FIU CHM 4130 - Chapter 20 Xiao_Molecular Mass Spectrometry_2018 (75 pages)

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Chapter 20 Xiao_Molecular Mass Spectrometry_2018



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Molecular mass spectrometry Chapter 20 The study of molecular ions M e M 2e Mass Spectrum of Ethyl Benzene M e M 2eBar type graph Each line represents a specific m z Interpretation of Spectra Ion Sources for Mass Spectrometers Unless you create an ion you won t see it Energy driven process HARD IONIZATION SOFT IONIZATION MS with Hard and Soft Sources A hard ionization source electron impact A soft ionization source chemical ionization Electron Impact Source Electrons from filament hit the sample Path of electrons and molecules are on a right angle This methods is applicable to all volatile compounds 103 Da and gives reproducible mass spectra with fragmentation to provide structural information Molecular mass 103 Da Why vacuum Ensure filament does not burn out Help to vaporize samples Reduce collision between formed ions and atmospheric gases Remove sample from instrument after analysis Electron Impact Source Electron Ionization EI M e 70 V M 2e Electron impact source Filament tungsten or rhenium Our source of 70 eV electrons Target anode used in association with the filament to produce electrons Repeller positively charged electrode used to push positive ions out of the ionization source Lens stack series of increasingly more negative electrodes used to accelerate our ions to constant kinetic energy Electron Ionization Process Typical Reactions during Electron Impact Energy 70eV 6700 kJ mol Typical bond energies 200 to 600 kJ mol EXTENSIVE FRAGMENTATION Electron Impact Spectra Isotope peaks Different molecules behave differently Good molecular ion little fragmentation C5H12O MW 88 No molecular ion extensive fragmentation Isotopes are extremely important Molecular ion isotopic cluster From undergraduate instrumental analysis by James W Robinson Chemical ionization Chemical ionization Source A soft ionization method Gaseous molecules of the sample are ionized by collision reactions with ions produced from a reagent gas Reagent gas methane propane isobutane ammonia



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