Crude oil is an unruly soup of tens of thousands of organic compounds, and this diversity makes it difficult to pick out individual molecules from the crowd for analysis using standard tools such as mass spectrometers. Despite the vast quantities of crude oil used globally each day, much remains unknown about its chemical composition, which can vary dramatically from one oil field to the next.
A method that separates crude oil into a dozen fractions based on their chemical properties now promises more details about composition: It could help chemists measure low levels of molecules that corrode pipelines or pinpoint the most toxic compounds in an oil spill (Anal. Chem. 2017, DOI: 10.1021/acs.analchem.6b04202).
Fractionation is not a new approach to simplifying oil analysis. One of the most common methods, dubbed SARA, uses chromatography to split oil into four broad classes: saturates, aromatics, resins, and asphaltenes. But this separation is based largely on the molecules’ solubilities in the solvent being used, and many chemical classes remain obscured within the mélange in each fraction.
In contrast, the new method developed by Steven J. Rowland of the University of Plymouth and coworkers is particularly good at teasing apart a mixture of polar compounds containing nitrogen, sulfur, or oxygen—often responsible for poisoning oil-processing catalysts—which conventional analytical methods struggle to identify.
The procedure is not based on radical innovation. It relies on a series of columns filled with commercial ion exchange resins and silica, making the method reproducible, relatively simple, and inexpensive. “The real novelty is putting it all together,” says Ryan P. Rodgers, director of the Future Fuels Institute at Florida State University, who was not involved with the work. By deploying the separation columns in the right order and eluting the crude oil with a series of increasingly polar solvents, the method isolates molecules depending on how well their functional groups stick to each type of column. This process yields fractions that are each dominated by a particular chemical class: sulfoxides, quinolines, carbazoles, fluorenones, and more.
After analyzing each fraction with techniques such as gas chromatography/mass spectrometry, the team identified dozens of specific compounds. Some of them, such as thioxanthones, were previously unknown in crude oil. The method achieves “a better separation between different classes of chemicals,” says Sonnich Meier of the Institute of Marine Research. “It’s the best I’ve seen.”
Meier has been working with Rowland’s team for the past three years and plans to use the technique to single out the compounds in crude oil that are toxic to fish embryos, which can be contaminated during an oil spill.
Meanwhile, the oil industry increasingly wants to know the precise composition of a crude oil before investing in extracting it. Declining production of low-sulfur, sweet crude, oil has the industry relying on heavier crudes requiring more refining and posing greater risks of pipeline corrosion or blockages.