Low energy charge exchange reactions and applications of membrane introduction mass spectrometry
Abstract
Charge exchange between a doubly-charged polyatomic ion and neutral organic targets in the low energy regime results to the ionization of the neutral target, partial neutralization and fragmentation of the projectile ion. The exothermicity is deposited into both the tungsten hexacarbonyl projectile and the target molecules because both exhibit fragmentation. Double extrapolation of the calculated internal energy distributions show that the average internal energy deposited in W(CO)$\sb6\sp{+.}$ is 1.0, 0.9, 1.3 and 2.3 eV for the targets isobutane, n-pentane, n-heptane and toluene, respectively. Based on energy equipartitioning, the energy deposited in these target molecules is expected to be 1.1, 1.1, 2.3 and 2.7 eV respectively. The sum of the internal energy deposited into the collision partners is about 2 eV (1.2-2.6 eV) less than the exothermicity. The lost energy is attributed to Coulombic repulsion between the two charged products. Charge exchange in this system is suggested to occur by a Landau-Zener curve crossing mechanism and the electron transfer occurs at internuclear distances of the order of $7 \pm 2$ A. Membrane introduction mass spectrometry is used to monitor and identify mixed haloamines in three different sets of reaction mixtures, to analyze ethanol, acetic acid and ammonia (as monochloramines) under EI conditions, to monitor the decomposition of cyanogen bromide in basic medium and to identify contaminants in a commercially available lens wetting agent. Silicone membranes are used in all these cases. The performance of microporous membranes is evaluated and compared with silicone membrane for the analysis of environmentally significant organic compounds. Both glow discharge and conventional thermionic electron emission are used as ionization methods for the microporous membrane. Glow discharge works very well with microporous membranes because it can tolerate the large amount of water entering the mass spectrometer. The design and application of new membrane interface is described and evaluated for environmental analysis. The interface consists of a helium purged membrane/jet separator system. The results show that the interface yields low detection limits (low ppt level) for volatile organic compounds.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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