Model Compound Fragmentation Pathways as an Entry to Structural Analysis of Crude Oil

Abstract

Introduction (117/120 words) The detailed fragmentation chemistry of the medium and larger mass compounds found in oil samples has received relatively little attention, primarily due to the enormous complexity of the source material which usually comprises multiple, empirically and chemically distinct ions at each nominal mass-to-charge (m/z) ratio [1]: i.e., a large number of isotopologues (distinguishable in MS1 by FT-ICR MS) and structural isomers (indistinguishable in MS1). Interpretation of the fragmentation pattern of within an m/z window is thus hampered by the contributing elemental compositions and structural isomers. A greater understanding of the fragmentation chemistry of the most abundant components should aid subsequent interpretation. Toward that goal, we investigate a series of model compounds based on FT-ICR MS and theoretical calculations. Methods (106/120 words) Peptide analytes were prepared in toluene/methanol/formic acid solution (50%/50%/1%) at 1-10 μM. Samples were introduced into the mass spectrometer by nano-electrospray. Following quadrupole isolation, the anlayte ions were either collisionally fragmented prior to transfer to the FT-ICR cell for detection or transferred without activation for subsequent CO2 laser activation. IRMPD was performed as a function of irradiation period. Density functional and MØller-Plesset calculations were performed on model peptide precursor ions, complexes, and product species by use of Gaussian 03/09. Transition structures were calculated and the connecting minima determined from intrinsic reaction coordinate calculations with the 6-31+g(d,p) and 6-311+g(2d,p) basis sets. MP2 single-point energies were obtained for comparison. Preliminary data (238/300 words) Our experiments generally show initial loss of neutrals, resulting in closed shell products at lower levels of activation. As the degree of activation increases, radical cation species are detected at increased relative abundance. Preliminary transition structure calculations show that the barriers to fragmentation of the oils investigated thus far are substantially greater than for biological compounds with similar degrees of freedom: i.e., typically > 250 kJ mol-1. To a large extent, that difference is logical, because much of the fragmentation occurs via charge-remote rather than charge-directed pathways. The comparatively large amount of energy necessary in order to fragment the oils opens up numerous, competing possibilities including potential competition between direct loss of H● followed by further degradation of the new radical cation versus fragmentation from triplet electronic states on entropically favorable pathways. The extent to which each of these pathways are active is a function of the oil type, size, and composition. Acknowledgments We thank Prof. Igor V. Alabugin (Florida State University) and Prof. Murray R. Gray (University of Alberta) for providing some of the analytes. This work was supported by NSF Division of Materials Research through DMR-11-57490 and the State of Florida. References: 1. (a) Marshall, A. G.; Rodgers, R. P., Petroleomics: Chemistry of the underworld, Proc. Natl. Acad. Sci. U.S.A., 2008, 105(47), 18090-18095. (b) Rodgers, R. P.; McKenna, A. M., Petroleum Analysis, Anal. Chem. 2011, 83(12), 4665-4687. Novel Aspect (12/20 words) Experiment and theory help to elucidate the complex fragmentation chemistry of nitrogen-containing oil compounds.