The Mass Spectrometer

1. The Mass Spectrometer

2. http://www.tcd.ie/CMA/equipment.php

3. Basic Theory A moving object that encounters a sideways force will be deflected. What would you personally be able to deflect more, a cannonball or a tennis ball? A) Cannonball B) tennis ball Deflection depends on the mass: more mass, less deflection less mass, more deflection

5. Deflection of ions Ions are charged particles • Ions in motion can be deflected by magnetic fields • Deflection will depend on mass and the amount of charge.

6. Step 1 - Vaporization • Samples must first be vaporized (turned into a gas)

7. Step 2 – Ionization • The atoms are converted into ions • This is done by knocking off one or more electrons • Will the ions be positive or negative? • Positive B) negative Positive ions are called cations

8. Electrons are knocked off atoms by an electron beam

9. Step 3 – Acceleration • The ions are accelerated in an electric field. • The particles must be accelerated to the same kinetic energy (speed)

10. Step 4 - Deflection • The accelerated ions are sent through a magnetic field, this causes them to be deflected. Which is correct? 1. Particles with more mass are deflected A) more B) less? 2. Particles with more charge are deflected • more B) less? This is know as the mass/charge ratio.

11. The magnetic field causes the cations to be deflected.

12. Deflection continued • If all ions charge of +1 In the diagram below. Which ion has the greatest mass/charge ratio? A) B) C) In the diagram which particle has the most mass? A) B) C) The least mass? A) B) C)

13. Check your answers • A has the lightest • B the next lightest • C the heaviest

14. Step 5 – Detection • From the last diagram only stream B will be detected • It will be neutralized in the detector by hitting a metal. 1) Electrons move from the metal to the cations. 2) Electrons in the metal move to fill the space left. • This movement is a current which is detected

15. The flow of electrons creates a current, which can be recorded.

16. What happens to A and C? • The magnetic field is varied to get the particles of other mass to go through to the detector

17. How is mass measured? • Atomic mass is measured relative to what isotope? A) C-12 B) H-1 C) C-14 The mass spectrometer measures mass on the relative Carbon-12 scale Carbon-12 has a mass of 12.000 amu One amu is equal to 1/12 of carbon-12

18. Output • This is an example of the Molybdenum mass spec diagram • # isotopes? most common?

19. 100 Relative number of Ions 50 0 24 25 m/z (m = mass, z = charge) Practice - Spectra of Element E • How would you find the atomic mass of this element, E?

20. Here is the formula: (mass of isotope 1 x % isotope 1)/100 + (mass of isotope 2 x % isotope 2)/100 + …. = Average atomic mass

21. 100 Relative number of Ions 50 0 24 25 m/z (m = mass, z = charge) Use the data to find the isotopes • Find the amount of each isotope 100 of E-24 and 10 of E-25 • Total = 110

22. Then: • Find % of the total for each isotope: • % E-24 = 100/110 x 100 = 90.91% • % E-25 = 10/110 x 100 = 9.09%

23. 100 Relative number of Ions 50 0 24 25 m/z (m = mass, z = charge) Finally: • Determine mass of each and add together: • (24 * 90.91) + (25 *9.09) = 24.0909 100 100

24. Calculate the relative atomic mass of boron • Answer: 10.78 amu = 10.8 amu rounded to 3 significant figures

25. Review • What are the five steps of a mass spectrometer? • VIADD – how will you remember this? • What two steps control the amount of deflection? • Which of the two is usually most important? • Are the samples turned into cations or anions?

26. Review continued • What is used to deflect ions? • Masses are measured on what scale? • How do you know how many isotopes a sample has? • How do you know which sample is the most abundant?

27. A nice review video. Just watch the first part • MS explained on a video

28. Bibliography • http://www.youtube.com/watch?v=a5aLlm9q-Xc • http://www.youtube.com/watch?v=J-wao0O0_qM&feature=related • http://www.chemguide.co.uk/analysis/masspec/howitworks.html