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SELECTIVE IONISATION AND AFFORDABLE HIGH RESOLUTION MASS SPECTROMETRY IS REVOLUTIONISING MOLECULAR CHARACTERISATION IN THE PETROLEUM AND PETROCHEMICAL INDUSTRIES


Since its early days High Resolution Mass Spectrometry (HRMS) has been a key technique for the molecular characterisation of the complex mixtures experienced in the petroleum based industries and in recent years the power and application of the technique has increased dramatically.


In 2005 Marshall and Rogers (1)(2) wrote “Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry has recently revealed that petroleum crude oil contains heteroatom- containing (N,O,S) organic components having more than 20 000 distinct elemental compositions. It is therefore now possible to contemplate the ultimate characterisation of all of the chemical constituents of petroleum, along with their interactions and reactivity, a concept we denote as “Petroleomics”.


Selective resolution of the components of interest is the key to successfully analysing complex sample matrices by any technique and in mass spectrometry this can be achieved by a combination of the resolution of the mass analyser with selective ionisation which targets only the compound classes of interest. In addition, the components of interest can be resolved from matrix interferences by coupling with pre-separation techniques such chromatography or ion mobility spectrometry.


In Rodgers and McKenna’s (3) review of Petroleum Analysis in 2011 more than half the text discussed advances in mass spectrometry with the remainder covering all other analytical techniques. This dominance was attributed to developments in a range of atmospheric pressure ionisation techniques combined with the resolving power of Fourier Transform Ion Cyclotron Resonance Mass Spectrometers (FT- ICR-MS). FT-ICR-MS is still the gold standard for high resolution mass spectrometry but the instrumentation is expensive, requires signifi cant laboratory space and has large cryo-cooled superconducting magnets. FT-ICR-MS also requires a high degree of operator experience and as such is generally only found in universities or large corporate research facilities. However, in recent years signifi cant advances have been made in “bench-top” HRMS technology through the introduction of Time of Flight and Orbitrap™ type instruments (4) which have proved to be affordable, robust and easy to operate and as a result they have revolutionised the potential for routine application of HRMS in petro industry laboratories.


Why is the Mass Spectrometer Resolving Power So Important?


The mass spectrum shown in Figure 1(a) was acquired from a crude oil sample using electrospray ionisation(ESI) with FT-ICR- MS and comprises ~8000 peaks over a m/z range of circa 800. An expanded area from this spectrum covering a range of circa 0.15 in Figure 1(b) shows a number of components and illustrates why high resolution is critical to the detailed analysis of complex mixtures such as crude oil.


In addition to achieving individual compound resolution, HRMS systems capable of acquiring high resolution with high mass accuracy make it possible to determine elemental compositions from the exact mass data of the detected ions. This is illustrated for several of the peaks in Figure 1(b) with molecular formulae derived from the accurate mass for components containing different combinations of nitrogen, oxygen and sulphur.


Another benefi t of HRMS for petroleum analysis is the ability to easily identify homologous series and the presence of double bonds. For a saturated hydrocarbon homologous series, a unit spacing of m/z 14.0157 is observed for the addition of each CH2


Author Tom Lynch


Figure 1(a): Mass Spectrum containing circa 8000 peaks obtained from a crude oil sample by ESI-FT-ICR-MS (Courtesy Prof J G Langley, Southampton University, UK)


Figure 1(b): Expanded FT-ICR Mass Spectrum from Fig 1(a) showing resolution of components differing by 0.034m/z (Courtesy Prof J G Langley, Southampton University, UK)


unit and a spacing of m/z 2.0157 in the spectrum is equivalent to the mass of 2 hydrogen atoms. Thus, the addition of –CH2 increases the mass by m/z 14.0157 unit and the addition of ring or double bond lowers the mass by m/z 2.0157 units. These parameters can then be used to calculate the double bond equivalent (DBE) which can be directly correlated to the degree of


unsaturation and aromaticity of the molecular classes.


High mass resolution data analysis for is also simplifi ed by converting the data to the Kendrick mass scale (2) where the mass of a methylene CH2 is set to14.0000 instead of 14.01565 Daltons. This rescaling of the data aids in the identifi cation of homologous series according to alkylation, number of heteroatoms, and the


AUGUST / SEPTEMBER • WWW.PETRO-ONLINE.COM


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