Monitoring of Remaining Thiophenic Compounds in Liquid Fuel Desulphurization Studies Using a Fast HPLC-UV Method

separations

Article Monitoring of Remaining Thiophenic Compounds in Liquid Fuel Desulphurization Studies Using a Fast HPLC-UV Method

Vasiliki Kapsali 1 , Konstantinos Triantafyllidis 2 , Eleni Deliyanni 2 and Victoria Samanidou 1,*

1 Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] 2 Laboratory of Chemical and Environmental Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] (K.T.); [email protected] (E.D.) * Correspondence: [email protected]

Abstract: Thiophenic compounds constitute a class of sulfur compounds derived by thiophene, containing at least one thiophenic ring. Their presence in fuels (crude oil, etc.) is important and can reach 3% m/m. The combustion of fuels leads to the formation of sulfur oxides a severe source of environmental pollution issues, such as acid rain with adverse effects both to humans and to the environment. To reduce such problems, the EU and other regulatory agencies worldwide set increasingly stringent regulations for sulfur content in fuels resulting in the necessity for intense desulphurization processes. However, most of these processes are inefﬁcient in the total removal of

sulfur compounds. Therefore, thiophenic compounds such as benzothiophenes and dibenzothiophenes are still present in heavier fractions of petroleum, therefore, their determination is of great

Citation: Kapsali, V.; Triantafyllidis, importance. Until now, all HPLC methods applied in similar studies use gradient elution programs K.; Deliyanni, E.; Samanidou, V. that may last more than 25 min with no validation results provided. To fill this gap, the aim of the Monitoring of Remaining Thiophenic present study was to develop and validate a simple and fast HPLC-UV method in order to be used as Compounds in Liquid Fuel a useful monitoring tool in the evaluation studies of novel desulfurization technologies by means of Desulphurization Studies Using a simultaneous determination of dibenzothiophene (DBT) and 4,6-dimethyl-dibenzothiophene and Fast HPLC-UV Method. Separations dibenzothiophene sulfone in the desulfurization effluents. 2021, 8, 48. https://doi.org/10.3390/ separations8040048 Keywords: HPLC; thiophene compounds; fuels; desulphurization; dibenzothiophene (DBT); 4,6-dimethyl-dibenzothiophene; dibenzothiophene sulfone Academic Editors: George Zachariadis and Rosa Peñalver 1. Introduction Received: 1 March 2021 Accepted: 9 April 2021 Organosulfur compounds such as sulfides, disulfides, thiophenes, benzothiophenes, Published: 11 April 2021 etc. contained in fossil fuels (e.g., petroleum, gasoline, diesel oil, etc.) can cause severe environmental pollution issues upon combustion due to the formation of sulfur oxides, Publisher’s Note: MDPI stays neutral which may cause air pollution and acid rain. Moreover, sulfur content may lead to the with regard to jurisdictional claims in poisoning of automotive catalysts used to reduce emissions of pollutant hydrocarbons, published maps and institutional affil- carbon monoxide, and nitrogen oxides from gasoline- and diesel-fueled vehicles [1]. iations. The growing environmental awareness resulted in a high necessity of efficient desulfurization processes. Strict environmental regulations by the U.S. Environmental Protection Agency (EPA) and other environmental regulatory agencies worldwide, require the total removal of sulfur content which is not feasible to be achieved by conventional desulfurization

Copyright: © 2021 by the authors. procedures [2,3]. Licensee MDPI, Basel, Switzerland. European Community Member States shall ensure that gas oils are not used within This article is an open access article their territory if their sulfur content exceeds 0.10% by mass; heavy fuel oils are not used if distributed under the terms and their sulfur content exceeds 3% by mass; marine fuels are not used within their territory conditions of the Creative Commons if their sulfur content exceeds 0.50% by mass, except for fuels supplied to ships using Attribution (CC BY) license (https:// emission abatement methods as reported in Directive (EU) 2016/802 [4] and Directive (EU) creativecommons.org/licenses/by/ 2015/1513 [5]. 4.0/).

Separations 2021, 8, 48. https://doi.org/10.3390/separations8040048 https://www.mdpi.com/journal/separations Separations 2021, 8, x FOR PEER REVIEW 2 of 8

Separations 2021, 8, x FOR PEER REVIEW 2 of 8 To that end, efficient desulfurization is an essential step in the petroleum refining Separations 2021, 8, 48 process. Hydrodesulfurization (HDS) is the main process widely used to eliminate2 of 8sulfur content. HDS processes are not economical and have a high energy demand since H2 and catalystsTo thatsuch end, as CoMo/Al efficient 2desulfurizationO3 and/or NiMo/Al is an2O essential3 are used step at temperaturesin the petroleum from refining 300 to 400process. °C Toand Hydrodesulfurization that high end, pressures efficient desulfurization[6]. (HDS)Besides, is amthe isong anmain essential thiophenic process step widely compounds in the used petroleum to that eliminate refiningare derived sulfur fromcontent.process. thiophene HDS Hydrodesulfurization processes containing are at not least (HDS) economical one is thethiophenic main and processhave ring, a widelyhigh DBT energy and used especially to demand eliminate 4,6-DMDBT,since sulfur H2 and content. HDS processes are not economical and have a high energy demand since H and thecatalysts sulfur-containing such as CoMo/Al compounds2O3 and/or in diesel NiMo/Al oil that2O3 areare used considered at temperatures as refractory from2 species 300 to catalysts such as CoMo/Al2O3 and/or NiMo/Al2O3 are used at temperatures from 300 since400 °C they and◦ resist high ordinary pressures methods [6]. Besides, of treatm amongent, thiophenic cannot be compoundscompletely removedthat are deriveddue to fromto 400 thiopheneC and high containing pressures at [ 6least]. Besides, one thiophenic among thiophenic ring, DBT compounds and especially that are derived4,6-DMDBT, theirfrom structure thiophene (their containing chemical at leaststructures one thiophenic are shown ring, in DBTFigure and 1). especially This fact 4,6-DMDBT, makes the cur- the sulfur-containing compounds in diesel oil that are considered as refractory species rentthe target sulfur-containing of a low level compounds of 30 ppmw in dieselsulfur oil for that diesel are consideredfuel, set by as the refractory Environmental species Pro- tectionsincesince they Agency they resist resist (EPA) ordinaryordinary requirements, methods methods of treatment, toof betreatm a difficult cannotent, cannot betask. completely be completely removed removed due to their due to theirstructure structure (their (their chemical chemical structures structures are shown are shown in Figure in1 Figure). This fact1). This makes fact the makes current the cur- renttarget target of aof low a low level level of 30 of ppmw 30 ppmw sulfur sulfur for diesel for fuel, diesel set byfuel, the set Environmental by the Environmental Protection Pro- tectionAgency Agency (EPA) requirements,(EPA) requirements, to be a difficult to be a task. difficult task.

Dibenzothiophene 4,6-Dimethyldibenzothiophene Dibenzothiophene sulfone (DBT) (4,6-DMDBT) (DBT sulfone)

Figure Dibenzothiophene 1. Chemical structures 4,6-Dimethyldi of the studiedbenzothiophene thiophenic compounds: Dibenzothiophene dibenzothiophene sulfone (DBT), 4,6- dimethyl-dibenzothiophene (4,6-DMDBT), and dibenzothiophene sulfone. (DBT) (4,6-DMDBT) (DBT sulfone)

FigureFigureFor 1. these 1.ChemicalChemical reasons, structures structures alternative of of the the studied studiedand/or thiophen thiophenicsupplementaryic compounds: compounds: methods dibenzothiophene are needed, (DBT), (DBT),including 4,6- hydrodesulfurizationdimethyl-dibenzothiophene4,6-dimethyl-dibenzothiophene on Co/Ni (4,6-DMDBT), (4,6-DMDBT), catalysts and and[7,8], dibenzothiophene dibenzothiophene oxidation/ph sulfone.oto-oxidationsulfone. [9,10], and adsorption [11] with adsorptive desulfurization to be of great importance especially when a For these reasons, alternative and/or supplementary methods are needed, including removalFor targetthese reasons,of a level alternative less than 30 and/or ppmw su sulfurpplementary is considered. methods are needed, including hydrodesulfurization on Co/Ni catalysts [7,8], oxidation/photo-oxidation [9,10], and hydrodesulfurizationAmong adsorbent on materials, Co/Ni catalysts activated [7,8], carbons oxidation/ph have beenoto-oxidation proven to be [9,10], suitable and ad-ad- sorptionadsorption [11] [with11] with adsorptive adsorptive desulfurization desulfurization to bebe ofof great great importance importance especially especially when when a sorbentsa removal of dibenzothiophenic target of a level less compounds than 30 ppmw [12–19]. sulfur is Besides, considered. their acidic oxygen functional removal target of a level less than 30 ppmw sulfur is considered. groups Amongon their adsorbent surface materials,lead to specific activated ad carbonssorption have of dibenzothiophenic been proven to be suitable species adsor- via oxygen–sulfurbentsAmong of dibenzothiophenicinteractions adsorbent [20]materials, and compounds acid-base activated [12 interactions– 19carbons]. Besides, have with their been the acidic slightlyproven oxygen tobasic functionalbe suitablethiophenes ad- [21].sorbentsgroups of on dibenzothiophenic their surface lead to speciﬁc compounds adsorption [12–19]. of dibenzothiophenic Besides, their acidic species oxygen via oxygen– functional groupssulfurAdditionally, on interactions their surface either [20] andlead oxygen acid-base to specific of these interactions ad carbons’sorption with ofsurface the dibenzothiophenic slightly oxygen basic functional thiophenes species groups [21 via]. oxy- or chemisorbedgen–sulfurAdditionally, interactions oxygen either may [20] take oxygen and part acid-base of in these reactions carbons’ interactions that surface lead with to oxygen the the oxidation functionalslightly ofbasic groupsadsorbed thiophenes or DBT and[21].chemisorbed 4,6-DMDBT oxygen on the may carbon’s take part surface in reactions to thei thatr sulfoxides lead to the oxidationand finally of adsorbedsulfones DBT with the and 4,6-DMDBT on the carbon’s surface to their sulfoxides and ﬁnally sulfones with the portionAdditionally, of 4,6-DMDBT either oxidation oxygen to of be these less carbons’compared surface to DBT ox oxidationygen functional [17,18,20,22]. groups The or chemisorbedportion of 4,6-DMDBT oxygen may oxidation take part to bein lessreactions compared that tolead DBT to oxidationthe oxidation [17,18 of,20 adsorbed,22]. The DBT DBTDBT sulfone sulfone is isformed formed according according to thethe oxidationoxidation reaction reaction route route shown shown in Figure in Figure2[ 23 ,224 [23,24].]. and 4,6-DMDBT on the carbon’s surface to their sulfoxides and finally sulfones with the portion of 4,6-DMDBT oxidation to be less compared to DBT oxidation [17,18,20,22]. The DBT sulfone is formed according to the oxidation reaction route shown in Figure 2 [23,24].

Figure 2. DBT sulfone formation. Figure 2. DBT sulfone formation. For this reason, DBT, 4,6-DMDBT, and DBT-sulfone can be found in ﬁnal efﬂuent after adsorptionFor this ofreason, dibenzothiophene DBT, 4,6-DMDBT, (DBT) and and 4,6-dimethyldibenzothiophene DBT-sulfone can be found (4,6-DMDBT) in final effluent in desulfurization studies, making their analytical determination during desulfurization afterFigure adsorption 2. DBT sulfone of formation.dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6- studies and the use of fast validated HPLC methods [25–27] of great importance. DMDBT) in desulfurization studies, making their analytical determination during desul- furizationFor this studies reason, and DBT, the 4,6-DMDBT,use of fast vaandlidated DBT- HPLCsulfone methods can be found [25–27] in offinal great effluent importance.after adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6- DMDBT) in desulfurization studies, making their analytical determination during desulfurization studies and the use of fast validated HPLC methods [25–27] of great importance.

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Therefore, in order to evaluate the effectiveness of new technologies, an analytical method is necessary to determine the concentration of initial as well as adsorption/oxidation products. Although these compounds have been determined in several fuel samples during desulfurization studies there is a vacancy in the analytical ﬁeld concerning the development, optimization, and validation of a fast analytical method for their monitoring [28–33]. Various GC and HPLC methods based on UV detection are reported using gradient elution programs that may last more than 25 min. A comprehensive review has listed most of the published methods. However, all these methods are reported as monitoring tools where neither analytical performance characteristics are provided, nor validation criteria are applied [25,34]. The aim of this study was to develop and validate a simple and fast HPLC method for the simultaneous determination of DBT, DBT-sulfone, and 4,6-DMDBT in extracts after the oxidative desulfurization process of fuels. To the best of our knowledge, there is no published method for the determination of the examined thiophenic compounds.

2. Materials and Methods 2.1. Instrumentation The separation of the target compounds was performed by the High-Performance Liquid Chromatography (HPLC) method, using an HPLC column 250 × 4.6 mm (PerfectSil Target ODS-3 5 µm, MZ-Analysentechnik GmbH, Wohlerstrasse, Mainz, Germany) with a mobile phase consisted of a solution of acetonitrile and water (90% CH3CN: 10% H2O v/v) delivered isocratically at a ﬂow rate of 1.0 mL/min, using a Shimadzu (Kyoto, Japan) LC-10AD pump. The pressure observed was 80 bar. Sample injection was performed via a Rheodyne 7125 injection valve (Rheodyne, Cotati, CA, USA) with a 20 µL loop. Detection was achieved by an SSI 500 UV-Vis detector (SSI, State College, PA, USA) at a wavelength of 313 nm and a sensitivity setting of 0.002 AUFS. DAQ Software for Chemists, developed by Emeritus Professor P. Nikitas (Dept. of Chemistry, Aristotle University of Thessaloniki). The degassing of the mobile phase was performed in an ultrasonic bath Transonic 460/H (35 kHz, 170 W, Elma, Germany). All samples were ﬁltered prior to HPLC analysis using Q-Max RR Syringe Filters 0.22 µm Nylon, 13 mm Frisenette, Knebel, Denmark.

2.2. Chemicals and Reagents HPLC grade acetonitrile and methanol were purchased from PanReac AppliChem (Ot- toweg, Darmstadt, Germany). High purity water, obtained by a Milli-Q puriﬁcation system (Millipore, Bedford, MA, USA), was used throughout the entire study. Dibenzothiophene sulfone 97%, 4,6-dimethyldibenzothiophene (4,6 DMDBT, C14H12S), and dibenzothiophene (DBT, C12H8S) were supplied by JKchemical 1 (Beijing, China).

2.3. Preparation of Standard Solutions Standard solutions of each analyte were prepared in acetonitrile (200 ng µL−1), stored at 4 ◦C in the dark. Working standards solutions were prepared by further di- lution in acetonitrile covering the linear range from 0.1 to 30 ng µL−1. Aliquots of 20 µL were injected into the column and quantitative analysis was performed based on peak area measurements.

2.4. Method Validation Method validation was performed following the criteria set by Commission Decision 2002/657/EC [35]. In that aspect, selectivity, linearity, accuracy, precision (repeatability and between-day precision), as well as limits of detection (LODs) and quantiﬁcation (LOQs) were taken into consideration. Calibration curves were constructed by three replicates of standard solutions in the working interval of 0.1–30 ng µL−1. Separations 2021, 8, x FOR PEER REVIEW 4 of 8

Limits of detection (LODs) were calculated based on the following equation LOD = 3.3 S/N, where S = signal and N = noise. Limits of quantitation (LOQs) were calculated by Separations 2021, 8, 48 the equation LOQ = 10 S/N. 4 of 8 Accuracy was evaluated at three concentration levels of 0.5, 2, and 10 ng/μL. Preci- sion within-day (intra-day, n = 3) and between-day (inter-day n = 4 days by duplicate Limits of detection (LODs) were calculated based on the following equation LOD = 3.3 S/N, analyses) was studied at three concentration levels 0.5, 2, and 10 ng/μL. where S = signal and N = noise. Limits of quantitation (LOQs) were calculated by the equationPeak LOQ purity = 10 Swas/N. checked by standard addition of respective standards. Accuracy was evaluated at three concentration levels of 0.5, 2, and 10 ng/µL. Precision 2.5.within-day Adsorption (intra-day, of DBTn = 3)and and 4,6-DMDBT between-day (inter-day n = 4 days by duplicate analyses) was studied at three concentration levels 0.5, 2, and 10 ng/µL. PeakFor the purity adsorption was checked experiments by standard additiona solution of respective containing standards. the same concentrations of DBT and 4,6-DMDBT in acetonitrile, was prepared. The concentration of each compound in solution2.5. Adsorption was of20 DBT ppmwS. and 4,6-DMDBT The corresponding total sulfur concentration was 40 ppmwS. The massFor the of adsorption the carbon experiments adsorbent a solution was containing1.25 g/L theand same the concentrations adsorption ofprocess DBT was carried and 4,6-DMDBT in acetonitrile, was prepared. The concentration of each compound in out at ambient temperature. The remaining concentrations of thiophenes after adsorption solution was 20 ppmwS. The corresponding total sulfur concentration was 40 ppmwS. The weremass ofdetermined the carbon adsorbent by HPLC was at 1.25 a wavelength g/L and the adsorptionof 231 nm. process was carried out at ambient temperature. The remaining concentrations of thiophenes after adsorption were 3.determined Results by HPLC at a wavelength of 231 nm.

3.1.3. Results HPLC Method 3.1. HPLCIsocratic Method elution was optimized in terms of mobile phase constituents and ratios. Ac- etonitrileIsocratic as the elution organic was optimized modifier inled terms to better of mobile peak phase shape. constituents The ratio and of 90:10 ratios. v/v of acetoni- trileAcetonitrile and water as the mixture organic modiﬁerprovided led the to bettersufficient peak peak shape. resolution The ratio of and 90:10 fastv/v analysis.of acetonitrile and water mixture provided the sufﬁcient peak resolution and fast analysis. AA typicaltypical HPLC HPLC chromatogram chromatogram at two mobileat two phasemobile ratios phase is illustrated ratios is in illustrated Figure3. in Figure 3.

FigureFigure 3.3.A A typical typical HPLC HPLC chromatogram chromatogram under different under eluentdifferent conditions: eluent (90% conditions: CH3CN: 10%(90% H2 CHO 3CN: 10% Hv/2Ov, peaks:v/v, peaks: DBT-sulfone DBT-sulfone 3.89 min, DBT3.89 9.59min, min, DBT and 9.59 4,6-DMDBT min, and 18.26 4,6-DMDBT min) and (80% 18.26 CH3 CN:min) 20% and (80% H O v/v, peaks: DBT-sulfone 3.33 min, DBT 6.25 min, and 4,6-DMDBT 10.20 min). CH2 3CN: 20% H2O v/v, peaks: DBT-sulfone 3.33 min, DBT 6.25 min, and 4,6-DMDBT 10.20 min). 3.2. Calibration Curves 3.2. CalibrationCalibration curvesCurves were constructed by least-squares linear regression analysis. All calibrationCalibration data obtained curves by were standard constructed solution analyses by least-squares are shown in Tablelinear1. Regressionregression analysis. All equations presented satisfactory correlation coefﬁcients ranging between 0.9925 and 0.9993 calibration data obtained by standard solution analyses are shown in Table 1. Regression over the examined range. equations presented satisfactory correlation coefficients ranging between 0.9925 and 0.9993Table 1. overLinearity the data examined of developed range. method.

Analyte Slope Intercept Correlation Coefﬁcient (R) Table 1. Linearity data of developed method. DBT 0.0136 0.009 0.9925 4,6-DMDBTAnalyte 0.0135Slope Intercept 0.0019 Correlation 0.9953 Coefficient (R) DBT Sulfone 0.0670 −0.0414 0.9993 DBT 0.0136 0.009 0.9925 4,6-DMDBT 0.0135 0.0019 0.9953 DBT Sulfone 0.0670 −0.0414 0.9993

The limit of detection was found to be 0.16 ng/μL, while the limit of quantitation was found 0.5 ng/μL.

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Selectivity was studied by the absence of peaks in the same Rt of analytes. The limit of detection was found to be 0.16 ng/µL, while the limit of quantitation was Accuracy and precision data at three concentration levels are summarised in Tables found 0.5 ng/µL. 2 and 3 with regards to within-day and between-day repeatability respectively. Selectivity was studied by the absence of peaks in the same Rt of analytes. Accuracy and precision data at three concentration levels are summarised in Tables2 and3 Table 2. Accuracy and repeatability using CH3CN-H2O 90:10 v/v as mobile phase n = 3. with regards to within-day and between-day repeatability respectively. Analyte Added (ng/μL) Found ± SD (ng/μL) R% RSD% Table 2. Accuracy and repeatability 0.5 using CH3CN-H 0.462O 90:10± 0.04 v/v as mobile 92.0 phase n = 3. 8.7 AnalyteDBT Added (ng/ 2 µL) Found ± 1.94SD (ng/± 0.20µL) R% 97.0 RSD% 10.3 10 9.8 ± 1.1 98.0 11.2 0.5 0.46 ± 0.04 92.0 8.7 DBT 2 0.5 1.94 0.47± 0.20 ± 0.05 97.0 94.0 10.3 10.6 4,6-DMDBT 10 2 9.8 ± 2.01.1 ± 0.2 98.0 100.0 11.2 10.0 0.5 10 0.47 10.2± 0.05 ± 0.87 94.0 102.0 10.6 8.5 4,6-DMDBT 2 0.5 2.0 0.52± 0.2 ± 0.06 100.0 104.0 10.0 11.5 10 10.2 ± 0.87 102.0 8.5 DBT Sulfone 0.5 2 0.52 ± 1.880.06 ± 1.3 104.0 94.0 11.5 6.9 DBT Sulfone 2 10 1.88 ± 9.81.3 ± 1.1 94.0 98.0 6.9 11.2 10 9.8 ± 1.1 98.0 11.2 Table 3. Accuracy and between-day precision using CH3CN-H2O 90:10 v/v as mobile phase (duplicate analyses in four days). Table 3. Accuracy and between-day precision using CH3CN-H2O 90:10 v/v as mobile phase (duplicate analysesAnalyte in fourAdded days). (ng/μL) Found ± SD (ng/μL) R% RSD% Analyte Added 0.5 (ng/ µL) Found ± 0.53SD ± (ng/ 0.06µL) R% 106.0 RSD% 11.3 DBT 2 2.08 ± 0.25 104.0 12.0 0.5 0.53 ± 0.06 106.0 11.3 DBT 10 2 2.08 9.49± 0.25 ± 0.84 104.0 94.9 12.0 8.8 0.510 9.49 0.5± 0.84± 0.06 94.9 100.0 8.8 12.0 4,6-DMDBT 0.5 2 0.5 2.16± 0.06 ± 0.3 100.0 108.0 12.0 13.9 4,6-DMDBT 10 2 2.16 9.79± 0.3 ± 0.7 108.0 97.9 13.9 7.2 10 9.79 ± 0.7 97.9 7.2 0.50.5 0.49 0.49± 0.05 ± 0.05 98.0 98.0 10.2 10.2 DBTDBT Sulfone Sulfone 2 2 1.92 1.92± 0.15 ± 0.15 96.0 96.0 7.8 7.8 1010 9.4 9.4± 0.8 ± 0.8 94.0 94.0 8.5 8.5

3.3. Application in Real Sample The methodmethod waswas applied applied to to samples samples derived derive fromd from desulfurization desulfurization studies studies as described as de- scribedin the experimental in the experimental part. part. A typical chromatogram chromatogram of of sample sample obtained obtained as as described described in in Section Section 2.5 2.5 after after 24 24 h is h shownis shown in Figure in Figure 4, 4which, which illustrates illustrates the thepeaks peaks of remaining of remaining DBT DBT and and4,6 DMDBT 4,6 DMDBT compounds,compounds, as well as well as the as thepeak peak of ofthe the polar polar DB DBTT sulfone, sulfone, which which is is present present in in the efﬂuenteffluent eluted in the polar acetonitrile solvent.

Figure 4. HPLCHPLC chromatogram chromatogram after after a a24 24 h hadsorption adsorption study study where where peaks peaks of ofremaining remaining DBT, DBT, 4,6- 4,6- DMDBT, asas wellwell asas thethe polarpolar DBTDBT sulfone sulfone present present in in the the efﬂuent effluent due due to to elution elution by by acetonitrile, acetonitrile, peaks: peaks:DBT-sulfone DBT-sulfone 3.31 min, 3.31 unknown min, unknown peak 3.76, peak DBT 3.76, 6.23 DBT min, 6.23 and min, 4,6-DMDBT and 4,6-DMDBT 10.24 min. 10.24 min.

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3.4. Comparison with Other Methods So far various GC and HPLC methods based on UV detection are reported using gradient elution programs that may last more than 25 min. All previously reported methods were applied as monitoring tools where neither analytical performance characteristics are provided, nor validation criteria are applied. The herein developed method is faster since the analysis is completed within 10 min and no equilibration time is necessary in the intervals as a beneﬁt of isocratic elution. Therefore, six samples can be analyzed per hour increasing the productivity of the method. The cost of the analysis is also lower since no highly sophisticated instrumentation is needed.

4. Conclusions It is vital to reduce the content of benzothiophene in liquid fuels—a process known as desulfurization. Adsorption and catalytic oxidation can be used for the removal of dibenzothiophene (DBT). Since this process is not complete, some amounts are still present in the final product, thus, it is necessary to quantitate the remaining as well as other thiophenic products in the final result. Herein, a fast HPLC method was developed and validated to serve as an analytical tool in desulfurization studies. The analysis is completed in ca 10 min and gives results for three thiophenic compounds. The proposed method has been proved to be a useful tool in the environmental technology field providing accurate, repeatable, and reliable analytical results when applied in the respective evaluation studies.

Author Contributions: Conceptualization, K.T., E.D. and V.S.; methodology, E.D., K.T. and V.S.; validation, V.K. and V.S.; investigation K.T., E.D. and V.S.; experimental: V.K.; writing—original draft preparation, E.D. and V.S.; writing—review and editing, E.D., V.S. and K.T.; supervision, E.D. and V.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available upon request. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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