1. SCC Science Dept F W Aston 1875 - 1945

2. SCC Science Dept Aston’s Mass Spectrometer

3. SCC Science Dept

4. Mass Spectrometer Invented by F W Aston in 1919 Working on separating the isotopes of Neon Discovered two types of atom One type of mass 20 Another type of mass 22

5. SCC Science Dept Both had 10 protons The first type had 10 neutrons Second type had 12 neutrons Isotopes [Greek for same place] Atoms of the same element that have the same atomic number but different mass numbers due to different numbers of neutrons Fredrick Soddy coined phrase working with lead 206,207 and 208 formed when uranium decayed radioactively – Nobel Prize 1921

6. SCC Science Dept The Mass Spectrometer works on the principal that positive ions are separated by mass when moving in a magnetic field

7. SCC Science Dept Structure of the Mass Spectrometer

8. SCC Science Dept Sample Inlet Ionisation Chamber Accelerator Separation Unit Ion detector Amplifier Recorder Vacuum Pump Electron Gun Negatively charged plates Analyser Unit Variable magnetic field Intermediate mass particles Lighter particles Heavier particles

9. SCC Science Dept 5 Stages 1)Vaporisation 2)Ionisation 3)Acceleration 4)Separation 5)Detection, Amplification and Display

10. SCC Science Dept 1. Vaporisation Sample must be vaporised There is a Vacuum in the instrument so liquids vaporise easily Solids may have to be heated in a special unit

11. SCC Science Dept Sample in syringe injected into chamber Vaporisation Chamber Sample may need to be heated to vaporise it Sample inlet A

12. SCC Science Dept 2. Ionisation Turns atoms and molecules into ions Using an “electron gun” Fires high energy electrons at atoms These high energy electrons knock off normal electrons forming positive ions X + e - = X + + 2 e - These ions can then be accelerated and deflected using electric charge and a magnetic field

13. SCC Science Dept ▬ This is the IONISATION CHAMBER where atoms are turned into ions Hot wire is negatively charged and boils off electrons which are repelled by negative and attracted to the anode at high speed Now vaporised atoms are sprayed into ionisation chamber and are hit by the high speed electrons These electrons knock other electrons from the atom turning it into a positive ion + + The positive ions are attracted by the negative charge of the accelerator plates and those which pass through the hole are accelerated to high speeds and fired into the variable magnetic field AcceleratorAccelerator

14. SCC Science Dept 3. Accelerator Made of a series of Negative plates with a hole in the centre These attract the positive ions Some of these ions pass through the holes in the centre of the plates And as they do so are accelerated to high speeds This produces a fine beam of positive ions which pass into analyser Vacuum allows the ions to move [no air molecules to stop or deflect them]

15. SCC Science Dept 4. Separation Magnetic field does the separation. In the Analyser All ions have the same kinetic energy light particles are deflected most heavy particles are deflected least Beam of particles is separated according to their masses

16. SCC Science Dept Effect of the magnetic field in the analyser unit on ions of differing Mass

17. SCC Science Dept + Separator or Analyser Unit Magnetic Field off Light Ion Magnetic Field on No magnetic field Light Ion Heavy Ion Large deflection No deflection Small deflection

18. SCC Science Dept Effect of changing the Magnetic Field

19. SCC Science Dept + Separator or Analyser Unit Ion of one mass Magnetic Field No Magnetic Field Strong Field Weak Field OFFSTRONG WEAK No deflection Small deflection Large deflection

20. SCC Science Dept 5a. Detector Very sensitive instrument. Responds to the number of ions hitting it. The more ions that hit it the bigger the reading Changing the magnetic field brings particles of different masses to focus on the detector. [Stronger the field the greater the particle mass] Detector needs to be calibrated

21. SCC Science Dept 5b. Amplifier The signal is amplified [made bigger] electronically

22. SCC Science Dept 5c. Display On computer screen Trace recorded as a series of peaks Trace called a Mass Spectrum When printed called a Mass Specrtrogram

23. SCC Science Dept Mass Abundance 0 Isotope 1 Isotope 2 Isotope 3

24. SCC Science Dept Aston discovered 35 Cl and 37 Cl Determined the percentage of each isotope present Won the Nobel Prize for Chemistry 1922 Discovered 212 of the 287 naturally occurring nuclides

25. SCC Science Dept Uses Determinig the Relative molecular mass of atoms [isotopes] till 1970 Determining mass of unknown organic compounds and thus help determine their structures – Gases from waste dumps – Trace organic pollutants in water Click here for a brief biography of F W Astonbiography