Qualitative and quantitative analysis of organosulphur compounds in selected fuel samples

Abstract

Crude oil is a complex mixture of hydrocarbons, non-hydrocarbons and heteroatom-containing compounds. This fossil fuel undergoes refining processes to produce different fuel oils, which contributes one third to one-half of the world’s energy supply. The presence of organosulphur compounds (OSCs) in fuel oils is undesirable because these compounds emit sulphur oxide (SOx) gases to the atmosphere, corrode vehicle engines and deactivate catalyst during the refinery processes of crude oil. On the other hand, the identification and quantification of OSCs in fuel oils has been a challenging task. This is due to their volatile nature, the substituents structure of some OSCs such as polycyclic aromatic sulphur heterocycles (PASH) and the presence of other polycyclic hydrocarbons in high concentration, which causes interferences during the chromatographic analysis. Therefore, there has been a progress on development of various analytical methods for both qualitative and quantitative determination of OSCs in fuels. Therefore, this study reports a rapid and efficient method for the qualitative analysis of OSCs in fuel oil samples. A simple sample dilution with dichloromethane was conducted for qualitative analysis of the OSCs in crude oil, kerosene, gasoline and diesel samples using gas chromatographytime of flight-mass spectrometry (GC-T-TOFMS). Diesel sample showed the largest number of OSCs (26) namely: Thiophene, 3-methyl; 2-Thiophenecarboxylic acid, 4-nitrophenyl ester; Acetic acid, mercapto-; Thiazol-2-amine, N-(4-dimethylaminobenzyl)-; benzo[b]thiophene; Thieno [3,4-d]-1,3-dioxole, tetrahydro-2,2-dimethyl-, 5,5-dioxide; dibenzothiophene ; Sulfurous acid, octyl 2-propyl ester; Benz(1,4) oxathiino (2,3-c)pyridine; 3-hydroxy-2-thiabutane; 2-Thiopheneacetic acid, isopropyl ester; 3H-[1,3,4]Oxadiazole-2-thione, 5-(4-fluorophenyl)-3-(piperidin-1-yl)methyl-; 2-Butanone, 3,3-dimethyl-1-thiocyanato-; 3-Acetyl-2,5-dichlorothiophene; 2(3H)-Thiazolethione, 4-methyl-; 1,3-Diphenyl-4H-1,2,4-triazoline-5-thione; Benzothiazole, 2-(m-tolyl)-; 1,3-Diphenyl-4H-1,2,4-triazoline-5-thione; Ethylamine, N,N-dinonyl-2-phenylthio; 1,3-Diphenyl-4H-1,2,4-triazoline-5-thione; 1,3-Diphenyl-4H-1,2,4-triazoline-5-thione; 10H-Phenothiazine, 2-(trifluoromethyl)-; 1,3-Diphenyl-4H-1,2,4-triazoline-5-thione; 1-(4-Methyl-dibenzothiophen-2-yl)-piperidine; 2,2-Dimethyl-propyl 2,2-dimethyl-propane-thiosulfinate; Difluoro(methylamino)phosphine sulfide followed by crude oil (10), kerosene (11) and gasoline with 8 OSCs, namely: Thiophene, 2-(1,1-dimethylethyl); 3-Picoline, 2-(tert-butylthiol); 2-Thiophenecarboxylic acid, 4-nitrophenyl ester; methanesulfonamide, N-[2-(octylamino)ethyl]; Tetramethyl diphosphane-oxide-sulfide; Sulfurous acid, 2-ethylhexyl hexyl ester; 2-Thiophenecarboxylic acid, 5-tert-butyl and Thiophene, 2-nitro. The study also describes the development of environmentally friendly, effective, and selective pre-concentration method of OSCs in fuels, followed by GC-TOF-MS for both qualitative and semi-quantitative analysis. The magnetic solid phase extraction (m-SPE) based on Au-Fe3O4 adsorbent was selected as the preconcentration method for selected OSCs (thiophene; 3-methylthiophene; benzothiophene and dibenzothiophene). The Au-Fe3O4 NPs were synthesized using a co-precipitation method and characterized using XRD which produced diffraction peaks for gold coated magnetite with miller indices of Au (111), (200),(220), (311) and (222). UV-Vis was also used and a surface plasmon resonance peak at around 520 – 523 was produced. TEM and SEM confirmed the spherical shape and the size of 100 nm of the nanoparticles. Lastly, FTIR confirmed the particles with vibrational peaks at 500 cm-1 for Fe-O bond, 1500 cm-1 and 1750 cm-1 for C=O and –OH groups. The most influential parameters (mass of the adsorbent, extraction time, eluent volume and pH of the sample) affecting m-SPE were investigated by using multivariate mathematical tools. The best optimum conditions were found to be 150 mg mass of adsorbent, 100 μL eluent volume of acetonitrile, 50 minutes extraction time and 6,5 pH of the sample. These optimum conditions showed relatively low limits of detection (0,02-0,199 μg/L) and limit of quantification (0,126-0,602 μg/L) for all four of the investigated compounds. Furthermore, precision of 0,2 - 0,8% was obtained with good linearity of 0,9816-0,9961. The extraction efficiency of the proposed m-SPE method ranged from 76 to 95% for the spiked samples (4- 12 μg/L spike concentration level). The optimized m-SPE method was then applied toreal fuel oil samples and the concentration levels of non-target OSCs in crude oil, gasoline, diesel and kerosene ranged from 0,126 – 3,06 μg/L. Therefore, the developed m-SPE method prior to GC-HR-ToF-MS analysis can be applied as an alternative for quantitative determination of OSCs in oily matrices.