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Abstract
We introduce vacuum resonance-enhanced multiphoton ionization (REMPI) with high-resolution Orbitrap Fourier transform mass spectrometry (FTMS) for analyzing silylated polar compounds. UV laser radiation at 248 nm sensitively and selectively targets aromatic constituents, while high-resolution mass spectrometry (HRMS) enables high-performance mass spectrometric detection. This workflow enhances the detection confidence of polar constituents by identifying unique isotopologue patterns, including at the isotopic fine structure (IFS) level, in analytical standards and complex bio-oils. A direct and derivatized gas chromatography (GC) procedure on a polar standard component mixture allowed us to explore the general ionization and detection characteristics of REMPI FTMS. HRMS enabled the examination of the complex isotopologue profiles, revealing distinct patterns for the CHOxSiy-class compounds. Particularly in complex mixtures, this isobaric complexity exceeded the classical mass resolution capabilities of the employed Orbitrap D30 series and prompted the usage of prolonged transients via an external data acquisition system. This procedure substantially improved mass spectrometric results by recording the unprocessed time-domain transient data for up to 2 seconds. Notably, the ability to distinguish diagnostic isotopic pairs, such as 12C/29Si vs. 13C/28Si with a mass split of ∼3.79 mDa and 13C12C/28Si29Si vs. 13C2/28Si2, with an approximate mass difference of ∼3.32 mDa, demonstrates a significant and expected performance improvement. Finally, we benchmark the GC HRMS methodology to identify silylated oxygenated and nitrogen-containing constituents in ultra-complex bio-oil samples.
Supplementary materials
Title
Supplemental Material
Description
List of compounds in the standard mixtures with distributors and the chemical grade (Table S1); An expanded tabular list of standard compounds, including both silylated and non-silylated forms, with details on their structure, molecular formulas and detectability (DET.) using REMPI at a wavelength of 248 nm. a Abbreviations: +, moderate; ++, good; +++, very good, ND, Not detected. *Phenyl acetic acid and its TMS derivative were identified. ** Isoeugenol (an impurity found in standard Eugenol) (Table S2); A Simulation of the Silicon isotopic pattern with M+1-M+2 M+3 (Figure S1); Theoretical mass splits and expected isotopic pairs for silylated compounds in complex mixtures (Table S3), Mass spectra of three derivatized and non-derivatized compounds in the standard mixture. Grey marker at m/z 202 indicated radical cation contaminant (Figure S2), Complex bio-oil samples mass spectra of the averaged GC run with derivatized and non-derivatized Heavy Algae sample (BIC) (Figure S3).
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