Plasma-Assisted Functionalization of Hydrocarbons for Enhanced Mass Spectrometric Detection.

The detection and structural analysis of hydrocarbons by mass spectrometry (MS) are challenging because of poor ionization. Here, we report a plasma-based strategy that selectively converts nonpolar hydrocarbons to derivatives that are mass spectrometry-friendly by controlled oxygen and nitrogen insertion reactions. A subatmospheric pressure plasma facilitates stable discharges under mild conditions, allowing the formation of ketones, unsaturated ketones, and imines without structural fragmentation or extensive oxidation. The reaction pathways can be tuned by adjusting the carrier gas and sample introduction mode, allowing controlled switching between simple oxidation, dehydrogenation, and nitrogen insertion. Mass spectrometric analysis reveals well-defined molecular ions that show characteristic fragmentation behavior in tandem MS, facilitating improved detection and structural interpretation across linear, branched, cyclic, and aromatic hydrocarbons. Furthermore, a scalable method has been developed to achieve preparative levels of modified samples for NMR analysis, and this confirmed ketone formation as the dominant oxygen-insertion pathway. Overall, this work demonstrates a simple and versatile plasma-based approach for enhancing the detection and structural characterization of otherwise challenging hydrocarbon species.