Deciphering Isotopic Fine Structures of Silylated\nCompounds in Gas Chromatography–Vacuum Photoionization Orbitrap\nMass Spectrometry of Bio-Oils

We introduce vacuum\nresonance-enhanced multiphoton ionization (REMPI)\nwith high-resolution Orbitrap Fourier transform mass spectrometry\n(FTMS) for analyzing silylated polar compounds. UV laser radiation\nat 248 nm sensitively and selectively targets aromatic constituents,\nwhile high-resolution mass spectrometry (HRMS) enables high-performance\nmass spectrometric detection. This workflow enhances the detection\nconfidence of polar constituents by identifying unique isotopologue\npatterns, including at the isotopic fine structure (IFS) level, in\nanalytical standards and complex bio-oils. A direct and derivatized\ngas chromatography (GC) procedure on a polar standard component mixture\nallowed us to explore the general ionization and detection characteristics\nof REMPI FTMS. HRMS enabled the examination of the complex isotopologue\nprofiles, revealing distinct patterns for the CHO<sub><i>x</i></sub>Si<sub><i>y</i></sub>-class compounds. Particularly\nin complex mixtures, this isobaric/isonucleonic complexity exceeded\nthe classical mass resolution capabilities of the employed Orbitrap\nD30 series and prompted the usage of prolonged transients via an external\ndata acquisition system. This procedure substantially improved mass\nspectrometric results by recording the unreduced time-domain transient\ndata for up to 2 s. Notably, the ability to distinguish diagnostic\nisotopic pairs, such as <sup>12</sup>C/<sup>29</sup>Si vs <sup>13</sup>C/<sup>28</sup>Si with a mass split of ∼3.79 mDa and <sup>13</sup>C<sup>12</sup>C/<sup>28</sup>Si<sup>29</sup>Si vs <sup>13</sup>C<sub>2</sub>/<sup>28</sup>Si<sub>2</sub>, with an approximate mass\ndifference of ∼3.32 mDa, demonstrates a significant and expected\nperformance improvement. Finally, we benchmark the GC HRMS methodology\nto identify silylated oxygenated and nitrogen-containing constituents\nin ultracomplex bio-oil samples. The presented approach of utilizing\nthe silicon isotope pattern and unique isotopologue mass splits for\nincreasing attribution confidence can be applied beyond bio-oils toward\nthe general GC analyses of polar oxygenates.