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CHMB42 Mass Spectrometry and IR Spectroscopy

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University of Toronto Scarborough
Wanda/ Lana

Chapter 12 Mass Spectrometry and Infrared SpectroscopyMass SpectrometryMass Spectrometry allows us to determine the molecular mass and the molecular formula of a compound as well as some of its structural featuresWhen the electron beam hits a molecule it knocks out an electron producing a molecular ion a radical cation a species with an unpair electron and a positive chargeAll the positively charged fragments of the molecule are drawn between two negatively charged plates which accelerate the fragments into an analyzer tubeNeutral fragments and negative ions are not attracted to the negatively charged plates and therefore are pumped out of the spectrometer only positive remainsThe path of a fragment with a smaller masstocharge ratio mz value will bend more than that of a heavier fragment particles with the same mz are separated from all the othersThe more stable the fragment the more likely it is to arrive at the collector without breaking down further the more stable the smaller mz ratioMass Spectrum a graph of the relative abundance of each fragment plotted against its mz value z on most fragments is 1 so mz is the molecular mass of the fragmentit only records positively charged fragmentsThe Mass SpectrumFragmentationNominal Molecular Mass the molecular mass to the nearest whole numberThe peak with the highest mz value in the spectrum corresponds to the fragment that results when an electron is knocked out of a molecule of the injected sample and represents the molecular ion MThe mz value of the molecular ion gives the molecular mass of the compoundFragment Ion Peaks peaks with smaller mz values represent positively charged fragments of the moleculeBase Peak the tallest peak due to its having the greatest relative abundance it is assigned a relative abundance of 100A mass spectrum gives us structural information about the compound because the mz values and relative abundances of the fragments depend on the strength of the molecular ions bonds and the stability of the fragmentsFragmentation that leads to a primary carbocation and a primary radical is more stable than the fragmentation to a primary carbocation and a methyl radicalPeaks are commonly observed at mz values two units below the mz values of the carbocations because the carbocations can lose two hydrogen atomsIsotopes in Mass SpectrometryM1 Peak contains a naturally occurring isotope of the element C13 for carbonNumber of C Atomsrelative intensity of M1 peak 0011 x relative intensity of M peakM2 Peak a contribution from O18 or from having two heavy isotopes in the same molecule like C13 and H2 or two C13soThe presence of a large M2 peak is evidence of a compound containing either chlorine or bromineoIf the M2 peak is onethird the height of the molecular ion peak then
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