1. THE MASS SPECTROMETER WHAT IS A MASS SPECTROMETER
An instrument to accurately determine the relative atomic mass
Relative atomic mass (Ar) is the average mass of atoms of an element relative to an atom of carbon – 12
Separates atoms or molecules according to their charge and mass.
This can be used to identify substances
 e.g. illegal drugs

2. THE MASS SPECTROMETER THE LAYOUT 5 key stages: Vaporisation Ionisation
Acceleration
Deflection
Detection
---- = heavy ions
---- = ions reaching detector
---- = light ions

3. THE MASS SPECTROMETER SUMMARISING WHAT HAPPENS:
1. Ionisation: Atoms are converted to ions
2. Acceleration: Ions are accelerated
3. Deflected: Deflected according to their mass & charge
4. Detection: They arrive at a detector
CONDITIONS:
Vacuum  so ions do not collide with air molecules (might stop them reaching the detector)
Gaseous State  solids are vaporised before being injected

4. THE MASS SPECTROMETER LOOK IN MORE DETAIL: Stage 1: Ionisation
Beam of electrons knocks electrons from atoms or molecules in the sample.
This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example).
Nearly all lose just one electron (~5% will lose two)
Mass spectrometers always work with positive ions!!

5. THE MASS SPECTROMETER Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection
The ions are then deflected by a magnetic field according to the ratio of their mass to charge (m/z), where z is the charge (usually +1)
Heavier ions are deflected less than light ones
2+ ions are deflected twice as much as 1+ ions

6. THE MASS SPECTROMETER Stage 4: Detection
Magnetic field is gradually increased  increases deflection
This allows ions of increasing mass to enter the detector
On striking the detector ions accept electrons, lose their charge and create a current
Current created is proportional to the abundance of each ion

7. Mass spectra of zirconium
THE MASS SPECTROMETER
MASS SPECTRA OF ELEMENTS
From the strength of the magnetic field at which a particular ion hits the detector the value of the mass to charge ratio (called m/z) is calculated
A graph is produced
(mass spectra) showing the relative abundances of each ion type
Mass spectra of zirconium

8. THE MASS SPECTROMETER MASS SPECTRA OF ELEMENTS
We can use the mass spectrometer to identify the different isotopes making up an element
Each isotope is detected separately because they have different masses
To calculate an elements relative atomic mass (which is given in the periodic table) you must take account of the relative abundances of each isotope
Zirconium has 5 isotopes!

9. THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS
This is the mass spectra for chlorine
We have 2 isotopes with relative isotopic masses of 35 and 37, detected in a ratio of 3:1
(or 75%:25%)
To calculate the relative atomic mass of chlorine:
(35 x 75) + (37 x 25)
100
= 35.5
Check the Ar of Chlorine in your periodic table

10. THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS RAM of Cl = 35.5
Notice there is no line at 35.5 on the
mass spectra. No atoms of Cl actually
have this mass. It is the average of all
the isotopes and their abundances!
STEPS:
Multiply the m/z value by the relative abundance % for each peak
Add results for each peak together
Divide by total relative abundance

11. THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS
Calculate the RAM of the element from its mass spectra:
Boron
Zirconium
100
51.5 23
17.1
17.4
11.2
2.8
Most abundant assigned 100
Use the percentage detected OR

12. THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS
(10 x 23) + (100 x 11) 123
RAM = 10.8
Boron
100 23

13. THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS RAM = 91.3
(90 x 51.5) + (91 x 11.2) + (92 x 17.1) + (94 x 17.4) + (96 x 2.8) 100
RAM = 91.3
51.5
17.1
17.4
11.2
2.8

14. THE MASS SPECTROMETER DOUBLY CHARGED IONS:
It’s not always as simple as we’ve just made it
During ionisation a small number of ions with a 2+ charge are created
These are deflected more than singly charged ions (behave as if they have half the mass)
A closer look Cl shows peaks at m/z 17.5 and 18.5.
Because double ionisation is rare these peaks have very small abundances

15. THE MASS SPECTROMETER CHLORINE GETS TRICKIER…
Chlorine can form a molecule  Cl2
This can also be ionised, creating molecular ions
Its spectra will therefore show 3 other peaks:
- At m/z 70, due to 35Cl 35Cl
- At m/z 72, due to 35Cl 37Cl
- At m/z 74, due to 37Cl 37Cl

16. THE MASS SPECTROMETER SPECTROMETERS IN SPACE
Space probes will carry spectrometers to identify elements in rocks
E.g. Giotto Space Probe passing close to Halley’s comet
Remember: we need a vacuum for a mass spectrometer to work. This is easy for a space probe as space is a vacuum!!

17. THE MASS SPECTROMETER DATA FOR HALLEY’S COMET
Greenburg J. M (2002). Cosmic Dust and Our Origins. Surface Science. 500: