Membrane introduction mass spectrometry and thermochemical studies by the kinetic method
Abstract
Membrane introduction mass spectrometry is used for the direct detection of volatile and semi-volatile organic compounds in aqueous solutions, and on-line monitoring of a photolysis reaction. A microporous membrane/jet separator interfaced to an ion trap mass spectrometer is developed. Both non-porous silicone and microporous polytetrafluoroethylene (PTFE) membranes provide detection limits in the parts per trillion (pptr) to parts per billion (ppb) range with a linear dynamic range of 3 orders of magnitude for some volatile organic compounds. A direct insertion membrane probe interfaced to a triple quadrupole mass spectrometer is used to monitor on-line the concentration of reactants and products of photolysis (254 nm) of benzyl acetate and 3,5-dimethoxybenzyl acetate in water. The results are consistent with earlier experiments that the photolysis proceeds through both ion and neutral radical pair intermediates. Thermochemical measurement by the kinetic method is performed on a pentaquadrupole mass spectrometer. Studies include the determination of ionization energies of organic compounds and stereoelectronic effects on cation affinities of pyridines. The ionization energy of 3-iodobenzonitrile is determined as 9.39 $\pm$ 0.05 eV. The cations studied include $\rm SiCl\sb3\sp+,\ SiCl\sp+,\ SF\sb3\sp+,\ Fe\sp+,\ Co\sp+,\ Ni\sp+,\ FeC\sb5H\sb5\sp+,\ CoC\sb5H\sb5\sp+$ and $\rm NiC\sb5H\sb5\sp+.$ Linear correlations between the logarithm of fragment ion abundance ratio and the proton affinities of the meta- and para-substituted pyridines are observed. Deviations from the linear correlations are observed in the cases of ortho-substituted pyridines. The deviations, the S$\rm\sp{k}$ values, are attributed to a combination of both steric and agostic effects. Agostic bonding is three-center two-electron bonding (M-H-L) in which the two electrons in the ligand L-H are formally donated to the metal center M. The S$\rm\sp{k}$ values for the cations are ordered and interpreted in terms of the stereoelectronic effects of the cation including the cation affinity, the size of the cation and the geometry of the cluster ion.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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