Exploratory Analysis of Crude Oil Thermal Evolution by Ultra-High-Resolution Mass Spectrometry
Energy Fuels 2024, 38, 22, 21932–21940: Exploratory Analysis of Crude Oil Thermal Evolution by Ultra-High-Resolution Mass Spectrometry
This study investigates the thermal stability of polar compounds in crude oils using ultra-high-resolution FT–MS with electrospray ionization (ESI). By monitoring the MS profiles of 14 key compounds subjected to micropyrolysis at temperatures up to 500 °C, we aim to better understand their thermal behavior.
A new "MS ratio" is introduced, based on ion abundance changes during thermal modification, and is compared to established GC–MS diagnostic ratios. The results demonstrate a correlation between the proposed MS ratio and classical thermal evolution indicators, providing a new approach for assessing crude oil maturity.
The original article
Exploratory Analysis of Crude Oil Thermal Evolution by Ultra-High-Resolution Mass Spectrometry
Arquimedes M. S. Coutinho, Marília Gabriela A. Pereira, Pedro G. C. Lucena, Jhonnatas de C. Carregosa, Wenes R. da Silva, Danielle M. M. Franco, Gesiane da S. Lima, Boniek G. Vaz, Alberto Wisniewski and Jandyson M. Santos
Energy Fuels 2024, 38, 22, 21932–21940
https://doi.org/10.1021/acs.energyfuels.4c03496
licensed under CC-BY 4.0
The chemical characterization of oils can be performed on the basis of the identification of biomarkers, which are compounds that allow inferences to be made concerning the geochemical features of oils, such as the type of original organic matter, the degree of thermal evolution, and the environmental origin. (3) Gas chromatography coupled with mass spectrometry (GC–MS) is the most common analytical technique used to identify biomarkers, enabling the calculation of diagnostic ratios to infer the degree of thermal evolution of the oil. (2,3) Additionally, the use of gas chromatography with flame ionization detection (GC–FID) is notable for its versatility and excellent response to hydrocarbons. Another valuable technique is comprehensive two-dimensional gas chromatography (GC × GC), which serves as an important tool for addressing coelution problems, a phenomenon commonly observed in the chromatographic analysis of oils. (4,5) The thermal evolution of an oil is a geochemical parameter that is related to its thermodynamic stability and depends upon the thermal stages in which it was formed as well as the temperature levels of its formation process. (2) The biomarker ratios most commonly used to evaluate thermal evolution are Ts/Tm, Ts/(Ts + Tm), and C32S/(S + R). (2,6−8) The compound 18α-22,28,30-trisnorneohopane (Ts) is more stable than 17α-22,29,30-trisnorhopane (Tm) at high temperatures; therefore, the Ts/Tm and Ts/(Ts + Tm) ratios increase with increasing degree of thermal evolution. (2,9) The same behavior occurs for the C32S/(S + R) ratio based on the ratio between the 17α(H),21β(H)-homohopane C32H56 isomers, involving conversion of the 22R isomer in the mixture of R and S isomers. (2,7)
The thermal behavior of oils can be evaluated by pyrolysis, where thermal decomposition reactions of the material occur in an inert environment, with the resulting compounds being characterized and related to the original molecule. (10)
Gas chromatography–mass spectrometry (GC–MS) is widely recognized as a fundamental technique for the analysis of hydrocarbon biomarkers of petroleum. Its significance is particularly evident in the separation of isomeric biomarkers, such as the previously mentioned Ts and Tm, allowing the determination of diagnostic ratios with high potential for inferring petroleum geochemical information. (11,12) Alternatively, ultra-high-resolution Fourier transform mass spectrometry (FT–MS) allows analysis to be performed by direct infusion of a simple oil solution into an electrospray ionization (ESI) source coupled to a mass analyzer, such as ion cyclotron resonance (ICR MS) or Orbitrap (Orbitrap MS) devices, enabling the detection and identification of thousands of components, in a single analysis and within only a few minutes. (3,10,13−15)
The main aim of this study was to evaluate the presence of typical compounds of the polar fraction of crude oils and to correlate them with data from micropyrolysis (Py) experiments, enabling the identification of potential biomarkers for use in geochemical thermal evolution diagnostics. The alternative analytical tool developed here is simpler and faster when compared to the classical biomarker ratios determined using GC–MS.
Experimental
Analytical Micropyrolysis (Py)
The 14 standard compounds described in Figure S1 of the Supporting Information were submitted to Py, according to the methodology of Maciel et al. (16) A 10 mg portion of each compound was inserted into a reactor tube with glass wool as an adsorbent material. The reactor tube was purged for 30 s with a 2 mL min–1 flow of N2, followed by heating to 500 °C in a Ni–Cr wire oven, resulting in a total reaction time of 3 min. The solvent used for final elution of the reactor tube contents was a methanol/toluene (1:1, v/v) mixture with a final volume of 1 mL. The solution obtained was analyzed by ESI(±) FT–MS.
Ultra-High-Resolution Fourier Transform Mass Spectrometry (FT–MS)
The FT–MS analyses were performed using an Orbitrap mass spectrometer (Exactive Plus, Thermo Scientific, U.S.A.). The standard compounds, their Py products, and the crude oils were prepared by dissolution to a final concentration of 1 mg mL–1 in a mixture of toluene/methanol (1:1, v/v). Formic acid was added in a proportion of 0.1% and ammonium hydroxide in a proportion of 0.4% as ionization promoters for ESI(+) and ESI(−) analyses, respectively. The samples were introduced by a direct infusion. The operating conditions were a flow rate of 5 μL min–1, capillary voltage at –3.2 kV in ESI(−) mode and +4.0 kV in ESI(+) mode, capillary temperature of 280 °C, and N2 as sheath, auxiliary, and spare gases at 5, 5, and 2 (arbitrary units), respectively.
The acquisition and interpretation of mass spectra were performed using Xcalibur 2.0 software (Thermo Scientific, U.S.A.), in the m/z range of 100–1000, which in a total of 100 μscans were accumulated to obtain the final average mass spectrum. The software determined the exact mass, inferring molecular formulas, indicating the assignment error for each molecular formula, providing double bond equivalent (DBE) values, and the total abundance of ions that were used in the MS ratio proposed. Confirmation of peak attribution considered agreement between the experimental and theoretical m/z values together with the isotopic pattern.
Sequential mass spectrometry analyses (MS/MS) were performed by FT-ICR MS (solariX 9.4 T, Bruker Daltonics, Germany), using ESI(−) ionization mode and a collision energy of 25 eV, for specific ions from the Py product of the 2,6-dimethyl-4-pyridinol standard compound. The ESI conditions of the FT ICR–MS/MS analysis were analogous to those described above for the Orbitrap FT–MS analyses.
Analysis by GC-MS
Crude oil samples (5 g) were fractionated into saturate, aromatic, resin, and asphaltene (SARA) compounds, according to the conditions of Santos et al. (17) Briefly, n-heptane (in the proportion of 30 mL/g of crude oil) was added to the crude oil sample, and the mixture was refluxed for 1 h. Subsequently, the mixture was cooled and stored in the dark overnight, resulting in precipitation of the asphaltenes. The maltene fractions soluble in n-heptane were separated using open-column chromatography (3 × 40 cm) after previously being adsorbed on activated neutral alumina (80–200 mesh, Brockmann I, Sigma-Aldrich, Steinheim, Germany). Cyclohexane was used to elute the saturated fraction, which was analyzed by GC–MS, using a gas chromatography/triple quadrupole mass spectrometry system (GCMS-TQ8040, Shimadzu, Japan).
GC–MS was operated in electron ionization (EI) mode, following the conditions of Carregosa et al. (18) Briefly, the chromatographic separation was performed with a SH-RTX5SilMS capillary column [crossbond 1,4-bis(dimethylsiloxy)phenylene dimethylpolysiloxane, 30 m × 0.25 mm inner diameter, 0.25 μm film thickness, Restek, U.S.A.]. A 1 μL volume of the solution was injected in the splitless mode. The injector temperature was 300 °C. The oven temperature was programmed from 70 to 325 °C, at 3 °C min–1, resulting in a total chromatographic run time of 85 min. Helium (99.995% purity) was used as the carrier gas at a constant flow rate of 1 mL min–1. The mass spectrometer ion source and interface temperatures were 280 and 290 °C, respectively. The collision gas was Ar (99% purity), and the collision energy was set at 12 eV.
Multiple reaction monitoring (MRM) GC–MS analysis was performed to identify terpane and sterane compounds that could be employed as thermal evolution biomarkers. The m/z transitions monitored were m/z 370 > 191, to calculate Ts/Tm and Ts/(Ts + Tm) ratios, and m/z 412 > 191, to calculate C32S/(S + R) ratios.
Results and Discussion
ESI(−) FT–MS of the Crude Oils
The six crude oils analyzed by ESI(−) FT–MS presented a complex chemical mixture, with a wide range of polar compounds, as also described by Santos et al. (17) A representative MS spectrum is shown for the AL crude oil (Figure 4a), with expansion to visualize the deprotonated ions of 2,6DPO (Figure 4b) and 1DP (Figure 4c), to highlight that only the standard compound and its 1DP product from Py were naturally present in the chemical composition of the oil.
Energy Fuels 2024, 38, 22, 21932–21940 - Exploratory Analysis of Crude Oil Thermal Evolution by Ultra-High-Resolution Mass Spectrometry - Figure 4. ESI(−) FT–MS spectrum of (a) AL, with expansions at (b) m/z 121–123, highlighting 2,6DPO, and (c) m/z 241, highlighting 1DP.
The ESI(−) FT–MS data for all of the crude oils are shown in Figures S2–S6 of the Supporting Information for the CA, PI, SE, O3, and O16 samples, respectively. The presence of ions corresponding to 2,6DPO and its 1DP product from Py was found for all of the crude oils analyzed. The m/z, molecular formula, DBE, and structures are summarized in Table 2.
Energy Fuels 2024, 38, 22, 21932–21940 - Exploratory Analysis of Crude Oil Thermal Evolution by Ultra-High-Resolution Mass Spectrometry - Table 2. ESI(−) FT–MS Data for the Six Crude Oils Related to the 2,6DPO and 1DP Compounds
Both compounds were naturally present in all of the crude oils but in different abundances, reinforcing that a relation between the abundances of the ions could be explored in the geochemical context of thermal evolution.
Conclusion
The thermal behaviors of 14 typical petroleum compounds were evaluated on the basis of ESI(±) FT–MS analyses, to identify a potential geochemical biomarker of thermal evolution among the polar compounds present in the oils. It was found that pyrolysis of 2,6-dimethyl-pyridin-4-ol (2,6DPO) led to the formation of product ions, which were also observed in the FT–MS analyses of the crude oils. MS/MS analysis identified the main thermal product as 2,2-[(ethene-1,2-diyl)]bis(6-methylpyridin-4-ol), denoted 1DP. These ions were used in a proposed indicator, denoted the MS ratio, which could be used to evaluate the degree of thermal evolution of oils based on FT–MS analysis. The resulting MS ratio values were directly correlated with the values of classical diagnostic ratios [Ts/Tm, Ts/(Ts + Tm), and C32S/(S + R)], obtained by GC–MS, and showed the same order of thermal evolution for the six crude oils evaluated. Therefore, the MS ratio is a tool that can be used to infer the thermal evolution of oils, focusing on the polar compounds, as an alternative to the traditional methods based on GC–MS, with the advantages of simpler direct analysis and faster data acquisition. Furthermore, the MS ratio offers potential advancements in geochemical research by enhancing the tracking of polar molecules, which are often neglected in traditional biomarker analysis. This method has practical implications for geochemical studies, enabling rapid and accurate evaluations of oil maturity degrees.
