Elemental Analysis By Mass Spectrometry Using An Atmospheric Pressure Microwave Induced Plasma As An Ion Source PDF Download

The expanding use of organochlorines and organofluorines in pesticides, pharmaceuticals, and industrial applications, along with environmental contamination by these compounds require the development of analytical methods for sensitive non-targeted characterization in complex sample matrices. Elemental mass spectrometry (MS) coupled to chromatography offers an advantage for such analyses by detecting Cl and F in eluates regardless of the analytes chemical structure. However, the low ionization efficiencies to form Cl+ and F+ via thermal ionization in conventional elemental MS have hindered progress in this realm. In this dissertation, novel elemental ionization methods are developed based on plasma-assisted reaction chemical ionization (PARCI) to alleviate the shortcomings of current elemental methods for F and Cl detection. A general strategy of plasma reactions for breakdown of analytes followed by post-plasma chemical ionization is utilized in our studies. In one investigation, we demonstrate the advantages of this approach using gas chromatography-low-pressure microwave-induced plasma and post-plasma formation of Cl- and F-. Specifically, high-sensitivity quantification of multi-residue presides is achieved with a single elemental standard in a complex food matrix.To facilitate the integration of the new elemental method with molecular MS toward a comprehensive analytical platform, we further expand the PARCI strategy to atmospheric pressure plasmas. We demonstrate that a dielectric barrier discharge (DBD) serves as a reaction plasma and an electron source for Cl- formation from organochlorines, providing compound-independent elemental ionization efficiencies. The analytical utility of this approach is also evaluated by quantifying a range of pesticides in a juice sample.Detection of F using the same strategy, however, faces difficulties due to thermodynamically favorable reactions between F- and other plasma products. Therefore, a novel ionization pathway is developed, where HF is generated from fluorinated compounds within a DBD. The subsequent gas-phase ion-neutral reactions between HF and sodiated reagent ions from nano-electrospray ionization (ESI) of sodium salts results in formation of Na2F+, offering completely decoupled plasma reactions and post-plasma ionization for robust analyses. Importantly, this method opens an avenue for simultaneous detection of Cl and F. Lastly, the combination of elemental and molecular ionization in the same chromatography-MS platform is shown for non-targeted analysis of organofluorines.