KEY CONCEPTS
• Complex petroleum samples were analyzed using a technique known as FT-ICR MS.
• This technique identified 244,749 peaks in a non-distillable fraction known as maltenes.
• Petroleomics software assigned molecular compositions for the peaks in the non-distillable fraction. 

Mineral oils, whether they are Group I, Group II, Group III or naphthenic based, are known to be complex mixtures of hydrocarbons. Gaining insight into their structure may provide a better understanding of how to use them in the formulation and application of lubricants. 

Improving analytical techniques are enabling researchers to understand in more detail the composition of complex mixtures. An example can be found in a recent TLT article that discusses the use of grazing incidence wide-angle X-ray scattering (GIWAXS) in determining the molecular composition of reverse osmosis membranes (1). Employment of GIWAXS enabled the researchers to better understand two molecular structures in the polyamide that makes up the reverse osmosis membrane. Further analysis showed that one of these structures preferably facilitates water diffusion. The hope is this analysis will lead to the production of more energy efficient reverse osmosis membranes.

One of the most important analytical tools available to determine the composition of mixtures is mass spectrometry. Mark Barrow, associate professor in the department of chemistry at the University of Warwick in Coventry, UK, says, “With respect to highest performance, the state-of-the-art mass spectrometry used by researchers is known as a Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Instead of detecting ions by them hitting a detector as was done using earlier mass spectrometers, the ions from the sample are sent through to a cylindrical ICR cell that is contained within a superconducting magnet where the ions orbit.”

Barrow continues, “The clouds of ions of different mass-to-charge ratios (m/z values) all orbit at the same time within the magnetic field. A frequency sweep is used to excite the ions to larger orbit radii so that they can be detected. As ions pass two detection plates, they induce an image current, providing non-destructive detection. The frequency of a specific ion is inversely proportional to its m/z. This means we can use a Fourier transform to go from the raw data (time domain) to a frequency domain spectrum. Knowing the relationship between frequency and m/z, calibration is used to convert to m/z, leading to the mass spectrum, which is the final form of the data.

"From each peak in the mass spectrum we can determine a molecular formula (compositional assignment) and therefore we can compile a list of detected formulae for a complex sample,” Barrow adds. “FT-ICR mass spectrometers can offer mass resolving powers of up to many million, while benchtop varieties of mass spectrometer may only offer thousands or tens of thousands. This makes FT-ICR mass spectrometry a cutting-edge tool for the analysis of highly complex mixtures.”

Typical FT-ICR cells are a few centimeters in diameter. Figure 1 shows an image of a FT-ICR MS. 


Figure 1. Researchers used an FT-ICR MS to determine the composition of a non-distillable fraction of mineral oil known as maltenes. (Figure courtesy of University of Warwick.)

In the attempt to better understand a complex mixture such as crude oil, FT-ICR MS has its limitations. Dr. Diana Catalina Palacio Lozano, Research Fellow at the University of Warwick, says, “One of the challenges in analyzing petroleum and other complex mixtures is that the compounds present do not ionize equally well. This means that some compounds’ ionization efficiencies can be low leading to the generation of a small number of mass-to-charge-ratio peaks, and also that multiple ionization methods must be used to provide complementary information. In some cases, they also are too many components present in petroleum samples to obtain a clear understanding of their composition and there needs to developments of better methods for characterizing the most complex samples.”

Another challenge facing researchers is the inverse relationship between mass resolving power and mass-to-charge ratio. For a complex mixture with components that exhibit a wide range of molecular weights, this means that resolving power will decrease as the mass-to-charge ratio increases. As a result, species with high molecular weights sometimes cannot be adequately resolved. 

A new technique has now been developed to more effectively utilize FT-ICR MS to characterize the most complex petroleum samples. 

OCULAR method
Barrow and Palacio Lozano determined that by segmenting the complex mixture into distinct mass-to-charge ratio ranges and designing the experiments carefully, ultrahigh resolving power can be maintained at a constant value, unlike traditional experiments. Each segment can then be stitched together to produce a complete mass spectrum.

This technique is known as the operation at constant ultrahigh resolution (OCULAR) method. 

The researchers evaluated this approach by analyzing the heaviest and most complex mixture produced during the refining of crude oil, maltenes of a truly non-distillable fraction of crude oil (evaporates above 687 C). This fraction contains not only hydrocarbons but a significant percentage of heteroatoms (nitrogen, sulfur, oxygen and traces of metals). 

A lower molecular weight fraction (also known as a light fraction) was produced by supercritical fluid extraction and was initially evaluated by the researchers as a model system, as it could be analyzed (with different levels of success) using both traditional and OCULAR methods. In comparison with the conventional broadband mass spectrum approach, the OCULAR technique produced peaks with greater resolution, which meant more components could be observed. A mean resolving power per segment of 1,792,000 at full width half maximum (FWHM) was achieved over the full mass range while the resolution in the broadband technique declined as the molecular weight increased. 

Palacio Lozano says, “Once the OCULAR method had been tested, we then isolated the maltenes of the non-distillable fraction by Soxhlet extraction using heptane and evaluated it using the OCULAR method. A total of 244,779 peaks were assigned and the resolving power was maintained at more than three million FWHM.” More than 88% of the peaks could be assigned molecular formulae. 

Barrow says, “We used petroleomics software to assign molecular compositions for the peaks in the non-distillable fraction. This complex sample included compounds that were just polycyclic aromatic hydrocarbons in nature and many more that contained heteroatoms. We observed compounds of very high double bond equivalents, such as over 50, and we obtained peaks with mass-to-charge ratios approximately between 260 and 1500, although we could see hints of signal extending up to approximately m/z 1800. The accuracy of the mass assignments was a root mean square mass error of 0.11 ppm, with half of the assignments actually having a mass error less than 0.05 ppm.”

The researchers believe this is the most complex petroleum fraction that has been successfully characterized to date without the use of chromatography.

Barrow says, “We are now in the process of using the OCULAR method to evaluate other complex samples, including bitumen and samples obtained from the oil sands in the Athabasca region of Alberta, Canada. Other complex mixtures being studied include petrochemical products and pyrolysis bio-oils.”

Additional information can be found in a recent article (2) or by contacting Barrow at M.P.Barrow@warwick.ac.uk. 

REFERENCES
1. Canter, N. (2019), “Molecular structure of reverse osmosis membranes,” TLT, 75 (7), pp. 14-15.
2. Lozano, D., Gavard, R., Diaz, J., Thomas, M., Stranz, D., Ospino, E., Guzman, A., Spencer, S., Rossell, D. and Barrow, M. (2019), “Pushing the analytical limits: new insights into complex mixtures using mass spectra segments of constant ultrahigh resolving power,” Chemical Science, 10 (29), pp. 6966-6978.