1. 2.2 The Mass Spectrometer

2. Assessment Objectives 2.2.1 Describe and explain the operation of a mass spectrometer. 2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12C scale. 2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.

3. Reference Textbook on Moodle (Topic 2 Atomic Structure) Powerpoint presentation on Moodle Workbook

4. Key terms Mass Spectrometer Vapourise Ionisation Isotope

5. Starter Work out the number of protons, neutrons and electrons: 23 Na 11 56 Fe 26 119 Sn 50 19 F 9 27 Al 13

7. Starter Write the symbols for the following elements. Hydrogen Lithium Sulphur Sodium Potasium Write the names of the following symbols. O He B Cl

8. Assessment Objective 2.2.1 Describe and explain the operation of a mass spectrometer.

9. The Mass Spectrometer An instrument that separates particles according to their masses and records the relative proportions of each. Powerpoint presentation of functioning of the mass spectrometer.

10. How it Works………… 1. The material is VAPOURISED (turned into a gas) by heating (if already a gas then used at low pressure). 2. The gas is then turned from a neutral atoms to positive ions. This is done by passing the gas through a stream of electrons between two horizontal plates. These electrons collides with electrons on the atoms and knocks them off. X + e -  X + + 2e - The Particles become IONISED (charged) 3. These charged particles are then ACCELERATED between two vertical plates with high electrical potential. A narrow stream of these particles are made by passing them through a hole at the end plate (Note: this is done in a vacuum to stop particles colliding with each other).

11. 4. The ionised particles are the passed through a magnetic field which alters their direction according to their mass. The lighter the particle the more it will be deflected. 5. By increasing the magnetic field strength the particles of certain weights can be brought to FOCUS of the detector and measured. 6. The detector measures the strength of the current produced when these charged particles collide with it. The more particles the higher the current.

12. Summary Vapourised sample. Ionisation with electron bombardment Positive ions accelerated be electrical field Ions deflected by a magnetic field Detector records ions with particular masses Vacumm prevents ions colliding.

13. Assessment Objectives 2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12C scale.

14. “Weighing” atoms: The mass spectrometer + + + + + + + + Atoms are given a + charge These charged atoms (ions) are accelerated past a magnet The deflected ions are detected on a screen or film. The magnet deflects this ion. The lighter the ion the more it is deflected + + + + + + + + + The more mass the atom has, the more nearly it travels in a straight line.

15. Relative Atomic Mass 1 The deflection in the mass spectrometer varies with the mass of the atom. However, this does not tell us the mass in grams. relative atomic mass (RAM) What it tells us is the relative masses of atoms – or relative atomic mass (RAM) Carbon is given a RAM value of 12 The element carbon is the atom against which the mass of all other atoms are compared. Carbon is given a RAM value of 12. Relative atomic mass = 12 C

16. Relative Atomic Mass 2 The lightest atom is hydrogen. It has one twelfth the mass of carbon and so has a RAM of 1. Magnesium is twice as heavy as carbon. It has a RAM of 24. Mg C C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H C 24 x 1 = 12 x 2 1 x 12 = 12 x 1

17. Relative Atomic Mass The Table shows the mass of various atoms relative to carbon. Calculate their relative atomic mass. ElementSymbolTimes as heavy as carbonR.A.M HeliumHeone third BerylliumBethree quarters MolybdenumMoeight KryptonKrseven OxygenOone and one third SilverAgnine CalciumCathree and one third 4 9 96 84 16 108 40 Activity

18. How does it Work??? m/Z % 242526 10 0 75 50 25 100 12.814.4 The isotope with the highest abundant is given a value of 100. 24 Mg = 100 x 100/127.2 = 78.6% 25 Mg = 100 x 12.8/127.2 = 10.0% 26 Mg = 100 x 14.4/127.2 = 11.3% The relative atomic mass of magnesium = (24 x 0.786) + (25 x 0.10) + (26 x 0.113) = 24.3

19. Try These…………. The element nitrogen has two isotopes with masses of 14 (99.5%) and 15 (0.5%). Calculate the average atomic mass of nitrogen. (show your work) The element chlorine has two isotopes with masses of 35 (75.5%) and 37 (24.5%). Calculate the average atomic mass of chlorine. (show your work)

20. Germanium (atomic number 32) contains 20% germanium-70, 27% germanium-71, 8% geramium- 72, 37% germanium-73 and 8% germanium-74. Draw a graph of the mass spectrum that you would expect germanium to produce. If an atom of germanium-70 lost two electrons to become a doubly charged ion, at what m/z would it appear?

21. Assessment Objectives 2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.

22. Plenary: Exam Question Silicon has three stable isotopes, 28Si, 29Si and 30Si. The heaviest isotope, 30Si, has a percentage abundance of 3.1 %. Calculate the percentage abundance of the lightest isotope to one decimal place. (Total 2 marks)

23. Plenary: Exam Question Answer 2809 = 3.10 × 30 + 28x + 29(96.9 – x); % 28Si = (93 + 2810.1 – 2809) = 94.1 %;2 Award [2] for correct final answer.

24. Homework Learn the symbols for the following elements. Bromine (Br) Chromium (Cr) Manganese (Mn) Iron (Fe) Cobalt (Co) Nickel (Ni) Copper (Cu) Zinc (Zn)