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Gaseous-Ion Fragmentation Mechanisms in Chlorobenzenes by GC/MS and GC/MS/MS: A Physical-Chemical Approach for Undergraduates
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Abstract
This experiment involves a variety of physical-chemical concepts. Students use a tabletop PC-controlled ion-trap mass spectrometer with a GC inlet to obtain positive-ion electron-ionization mass spectra and collision-induced-dissociation mass spectra (MS/MS) for chlorobenzene and 1,2- and 1,4-dichlorobenzene. They interpret the mass spectra to identify the ions giving rise to the major peaks and to establish ion fragmentation mechanisms, for which such direct evidence has not previously been reported. Thus the molecular ion of chlorobenzene loses neutral Cl and then C2H2; a competing path shows loss of C2H3Cl with rearrangement. The molecular ion of each dichlorobenzene loses Cl and then HCl; a competing path shows loss of C2H2Cl2. From the chromatographic results, students consider such quantities as gas viscosity, retention time, retention factor, distribution constant, and Gibbs energy of adsorption. From the mass spectra they can appreciate the mass spectrometer as a chemical reactor for unimolecular and collision-induced processes and investigate isotope distributions, average atomic mass, fragmentation patterns, and gaseous ion reaction mechanisms. For these chemically simple systems they are led to discover the even-electron rule and a special case of the nitrogen rule of mass spectrometry.
Keywords (Audience):
Upper-Division UndergraduateKeywords (Domain):
Laboratory InstructionKeywords (Pedagogy):
Hands-On Learning / ManipulativesKeywords (Subject):
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- Received: August 03, 2009
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