1. The Mass Spectrometer

2. Isotopes Most elements occur as mixtures of isotopes
Isotopes of the same element have different masses.
The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to the mass of carbon-12 (exactly amu)
Individual atomic masses are determined by mass spectrometry

3. F W Aston ~ Invented the 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

4. F W Aston ~ Invented the Mass Spectrometer
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

5. Aston’s Mass Spectrometer
ITS Chemistry

6. ITS Chemistry

7. Principle on which the Mass Spectrometer is based
The Mass Spectrometer works on the principal that positive ions are separated by mass when moving in a magnetic field

8. Structure of the Mass Spectrometer
Sample Inlet
Negatively charged plates
Ionisation Chamber
Accelerator
Electron Gun
Variable magnetic field
Separation
Unit
Lighter particles
Recorder
Ion detector
Heavier particles
Amplifier
Vacuum Pump

9. Five Stages in mass spectrometry
Vaporisation
Ionisation
Acceleration
Separation
Detection, Amplification and Display

10. 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. Sample may need to be heated to vaporise it
Vaporisation Chamber
Sample inlet A
Sample in syringe injected into chamber
Sample may need to be heated to vaporise it

12. 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 e-
These ions can then be accelerated and deflected using electric charge and a magnetic field

13. where atoms are turned into ions+
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
These electrons knock other electrons from the atom turning it into a positive ion
Accelerator
This is the
IONISATION CHAMBER
where atoms are turned into ions
Now vaporised atoms are sprayed into ionisation chamber and are hit by the high speed electrons +
Hot wire is negatively charged and boils off electrons which are repelled by negative and attracted to the positive port at high speed ▬

14. 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. 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. Separator or Analyser Unit
Magnetic Field on
Magnetic Field off
Heavy Ion
Light Ion
No magnetic field
No deflection +
Small deflection
Heavy Ion
Large deflection
Light Ion

17. 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

18. Display On computer screen Trace recorded as a series of peaks
Trace called a Mass Spectrum
When printed called a Mass Specrtrogram

19. Isotope 2
Abundance
Isotope 1
Isotope 3
Mass

20. Structure of the Mass Spectrometer
Sample Inlet
Negatively charged plates
Ionisation Chamber
Accelerator
Electron Gun
Variable magnetic field
Separation
Unit
Lighter particles
Recorder
Ion detector
Heavier particles
Amplifier
Vacuum Pump

21. Relative Atomic Mass
Because most elements have multiple isomers, the masses on the periodic table cannot describe only 1 isotope’s individual atomic mass.
Therefore, the mass numbers on the periodic table are relative atomic masses:
Relative atomic mass is the average mass of the individual isotopes of an element, taking into account the naturally occurring relative abundance of each.
To find the relative atomic mass for an element, sum the mass contributions from each isotope of the element.
2 -

22. Relative atomic mass of Cl = = 35.48
What is the relative atomic mass of Cl?
The relative abundances of Cl-35 and Cl-37 are and respectively
Relative atomic mass of Cl =
= 35.48

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

24. Uses of the mass spectrometer
Determining the Relative molecular mass of atoms [isotopes] till 1970
Determining mass of unknown organic compounds and thus help determine their structures
Gases from waste dump
Trace organic pollutants in water