Molecular Mass Spectroscopy

1. Molecular Mass Spectroscopy • Molecular structure • Composition of mixtures • Molecular mass spectra • Ion Source • Mass Spectrometers • Applications

2. Molecular Mass Spectra • Removal of electron by electron bombardment • In vapor phase • Charged species is the molecular ion • Electron causes excitation and fragmentation • Major product is base peak • Assigned 100% relative abundance • Smaller fragments also form

3. Ion Sources • Ion source has profound effect on spectra • Gas phase source • Vaporized then ionized • Desorption source • Conversion of liquid or solid to gas • Hard source • Ion in excited state • Fragments produced • Soft source • Little fragmentation • Mainly ion of molecule

6. Electron-Impact Source • Sample as vapor • Ionized by beam of electrons • W or Re filament • 70 V potential • 1E-6 effective • M+e- ->M++2e- • High potentials in accelerating region • 1E3 to 1E4 volts • KE of ion in 1000 V • KE=qV=zeV • KE=1*1.6E-19 C*1000 V • 1.6E-16 J • KE independent of mass • Velocity varies with mass • KE=0.5mv2

7. Electron Impact Spectra • Energy from e- accelerated by 70V • Find in J/mol to compare bond energy • KE=eV • 1.6E-19 C *70 V=1.12E-17 CV/e- • For a mole • 1.12E-17 J*6.02E23 =6.7E6 J/mol • Bond energy 200 to 600 kJ/mol

8. Electron Impact Spectra

9. Electron Impact Spectra

10. Electron Impact Spectra • Sensitive method • Fragments useful in identification • Lack of molecular ion peak • M+, difficult to identify specie • Molecules must be in vapor phase • Stability issues in vapor phase • MW<1000 dalton

11. Isotopics • Isotopic variation can impact spectra

12. Chemical Ionization • Sample ionized by secondary ionization • Reagent gas ionized by electrons, then ionized reagent gas reacts with sample gas • Reagent to sample ratio • 1E3 to 1E4 • Methane most common reagent gas • CH4+ and CH3+ (about 90%), CH2+

13. Chemical Ionization • Produces ions that are 1 proton more or 1 proton less than molecule • Transfer of C2H5+ give M+29 peak • Field Ionization • Large electric field • 1E8 V/cm • Mainly produces M and M+1 peaks

14. Comparison of spectra • a-electron impact • b-field ionization • c-desorption

15. Large molecule desorption • Solid or liquid samples directly energized into gas phase • Molecular or protonated ion • Matrix Assisted Laser Desorption/Ionization (MALDI) • Soft Ionization • Sample dissolved in solution containing UV-absorber and solvent • Solution evaporated and precipitate formed • Pulsed laser used to excite precipitate • Molecular ion desorbed from surface of precipitate

17. Electrospray Ionization • Solution pumped through a needle • Needle is at kV potential compared to surrounding electrode • Droplets become charged • Solvent evaporates, droplets shrink and charge density increases • Can be combined with a number of methods • Useful for large molecules • M+, M2+

18. Electrospray MS spectra

19. Fast Atom Bombardment • Samples in glycerol • Bombarded by Xe or Ar atoms • Several keV • Atoms and ions sputtered from surface • Production of fragments

20. Mass Spectrometers

21. Magnetic sector

22. Ion Trap Analyzer • Variable radio frequency voltage applied to the ring electrode • Ions of appropriate m/z circulate in stable orbit • scan radio frequency • heavier particles stable • lighter particles collide with ring electrode • ejected ions detected by transducer as an ion current

23. Ion Trap

24. Applications • Identification of Pure Compounds: • Nominal M+ peak (one m/z resolution) (or (M+1)+ or (M-1)+) • gives MW (not EI) • Exact m/z (fractional m/z resolution) can give stoichiometry but not structure (double-focusing instrument) • Fragment peaks give evidence for functional groups • (M-15)+ peak methyl • (M-18)+ OH or water • (M-45)+ CO2H • series (M-14)+, (M-28)+, (M-42)+..sequential CH2 • loss in alkanes • Isotopic peaks can indicate presence of certain atoms • Cl, Br, S, Si • Isotopic ratios can suggest plausible molecules from M+, • Comparison with library spectra