From bulk to spatial Petroleomics: Molecular characterization of rock sample by UHRMS and Mass Spectrometry Imaging

Molecular-level characterization of complex organic matrices has been significantly advanced by ultra-high resolution mass spectrometry (UHRMS), particularly through the development of petroleomics. Conventional petroleomics approaches typically rely on liquid samples or extracts, providing a comprehensive molecular profile, but losing information regarding the spatial distribution of chemical species across the sample surface. In this context, mass spectrometry imaging (MSI) emerges as a complementary approach for petroleomics strategies, allowing molecular composition to be associated with spatial location within the sample. Here, bulk analysis and MSI were compared to investigate their complementarity in the molecular characterization of a rock sample provided by CENPES/PETROBRAS. A dichloromethane:methanol extract was analyzed by direct infusion using positive-mode electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry (ESI(+)-FT-ICR MS). In parallel, the rock surface was analyzed using laser ablation electrospray ionization coupled to Orbitrap mass spectrometry imaging (LAESI-FT-Orbitrap MSI). Molecular formulas for the ESI(+)-FT-ICR MS bulk extract were assigned using Composer®, whereas LAESI-FT-Orbitrap MSI data were processed using coreMS, with molecular assignments performed individually for each pixel spectrum considering only signals with signal-to-noise ratio greater than 4. Bulk analysis revealed a predominance of nitrogen-containing compounds from the N1 class, with lower contributions from the N2 and N1O1 classes. In contrast, LAESI-MSI accessed a broader range of heteroatom classes, including N1, N1O1, N1O2, and N2 species, as well as O1, O2, O3, and O4 species, which were not observed by ESI(+)-FTICR MS. The approximately normal distribution of assignment errors centered near 0 ppm reinforced the consistency of the assignments obtained from LAESI-MSI data. Chemical class-based image reconstruction showed a relatively homogeneous overall distribution of Nx and Ox species across the analyzed surface. However, ions specific images reconstructed from molecular formulas compatible with pyridines, quinolines, and acridines type species, revealed localized and heterogeneous spatial patterns. Pyridine-type species were concentrated in specific regions of the sample, while the other selected species exhibited distinct spatial distributions. The differences observed between bulk analysis and MSI likely reflect both extraction selectivity and differences in the ionization mechanisms involved. Overall, the integration of bulk UHRMS and MSI provides complementary molecular information, combining the comprehensive compositional profile obtained from extracts with the chemical heterogeneity preserved on the rock surface. These results highlight the potential of MSI to introduce spatially resolved molecular information into petroleomics, expanding conventional analysis toward spatial petroleomics.