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Introduction to Mass Spectrometry

Introduction to Mass Spectrometry. March 2008. What is a Mass Spectrometer?. A Mass Spectrometer is a machine that weighs molecules !. 0 units. What is a Mass Spectrometer?. A Mass Spectrometer is a machine that weighs molecules !. 12 units. What is a Mass Spectrometer?.

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Introduction to Mass Spectrometry

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  1. Introduction to Mass Spectrometry March 2008

  2. What is a Mass Spectrometer? A Mass Spectrometer is a machine that weighs molecules ! 0 units

  3. What is a Mass Spectrometer? A Mass Spectrometer is a machine that weighs molecules ! 12 units

  4. What is a Mass Spectrometer? A Mass Spectrometer is a machine that weighs molecules ! 12 units 8 9 10 11 12 13 14 15 16

  5. What is a Mass Spectrometer? A Mass Spectrometer is a machine that weighs molecules ! 14 units 8 9 10 11 12 13 14 15 16

  6. What is a Mass Spectrometer? A Mass Spectrometer is a machine that weighs molecules ! Number of counts 12 units 8 9 10 11 12 13 14 15 16 mass

  7. Outline • Basic Chemistry • Analytical Chemistry • Mass Spectrometry • Types of Ion Sources • EI, CI, ESI, APCI, APPI, MALDI • Types of MS • Ion Traps, Quads, FT-ICR, TOF, Sector • MS/MS • Performance Comparisons • Market Segments

  8. Basic Chemistry • Everything is made of Atoms • Atoms are made of protons, neutrons, and electrons • Many atoms together make up molecules ATOM U

  9. Carbon Nitrogen Hydrogen Oxygen

  10. Carbon Atom 6 protons (+) 6 neutrons 6 electrons(-)

  11. Carbon

  12. More Carbon • 6 protons (1 mass unit each) + 6 neutrons (1 mass unit each) = 12 mass units • Electrons are negligible ( 1/3600 of mass unit) • Some carbon (about 1%) has 7 neutrons so weigh 13 units

  13. 12.00 x 99%+13.00 x 1% = 12.01 amu

  14. But how much does an atom weigh ? • It was found that 12 grams of carbon contains 6.02 x 1023 atoms of carbon. ( 1023 seconds have not elapsed since the beginning of time !) • So one atom of carbon weighs 1.99 x 10-23 grams !

  15. Caffeine C8H10N4O2 Total Mass 194 Daltons 3.22x10-22 grams

  16. Caffeine C8H10N4O2 Total Mass 194 Daltons 3.22x10-22 grams So we must devise a machine which can measure ~ 10-22 grams.

  17. Analytical Chemistry Chemical Methods Instrumental Methods • Titration • Gravimetric Analysis • Solution Chemistry Mass Spectrometry Spectroscopy Optical Emission • FT-ICR • TOF • Quadrupole • Ion Trap • Linear Trap • Magnetic Sector NMR Microwave Optical Absorption

  18. 3 Elements to Mass Spectrometry (J.J. Thomson ~ 1910) Gas Phase/ Ionize Separate Based on Mass/Charge Sample Why Ionize ? Difficult to manipulate neutral particles on molecular scale. If they are charged, then we can use electric fields to move them around. Detector

  19. 3 Elements to Mass Spectrometry (J.J. Thomson ~ 1910) Gas Phase/ Ionize • Electron Impact (EI) • Chemical Ionization (CI) • Electrospray (ESI) • Atmospheric Pressure Chemical Ionization (APCI) • Photo-ionization (APPI) • Matrix Assisted Laser Desorption and Ionization (MALDI) Separate Based on Mass/Charge Sample Detector

  20. 3 Elements to Mass Spectrometry (J.J. Thomson ~ 1910) Gas Phase/ Ionize • Scanning (Filter) • Linear Quadrupole • Sector • Pulsed (Batch) • Ion Trap • FT-ICR • Time-of-Flight Separate Based on Mass/Charge Sample Detector

  21. 3 Elements to Mass Spectrometry (J.J. Thomson ~ 1910) Gas Phase/ Ionize • Faraday Cup • Discrete Dynode • Continuous Dynode • Multi-channel Plate Separate Based on Mass/Charge Sample Detector

  22. 3 Elements to Mass Spectrometry (J.J. Thomson ~ 1856-1940) Gas Phase/ Ionize Separate Based on Mass/Charge Sample So, we could come up with 6x5x4 = 120 Unique Mass Spectrometers. In reality, not all combinations make sense, but many extra “hybrid” MS systems have value. For example Q-TOF’s and LT-FT-ICR Detector

  23. 6 Types of Ion Sources

  24. Ion Source Depends on Sample Solid Sample Liquid Sample Gas Sample Make into Solid ? Make into Solution ? Turn into Gas? Chemical Properties of analyte in gas phase ? Chemical Properties of analyte in solution phase ? MALDI APCI APPI ESI CI EI

  25. Polarity, MW and Volatility

  26. Polarity, MW and Volatility Caffeine

  27. Gas Phase Ionization • EI and CI are gas phase ionization techniques • Sample is heated to cause volatilization • The molecule must have a low enough MW and polarity so that: • TBoil< TDecomposition

  28. Electron Impact e- e- e- M M(g) + e- M+(g) + 2e- This reaction creates the molecular ion so is very useful. However, the excess energy from the electron can cause the molecular ion to fall apart:

  29. Neutral Molecule IP2 Excess Energy get redistributed throughout ion to cause fragmentation. s1 IP s0 Ionized Molecule s1 s0

  30. Electron Impact e- e- e- M M(g) + e- M+(g) + 2e- M+(g)  A+Fragment 1 (g) + BFragment 2 (g) • Electron energy is chosen by compromise. • Fragment Information is useful. It can help structural determination. However, many ions produce only fragments with no molecular ion remaining. Molecular ion often very unstable. • 70 eV “Classical Spectra” to be used for comparisons

  31. MW 194

  32. 109 m/z

  33. 55 m/z

  34. Chemical Ionization • EI is not appropriate for some molecules (it causes too much fragmentation) • Instead, ionize a reagent gas (by EI) then react it with a analyte molecules • Typically use methane or ammonia for reagent gas

  35. CI: Form Reagent Ions First • For Example - Methane CI • electron ionization of CH4: • CH4 + e-  CH4+ + 2e- • Fragmentation forms CH3+, CH2+, CH+ • ion-molecule reactions create stable reagent ions: • CH4+ + CH4 CH3 + CH5+ • CH3+ + CH4 H2 + C2H5+ • CH5+ and C2H5+ are the dominant methane CI reagent ions

  36. Methane CI Reagent Ions • Ions at m/z 17, 29, and 41 are from methane; • H3O+ is also formed from water vapor in the vacuum system

  37. Reagent Ions React with Analytes • Several Types of Reactions May Occur • Form Pseudomolecular Ions (M+1) • CH5+ + M  CH4 + MH+ • M+1 Ions Can Fragment Further to Produce a Complex CI Mass Spectrum • Form Adduct Ions • C2H5+ + M  [M + C2H5]+ M+29 Adduct • C3H5+ + M  [M + C3H5]+ M+41 Adduct • Molecular Ion by Charge Transfer • CH4+ + M  M+ + CH4 • Hydride Abstraction (M-1) • C3H5+ + M  C3H6 + [M-H]+ • Common for saturated hydrocarbons

  38. EI Spectrum of Cocaine • Extensive Fragmentation • Molecular Ion is Weak at m/z 303

  39. Methane CI of CocainePseudo molecular Ion and Fragment Ions

  40. Isobutane CI of Cocaine • Soft Reagent - Less Fragmentation

  41. Polarity, MW and Volatility

  42. Liquid Techniques • Depending on solvent composition and molecular properties either • APPI • ESI • APPI

  43. APPI

  44. APPI • Lamp Wavelength chosen to only excite analytes not solvent/background • Low amount of photo dissociation results • New technique with few novel applications • Less universal than electrospray

  45. APCI Principles • Rapidly vaporize entire liquid flow • Ionize solvent molecules in corona discharge • CI process ionizes sample molecules • Positive mode: proton transfer or charge exchange • Negative mode: proton abstraction or electron capture

  46. APCI – Cut Away View

  47. What applications need APCI? • APCI works well for small molecules that are moderately polar to non-polar • APCI works well for samples that contain heteroatoms • Avoid samples that typically are charged in solution • Avoid samples that are very thermally unstable or photosensitive

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