Mass spectrometric characteristics and preliminary figures of merit for polyaromatic hydrocarbons via the liquid sampling-atmospheric pressure glow discharge microplasma
The liquid sampling-atmospheric pressure glow discharge (LS-APGD) has been demonstrated as a combined atomic and molecular (CAM) ionization source, through the analysis of elemental species, small polar molecules, and proteins. Operation using 2% HNO3 as the electrolytic carrier solution is employed for elemental analysis, while simply changing to a MeOH : H2O solution results in mass spectra reflecting the protonation of polar molecular species. In an effort to expand the already diverse sampling capabilities of the LS-APGD, the determination of low-polarity polyaromatic hydrocarbons (PAHs) was investigated. These molecules contain none of the common polar sites for protonation, typically undergoing ionization via charge transfer or electron ionization, forming radical cations. While LS-APGD ionization is believed to involve both electron and Penning ionization, as well as solvent-derived gas phase proton transfer reactions, it was expected that a radical cation formation would be the sole product for PAHs. Interestingly, both radical cation and protonated molecular ions were observed for each of the PAHs studied, suggesting concurrent electron/Penning ionization and atmospheric pressure chemical ionization (APCI). In order to assess the potential analytical utility of the approach, preliminary detection limits of as low as 270 pg mL−1 (5.4 pg, absolute) have been realized. The addition of PAH species to the microplasma's analyte portfolio expands the concept of CAM ionization, finding utility in diverse fields, including environmental analysis and petroleomics.