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Mass Spectrometry

Mass Spectrometry. Chapter 2 Pg 48. http://www.chemguide.co.uk/analysis/masspec/elements.html#top. Isotopes. Atoms of the same element (same number of protons) that differ in the number of neutrons. Ex: Neon-20, Neon-21, Neon-22 Atomic Mass- average mass of all the isotopes

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Mass Spectrometry

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  1. Mass Spectrometry Chapter 2 Pg 48 http://www.chemguide.co.uk/analysis/masspec/elements.html#top

  2. Isotopes • Atoms of the same element (same number of protons) that differ in the number of neutrons. • Ex: Neon-20, Neon-21, Neon-22 • Atomic Mass- average mass of all the isotopes • But it’s not a normal average!!!

  3. Mass Spectrometer • The most direct and accurate means for determining atomic and molecular weights. • Use the gas form of the element or compound.

  4. Basic Idea • Suppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose-pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.

  5. Basic Idea • But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection. • The same idea can be applied to atomic sized particles. • The lighter the atom, the more it’s deflected and so on.

  6. An outline of what happens in a mass spectrometer • Stage 1: Ionisation • The atom is ionised by knocking one or more electrons off to give a positive ion. • Mass spectrometers always work with positive ions.

  7. An outline of what happens in a mass spectrometer • Stage 2: Acceleration • The ions are accelerated so that they all have the same kinetic energy.

  8. An outline of what happens in a mass spectrometer • Stage 3: Deflection • The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

  9. An outline of what happens in a mass spectrometer • Stage 4: Detection • The beam of ions passing through the machine is detected electrically.

  10. Mass Spec

  11. Mass Spectrometer http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm

  12. What does the output look like? Relative Abundance for Molybdenum (of the different isotopes)

  13. Relative Abundance of Molybdenum • As you will see from the diagram, the most common ion has a mass/charge ratio of 98. • That means that molybdenum consists of 7 different isotopes.

  14. Mass Spec of Zirconium

  15. The Number of Isotopes • The 5 peaks in the mass spectrum shows that there are 5 isotopes of zirconium - with relative isotopic masses of 90, 91, 92, 94 and 96 on the 12C scale.

  16. The Abundance of the Isotopes • This time, the relative abundances are given as percentages. • zirconium-90 (51.5%), zirconium-91 (11.2%), zirconium-92 (17.1 %)zirconium-94 (17.4%), zirconium-96 (2.8%)

  17. How to Calculate! • To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. • The total mass of these 100 typical atoms would be • (0.515 x 90) + (0.112 x 91) + (0.171 x 92) + (0.174 x 94) + (0.028 x 96) = 91.3

  18. Practice • Naturally occurring magnesium has the following isotopic abundances: • What is the average atomic mass of Magnesium? • Sketch the mass spectrum of Mg.

  19. Did you know that mass spectrometry is used to... • Detect and identify the use of steroids in athletes • Monitor the breath of patients by anesthesiologists during surgery • Determine the composition of molecular species found in space • Determine whether honey is adulterated with corn syrup • Locate oil deposits by measuring petroleum precursors in rock • Monitor fermentation processes for the biotechnology industry • Detect dioxins in contaminated fish • Determine gene damage from environmental causes • Establish the elemental composition of semiconductor materials http://www.asms.org/whatisms/p1.html

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