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Gas Chromatography and Mass Spectrometry

Gas Chromatography and Mass Spectrometry. Chem 2411L Dr. Clower. Gas Chromatography. Mobile phase = stream of inert gas Stationary phase = high boiling liquid film Polarity similar to compounds in mixture Polymers Supported on a solid Packed in a column Heated glass or metal

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Gas Chromatography and Mass Spectrometry

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  1. Gas Chromatography and Mass Spectrometry Chem 2411L Dr. Clower

  2. Gas Chromatography • Mobile phase = stream of inert gas • Stationary phase = high boiling liquid film • Polarity similar to compounds in mixture • Polymers • Supported on a solid • Packed in a column • Heated glass or metal • 2-3 m in length; 2-5 mm diameter • Sample = injection from syringe (1 mL) • Sometimes add 1 mL air

  3. GC, cont. • Affinity = vapor vs. liquid • Separated components • Pass by detector • Send signal to electronic recorder • Can be collected for further analysis (rare) • Signal display = peaks • Area under peak = % of mixture • ½ b x h • Massed on balance • Integrated • Analysis • Retention time (min.), Area, % of mixture

  4. Gas Chromatogram • Retention time can be used to ID a component if compared to known sample under same conditions • Flow rate, column size, temperature, etc.

  5. Analysis of GC • What is the percentage of each component in the mixture? • Which component has the higher boiling point? • How many components are in this mixture?

  6. MassSpectrometry • Used with GC • Mixture of compounds separated by gas chromatography, then identified by mass spectrometry • Determines MW and provides information about molecular formula • Does not involve the absorption or emission of light • A beam of high-energy electrons breaks molecules into ions (fragments) M → M•+ + e- M•+→ A+ + X A+→ B+ + Y etc. • Ions are separated and detected; mass determined

  7. Mass Spectrometer

  8. The Mass Spectrum • Plot relative abundance vs. mass-to-charge ratio • Charge = +1

  9. C3H8 C2H4OCO2 CN2H4 44.06260 44.02620 43.98983 44.03740 High Resolution MS • Masses measured to 1 part in 20,000 (more precise) • A molecule with mass of 44: • C3H8 • C2H4O • CO2 • CN2H4 • If a more exact mass is 44.029, which is the correct structure?

  10. 81Br Molecules with Heteroatoms • Isotopes: present in their usual abundance • Hydrocarbons contain 1.1% 13C, so there will be a small M+1 peak • If S is present, M+2 will be 4% of M+ • If Cl is present, M+2 is one-third of M+ • If Br is present, M+2 is equal to M+ • If I is present, peak at 127; large gap

  11. Mass Spectrum with S

  12. Mass Spectrum with Cl

  13. Mass Spectrum with Br

  14. Mass Spectrum with I

  15. Mass Spectra of Alkanes • More stable carbocations will be more abundant

  16. Mass Spectra of Alkenes • Resonance-stabilized cations favored

  17. Mass Spectra of Alcohols • Alcohols usually lose a water molecule • M+ may not be visible

  18. 105 77 136 51 Methyl Benzoate

  19. Essential Oil Experiment • IR of oil • GC of oil • MS of components of oil

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