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Analytical Instrumentation


DBE. Kendrick normalisation gives series that appear as horizontal rows in a plot of DBE versus Kendrick mass thus facilitating visualisation of complex data sets. The data can also be plotted as a 3D heat-map to indicate the relative intensity of the mass spectral peaks. From the Kendrick plot, the species with peaks in the mass spectrum can be sorted into compound classes by the number of nitrogen, oxygen and sulphur heteroatoms. Example Kendrick plots for only two of the many compound classes in a crude oil are shown in Figure 2.


Exploiting the Power of Different Ionisation Techniques


In recent years a range of ambient ionisation techniques have been developed for use with mass spectrometry which have significantly increased the routine applicability of HRMS in the petroleum industry (1)(2)(3)(5) and there are currently >80 different techniques listed in Wikipedia as ambient ionisation techniques. Many of these have specific attributes which can be valuable tools for the petro industry analyst. For example, techniques such as desorption electrospray ionisation (DESI) and direct analysis in real time (DART) can be employed for the direct analysis of molecules present on surfaces which can be particularly useful for the analysis of deposits and stains on samples such as pipes and engine components. The application of DESI for the quantitative analysis of lubricant additives (6) on surfaces and also the analysis of corrosion inhibitors (7) on metal surfaces has been reported by De Costa et al.


Petroleomics aims to fully characterise all of the chemical constituents of petroleum samples including crude oil. However, it soon became clear that no single ionisation technique would ionise all of the species in a crude oil. For example, ESI is most efficient for polar molecules but not for hydrocarbons so the ~8000 peaks shown for crude oil in Fig 1(a) will result from heteroatom components which typically may account for only circa 10% of the species in the crude. However other ionisation techniques such as field ionisation (FI), field desorption(FD) and atmospheric pressure photo-ionisation (APPI) can be applied to access many of the remaining 90% of components in a crude. Marshall and Rogers reported that an APPI mass spectrum of a crude typically contains circa 5 times as many peaks as an ESI mass spectrum of the same sample (2).


Although the lack of a universal ion source may be an issue for Petroleomics it is a major benefit for many application areas which require the characterisation of specific chemical classes of compounds present in complex mixtures. By selecting an ionisation technique with high sensitivity and selectivity for the chemical class of interest and combining it with HRMS to give detailed molecular characterisation we have a powerful tool applicable to many areas of the petroleum and petrochemical industries.


The Analysis of Hetero species in Petroleum Based Samples


As discussed, electrospray ionisation is especially efficient in generating gas phase ions from hetero atom species without ionising the hydrocarbon matrix and this can be a powerful tool for the petroleum industry chemist whose day to day work requires the detailed characterisation of a wide range of hetero containing components. For example, differentiated products, such as fuels and lubricants, which claim enhanced performance in the field, are now a major revenue generator for many companies. Product differentiation is usually achieved using functionalised additive molecules such as detergents, dispersants antioxidants etc. which are often present at only trace levels in the final product. ESI coupled with HRMS has provided new possibilities in the analysis of such samples allowing the characterisation and identification of active species at the molecular level. The techniques can be applied through product research and development to develop structure property relationships and identify the key molecular species which are contributing to enhanced performance. They can also be applied through the manufacturing and supply chain to study feedstock variations, product stability and degradation and in customer support to identify field issues such as cross contamination and adulteration.


In recent years the drive towards sustainability and environmental protection has seen a dramatic increase in the development of biofuels and bio lubricants. These are usually derived from plant oils or synthetic esters manufactured from sustainable feedstocks and as such usually contain oxygen. The plant derived feedstock


Figure 3: Mass spectrum obtained with FI from a crude oil sample showing the hydrocarbon species detected as molecular ions. (Courtesy Joel UK Ltd)


Author Contact Details Tom Lynch CSci, CChem FRSC, Independent Analytical Consultant, Cricket House, High St, Compton, Newbury, RG20 6NY • Email: tomlynch.lynch@ btinternet.com


AUGUST / SEPTEMBER • WWW.PETRO-ONLINE.COM


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Figure 2: Example Kendrick plot heat maps for 2 compound class distributions from a crude oil sample using ESI-HRMS (Courtesy Dr C Wicking, BP Pangbourne UK)


composition can vary due to many external factors such as the location and weather and therefore it can be difficult to maintain final product consistency at the molecular level. Also, many of these products are blended with conventional hydrocarbons for field use and ESI in combination with HRMS is the ideal tool to characterise and identify bio derived molecules of interest in blends. For example, in biodiesel production trace levels of sterol glucosides may be present which can accelerate precipitation problems and cause filter blocking issues and the sensitivity and selectivity of ESI-HRMS is ideal for detecting and quantifying such components in blended fuels.


Soft Ionisation for Hydrocarbons


For hydrocarbon only molecules, and especially saturated hydrocarbons, most ionisation techniques produce extensive fragmentation but field ionisation and field desorption ionisation techniques have been successfully applied to produce molecular ions with minimal fragmentation for hydrocarbon type analysis. The 4th generation Jeol Accutof GCx instrument can be equipped with a range of ionisation options including electron ionisation (EI), chemical ionisation (CI), field ionisation (FI), desorption electron ionisation (DEI), desorption chemical ionisation (DCI), and field desorption (FD) ionisation using specialised direct sample inlet probes. The TOF-MS has a mass resolution >10,000 (FWHM) with a mass accuracy: <1.5mDa or 4 ppm (RMS) and a mass range: m/z 4 to 6,000.


The optional EI/FI/FD combination ion source is a single ion source that supports sample analysis using 3 different ionisation techniques. An ionisation mode is selected by replacing the probe which can be changed while keeping the ion source in high vacuum thus minimising system downtime for an ionisation mode switch.


In addition, up to 50 spectra/sec can be collected which means the instrument is ideally suited for use with both 1D and Comprehensive 2D GC using the EI/FI/FD combination ion source to further expand the application area of the technique for complex petrochemical samples. The FI mass spectrum of hydrocarbon molecular ions from a crude oil sample shown in Figure 3 was obtained by accumulating the data from an entire GC run. The FI and FD modes of the instrument are particularly useful in generating Kendrick Mass Defect Plots for hydrocarbon group type analysis as in the characterisation of fuels and lubricant base oils. Jeol have produced a comprehensive applications book (8) which highlights many areas where the different ionisation techniques can be applied to provide unique information.


In Conclusion


Recent developments in lower cost, robust HRMS instrumentation combined with the plethora of ionisation techniques have provided a huge opportunity for the petroleum industry to generate comprehensive chemical characterisation of complex mixtures at the molecular level. The data generated can be used to develop new structure property relationships which in turn can lead to enhanced product performance. By selecting an ionisation technique with high sensitivity and selectivity for the chemical class of interest relative to that for the sample matrix and combining it with HRMS to give detailed molecular characterisation we have a powerful tool applicable to many areas of the petroleum and petrochemical industries.


References


(1) Petroleomics: The Next Grand Challenge for Chemical Analysis: Marshall, A.G., Rodgers, R.P.: Acc. Chem. Res., 2004, 37 (1), pp 53–59


(2) Petroleomics: Chemistry of the Underworld: Alan G Marshall, Ryan P Rodgers; PNAS, 2008, V105, No47, pp18090-18095


(3) Petroleum Analysis: Rodgers, R. P., McKenna, A. M.; Anal Chem. 2011, 83, pp4665-4687.


(4) Evaluation of High-Field Orbitrap Fourier Transform Mass Spectrometer for Petroleomics: Konstantin O. Zhurov, Anton N. Kozhinov, Yury O. Tsybin; Energy Fuels, 2013, 27 (6), pp 2974–2983


(5) Ambient Ionisation Mass Spectrometry: Marek Domin and Robert Cody (Eds), Published by Royal Society of Chemistry; 2015, ISBN 978-1-84973926-9


(6) The quantitative surface analysis of an antioxidant additive in a lubricant oil matrix by desorption electrospray ionisation mass spectrometry: Caitlyn Da Costa, James C Reynolds, Samuel Whitmarsh, Tom Lynch, Colin S Creaser; Rapid Communications in Mass Spectrometry 2013, 27, pp2420-2424.


(7) Direct Analysis of Oil Additives by High-Field Asymmetric Waveform Ion Mobility Spectrometry-Mass Spectrometry Combined with Electrospray Ionization and Desorption Electrospray Ionization: Caitlyn Da Costa, Matthew Turner, James C. Reynolds, Samuel Whitmarsh, Tom Lynch, Colin S. Creaser; Anal. Chem., 2016, 88 (4), pp 2453–2458


(8) AccuTOF GC Series Applications Notebook, Edition April 2016, Jeol.


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