Mass spectrometry with hydrogen plasma driven chemical ionization for investigating atmospheric processes / vorgelegt von Md Fakrul Islam

Islam, Md Fakrul

This thesis presents the development, optimization, and analytical characterization of a hydrogen plasma driven chemical ionization (HPCI) source integrated into a triple quadrupole mass spectrometer (TQ MS) for the soft ionization of atmospheric relevant trace gases. The work addresses the limitations of classical filament based EI/CI sources—short filament lifetimes in oxygen rich matrices, elevated source temperatures, and extensive fragmentation—by replacing the filament with a low power helical resonator hydrogen plasma operated at 1 – 15 mbar. Hydrogen is used to generate H₃⁺ as a primary reagent ion, which is converted into secondary reagents such as N₂H⁺ and H₃O⁺ by controlled admixtures.A central part of the study is the systematic optimization of the plasma gas composition, pressure, and flow, as well as the conductance of the reaction chamber and the plasma source distance (PSD) relative to the reaction chamber. Using combined mass spectrometric and optical (UV–NIR) diagnostics, an optimum N₂ fraction of 9 – 12% in H₂ and plasma gas flows around 5.5 – 6.5 sccm were identified, maximizing N₂H⁺ formation at reaction chamber pressures near 8 mbar while suppressing parasitic NH₄⁺ formation via heterogeneous NH₃ chemistry. Reactant ion losses due to wall interactions in the glass transfer tube were quantified by varying PSD, analyte inlet position, and transfer line temperature, demonstrating that short ion paths, co localized reactant and analyte beams, and low transfer line temperatures favor high protonation efficiency.The ionization behavior of aromatic compounds (benzene, toluene, o xylene, BTX) with N₂/H₂ reagent gas is governed by proton transfer from N₂H⁺ and H₃O⁺, complemented by charge transfer and VUV driven photoionization from O₂⁺ and plasma emission. At optimized conditions, BTX yields comparable fractions of MH⁺ and M⁺, with fragmentation ((M(–H)⁺, M(–CH₃⁺)) largely controlled by the excess energy of H₃⁺ and N₂⁺ reactions, enabling soft but sensitive detection. Full scan (FS) and selected ion monitoring (SIM) measurements show that SIM profile data enhance sensitivity by roughly two orders of magnitude, yielding MH⁺ sensitivities up to 2.4×10^5 cps/ppbV and LODs of 3–7 ppbV for BTX.Small oxygenated VOCs could not be measured satisfactorily with N₂/H₂ alone, and a methanol/synthetic air dopant was therefore introduced to generate CH₃OH₂⁺ and methanol clusters as tailored reactants. This enabled adduct ion formation M·CH₃OH₂⁺ for ethanol, iso propanol, and n butanol, with linear calibration and ppbV level LODs for multi carbon alcohols, as confirmed by CID experiments in Q2. Detailed background studies revealed that high voltage Penning gauges and dopant related cluster signals dominate noise at critical m/z values, and the instrument must be operated with the Penning gauge off and with carefully chosen reagent conditions to achieve optimal performance. Overall, the HPCI TQMS system is shown to be a practical, filament free alternative to classical CI, capable of soft and sensitive analysis of aromatic and oxygenated compounds under atmospheric like conditions.

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