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Mass Spectroscopy - A Level Chemistry

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Published in: Chemistry
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Instrumental aspects of MS Theory of MS Identification of peaks and molecular ions peaks Ratio of heights of peaks Previous year questions related to Msss spectroscopy

Salman F / Doha

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Qualification: Post Graduate in Chemistry, BSc in Chemistry, MS in Chemistry, Bachelors degree in Education

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  1. Mass Spectroscopy 34 wnww.tachyonclasses.com 35. s
  2. HOW DOES A MASS SPECTROMETER WORK? 3. In the presence Of electric field, the +ve ion accelerate. electric field magnetic field 4. +ve charged ion deflected by Mag. Field". Lower the mass of +ve ion; higher is its deflection. A fig A A mass spectrometer. vaporised sample ionisation cham electron gun ion detector Gaseous sample is injected 2. Vaporized sample is bon barded with Electron gun and I-onns a positive ion C k cot 8 o 3S 40 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 mvw.tachyonclasses.com
  3. el 3. Calculate the relative atomic mass of a sample of magnesium that has the following isotopic composition: magnesium-24: 78.6% magnesium-25: 10.1% magnesium-26: 11.3% Give your answer to three significant figures. A sample of copper contains two isotopes Of relative isotopic mass 63.0 and 65.0. If the relative atomic mass of copper is 63.5, calculate the relative abundance of each isotope. 100 mvw.tachyonclasses.com
  4. The mass spectrum of bromine vapour. Br2, is shown in fig E below. Re,.ative intensity fig E 100 80 60 40 20 500 120 158 140 160 162 160 (a) (b) What are the relative isotopic masses of the two isotopes present in bromine? Al Identify the particles responsible for the peaks at m/z 158, 160 and 162. Deduce the relative abundance of the two isotopes and explain the relative heights of the three peaks at m/z 1 58, 160 and 162. mvw.tachyonclasses.com
  5. SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 Mass spectrometry is used to determine the relative atomic masses Of elements. The diagram shows the main features of a mass spectrometer. vaporised sample to vacuum pump detector (a) After vaporisation, the sample passes through three stages before reaching the detector. (i) The first stage is ionisation, which occurs at R. Describe fully the ionisation process for the element nickel, Ni, writing an equation to illustrate it. State symbols are not required. e_, (ii) Describe the processes occurring at S and T. S e Uuå---s-i c, el S-e-4 mvw.tachyonclasses.com
  6. (i) Calculate the relative atomic mass of nickel in this sample. Give your answer to one decimal place. The rnass spectrum Of a sample Of an element has only two peaks. 80 70 60 abundance 20 10 (iii) Explain why the sample needs to be ionised. (b) The tallest peak in a mass Spectrum (called the base peak) is given a height of 100 and the heights Of all the Other peaks are given relative to the base peak. A sample Of the element nickel is analysed in a mass spectrometer and found to have two significant peaks. Relative peak height 92.5 58 60 100 398 191 192 194 What is the approximate relative atomic mass Of the element in this sample? o B D 191S 1921) 193.0 (Total for Question 3 1 mark) mvw.tachyonclasses.com
  7. 10 Mass spectrometers are used in chemical analysis. (a) In which region of this mass spectrometer are the ions accelerated? Q vaporised sample A region p B region Q C region R D region S electromagnet to vacuum pump amplifier chart recorder (b) Four iron ions are present in a stream of ions in a mass spectrometer. The paths of these ions are shown. path 4 stream Of ions path 3 path 1 path 2 Which of these ions will follow path 1 through the mass spectrometer? s-c.f O D 5'Fe•• mvw.tachyonclasses.com
  8. (c) Naturally-occurring chlorine has two isotopes, 35Cl and 3 'Cl. How many peaks due to the singly-charged molecular ions are seen in the mass spectrum of chlorine, CC? eco 16 This question is about the Group 4 element germanium and some of its compounds. (a) Naturallyoccurring germanium consists of five stable isotopes. Explain what is meant by the term isotopes. (b) The mass spectrum of a sample of germanium is shown. 40 30 Percentage abundance 20 10 20.6 36.7 27.4 7.7 76 70 71 72 73 74 75 76 77 Calculate the relative atomic mass Of this sample Of germanium. 72.7 (c) Give the electronic configuration of a germanium atom.
  9. (ii) Describe the formation Of the molecular ion Of bromine in the mass spectrometer. Include an equation. State symbols are not required. On the mass spectrum grid, draw the peaks for the bromine molecular ions, showing the relative peak heights. The bromine isotopes in this sample have the same relative abundance. 90 abundance 30 156 157 158 159 160 161 162 163 164 2 Iodine exists as one isotope with mass number 127. Chlorine exists as two isotopes with mass numbers 35 and 37. How many molecular ion (ICC) peaks are there in the mass spectrum of ICI,? AS (Total for Question 2 = 1 ma mvw.tachyonclasses.com
  10. 3 The diagram shows a mass spectrometer. to Vacuum pump (a) Which process occurs in region R? A the sample is vaporised using a heater B electrons are removed from molecules Or atoms and— are formed C] C electrons are added to the molecules or atoms and negative ions C] D ions are accelerated by an electric field (b) Which statement is correct for region T? A ions With a greater mass have a smaller deflection B ions with a greater mass have a greater deflection C ions With a greater charge have a smaller deflection D ions are speeded up by a magnetic field 13 This question is about silicon and carbon. (a) Silicon is a semiconductcr. (i) Data obtained using the mass spectrum Of silicon are shown. = it.-f Isotope mass number Relative abundance 92.17 4.71 3.12 Calculate the relative atomic mass Of silicon to two decimal places. 28.11 (ii) Suggest a reason why there is a small peak in the mass spectrum of silicon atm/z=14

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