Dissertation Defense: Samuel White

Candidate: Samuel White

Major: Chemistry

Advisor: Kaveh Jorabchi, Ph.D.

Title: Development of Post-ICP Chemical Ionization Methods for Elemental Quantification of Fluorinated and Phosphorylated Compounds

Elemental mass spectrometry (MS) is capable of quantitation without requiring compound-specific standards and offers seamless coupling to chromatography. However, conventional elemental MS techniques face a major challenge when applied to fluorochemicals. Specifically, the low ionization efficiency of fluorine through thermal ionization limits the sensitivity of fluorine quantitation for trace concentration measurements. This fundamental limitation in turn hampers the applications of elemental analysis in pressing areas such as quantitation of environmental contaminants (e.g. perfluorinated forever chemicals) and drug metabolites where standards are not readily available.

This dissertation addresses the fundamental challenge of F detection by developing a novel ionization pathway via improvements to plasma-assisted reaction chemical ionization (PARCI). Specifically, fluorochemicals in a stream of solvent are introduced into an inductively coupled plasma (ICP), resulting in quantitative formation of HF. Novel chemistries are then developed to efficiently ionize HF via post-ICP ion-neutral reactions at atmospheric pressure. The developments result in a significantly higher sensitivity for F detection than that in conventional elemental MS techniques.

Building on these developments, further optimizations of the signal generation pathway are conducted, leading to the first high-sensitivity flow-injection method for quantifying total extractable organic fluorine (EOF), demonstrating the capabilities of the post-ICP ionization method in addressing pressing environmental analysis needs. This continuous-flow analysis method achieves detection limits competitive with the state-of-the-art batch analysis techniques, such as combustion ion chromatography, while providing improved compound-independent responses and rapid flow injection analysis.

In addition to fluorine quantitation, this dissertation extends PARCI’s standard-free quantitation capabilities to other elements, with an emphasis on phosphorous because of the prevalence of this element in biologically and environmentally significant compounds (e.g. phospholipids). An extensive investigation into ionization pathways is conducted for phosphorus detection via post-ICP formation of H3PO4 from P-containing compounds followed by sodium-ion mediated ionization reactions. Plasma operating parameters and ionization reactions for compound-independent responses without interference from H2SO4 formed from S-containing compounds are identified and solvent effects are investigated, providing the basis for a quantitative phosphorus detection method with chromatographic separation applicable to food, biomedical, and environmental analysis.

Overall, the results in this dissertation advance elemental analysis of difficult-to measure elements (specifically F and P), paving the way for a versatile, high-sensitivity elemental detection technique readily interfaced with liquid chromatography for rapid standard-free quantitation of compounds.