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

Abstract

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 RIMS ∼ 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.