62nd Annual Report on Research 2017

1. Problem and Approach.

Petroleum crude oil represents one of the most challenging and complex natural mixtures to characterize^1,2. Tandem mass spectrometry (MS/MS) potentially offers a means of identification of the elemental compositions (C_cH_hN_nO_oS_s) of the components and their structures. In theory this approach involves isolation of a single component of the sample, which is then activated and fragmented into diagnostic charged pieces which are then detected. Based on these data the individual component chemical’s structure is inferred. For petroleum, this approach is complicated by the large number of elemental compositions present in a typical isolation window and the isomeric complexity likely contained within each of these peaks. This problem can be reduced by temperature and/or chromatographic separation prior to MS/MS analysis. In the present approach we utilize series of standard compounds and theory to examine what the degree of resolvable spectral complexity is and how much an increased understanding of the fragmentation chemistries in play may mitigate this.

2. Synthetic Work

We began by examining like C_cH_hN_{n ­}compounds as these are high abundance components of the positive mode electrospray and atmospheric pressure photoionization mass spectra. Our initial synthetic work centers on acridine and anthracene-derivatives and their isomers and isobars (C₂₁H₂₁N, C₂₁H₂₃N, C₂₂H₁₈N, C₂₀H₂₂N, C₂₂H₂₄N, C₂₃H₂₀N, etc.). These model compounds have systematically varied location, size, number, and nature of substituent(s) to enable physical organic hypotheses of structural influence on fragmentation chemistry to be assessed. This part of the project initially progressed slowly, but is now moving along well. Additional, wider variation of the carbon number and degree and location of substituents is currently being undertaken. Nitrogen-centered, secondary amine species have also been examined.

3. Experimental Work

In addition to those already synthesized standards, a large suite of commercially available putative and known petroleum component molecules have been analyzed with energy-resolved tandem mass spectrometry. Both radical cation and protonated species have been investigated. Direct comparison between the two means of activation for those standards amenable to both ionization methods was also undertaken. A series of broad “rules” have been formulated based on these data ready for testing against the wider suite of synthesized chemicals. E.g., relative preponderance and nature of substituent loss(es); alkyl radical vs. alkene vs. other chemistries.

4. Parallel Theoretical Work

Theoretical work characterizing the various potential energy surfaces of our analytes has been undertaken. Additional modeling of fragmentation reaction pathways, potential isomerization’s and putative product ion and neutrals has also been undertaken. This work is ongoing and guided by the experimental findings. Density functional theory optimizations culminating at the M06­2X/6-311G(2d,2p) level of theory and supplementary single point ab initio calculations (MP2, with various large basis sets) form the basis of these comparisons. Comparative rate calculations utilizing Rice–Ramsperger–Kassel–Marcus (RRKM) theory have and continue to be utilized to compare reaction progress as a function of time and degree of activation.

5. What next?

We will continue the synthetic, experimental and theoretical work mentioned above. We will expand the experimental work to look at more complex mixtures of known solution content. This will probe the concomitant effects of ionization efficiency, compound abundance, and fragmentation chemistry. We are also in the process of combining our ongoing the experimental and theoretical data into a large database for broad statistical comparison. Manuscripts resulting from the fundamental physical chemistry examined and the more practical consequences will be written up shortly.

1- Marshall, A.G., Rodgers, R.P.: Petroleomics: Proc. Natl.Acad. Sci. 105, 18090–18095 (2008).

2- Marshall, A.G., Rodgers, R.P.: Marshall, A.G., Rodgers, R.P.: Acc. Chem. Res. 37, 53–59 (2004).