Direct and Rapid Quantitative Analysis of Alkyldibenzothiophenes in

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Direct and Rapid Quantitative Analysis of Alkyldibenzothiophenes in Deeply Hydrodesulfurized Diesel Fuel by Gas Chromatography Quadrupole Time-of-Flight Mass Spectrometry Xinyi Zhu,* Mingxing Liu, Zelong Liu, Ying Li, and Songbai Tian Research Institute of Petroleum Processing, Sinopec, Beijing 100083, People’s Republic of China S Supporting Information *

ABSTRACT: Polycyclic aromatic sulfur heterocycles are undesirable compounds in fuel and refined petroleum-based products. The sulfur compounds in diesel are limited strictly to an ultralow concentration according to environmental protection rules. Alkyldibenzothiophenes are recalcitrant species in the hydrodesulfurization process, and speciation of dibenzothiophenes in deeply hydrodesulfurized diesel fuel provides important information to the catalyst and process improvement. However, most of the existing analytical methods are unsuitable for identification of ultralow-sulfur compounds in deeply hydrodesulfurized diesel because of the limits of sensitivity. A new method for direct quantitative analysis of alkyldibenzothiophenes in deeply hydrodesulfurized diesel fuel was developed by gas chromatography quadrupole time-of-flight mass spectrometry, which exhibited good linear response and high precision and sensitivity. The practicability of the method was shown by an extensive qualitative and quantitative analysis of alkyldibenzothiophenes in hydrodesulfurized middle distillates at different sulfur levels.

1. INTRODUCTION As environmental regulations become more stringent, the sulfur content in diesel is limited strictly to a lower level, with some countries having set a low limit of 10 ppm. How to reduce the sulfur concentration of diesel to such a low level is a challenging task for the refiners, particularly because refiners are forced to use crude oils with higher sulfur contents as a result of shorter supplies of low-sulfur crude oils. Hydrodesulfurization (HDS) is the most commonly used process for sulfur removal in diesel fuel.1−5 Despite the high efficiency of HDS, it is known that not all sulfur species are equally easily desulfurized, so that a pattern of particular refractory sulfur compounds is left in the product after the hydrotreatment. Identifying such species will benefit the further improvement of HDS catalysts and processes. On the basis of the knowledge, the recalcitrant species are found to be alkyldibenzothiophenes. Detailed knowledge of the refractory-substituted dibenzothiophenes (DBTs) is useful for optimization of sulfur removal processes and development of novel catalysts. Various techniques, such as gas chromatography atomic emission detection,6−10 gas chromatography or comprehensive two-dimensional gas chromatography mass spectrometry sulfur chemiluminescence detection,11−15 comprehensive two-dimensional gas chromatography mass spectrometry,16−20 and Fourier transform ion cyclotron resonance mass spectrometry,21,22 have been applied for the analysis of sulfur compounds in diesel fuel. However, these existing methods reach their limits when the polycyclic aromatic sulfur heterocycles (PASHs) are present in deeply hydrodesulfurized diesel, with the concentration of individual PASHs lying in the sub-parts per million (ppm) range. Petroleum is one of the most complex mixtures known. The complex hydrocarbon matrix will interfere with the analysis of trace amounts of PASHs as a result of the fact that hydrocarbon compounds are several orders of magnitude larger in concentration than the sulfur © 2017 American Chemical Society

compounds. Pre-concentration or derivatization of sulfur compounds was used to deal with this problem, which lead to sample loss, quantification error, and tedious procedures.23−26 The prominent advantage of gas chromatography quadrupole time-of-flight mass spectrometry (GC−Q-TOFMS) is high selectivity and sensitivity, profiting from targeted tandem mass spectrometry (MS/MS) mode and high mass accuracy and resolution, which is especially suitable for analysis of trace amounts of target compounds in complex samples. Thus, we now report the first use of GC−Q-TOFMS in direct and rapid quantitative analysis of DBTs in deeply hydrodesulfurized diesel fuel with no pretreatment or derivatization. The method could be applied to extensive qualitative and quantitative analyses of alkyldibenzothiophenes in hydrodesulfurized middle distillates at different sulfur levels.

2. EXPERIMENTAL SECTION 2.1. Chemicals. p-Terphenyl-d14 (purity of 98% D) was purchased from Aldrich; toluene (GR) was purchased from Dikma, DBT (purity of 98%) and 4-methyldibenzothiophene (purity of 97%) were purchased from J&K; 4,6-dimethyldibenzothiophene (purity of 95%) was purchased from Acros; 2,4,8-trimethyldibenzothiophene and 2,4,6,8-tetramethyldibenzothiophene (purity of 99%) were purchased from University of Münster; and all diesel samples were from Sinopec. 2.2. Preparation of Standard Solution. A series of the standard solutions from 0.01 to 20 μg/mL DBT, 4-methyldibenzothiophene, 4,6-dimethyldibenzothiophene, 2,4,8-trimethyldibenzothiophene, and 2,4,6,8-tetramethyldibenzothiophene was prepared with toluene as the solvent. Additionally, p-terphenyl-d14 was added as an internal standard at a concentration of 2 μg/mL to each standard solution to obtain internal standard solutions. An internal standard calibration method Received: May 16, 2017 Revised: August 14, 2017 Published: August 21, 2017 9125

DOI: 10.1021/acs.energyfuels.7b01289 Energy Fuels 2017, 31, 9125−9131

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Figure 1. Analysis results of alkyldibenzothiophenes in deeply hydrodesulfurized diesel by GC−SCD, GC−MS/MS, and GC−Q-TOFMS. was used for quantification of alkyldibenzothiophenes in deeply dehydrosulfurized diesel. 2.3. GC−Q-TOFMS. Analysis of alkyldibenzothiophenes in deeply dehydrosulfurized diesel was carried out on an Agilent GC−QTOFMS system equipped with 7890B GC and 7200 Q-TOFMS, an Agilent HP-5ms column (30 m × 0.25 mm, 0.25 μm). Helium was used as a carrier gas at a flow rate of 1.1 mL/min. A split/splitless injector was used with 300 °C injector temperature. A 1.0 μL sample injection volume with 20:1 split ratio was used. The oven temperature was programmed initially at 60 °C for 2 min and then programmed to increase to 300 °C at a rate of 10 °C/min, ending with 5 min of isothermal at 300 °C. The electron ionization (EI) ion source temperature was maintained at 280 °C. The quadrupole temperature was maintained at 150 °C. The MS transfer line was maintained at a temperature of 300 °C. The solvent delay was 5 min. Mass spectra were recorded across the range m/z 50−300 amu, with accurate mass

measurement of all mass peaks using the targeted mode. The frequency of spectra acquisition of the TOF was 5 Hz. Data acquisition was controlled by MassHunter Qualitative Analysis B.06 (Agilent Technologies). 2.4. Quantification of DBTs. The quantification of DBTs was performed by internal calibration curves, using reference compounds selected on the basis of the principle of structure-related target analyte/standard (same alkyl carbon atom numbers). According to the internal standard selection principle, p-terphenyl-d14 was preferred as the internal standard. The concentration of target analyte (Ci) was calculated according to the expression

Ci = Ai /A s(ki + mi) where Ai and As are the peak areas of the target analyte and internal standard, respectively, ki is the response factor of linearity, and mi is the constant for the standard calibration curve. 9126

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Figure 2. Standard spectra of DBTs and p-terphenyl-d14. The method was validated in accordance with the requirements for new methods for linearity, sensitivity, precision, and repeatability. 2.4.1. Linearity. Standard calibration curves were prepared using the following standards: DBT, 4-methyldibenzothiophene, 4,6-dimethyldibenzothiophene, 2,4,8-trimethyldibenzothiophene, and 2,4,6,8-tetramethyldibenzothiophene. Standard solutions were prepared at appropriate concentrations of 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, and 20 μg/mL for the plotting of calibration curves. The linearity was obtained by plotting the ratio of peak areas of reference compounds and internal standard versus the corresponding concentrations (μg/ mL) of each analyte. The calibration curve of DBT was used to quantify DBT. The calibration curve of 4MDBT was used to quantify C1-DBTs. The calibration curve of 46DMDBT was used to quantify C2-DBTs. The calibration curve of 248TrMDBT was used to quantify C3-DBTs. The calibration curve of 2468TeMDBT was used to quantify C4-DBTs. The extent of linearity was examined with mass concentrations ranging from 0.01 to 20 μg/mL and proven to have good linearity, with a high correlation coefficient (R2 > 0.9920) of all calibration curves (as shown in Table S1 of the Supporting Information). 2.4.2. Limit of Detection (LOD) and Limit of Quantification (LOQ). The LOD and LOQ of standard stock solutions were determined by preparing dilute solutions of standards (five dilution points were used in each case), scanning these solutions in targeted MS/MS mode, and recording the signal-to-noise (S/N) ratio for peaks at each concentration. LOD and LOQ were determined at a S/N ratio of about 3 and 10, respectively. Data in Table S2 of the Supporting Information showed that the S/N ratio of all peaks at a concentration of 0.01 μg/mL could exceed 10, which totally meets the industrial demand, and there is no necessity to determine the lower LODs. 2.4.3. Precision and Repeatability. For the precision test, the standard solutions containing the five standard compounds at two concentrations of 0.5 and 5 μg/mL were analyzed 3 times. Precision of the method was evaluated by calculating the recovery of standard compounds. Repeatability of the method was evaluated by calculating the relative standard deviation (RSD) of three independent determinations. For the standard solutions at concentrations of 0.5 and 5 μg/mL, the recovery of C0−C4-DBTs were 93.1−107.8 and 85.1−111.7%, respectively, and the RSD of C0−C4-DBTs were 1.1−

3.4 and 0.6−3.4%, respectively (as shown in Table S3 of the Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Development of the Targeted MS/MS Method. The concentration of individual alkyldibenzothiophenes in deeply hydrodesulfurized diesel is low to sub-ppm, and the matrix is very complex. The analysis of PASHs seriously interfered with polycyclic aromatic hydrocarbon (PAH) compounds. PASHs and PAHs are difficult to be separated by GC as a result of their similar polarity and boiling points. Furthermore, it is impossible to be distinguished by lowresolution mass spectrometry because they share the same nominal mass, such as C14H12S/C16H20 (212 amu). GC−QTOFMS has selectivity and sensitivity high enough to reduce the interference when targeted MS/MS mode was applied. Figure 1 illustrates the analysis results of alkyldibenzothiophenes in deeply hydrodesulfurized diesel at a sulfur level of 9 ppm by GC−sulfur chemiluminescence detection (SCD), GC− MS/MS, and GC−Q-TOFMS. It showed that the LOD of the sulfur-selective detector is too high to detect ultralow DBTs in deeply hydrodesulfurized diesel directly; meanwhile, the selectivity of triple quadrupole mass spectrometry is not high enough to eliminate the matrix interference. The high selectivity of GC−Q-TOFMS benefits from the multiple selective monitor and high resolving power (RP) of TOFMS. The first quadrupole mass spectrometry is set to restrict the precursor ion of the target compound, which collided in a collision cell, and all product ions were recorded with accurate mass measurement by coupled high-resolution time-of-flight spectrometry. The RP of TOFMS is sufficient to resolve product ions of PAHs and PASHs, which share the same nominal mass but differ in exact masses. The selectivity of QTOFMS allows for consistently lower detection limits and eliminates the matrix interference. 9127

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Figure 3. EIC of C0−C4-DBTs in an unhydrotreated diesel sample.

Figure 4. Influence of the mass error tolerance (500 and 50 ppm) on qualification and quantification of DBTs.

Selection of precursor ions and product ions of target compounds and optimization of collision energy are critical for accuracy of qualification and quantification. Ions with higher mass and abundance are generally preferred as precursor ions to be detected. The structure of DBTs is stable and difficult to

be fragmented; thus, molecular ions are preferred to be the precursor ions of target DBTs. The same principle is applicable to the choice of product ions. The most abundant ion, except molecular ion, was preferred to be product ions. In our research, identification of alkylated DBTs was based on their 9128

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expansion for this chromatogram is set to 500 ppm, naphthalenes would be identified as target analytes and interfere with the quantification of DBTs. When the parameter is set to 50 ppm, the interference of naphthalenes would be avoided. Thus, in our work, 50 ppm was preferred to guarantee perfect qualification and quantification results of DBTs. 3.3. Analysis of Deeply Hydrodesulfurized Diesel Fuels. To illustrate the application of this method, various types of deeply hydrodesulfurized diesel fuels at different sulfur levels were analyzed by this method. The results in Table 1

alkyl substituents. Different types of DBTs were identified by both precursor and product ions. According to the standard sample spectra (as illustrated in Figure 2), molecular ions of DBT, C1-DBTs, C3-DBTs, C4-DBTs, and p-terphenyl-d14 were chosen as the precursor ions. The most abundant ions, except molecular ion, were usually chosen as the product ions, but C2DBTs are an exception, because the most abundant ions, except molecular ions of dimethylbenzothiophenes (211.0576 amu) and ethyldibenzothiophenes (197.0419 amu), are different; therfore, the second abundant ion of dimethylbenzothiophenes (197.0419 amu) was preferred as the product ion to be detected. The abundance of product ions is used for the quantification of alkyldibenzothiophenes and affected by the collision energy obviously. Optimization of the collision energy is critical to obtain abundant product ions to reduce quantification error. MS/MS spectra were obtained by applying a collision energy ramping program starting from 5 to 30 eV (5 eV intervals) over one MS analysis in the collision cell. Optimized collision energies of DBT, C1-DBTs, C2-DBTs, C3-DBTs, C4-DBTs, and p-terphenyl-d14 are 25, 15, 20, 20, 15, and 22 eV, respectively. In individual time segments, different scanning parameters could be applied for different compounds in one scan; furthermore, the S/N ratio could be improved, and a highquality peak shape could be obtained. To determine the retention time of DBTs with different alkyl substituents, a mass spectrum of an unhydrotreated diesel sample was obtained by analyzing C0−C4-DBTs in the whole scanning time window. From their extracted ion chromatogram (EIC) of product ions (as illustrated in Figure 3), the retention time of DBTs was determined. The optimized targeted MS/MS scan conditions of DBTs are listed in Table S4 of the Supporting Information. 3.2. Identification of DBTs. Characterization of DBTs was carried out via the accurate molecular masses of their product ions. The data obtained from targeted MS/MS scan were subsequently entered into the MassHunter Qualitative Analysis B.06 software. On the basis of these data, the software is able to define EICs of different DBTs by their precursor and product ion pairs. The mass error tolerance of product ions (defined as a parameter of “the single m/z expansion for this chromatogram” in the software) influences the EIC obviously (as shown in Figure 4). When the parameter is set to 500 ppm, obvious interference of corresponding PAHs could be observed in the EIC of DBTs. As an example, Figure 5 illustrates four compounds that overlap in nominal masses, The C2H8/S doublet existing commonly in product ions of PAHs and PASHs has a mass difference of 0.0905 amu. If the single m/z

Table 1. Quantitative Results of DBTs in Deeply Hydrodesulfurized Diesel (μg/g) compound

mixed hydrogenated diesel

hydrogenated straight-run diesel

hydrogenated FCC diesel

C0-DBT 4MDBT 3/2MDBT 1MDBT ∑C1-DBT 4EDBT 46DMDBT 24DMDBT 26DMDBT 14/16DMDBT ∑C2-DBT 4E6MDBT 246TrMDBT 146TrMDBT 346TrMDBT ∑C3-DBT 46DEDBT ∑C4-DBT ∑C0−C4-DBT

0.30 0.11 0.10 0.05 0.38 0.05 0.05 0.46 0.25 0.41 0.51 0.03 0.04 0.03 0.03 0.47 0.00 0.00 1.66

0.04 0.08 0.04 0.04 0.17 0.09 0.16 0.05 0.05 0.15 0.78 0.35 0.18 0.22 0.08 1.09 0.08 1.23 3.31

0.00 0.08 0.03 0.03 0.14 0.11 0.66 0.06 0.07 0.13 1.10 1.12 1.46 0.60 0.40 3.94 0.14 2.72 7.90

showed that this method could be applied for the analysis of all different types of diesel, such as straight-run diesel, fluid catalytic cracking (FCC) diesel, or mixed diesel at a sulfur concentration below 10 μg/g. Furthermore, some isomers of C1-DBT, C2-DBT, and C3-DBT could be identified by standard compounds and the literature report,1 e.g., 4MDBT, 3/ 2MDBT, 1MDBT, 4EDBT, 46DMDBT, 24DMDBT, 26DMDBT, 14/16DMDBT, 4E6MDBT, 246TrMDBT, 146TrMDBT, 346TrMDBT, 46DEDBT, etc. Such detailed analysis results of alkyldibenzothiophenes in deeply hydrodesulfurized diesel fuel could not be obtained directly by conventional methods.

4. CONCLUSION Direct analysis of DBTs in deeply hydrodesulfurized diesel is challenging because the concentration of individual alkyldibenzothiophenes is sub-ppm and the matrix interference is very serious. Fortunately, the extraordinary selectivity and sensitivity of GC−Q-TOFMS is suitable for analysis of trace amounts of target analyte in a complex matrix. However, there was little application of GC−Q-TOFMS to direct quantitative analysis of alkyldibenzothiophenes in deeply hydrodesulfurized diesel. In this work, such a method was developed, which exhibited good linear response and high precision and sensitivity. The practicability of the method was shown by direct and rapid

Figure 5. Mass difference and RP needed of [M − 15]+ of C3-DBT, C4-DBT, and corresponding naphthalenes. 9129

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analysis of alkyldibenzothiophenes in various types of hydrodesulfurized middle distillates at different sulfur levels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01289. Linearity of five DBT calibration curves (Table S1), S/N ratio of five DBTs in standard solution of 0.01 μg/mL (Table S2), recovery and precision of DBTs (Table S3), and optimized targeted MS/MS conditions of different DBTs and internal standard (Table S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinyi Zhu: 0000-0002-8055-2678 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program o f China (973 Pr ogram, 2012CB224801).

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NOMENCLATURE PASH = polycyclic aromatic sulfur heterocycle HDS = hydrodesulfurization GC−Q-TOFMS = gas chromatography quadrupole time-offlight mass spectrometry DBT = dibenzothiophene PAH = polycyclic aromatic hydrocarbon RP = resolving power LOD = limit of detection LOQ = limit of quantification S/N = signal-to-noise RSD = relative standard deviation FCC = fluid catalytic cracking

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REFERENCES

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