60th Annual Report on Research 2015

Petroleum is a complex mixture of molecules containing primarily carbon, hydrogen, nitrogen, oxygen, and sulfur. Characterizing this mixture is critical for processing petroleum into higher-value fuels and materials. The objective of this research is to use ambient temperature, low-pressure ion mobility mass spectrometry measurements in different gases and computational chemistry to understand the fundamental interactions between gases and petroleum ions. This fundamental investigation will (1) elucidate the optimal gas for separating petroleum ions, (2) reveal the extent to which independent measurements in different gases provide additional specificity for petroleum ion identification, and (3) more generally provide a framework for designing experiments for and interpreting the results from future investigations of petroleum samples using ion mobility mass spectrometry.

Technical Progress. Over our first year of funding, we made significant progress in characterizing the effects of different drift gases (He, Ar, N₂, CO₂, and N₂O) on the absolute collision cross sections of two classes of ions: a series of analogues of quinoline and the twenty common amino acids. Furthermore, we developed figures of merit and a framework for characterizing the performance and selectivity of ion mobility separations in different gases.

The analogues of quinoline were selected to probe the effects of gas polarizability on heterocyclic compounds that differ by the number of double bond equivalents (DBE), which is a critical classifier in petroleum analysis. Quinoline, 1,2,3,4-tetrahydroisoquinoline, 2,3,4,4a,5,6,7,8-octahydroquinoline, and perhydroisoquinoline have 7, 5, 3, and 2 DBE, respectively. The relative drift times of the protonated forms of these molecules depend on the drift gas used. For example, tetrahydroisoquinoline ions have shorter drift times than octahydroquinoline ions in He gas, but the opposite is true in N₂. Clearly, differences in long-range, ion/molecule interactions can have a very significant effect on the mobilities of these ions.

The twenty common amino acids were selected to probe the effects of a wide range of functional groups containing the same elements as petroleum. For each amino acid, the Ω increased with increasing polarizability of the gas. The Ω for all 20 amino acids in two different gases are correlated, but significant non-correlated differences were also observed. These non-correlated difference correspond to different specificities in different gases. For example, phenylalanine (F) and arginine (R) have indistinguishable drift times in He, but are separated in N₂O. These differences are attributed to interactions that the ion has with He (which has a polarizability of 0.2 ų and no dipole moment) relative to those that it has with N₂O (which has a polarizability of 3.0 ų and a dipole moment of 0.16 Debye). Using these results, we have evaluated the effects of drift gas selection on the peak capacities and selectivities of ion mobility separations. We find separations in N₂ have the best overall performance, but that separations in other gases (particularly CO₂ and N₂O) are complementary.

We are expanding these studies to include additional analytes that will enable us to probe additional aspects of petroleum ion structure. Using those experimental results and complementary computational approaches, we will characterize the specific long-range ion/molecule interactions that give rise to drift-gas specific effects in ion mobility spectrometry.

Impact of Research. The collision cross sections measured here can be used to calibrate other low-pressure, ambient-temperature ion mobility experiments, including those using traveling-wave and trapped ion mobility spectrometry. Furthermore, these results provide invaluable benchmarks for new theoretical approaches for calculating the collision cross sections of ions in a wide range of buffer gases. Long term, these results will help enable higher performance analyses of petroleum samples using ion mobility mass spectrometry experiments.

Impact on Career of PI. This research has significantly expanded two areas of investigation for the PI: ion mobility mass spectrometry of small molecular ions and the effects of drift gas selection on ion mobility separations. Based on the outcomes of this research, the PI is pursuing new directions for improving the selectivity of ion mobility separations and applying ion mobility mass spectrometry to new analytes.

Impact on Career of Student. This research is an excellent training experience for Kim Davidson, the graduate student who performed all of these experiments. Based on these results, she has been selected to speak at both the Lake Arrowhead Ion Chemistry and American Society for Mass Spectrometry conferences and will publish two first-author manuscripts in peer-reviewed journals. More generally, her experiences pursuing this research have positioned her well for the remainder of her Ph.D. and post-graduate career.