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Chem 3500 Mass Spectrometry Y. Zhao

Chem 3500 Mass Spectrometry Y. Zhao. Lecture 2 A Glimpse of Mass Spectrometry. Began from 1920s: Aston, Dempster, Thompson. MS is often an ideal starting point in organic structure analysis. C 7 H 7 NO M.W. 121. EI-MS of benzamide.

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Chem 3500 Mass Spectrometry Y. Zhao

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  1. Chem 3500 Mass Spectrometry Y. Zhao • Lecture 2 • A Glimpse of Mass Spectrometry Began from 1920s: Aston, Dempster, Thompson MS is often an ideal starting point in organic structure analysis C7H7NO M.W. 121 EI-MS of benzamide

  2. Chem 3500 Mass Spectrometry Y. Zhao What is Mass Spectrometry? A technique used to determine the masses of atoms or molecules in which electrical charges are placed on them and the resulting ions are separated by their mass-to-charge ratios (m/z). A Mass Spectrometer is an instrument that can separate charged atoms or molecules according to their mass-to-charge ratio.

  3. Chem 3500 Mass Spectrometry Y. Zhao Presentation of Data A Low-Resolution EI-MS of n-pentane. Each m/z value is the nominal molecular mass of the fragment (z = +1)

  4. Chem 3500 Mass Spectrometry Y. Zhao Species with a nominal mol. mass of 122 amu Exact mol. masses can be measured by using a High-Resolution Mass Spectrometer (HR-MS) The fate of a sample molecule in a mass spectrometer • Ionization—Molecular Ion (M+·) • Fragmentation and rearrangement—(fragment ions, radicals, neutral species)

  5. Chem 3500 Mass Spectrometry Y. Zhao • Principles and Instrumentation of MS Major components of an MS instrument Sample introduction Ionization Analyzer Detector Data system

  6. Chem 3500 Mass Spectrometry Y. Zhao Schematic of an electron-impact mass spectrometer with a single magnetic sector analyzer

  7. + + + + + + + m/z = 185 + + + B2r2 m 185 = 100% z 2V 173 305 A mass spectrum Chem 3500 Mass Spectrometry Y. Zhao • How does it work? A single-focusing, 180º sector mass analyzer Acceleration V Magnetic field B + - 305 185 173 + + Base peak Analyzer tube Detector m/z

  8. Chem 3500 Mass Spectrometry Y. Zhao Resolution Two adjacent peaks with almost equal intensity C15H24NO2 C16H26O2 250.1933 250.1807 height < 10% valley m/z 250.1933 = 20,000 R = 250.1933-250.1807 High-resolution mass spectrum (HRMS) can be used to measure exact masses

  9. Chem 3500 Mass Spectrometry Y. Zhao Why HRMS? Exact mass H 1.0078522 C 12.0000000 N 14.0080744 O 15.99491502 CO CH2=CH2 Nominal mass is 28 Nominal mass is 28 Exact mass 1  12.0000000 = 12.000000 1  15.99491502 = 15.99491502 27.994915 Exact mass 2  12.0000000 = 24.000000 4  1.00782522 = 4.03130 28.03130

  10. e e Chem 3500 Mass Spectrometry Y. Zhao Electron Impact/Ionization (EI) • EI is a “hard” ionization method • One electron is removed to form singly charged molecular ion, M+· • M+·carries excess internal energy and tends to fragment • Give reproducible spectra but limit to MW < 1,000 Da • Pressure < 10-5 Torr • ionization efficiency is high • electron energies ~ 20-70 eV Anode e e M + · Cathode

  11. Chem 3500 Mass Spectrometry Y. Zhao Chemical ionization (CI) Anode e M + R Cathode

  12. Chem 3500 Mass Spectrometry Y. Zhao Chemical ionization (CI) • CI is especially useful (“soft” ionization) technique when no molecular ion is observed in EI mass spectrum, and also in the case of confirming the mass to charge ratio of the molecular ion. • Reagent gas is first ionized by EI. • Sample molecules are ionized by interaction with reagent gas ions. • Reagent gas : sample molecules ~ 100 : 1 • Positive and negative ions are formed in the CI process. However, depending on the instrument setup, only positive ions or negative ions are recorded in the spectrum. • Pressure = 0.1 – 2 Torr

  13. Chem 3500 Mass Spectrometry Y. Zhao Chemical ionization (CI) Positive ion mode: G+· + M M+· + G charge transfer G-H+ + M M-H+ + G proton transfer TiCl2+ + M (M + TiCl2)+ adduct formation Negative ion mode: [G-H]- + M [M-H]- + G proton transfer M + e M-· electron capture

  14. + + + + Multiple desolvation steps + + Ion cluster + Chem 3500 Mass Spectrometry Y. Zhao Desorption ionization (DI) • Fast atom bombardment (FAB) Primary incident beam Xe Sample solution = sample molecule = solvent molecule

  15. + laser Chem 3500 Mass Spectrometry Y. Zhao Desorption ionization (DI) • Matrix assisted laser desorption/ionization (MALDI) substrate

  16. Chem 3500 Mass Spectrometry Y. Zhao Electrospray ionization (ESI) Drying gas needle + + + + + + + + + + + + + Sample solution + + + + + + + + + + Voltage ~3 – 5 kV Single ion in droplet theory: assume that increased charge density, due to solvent evaporation, causes large droplets to divide into smaller droplets eventually leading to single ions. Ion evaporation theory: assumes the increased charge density that results from solvent evaporation causes Coulombic repulsion to overcome the liquid’s surface tension, resulting in a release of ions from droplet surfaces.

  17. N + r s Chem 3500 Mass Spectrometry Y. Zhao Mass analyzer Magnetic sector Time of flight (TOF) V d detector Field free region m/z = 1.93 t2 V/d2 Acceleration region

  18. Chem 3500 Mass Spectrometry Y. Zhao Quadrupole mass filter + - + + - + Ion trap Ion cyclotron resonance (ICR)

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