in combination with Zr (Fe, Ni, Ru, Rh, Pd, and Au) and their influence in the GeS absorbance is shown in Fig. 2. None of these chemical modifiers
3.3. Determination of sulfur in heavy crude oils The concentration of sulfur in heavy crude oils was determined in a prepared micro-emulsion by adding a germanium solution as a chemical forming agent in a graphite furnace. Thus, by heating the graphite furnace, the transient molecule of GeS is formed, and its molecular spectra were monitored during the vaporization, and then used as an analytical signal. This method was calibrated by standard addition of an aqueous solution of (NH4)2SO4 on Sample 1. This sample was used as the standard for calibration due to its middle sulfur concentration compared with all other samples (known from ICP-OES results, Table 3). As shown in Fig. 1(B), the molecular spectrum of GeS is present as nine well-resolved absorption lines around the highest peak. These absorption lines are found at wavelengths 295.1414 nm, 295.1569 nm, 295.1724, 295.1891, 295.2057 nm (highest peak), 295.2227 nm, 295.2401 nm, 295.2582 nm, and 295.2764 nm. By integration of these absorption lines, it is possible to improve the sensitivity of the analytical method. However, due to the high sulfur concentration in the studied samples, measurements were done by monitoring only the highest absorption line. The analytical signal was monitored for the central pixel of the CCD camera with a width of 5 pixels. The figures of merit are summarized in Table 2. The obtained results for sulfur determination by our proposed method were compared to a routine analysis using ICP-OES to analyze the 10 heavy oil samples, and 2 standard reference materials. These results are summarized in Table 3. A relatively large standard deviation in HR-CS-GFMAS may be explained by the formation and loss of volatile sulfur compounds [25]. Also, a selected representative sample
0.10
0.05 Table 2 Figures of merit for an ammonium sulfate aqueous standard solution used in calibration for sulfur determination. GeS molecule λ = 295.2053 nm, Triton-X 2 μL 0.1% and 5 μL Ge 0.1%). Parameter Slope Coefficient R2 Linear range Characteristic mass, m0 Limit of detection LOD (absolute/relative)a Limit of quantification LOQ (absolute/relative)a
0.00
Zr
e Zr-F
i Zr-N
u Zr-R
h Zr-R
d Zr-P
u Zr-A
Modifiers Fig. 2. Evaluation of chemical modifiers. Error bars are reported as standard deviation of n = 3.
a
3
For 0.05 g sample.
0.380 s μg−1 0.9963 0.1–1.5 μg 7.5 ng 8.05 ng/0.16 mg kg −1 24.16 ng/0.48 mg kg −1
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Table 3 Results for sulfur determination in, heavy oil samples and standard reference materials (SRM NIST 1620c & 1622e) by HR-CS-GFMAS compared to an ICPOES method. Sample
ICP-OES
(%)a
HR-CSGFMAS
(%)b
CoAc
1 2 3 4 5 6 7 8 9 10 SRM NIST 1620c SRM NIST 1622e
3.13 5.17 2.66 1.66 4.15 1.53 3.35 0.61 1.39 0.27 4.66
± 0.09 ± 0.21 ± 0.13 ± 0.09 ± 0.12 ± 0.11 ± 0.11 ± 0.05 ± 0.06 ± 0.08 ± 0.14
3.26 4.24 2.44 1.19 5.05 1.75 3.72 0.67 1.27 0.22 4.30
± 0.16 ± 0.23 ± 0.29 ± 0.18 ± 0.09 ± 0.35 ± 0.12 ± 0.26 ± 0.06 ± 0.09 ± 0.19
– – – – – – – – – – 4.561
± 0.015
2.15
± 0.10
2.11
± 0.30
2.1468
± 0.0041
[2] [3] [4] [5]
[6] [7] [8]
[9]
[10] [11]
a
Uncertainties are reported as standard deviation of n = 3. b Uncertainties are reported as the relative standard deviation of n = 3 with a coverage factor of k = 2, corresponding to a level of confidence of 95%. c From certificates of analysis.
[12]
[13]
calibrated the method although heavy crude oil samples differ in their chemical composition. Nevertheless, results were accurate and reliable for fast screening of sulfur in complex heavy crude oils. Despite a higher standard deviation observed for the HR-CS-GFMAS compared to the ICP-OES measurements, no statistical difference was found for t critical α = 0.05 when performing a t-test for almost all samples. Samples # 2 and # 4 differ statistically from the results reported by the ICP-OES method. However, they are still in the same order of magnitude.
[14]
[15] [16]
[17]
4. Conclusion [18]
A method for a fast and accurate quantification of sulfur in heavy oils was developed by monitoring the transient molecular spectra of GeS using HR-CS-GFMAS. The preparation of a microemulsion was a convenient way for a fast and simple sample introduction into the graphite furnace. The application of a ZrO2 coating with a nanoparticulated precursor improves previous analytical methods by lowering the vaporization temperature. This probably avoids the release of gaseous sulfur species before the vaporization step. The avoidance of chemical modifiers in favor of the molecular forming agent is an advantage in comparison to other reported methods. Results presented herein are compatible with those obtained by routine analytical methods. The proposed method also provides a possible control mechanism of sulfur content to ensure compliance with US and European regulations [4,5].
[19]
[20]
[21]
[22]
[23]
Acknowledgments [24]
Financial support by the Bundesministerium für Wirtschaft und Energie and the Deutsche Forschungsgemeinschaft (DFG) within the School of Analytical Sciences Adlershof (SALSA) is gratefully acknowledged. This research was also financially supported by the Senatsverwaltung für Wirtschaft, Technologie und Forschung des Landes Berlin, and the Bundesministerium für Bildung und Forschung. We thank Analytik Jena AG for the support of this research with a HRCS-AS instrument. Chevron Energy Technology Company is acknowledged for permission to publish this work.
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