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 They produce different spectra It relies on our charge being transferred from a reagent molecule to our sample Reagent ion molecule molecular ion reagent ion This method gives molecular weight information and reduced fragmentation in comparison to EI Chemical Ionization Source CI REAGENT GAS Chemical Ioniza on MS Sources CH4 High Energy electrons Primary ions Secondary ions CH4 CH4 CH3 CH2 CH 4 CH 4 CH 5 CH 3 CH 3 CH 4 C2 H5 H 2 Sample Molecule MH Proton transfer CH 5 MH MH 2 CH 4 C2 H5 MH MH 2 C2 H 4 Hydride transfer C2 H5 MH M C2 H 6 Molecule Ions EI vs CI EI vs CI Mass spectrum of 1 decanol from a hard ionization source and a soft ionization source Why EI and CI not enough Sample must be in gas phase Not for nonvolatile or thermally unstable compounds Desorption Sources Electrospray Ionization Source Molecular mass 100 000 Da ELCTROSPRAY IONIZATION DETAILS ELCTROSPRAY IONIZATION PROCESS Iribarne Thomson Model Charge density increases Rayleigh limit Coulomb repulsion surface tension Coulomb explosion daughter driblets Evaporation of daughter droplets Desorption desolvation of ions from the droplets into the ambient gas IONS FORMED Electrospray Ioniza on MS of Proteins and Pep des Special Features of ESI Little fragment Multiple charges Linear relationship between AVG charge and molecular weight Coupling to HPLC and capillary electrophoresis columns directly Matrix Assisted Laser DesorptionIonization MALDI Matrix Assisted Laser Desorption Ionization MALDI Method where a laser is used to generate ions of high molecular weight samples such as proteins and polymers Analyte is embedded in to crystal matrix The presence of an aromatic matrix causes the large molecules to ionize instead of decomposing High mass range achievable Mechanism of MALDI uncertain Formation of a solid solution Matrix excitation Analyte ionization MALDI Matrix Matrix must meet the following properties and requirements Be able to embed and isolate analytes e g by cocrystallization Be soluble in solvents compatible with analyte Be vacuum stable Absorb the laser wavelength Cause co desorption of the analyte upon laser irradiation Promote analyte ionization MALDI Matrix MALDI More tolerant of salts and complex mixture analysis than ESI Important for huge molecules proteins polymers Spectra often contain multiple charged ions Fast Atom Bombardment Ionization FAB Liquid Secondary Ion Mass Spectrometry LSIMS 1 First reported 1980 Major extension in role of MS for analyses of biological materials 2 Use for compounds with MW less than 1500 with low volatility Peptides drugs natural products pollutants metalorganics 3 Samples should be easily charged in solution Acids bases highly polar compounds 4 Samples must be soluble in a liquid non volatile matrix such as glycerol 5 Use with solids probe No HPLC No automation 6 Samples should be relatively pure and free of additional salts buffers etc 7 Often used to confirm MW of compounds analyzed by EI or CI May give some fragments Suffers from high background signal No heating required Sample directly from solution FAB Ionization Fast Atom Bombardment SECONDARY ION MASS SPECTROMETRY SIMS FAB Ionization Xe Primary Ion Beam Atom Gun Xe0 Secondary ion Beam to MS Fast Atom Bombardment Source Soft ionization technique Samples in condensed state glycerol solution matrix are ionized by bombardment with energetic xenon or argon atoms Rapid sample heating little loss of compound Matrix absorbs part of energy Suitable for the analysis of low volatility species typically producing large peaks for the pseudo molecule ion species M H and M H along with structurally significant fragment ions and some higher mass cluster ions and dimers Choosing correct matrix is difficult Strengths of various ionization techniques MS Instrument Components More MS Analyzers Resolution The capability of a MS to differentiate between masses R m m m mass different between two adjacent peaks that are just resolved height of the valley 10 of the peak M the nominal mass of the first or the mean of the two peaks Resolution Estimate the accuracy of the measurement if resolution is known If R 5000 at mass 500 range 5000 500 m m 0 1 Determine R required C2H4 and CH2N have masses of 28 0313 and 28 0187 R 28 025 0 0126 2220 Sample Inlet Systems Batch inlet systems Direct probe inlets Chromatographic inlets Capillary electrophoretic inlets Gas Liquid Inlet System 350 oC Solid Matrix Inlet Systems Sample Mass Spectrometry Mass Analyzers Magnetic Sector Mass


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