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Mass Spectrometry

Mass Spectrometry. Unit 3. It Weighs How Much?!?. Atoms have very little mass. We cannot use a scale to weigh them. Any scale that were sensitive enough to weigh an atom would be affected by minute vibrations

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Mass Spectrometry

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  1. Mass Spectrometry Unit 3

  2. It Weighs How Much?!? • Atoms have very little mass. • We cannot use a scale to weigh them. • Any scale that were sensitive enough to weigh an atom would be affected by minute vibrations • Like the rolling tires of a truck on I-295 or the breathing of your neighbor (not to mention, your heartbeat).

  3. It’s beyond me • In order to determine the mass of atoms and molecules, • We have to use physics. • Sir Isaac Newton’s 2nd Law of Motion: • F = ma • Dr. Hendrik Lorentz’s Force Law: • F = Q(E + v x B)

  4. What’s that got to do with me? • Lorentz Force Law tells us that a charged particle will be acted upon by a force in an magnetic field. • Newton’s 2nd Law tells us that the force acting upon an object will be proportional to its mass. • If we could combine these two forces, we could determine the mass of really small things.

  5. Isaac, meet Hendrik • ma = F = Q(E+vxB) • (m/Q)a = E+vxB • m/Q = mass to charge ratio of a particle • a.k.a. m/z • Acceleration is proportional to charge and inversely proportional to the mass. • If we just had a way to add a charge to a particle, we could subject it to a magnetic field and calculate its mass…

  6. Atoms and ions • Atoms are normally electrically neutral (same # of protons as electrons). • When we strip electrons away (or add more), we get ions: Cation – positively charged (#p+ > #e-) Ex: Na  Na+ + e- Fe  Fe+2 + 2e- Anion – negatively charged (#p+ < #e-) Ex: Cl + e- Cl- O + 2e- O-2

  7. Nice! • Enter the mass spectrometer. • a.k.a. Mass-Spec or MS • The basics: 1. “Shoot” ions at a magnet. 2. Deflect the ions with the magnetic field. 3. “Catch” the ions in a detector.

  8. Bend it like…well, you know… • Mass spectrometers have 3 components: • Ion Source • Mass Analyzer (Magnet) • Detector

  9. The Perfect Couple • Most times, Mass-Specs are couple with a Gas Chromatograph • a.k.a. GCMS

  10. The Perfect Couple • The Gas Chromatograph separates mixtures into its components, • Which feed into the Mass-Spec, • Which determines the m/z ratio of the components • This allows you to determine the content and identity of compounds in mixtures (who poisoned the dead guy with what?)

  11. Deflection of Charged Particles • Assuming the same magnetic field and speed, • The amount of deflection depends on the charge and mass of the particle. • Small Mass = more deflection • Greater Charge = more deflection • MS readouts show relative intensity & m/z • Most of the time, z = +1, so m/z = m

  12. Say What, Now…?

  13. A little better… C7H7+ ion MW = 91 C7H8 MW = 92 Little bit of C7H8 with 14C MW = 93

  14. Predict the deflection + Greater Charge = More Deflection Sr+2 Sr+1 – Sr+2 Sr+1 Mass: 88 88 Q: +2 +1

  15. Predict the deflection + Greater Mass = Less Deflection Na+1 K+1 – Na+1 K+1 Mass: 23 39 Q: +1 +1

  16. Predict the deflection + Greater Charge = More Deflection O-2 O-1 – O-2 O-1 Mass: 16 16 Q: -2 -1

  17. Predict the deflection + Greater Mass = Less Deflection Cl-1 Br-1 – Cl-1 Br-1 Mass: 35 80 Q: -1 -1

  18. Predict the deflection +   Particles with no charge are not deflected! g n –   n Mass: 4 0 0 1 Q: +2 -1 0 0

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