Expanding the capabilities and utilities of flowing atmospheric pressure afterglow (FAPA) ionization source for mass spectrometry

Abstract

Finally, issues with reproducible sample introduction and poor spatial resolution were addressed by coupling FAPA with laser-ablation (LA) sampling. Aerosolized particles from the ablation event were carried to the FAPA source for desorption/ionization where resulting ions were analyzed with a high-resolution mass spectrometer. By raster scanning the laser across the sample surface, followed by appropriate data processing, chemical-specific images of molecular analytes were generated. Furthermore, emission spectra from the laser induced plasma formed during ablation event were collected, technique known as laser induced breakdown spectroscopy (LIBS), to obtain elemental information. This LIBS/LA FAPA MS system enabled simultaneous elemental and molecular mapping of sample surfaces. Pharmaceutical tablets and printed targets were used to demonstrate the capability of simultaneous elemental and molecular imaging.

Ambient desorption/ionization (ADI) sources have eliminated the requirements of extensive sample preparation and/or time-consuming chromatographic separations prior to mass spectrometric analysis. In ADI-MS, chemical species are directly desorbed and ionized from their native environment in the open air. Plasma based ADI sources, which employ a low-energy electrical plasma to desorb and/or ionize molecular species, have attracted considerable attention due to their easy construction, simple operation, and high sensitivity. However, the capabilities of these sources are often limited to small, polar organic molecules. Additionally, reproducible sample introduction and poor spatial resolution in imaging applications are still major challenges.

Furthermore, atmospheric pressure glow discharge (APGD) of FAPA is used as photoionization lamp to understand the contribution of photoionization in analyte ionization by sealing the discharge cell with MgF2 window to pass high energy photons from the discharge towards gas-phase analytes. Mass spectra of several analytes were acquired with this APGD photoionization lamp and were compared to those obtained with FAPA in ADI-mode. The influence of molecular gases, discharge current, and dopants on photoionization process were also studied.

In these studies, changes in discharge processes and ionization chemistry with respect to different operating parameters of the plasma based flowing atmospheric pressure afterglow (FAPA) was studied. Operating parameters such as discharge current, plasma-gas flow rate, and discharge-gas composition were varied, while the reagent and analyte ions populations were monitored with mass spectrometry. It was found that the type and distribution of reagent ions as well as the corresponding analyte ionization pathway of can be altered by adjustment of the working parameters. Higher abundance of charge-transfer reagent ions and improved ion signals for non polar analytes were obtained at higher discharge currents and lower helium flow rates. Additionally, the influence of discharge-gas composition was also explored via the addition of molecular gases (O2, H2, and N2) to helium discharge. Addition of 0.1% (v/v) O2 to the helium discharge gas increased the ion signal at least by 3.5 times for simple, polar analytes (e.g., acetone and methanol). However, aromatic compounds (e.g., benzene), undergo unique chemical modification to produce pyrylium-based species. In another case, N2 addition to the helium discharge gas improved analyte signals for nitrated explosives, such as RDX, while addition of H2 produced chemically cleaner mass spectra. To better understand the reasons for these observations, on the influence of molecular gas addition to the helium FAPA was also studied with optical emission spectroscopy. These emission spectra also provided other important plasma parameters such as rotational temperature and electron number density.