Analysis of Geologically Relevant Metal Porphyrins Using Trapped Ion Mobility Spectrometry Mass Spectrometry and Theoretical Calculations

The structural characterization of metal porphyrins has been traditionally challenging as a result of their large structural and compositional diversity. In the present paper, we show the advantages of gas-phase, postionization separations for the fast identification and structural characterization of metal octaethylporphyrins (Me-OEP) from complex mixtures using trapped ion mobility spectrometry (TIMS) coupled to ultrahigh-resolution mass spectrometry (FT-ICR MS). TIMS-FT-ICR MS allows for the separation of Me-OEP (Me = Mn, Ni, Zn, V=O, and Ti=O) within a crude oil sample based on accurate mass and mobility signatures (with a mobility resolving power of R-IMS similar to 150-250). Accurate collision cross sections are reported for Me OEP in nitrogen as bath gas (CCSN2). Inspection of the Me-OEP mobility spectra showed a single mobility component distribution for Me OEP (Me = Mn, Ni, and Zn) and a multi-component distribution for the two metal carbonyls, vanadyl (V= O) and titanyl (Ti=O) Me-OEP. Candidate structures were proposed at the DFT/B3LYP/6-31g(d) level for all Me OEP mobility bands observed. Inspection of the optimized Me-OEP candidate structures shows that manganese, zinc, and free OEP adopt a planar conformation, the nickel-complexed OEP structure adopts a ruffled conformation; and the metal oxide OEP adopts a dome conformation, with carbonyl pointing upward, perpendicular to the plane of the structure.