Crude Oil Quality can be Determined by Electron Spin Resonance Spectroscopy

Crude oil has many different applications, including the generation of electricity, transportation, and the manufacture of plastics. However, asphaltene, a complex mixture of compounds with polyaromatic rings and aliphatic side chains, combined with trace amounts of vanadium, reduces crude oil quality.

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These compounds can readily aggregate, increasing the viscosity of the oil, which can plug wellbores and flowlines during the extraction and processing. In addition, asphaltenes can also deactivate catalytic reactions during upgrading and refining processes. Therefore, analysis of asphaltene and vanadyl content in crude oil is essential.

Due to the complex mixture of hydrocarbons in crude oil, examining the asphaltene and vanadyl concentration is difficult. So analysis is commonly performed using model systems or in samples that have been diluted into simple solvents. However, this characterization does not give a true reflection of asphaltenes in situ.

Electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy is a non-destructive technique that requires no sample preparation, allowing for real-time analysis. This method provides high-resolution and quantitative data, which gives real dynamic information within the local environment.

Asphaltenes can be easily identified in the complex hydrocarbon mixture of crude oil by ESR, because the organic free radicals in their polyaromatic core give a characteristic signal at g = 2.0032. Vanadyl porphyrins also give a characteristic spectrum, appearing as eight broadly spaced, narrow peaks in a similar region to the asphaltene organic free radical signal.

These characteristic ESR spectra have enabled different types of crude oil to be distinguished from each other, but in recent years more detailed analysis of the changes to the chemical composition has been achieved.

Biodegradation decreases the concentration of saturated and aromatic hydrocarbons and increases the viscosity, acidity and metal content, such as vanadyl compounds. Recent work has measured the level of biodegradation in three Brazilian oils by ESR spectroscopy and compared the concentration of asphaltene organic free radicals and vanadyl species. Differentiation between the degraded samples and the non-biodegraded sample was possible, as well as differentiation between the different degraded samples.

Temperature also affects the quality of crude oil as it changes the behavior and structural properties of organic free radical species. By analyzing the concentration of asphaltene organic free radicals on different crude oil fractions over a temperature range using in situ ESR, these changes could be monitored. An increase in temperature led to an increase in radical species for all fractions. However, the mechanisms by which this process occurred varied depending on the fraction.

These studies highlight the wealth of information that can be obtained from crude oil samples using ESR spectroscopy. However, most ESR spectrometers are large, bulky instruments that cannot be easily incorporated into online, real-time monitoring.

The microESR from Bruker uses the latest advances in optical technology and wireless communications. It is the smallest ESR spectrometer on the market, with a 30.5 x 30.5 x 30.5 cm3 footprint and a mass of only 10 kg. The Bruker Xenon software allows the user to easily perform data acquisition, processing, and ESR spectra analysis.

By combining these recent advances in ESR techniques with the microESR, online monitoring of crude oil quality can be performed. This assessment enables real-time analysis of crude oil and can be used to anticipate and potentially resolve any problems during the exploration, production and refinement processes.


  1. De Abreu, C. R., et al. (202). Application of the Electron Spin Resonance Technique in the Characterization of Brazilian Oils: Correlation with Their Biodegradation Level and Polar Composition. Energy Fuels.
  2. Dappe, V., et al. (2020). Effect of Thermal Treatment of Different Petroleum Fractions: Characterization by In Situ EPR Spectroscopy. Energy Fuels.
  3. (2020). EPR Instruments. MicroESR.


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