1. KNOCKHARDY PUBLISHING
MASS SPECTROMETRY
A guide for A level students
2008 SPECIFICATIONS
KNOCKHARDY PUBLISHING

2. KNOCKHARDY PUBLISHING
MASS SPECTROMETRY
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
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3. MASS SPECTROMETRY CONTENTS Prior knowledge Background information
The basic parts of a mass spectrometer
The four stages of obtaining a spectrum
How different ions are deflected
Calculating molecular masses using mass spectra
Example questions
Test questions
Other uses of mass spectrometry
Check list

4. Before you start it would be helpful to…
MASS SPECTROMETRY
Before you start it would be helpful to…
know that atoms are made up of protons, neutrons and electrons
know that like charges repel

5. MASS SPECTROMETRY
The first mass spectrometer was built in 1918 by Francis W Aston, a student of J J Thomson, the man who discovered the electron. Aston used the instrument to show that there were different forms of the same element. We now call these isotopes.
In a mass spectrometer, particles are turned into positive ions, accelerated and then deflected by an electric or magnetic field. The resulting path of ions depends on their ‘mass to charge’ ratio (m/z).
Particles with a large m/z value are deflected least
those with a low m/z value are deflected most.
The results produce a mass spectrum which portrays the different ions in order of their m/z value.
Francis Aston
USES
Mass spectrometry was initially used to show the identity of isotopes.
It is now used to calculate molecular masses and characterise new compounds

6. PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTED
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTED

7. HOW DOES IT WORK? IONISATION
DETECTOR
ION SOURCE
ANALYSER
IONISATION
gaseous atoms are bombarded by electrons from an electron gun and are IONISED
sufficient energy is given to form ions of 1+ charge

8. HOW DOES IT WORK? IONISATION
DETECTOR
ION SOURCE
ANALYSER
IONISATION
gaseous atoms are bombarded by electrons from an electron gun and are IONISED
sufficient energy is given to form ions of 1+ charge
ACCELERATION
ions are charged so can be ACCELERATED by an electric field

9. HOW DOES IT WORK? IONISATION
DETECTOR
ION SOURCE
ANALYSER
IONISATION
gaseous atoms are bombarded by electrons from an electron gun and are IONISED
sufficient energy is given to form ions of 1+ charge
ACCELERATION
ions are charged so can be ACCELERATED by an electric field
DEFLECTION
charged particles will be DEFLECTED by a magnetic or electric field

10. HOW DOES IT WORK? IONISATION
DETECTOR
ION SOURCE
ANALYSER
IONISATION
gaseous atoms are bombarded by electrons from an electron gun and are IONISED
sufficient energy is given to form ions of 1+ charge
ACCELERATION
ions are charged so can be ACCELERATED by an electric field
DEFLECTION
charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
by electric or photographic methods

11. HOW DOES IT WORK? IONISATION
DETECTOR
ION SOURCE
ANALYSER
IONISATION
gaseous atoms are bombarded by electrons from an electron gun and are IONISED
sufficient energy is given to form ions of 1+ charge
ACCELERATION
ions are charged so can be ACCELERATED by an electric field
DEFLECTION
charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
by electric or photographic methods

12. HOW DOES IT WORK? - Deflection HEAVIER ISOTOPES ARE DEFLECTED LESS
20Ne
21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS
the radius of the path depends on the value of the mass/charge ratio (m/z)
ions of heavier isotopes have larger m/z values so follow a larger radius curve
as most ions are 1+charged, the amount of separation depends on their mass

13. HOW DOES IT WORK? - Deflection
20Ne
m/z values
ABUNDANCE
1+ ions
2+ ions
20Ne
22Ne
Doubling the charge, halves the m/z value
Abundance stays the same
21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS
the radius of the path depends on the value of the mass/charge ratio (m/z)
ions of heavier isotopes have larger m/z values so follow a larger radius curve
as most ions are 1+charged, the amount of separation depends on their mass
if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

14. WHAT IS A MASS SPECTRUM? 20Ne 90.92% MASS SPECTRUM OF NEON 21Ne 0.26%
20Ne %
21Ne 0.26%
22Ne 8.82%
MASS SPECTRUM OF NEON
In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes Ne, 21Ne and 22Ne.
positions of the peaks gives atomic mass
peak intensity gives the relative abundance
highest abundance is scaled to 100% and other values are adjusted accordingly

15. EXAMPLE CALCULATION (1)
Calculate the average relative atomic mass of neon using data on the previous page.
Out of every 100 atoms are 20Ne , are 21Ne and are 22Ne
Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) = Ans. =
100
TIP In calculations of this type... multiply each relative mass by its abundance
add up the total of these values
divide the result by the sum of the abundances
* if the question is based on percentage abundance, divide by 100 but if
it is based on heights of lines in a mass spectrum, add up the heights
of the lines and then divide by that number (see later).

16. EXAMPLE CALCULATION (2)
Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is 39.1.
Assume that there are x nuclei of 39K in every 100; there will then be (100-x) of 41K.
so x (100-x) = therefore x x = 3910
100
thus x = so x = 95 Ans % 39K and 5% 41K

17. TEST QUESTION
A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions.
Redraw the diagram with the most abundant isotope scaled up to 100%.
Calculate the average relative atomic mass.
What would be a) the m/z values and b) the abundance if 2+ ions had been formed?
100%
60%
40%
ABUNDANCE
m/z values
ANSWERS ON NEXT PAGE

18. TEST QUESTION
A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions.
Redraw the diagram with the most abundant isotope scaled up to 100%.
Calculate the average relative atomic mass.
What would be a) the m/z values and b) the abundance if 2+ ions had been formed?
100%
60%
40%
ABUNDANCE
m/z values
The new values are 100 and 66.7 (see diagram)
New scale atoms of mass 38; abundance = 100
atoms of mass 40; abundance = 66.7
Average = (100 x 38) + (66.7 x 40) =
166.7
By doubling the charge to 2+, m/z value is halved; new peaks at 19 and 20. The abundance is the same.
100%
100%
66.7%
ABUNDANCE
m/z values

19. OTHER USES OF MASS SPECTROMETRY - MOLECULAR MASS DETERMINATION -
Nowadays, mass spectrometry is used to identify unknown or new compounds.
When a molecule is ionised it forms a MOLECULAR ION which can also undergo FRAGMENTATION or RE-ARRANGEMENT to produce particles of smaller mass.
Only particles with a positive charge will be deflected and detected.
The resulting spectrum has many peaks.
The final peak (M+) shows the molecular ion (highest m/z value) and indicates the molecular mass. The rest of the spectrum provides information about the structure.
IONISATION
MOLECULAR ION
FRAGMENTION
RE-ARRANGEMENT
FRAGMENTION

20. MASS SPECTRUM OF C2H5Br
The final peak in a mass spectrum is due to the molecular ion. In this case there are two because Br has two main isotopes. As each is of equal abundance, the peaks are the same size.
molecular ion contains...79Br Br

21. IDENTIFY THE PEAKS

22. IDENTIFY THE PEAKS

23. IDENTIFY THE PEAKS

24. OTHER USES OF MASS SPECTROMETRY
- SPACE EXPLORATION -
Mass spectrometry is used on space probes to identify elements and compounds on the surface of planets.
In August and September 1975 the USA launched Viking 1 and 2.
Both probes entered Mars orbit to map the planet, dropping landers that transmitted pictures, acted as weather and scientific stations and analysed the Martian soil.
VIKING LANDER
THE MARTIAN SURFACE

25. What should you be able to do?
REVISION CHECK
What should you be able to do?
Recall the four basic stages in obtaining a mass spectrum
Understand what happens during each of the above four stages
Understand why particles need to be in the form of ions
Recall the the meaning of mass to charge ratio (m/z)
Explain how the mass/charge value affects the path of a deflected ion
Interpret a simple mass spectrum and calculate the average atomic mass
Understand how mass spectrometry can be used to calculate molecular mass
Recall that mass spectrometry can be used to in space exploration
Recall other uses of mass spectrometry
CAN YOU DO ALL OF THESE? YES NO

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27. Try some past paper questions
WELL DONE!
Try some past paper questions

28. © 2008 KNOCKHARDY PUBLISHING
MASS SPECTROMETRY
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© 2008 KNOCKHARDY PUBLISHING