Determination of Chlorine in Crude Oil by High-Resolution Continuum

Dec 16, 2015 - Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas, Pelotas, Rio Grande do Sul. 96010-610, B...
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Determination of Chlorine in Crude Oil by High-Resolution Continuum Source Graphite Furnace Molecular Absorption Spectrometry Using AlCl, InCl, and SrCl Molecules Michele S. P. Enders,† Alexandre O. Gomes,‡ Raphael F. Oliveira,‡ Regina C. L. Guimaraẽ s,‡ Marcia F. Mesko,§ Erico M. M. Flores,† and Edson I. Müller*,† †

Departamento de Química, Universidade Federal de Santa Maria, Santa Maria, Rio Grande do Sul 97105-900, Brazil Tecnologia de Avaliaçaõ em Petróleos, Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello (CENPES), Petrobras, Rio de Janeiro, Rio de Janeiro 21941-915, Brazil § Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas, Pelotas, Rio Grande do Sul 96010-610, Brazil ‡

ABSTRACT: In this work, high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS) using AlCl, InCl, and SrCl molecules was proposed for direct Cl determination in crude oil. Crude oil analysis was performed by pipetting masses of crude oil directly on the graphite platform. Calibration was performed using aqueous standard solutions containing chlorine. A solution containing Sr2+ ions should be used to increase the sensitivity for AlCl and InCl molecules. For SrCl, only Sr2+ solution was used and the graphite platform should be covered with a Zr permanent modifier. Accuracy was evaluated using certified reference material (NIST 1634c, fuel oil) and also by comparison of results obtained with HR-CS GF MAS to those obtained by microwave-induced combustion (MIC), microwave-assisted extraction (MAE), ASTM D6470-99, and neutron activation analysis (NAA). Significant differences were not observed by comparison of results obtained by HR-CS GF MAS to the certified reference value and the other methods for Cl determination (95% confidence level). The AlCl molecule provided the lowest characteristic mass (0.28 ng). Limits of detection (LODs) of 1.4 and 0.5 mg kg−1 of Cl were obtained for AlCl and SrCl molecules, respectively. Sample preparation was minimized using direct analysis of crude oil combined with HR-CS GF MAS providing better LOD when compared to conventional methods for Cl determination. HR-CS GF MAS using AlCl and SrCl molecules was suitable for quality control of crude oil for Cl determination in compliance with the requirements of crude oil industries. combustion methods,12−16 pyrohydrolysis,17−22 neutron activation analysis (NAA),23,24 X-ray fluorescence spectrometry (XRF),23,24 and, more recently, high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS).7 The most common method employed by crude oil companies is ASTM D6470-99 that includes the dissolution of the sample in xylene and a subsequent extraction of chlorine using different solvents. Finally, the water extract is analyzed using potentiometric titration, allowing the analysis of crude oil samples with the chlorine concentration ranging from 5 to 1500 mg kg−1 of NaCl. The main drawbacks of this method are related to the high risk of contamination as a result of the high consumption of different solvents, many steps of the analytical procedure, excessive waste generation, and exposition of the analyst to toxic solvents.8 Nowadays, the use of high-resolution continuum source (HR-CS) atomic absorption spectrometry (AAS) was reported

1. INTRODUCTION Salts are impurities present in crude oil emulsion that should be removed as complete as possible before the refining process. However, the removal of salts in the first stage of the crude oil treatment process could not be performed at the desired levels to refine the crude oil, and drawbacks could be observed during refining. Even in small concentrations, salts may be accumulated in stills, heaters, and exchangers, leading to clogging of pipelines. Additionally, salts can also undergo hydrolysis at high temperatures forming of hydrochloric acid that causes the corrosion of the head systems at the refineries. Most of the salts are present as NaCl and are dissolved in the emulsified water of crude oil.1−5 The salt content in crude oil emulsion can be defined as the amount of sodium chloride (NaCl), and commonly, the maximum concentration of salt, according to the refinery requirement, is 5 mg kg−1 of NaCl (about 3 mg kg−1 of Cl).5,6 In this sense, many analytical methods are available for quality control of the chlorine content in crude oil. However, chlorine determination methods are prone to several drawbacks, such as the risk of contamination and losses during sample preparation that affect the accuracy and precision of final results. In this way, the determination of chlorine in crude oil samples is a challenging task.7 The main methods used for chlorine determination are based on extraction procedures,8−11 © XXXX American Chemical Society

Special Issue: 16th International Conference on Petroleum Phase Behavior and Fouling Received: September 15, 2015 Revised: December 16, 2015

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microbalance (Sartorius, Göttingen, Germany) with a resolution of 0.001 mg. Transversely heated pyrolytically coated graphite tubes using platforms specially designed for solid sampling (Analytik Jena) were used throughout. For the comparison of results, crude oil samples were digested by MIC using a microwave oven (Multiwave 3000, Anton Paar, Graz, Austria) equipped with up to eight pressurized quartz vessels with 80 mL of internal volume with a maximum pressure of 80 bar. Other experimental conditions were performed according to a previous publication.32 Additionally, crude oil samples were also analyzed using MAE that was carried out using a microwave oven (Multiwave 3000, Anton Paar, Graz, Austria) with the same quartz vessels used for MIC digestion. For the MAE, sample mass up to 10 g, 20 mL of water and five glass spheres (to avoid sample projection during heating) were transferred to the quartz vessel. Vessels were positioned in a rotor that was placed inside of the microwave cavity, and the heating program was started. The maximum power of 800 W for 60 min was used to extract Cl of the crude oil sample. The water phase was separated from crude oil and diluted up to 50 mL with water.9 MIC digests and MAE water extracts were analyzed by an ion chromatographic (IC) system (model 850 Professional IC Ion Analysis, Metrohm, Switzerland) equipped with a pump (IC liquid handling unit), compact autosampler (model 858 Professional Sample Processor, Metrohm) with 112 positions, and conductivity detector (model 819). Determination was performed using a Metrosep A Supp 5 column (150 × 4 mm inner diameter) based on poly(vinyl alcohol) with quaternary ammonium groups with a particle size of 5 μm and a guard column (Metrosep A Supp 15 Guard, 5 × 4 mm inner diameter) with the same packing material and particle size of the analytical column. The mobile phase used was a mixed solution of 3.2 mmol L−1 Na2CO3 and 1 mmol L−1 NaHCO3, in a flow rate of 0.7 mL min−1. A suppressor column (model 833 Suppressor Unit, Metrohm) used to reduce the conductivity of the mobile phase was regenerated periodically with water and 100 mmol L−1 H2SO4. In addition, the system was equipped with a dialysis cell with a cellulose acetate membrane with a pore size of 0.25 μm and a sample loop of 100 μL. Finally, a selected crude oil sample “L” was characterized for Cl content using NAA at the Instituto de Pesquisas Energeticas e Nucleares (São Paulo, Brazil). Sample masses up to 450 mg were irradiated using a neutron flux of 3 × 1012 neutrons cm−2 s−1 during 50 s for 38Cl (1642.7 keV) radionuclide. Detection of γ radiation was achieved using a Ge detector (GX 2020, Canberra) having a resolution of 1.70 keV at the 1332.49 keV peak of 60Co. 2.2. Reagents, Solutions, and Samples. Water was purified in a Milli-Q system (18.2 MΩ cm, Millipore Corp., Billerica, MA). All regents were of analytical-grade and were purchased from Merck (Darmstadt, Germany). Calibration standard solutions were prepared daily by serial dilution of Cl stock standard solution (5 g L−1 NaCl). Other Cl solutions were prepared from HCl and chlorobenzene to evaluate different chlorine species for the formation of diatomic molecules. Stock solutions of Al3+ (0.2 g L−1), In3+ (0.2 g L−1), and Sr2+ (10 g −1 L ), used as chlorine-molecule-forming reagents, were prepared by suitable dissolution of Al(NO3)3·9H2O, In(NO3)3, and Sr(NO3)2, respectively, in water. It is important to mention that Al(NO3)3·9H2O from Sigma-Aldrich (99.997% trace metal basis) was also evaluated to minimize the blank values for the AlCl molecule. Additionally, Sr solution was also used as a modifier for AlCl and InCl diatomic molecules, allowing for better sensitivity. Salt of Zr(NO3)4 was used to prepare 1000 mg L−1 Zr that was used as a permanent chemical modifier. Interferences promoted by alkaline and earth alkaline element solutions containing 10 g L−1 of each element were prepared by the dissolution of Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, and KNO3. Absorbing solution used for decomposition of samples by MIC was 50 mmol L−1 NH4OH, prepared by dilution with water of a 25% NH4OH solution. Ammonium nitrate was used as an igniter for the MIC method (6 mol L−1 solution in water). Oxygen (99.6%, White Martins, São Paulo, Brazil) was used for the digestion procedure by

as an alternative for the determination of Cl in several matrices.25 However, until now, no application for crude oil has been reported in the literature. For analysis using HR-CS AAS, Cl should react with other elements in the atomizer (flame or graphite furnace) to form small diatomic molecules that will absorb radiation. The most common molecules used in molecular absorption spectrometry (MAS) for Cl determination are AlCl and InCl.26−29 The combination of HR-CS with the graphite furnace (GF) atomizer is a powerful arrangement for trace analysis, allowing the direct analysis of the sample and minimizing the risk of contamination of the analyte because the sample handling is reduced at a minimum.28,29 Additionally, the correct optimization of the pyrolysis temperature that minimizes the possibility of interferences during the vaporization step, the possibility of calibration with aqueous standard solution, even for direct analysis, and no sample dilution can be appointed as advantages for Cl determination using the GF atomizer.28−30 Finally, a new molecule of SrCl with narrower absorption bands in comparison to AlCl and InCl was recently proposed for the determination of biological samples using solid sampling graphite furnace analysis. Determinations using SrCl were carried out at 635.862 nm, and the limit of detection (LOD) was 1.0 ng (0.67 mg kg−1, considering the maximum sample mass introduced in the atomizer).31 Thus, the present work describes the use of HR-CS GF MAS for Cl determination in the crude oil using direct analysis to minimize sample preparation and the risk of contamination. AlCl, InCl, and SrCl diatomic molecules were compared for Cl determination in crude oil. Pyrolysis and vaporization temperatures were optimized for each molecule to minimize spectral interferences promoted by the crude oil matrix. Additionally, calibration using aqueous standard solutions was also investigated. Accuracy was evaluated by comparison of the results obtained by HR-CS GF MAS to those obtained by other methods, such as microwave-induced combustion (MIC), microwave-assisted extraction (MAE), ASTM D6470-99, and NAA. Last but not least, the certified reference material (CRM) of fuel oil (NIST 1634c) was also used to evaluate the accuracy of the proposed method.

2. EXPERIMENTAL SECTION 2.1. Instrumentation. Determination of chlorine in crude oil samples was performed using a high-resolution continuum source graphite furnace molecular absorption spectrometer, model ContrAA 700 (Analytik Jena, Jena, Germany) equipped with a manual solid sampling system model SSA 600 (Analytik Jena). The optical system comprises a xenon short-arc lamp (XBO 301, GLE, Berlin, Germany) operating in “hot-spot” mode as the radiation source, which emits a high-intensity continuum spectrum within the wavelength range from 180 to 900 nm. The background correction was performed in interactive background correction mode for diatomic molecules (IBCm) using the least-squares algorithm supplied by the equipment software that was used to eliminate the spectral interference. Integration times of 10, 5, and 10 s were used for AlCl, InCl, and SrCl molecules, respectively. For all of the evaluated molecules, the integrated absorbance corresponds to the sum of the integrated absorbances of five pixels (the central pixel plus the adjacent pixels, CP ± 2) for each molecule. The spectrometer has a prism as a premonochromator and an echelle monochromator to provide the high resolution required for HR-CS GF MAS analysis. The resolution is about 1.2 pm per pixel at 200 nm with a linear charge-coupled device (CCD) array detector with 588 pixels (only 200 pixels are used for analytical purposes). Samples were weighed using a model M2P B

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Energy & Fuels MIC. Small disks of paper (15 mm of diameter) with a low ash content (Black Ribbon Ashless, Schleicher and Schuell GmbH, Dassel, Germany) were used as an aid for the ignition process for MIC. The paper was previously decontaminated by immersion in ethanol for 2 cycles of 20 min followed by 2 cycles with water for 20 min in an ultrasonic bath (45 kHz), washed with water, and dried in a class 100 laminar bench (CSLH-12, Veco, Brazil). Argon with a purity of 99.998% (White Martins, São Paulo, Brazil) was used as protection and purge gas in determination by HR-CS GF MAS. A CRM of fuel oil, NIST 1634c (National Institute of Standards and Technology, Gaithersburg, MD), was used for accuracy evaluation. A total of 11 crude oil samples donated by PETROBRAS company [American Petroleum Institute (API) gravity higher than 25°] were used for determination of Cl by HR-CS GF MAS. 2.3. Procedure. The chlorine measurements were performed in quintuplicate, and integrated absorbance mode was used for the evaluation of absorption signals at the wavelengths of 261.4180, 267.2181, and 635.8620 nm for the molecules of AlCl, InCl, and SrCl, respectively. The graphite furnace temperature program used for the measurement is shown in Table 1.

the platform-containing sample was again introduced in the graphite tube, and the complete heating program was carried out (Table 1). Argon (99.998%, White Martins, São Paulo, São Paulo, Brazil) was used as the carrier gas at a flow rate of 2.0 L min−1 during all stages, except for the vaporization process, during which the flow was interrupted for accomplishment of the measurement. The influence of the modifier and molecule-forming reagents was investigated by pipetting different aliquots of solution containing these reagents. Calibration curves used for quantification were obtained by manual pipetting of different volumes of Cl aqueous standard stock solution on the platform. Blank values were determined by carrying out the complete analysis cycle, including the transport of the empty platform to and from the balance. LOD and limit of quantification (LOQ) were determined on the basis of blank values, and the respective standard deviation was obtained by carrying out the complete heating cycles, including taking out the platform from the graphite furnace, transporting it to and from the balance, and introducing into the furnace, but without application of any sample.

3. RESULTS AND DISCUSSION 3.1. Preliminary Results. Initial experiments were carried out to evaluate the response of solutions containing different species of Cl, such as HCl, NaCl, and C6H5Cl, using HR-CS GF MAS with AlCl, InCl, and SrCl molecules. Significant differences were not observed when compared to the integrated absorbances for NaCl and HCl for all investigated molecules (t test, 95% confidence level). The equivalent sensitivity for both NaCl and HCl is probably due to thermal stabilization of different Cl species provided by the modifier and moleculeforming reagents used in HR-CS GF MAS. A similar behavior related to equivalent sensitivity by different chlorine-containing substances was also observed by Nakadi et al.33 In this work, authors observed that diclofenac sodium, HCl, KCl, NH4Cl, and NaCl presented equivalent sensitivity using HR-CS GF MAS and the AlCl molecule.33 Despite sodium diclofenac being an organic Cl species, it is less volatile than the chlorinated organic species present in crude oil, allowing the determination by HR-CS GF MAS. In this sense, integrated absorbances for C6H5Cl solution were very low (lower than 0.0010 s) for all molecules using HR-CS GF MAS. These low values of integrated absorbance for C6H5Cl probably occurred as a result of losses of the molecule by vaporization before reaction with molecule-forming reagents. Because the majority of Cl in crude oil is present in inorganic form, only NaCl was used for subsequent experiments and construction of the analytical curve.1,2 According to the literature, modifiers, such as Sr2+, should be mixed with molecule-forming reagents (Al3+ or In3+) to improve the sensitivity for AlCl and InCl molecules using HR-CS GF MAS.31 In this sense, masses of Sr2+ ranging from 0 to 100 μg were transferred to the graphite platform with Cl and respective molecule-forming reagents to achieve the most sensitive condition for AlCl and InCl molecules. Results showed an increase of 3.5 and 2.9 times in integrated absorbance using solutions with Sr2+ for AlCl and InCl, respectively, when compared to results without Sr2+. Additionally, the optimized masses of Sr2+ necessary to achieve the maximum integrated absorbance for AlCl and InCl molecules were 20 and 60 μg of Sr2+, respectively. These masses were used for the subsequent experiments carried out with AlCl and InCl molecules. According to Dittrich et al.,34 the increase of the signal for the AlCl molecule is caused by a reduction of the volatilization of Cl by the Sr2+ ions. Therefore, Cl evaporates later, and the maximum concentration in the vapor phase of Cl

Table 1. Temperature Program Used for Cl Determination by HR-CS GF MAS molecule stage temperature (°C) ramp (°C s−1) hold time (s) Ar flow rate (L min−1) temperature (°C) ramp (°C s−1) hold time (s) Ar flow rate (L min−1) temperature (°C) ramp (°C s−1) hold time (s) Ar flow rate (L min−1) temperature (°C) ramp (°C s−1) hold time (s) Ar flow rate (L min−1)

AlCl

InCl

Drying 130 130 10 10 130 130 2 2 Pyrolysis 600 700 5 5 150 150 2 2 Vaporization 2300 2000 2000 2000 10 10 0 0 Cleaning 2600 2600 500 500 4 4 2 2

SrCl

Zr coating

130 10 130 2

120 10 100 2

600 5 150 2

350 100 25 2

2000 2000 10 0

1100 300 5 0

2600 500 4 2

2300 500 5 2

For analysis of aqueous standard solution using the AlCl molecule, the program was started after the pipetting of 10 μL of Cl standard solution (50 ng of Cl) and aliquots of stock solutions containing 2 and 20 μg of Al3+ and Sr2+, respectively, on the platform. Analysis using the InCl molecule was carried out using 50 ng of Cl and aliquots of stock solutions containing 2 and 60 μg of In3+ and Sr2+, respectively. On the other hand, for the determination of Cl using the SrCl molecule, platforms were first coated with a layer of Zr. For this purpose, 40 μL of a stock solution containing 1000 μg L−1 Zr was pipetted on the platform, which was then inserted into the graphite tube, and the heating program for Zr coating was executed (Table 1). This procedure was repeated 15 times, resulting in the deposition of a total of 600 μg of modifier on the platform. After coating using Zr, for analysis using SrCl, only an aliquot containing 100 μg of Sr2+ was pipetting on the platform together with 50 ng of Cl. For crude oil analysis, modifier and molecule-forming reagent solutions were pipetted on the graphite platform and the drying stage of the temperature program (Table 1) was performed. The platform was removed from the graphite furnace, and the crude oil sample was directly weighted on this platform using a micropipette. Subsequently, C

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allowed for a mass ratio of Al3+/Cl of at least 40 was selected for further experiments with Cl aqueous standard solutions and crude oil samples. However, blank values for AlCl molecules were not negligible, even using analytical-grade reagent to prepare Al3+ solution. Commonly, for the aliquot used for AlCl molecule formation (2 μg of Al3+), the integrated absorbances for the blanks were lower than 0.050 s. These blank values caused a deleterious effect on the detection limit obtained for the AlCl molecule. On the other hand, a minimum mass of 2 μg of In3+ (mass ratio of In3+/Cl of 40) was required to provide the most sensitive absorbance signal for the InCl molecule and was used for the subsequent experiments. Finally, for the SrCl molecule, the minimum mass of Sr2+ required to achieve higher sensitivity was 100 μg (mass ratio of Sr2+/Cl of 200). In this case, the mass ratio of the molecule-forming reagent and Cl is higher when compared to the other molecules, probably as a result of the fact that, for SrCl, only Sr2+ was used to react with Cl. Therefore, 100 μg was adopted for further tests carried out with the SrCl molecule. 3.3. Evaluation of Pyrolysis and Vaporization Temperatures. The pyrolysis curve was constructed for Cl aqueous standard solution and crude oil sample “J” (about 1 mg) with the temperature ranging from 400 to 800 °C (Figure 2). Sample J was selected because it was previously wellcharacterized for the Cl concentration using different methods (MIC, MAE, and ASTM D6470-99). A similar procedure was performed for vaporization curve construction, with the temperature ranging from 1800 to 2500 °C (Figure 2). The integrated absorbance values for the crude oil sample were normalized for 1 mg. The correct optimization of pyrolysis and vaporization temperatures that can result in an equivalent sensitivity for Cl in aqueous standards and in the samples allows for the calibration using aqueous standard solution containing Cl. Initially, it was possible to observe that an aqueous standard solution containing Cl and the crude oil sample presented a similar behavior for all investigated molecules. For the AlCl molecule (Figure 2a), the best pyrolysis and vaporization temperatures (for both standard solution and crude oil samples) were 600 and 2300 °C, respectively. On the other hand, for InCl, the optimized temperatures were 700 and 2000 °C for pyrolysis and vaporization temperatures, respectively (Figure 2b). Last but not least, a similar thermal behavior was observed for SrCl that required 600 and 2000 °C for best pyrolysis and vaporization temperatures, respectively. These above-mentioned temperatures were used in all subsequent experiments using HR-CS GF MAS. 3.4. Interferences. The presence of common cations present in crude oil emulsion was evaluated to determine their effect on the formation of molecules investigated in this work. In this sense, Ca, K, and Mg were investigated as interference for AlCl, InCl, and SrCl molecule reactions. It is well-reported in the literature that halogens as F−, Br−, and I− can also cause interference in Cl diatomic molecules, but these ions were usually not present in significant amounts in crude oil emulsions. Therefore, the interference of these anions was not evaluated in this work. For investigation of interference caused by Ca, K, and Mg, aliquots of standard solution containing up to 100 ng of each individual metal (which corresponds to 100 mg kg−1 of each cation using 1 mg of crude oil) were evaluated for the three diatomic molecules and significant differences were not

and the less volatile Al occurs closer in time inside the graphite furnace. The same phenomenon probably occurs with the InCl molecule.34 It is important to mention that, for the SrCl molecule, only Sr2+ solution was used and the platform should be previously coated with Zr as a permanent modifier. The use of the platform coated with Zr allowed an improvement of 35% in integrated absorbance when compared to experiments without a coated platform. All of the above-mentioned experiments were carried out using aqueous standard solution containing Cl and crude oil samples. 3.2. Optimization of the Molecule-Forming Reagent Concentration. Figure 1 presents the masses of molecule-

Figure 1. Influence of Al3+, In3+, and Sr2+ masses on the AlCl, InCl, and SrCl absorption signal (in integrated absorbance), respectively. Masses of 50 ng of Cl (10 μL of a 5000 μg L−1 Cl aqueous standard solution), 20 μg of Sr2+ for AlCl, and 60 μg of Sr2+ for InCl were used. Pyrolysis and vaporization temperatures were 600−2300, 700−2000, and 600−2000 °C for AlCl, InCl, and SrCl, respectively. The error bars are the standard deviation of five replicates.

forming reagents (Al3+, In3+, and Sr2+) necessary for the quantitative reaction with 50 ng of Cl and production of AlCl, InCl, and SrCl molecules. Initially, it is possible to observe that the mass of molecule-forming reagents should be at least 40 times higher than the analyte mass. For the AlCl molecule, significant differences in integrated absorbances were not observed for Al3+ masses higher than or equal to 2 μg, which is equivalent to the mass ratio of Al3+/Cl of 40. A similar mass ratio of Al3+/Cl was achieved by Nakadi et al. in Cl isotope determination.33 In this sense, a mass of 2 μg of Al3+ that D

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observed in integrated absorbance when compared to results without the presence of these cations [analysis of variance (ANOVA), 95% confidence level]. All possible mixtures of these cations (masses up to 300 ng of interferences) were evaluated, and significant differences in integrated absorbances were not observed. In this sense, it is possible to state that interferences caused by alkaline and earth alkaline metals probably do not occur for crude oil analysis by HR-CS GF MAS, because the concentration of these metals is usually lower than the concentrations evaluated in this work. 3.5. Determination of Chlorine in Crude Oil by HR-CS GF MAS. After optimization of all important variables, 11 samples with Cl concentrations ranging from 3.25 to 181 mg kg−1 were analyzed using HR-CS GF MAS with AlCl, InCl, and SrCl molecules (Table 2). Sample masses up to 1.5 mg of crude oil were directly weighed in the graphite platform, and calibration was carried out using aqueous standard solutions containing Cl. All crude oil samples were quantified for Cl using the three molecules, with exception of samples “G”, “H”, and “I” that could not be analyzed using the InCl molecule because the concentration of Cl was lower than the LOQ. Relative standard deviations (RSDs) were lower than 20% for all three diatomic molecules. Significant differences were not observed in comparison to the results obtained for all molecules and the certified reference value of NIST 1634c. Additionally, accuracy was also evaluated by comparison of results obtained by HR-CS GF MAS using AlCl, InCl, and SrCl molecules to those obtained using MAE, MIC, ASTM D6470-99, and NAA, and significant differences were not observed in all comparisons (t test, 95% confidence level). 3.6. Figures of Merit and Signal Profile. The figures of merit for HR-CS GF MAS using AlCl, InCl, and SrCl are summarized in Table 3. The linearity spread in a wide range up to 50 ng of Cl for all molecules analyzed. The LOD and LOQ were calculated as 3σ/S and 10σ/S (n = 10), respectively, where σ is the standard deviation of 10 blank measurements and S is the slope of the calibration. In comparison of the LOD and LOQ obtained for the three molecules, it is possible to observe that the SrCl molecule presented the lower values (0.7 ng). The LOD for AlCl was higher than the LOD for SrCl, probably by the blank values obtained using only the molecule-forming reagent (Al3+) solution. LODs of 1.4 mg kg−1 of Cl (AlCl) and 0.5 mg kg−1 of Cl (SrCl) are comparable to the LOD obtained by the ASTM D6470-99 method (LOD = 1.2 mg kg−1 of Cl).

Figure 2. Pyrolysis and vaporization curves for (a) AlCl, (b) InCl, and (c) SrCl molecules using optimized conditions. Integrated absorbance for the aqueous standard solution (left axis, integrated absorbance) and the crude oil sample (right axis, normalized integrated absorbance). Curves were constructed using 50 ng of Cl (10 μL of 5000 μg L−1 solution) and about 1 mg of crude oil sample “J” containing 40 ng of Cl. (a) For the AlCl molecule, experiments were carried out using 2 μg of Al3+ and 20 μg of Sr2+. (b) For the InCl molecule, experiments were carried out using using 2 μg of In3+ and 60 μg of Sr2+. (c) For the SrCl molecule, experiments were carried out using using 100 μg of Sr2+ and Zr as the permanent modifier. The error bars are the standard deviation of five replicates. (○) Aqueous standard solution and (●) crude oil sample.

Table 2. Chlorine Determination by HR-CS GF MAS in Crude Oil (mg kg−1 of Cl; Mean ± Standard Deviation; n = 5) MAS sample A B C D E F G H I J L NIST 1634cb a

AlCl

InCl

± ± ± ± ± ± ± ± ± ± ± ±

13.6 ± 0.3 27.9 ± 2.1 22.9 ± 2.5 35.5 ± 5.2 23.7 ± 0.7 43.5 ± 5.9