Mass spectroscopy ( Mass spec )

Transcription

1 Mass spectroscopy ( Mass spec ) Topics covered in this module: 1. Mass spectrometry 2. Mass spectrometry in organic chemistry 3. Mass spectroscopy: fragmentation patterns 1 P a g e

2 1. Mass spectrometry Module 2 - Alcohols, halogenoalkanes and analysis Modern analytical techniques Done Outline the early developments and use of mass spectrometry: (i) (ii) in the determination of relative isotopic masses as a method for identifying elements, i.e. for use in the Mars space probe and in monitoring levels of environmental pollution such as lead. Interpret mass spectra of elements in terms of isotopic abundances. Key areas to concentrate on You need to be able to: identify molecular ion peaks and fragment peaks in mass spectra (remember, you do not have to identify every peak in order to discern a compound). Take care with units relative atomic mass doesn t have any! When identifying ions, always remember the + charge e.g. 86 Sr + 2 P a g e

3 Revision notes 3 P a g e

4 2. Mass spectrometry in organic chemistry Module 2 - Alcohols, halogenoalkanes and analysis Modern analytical techniques Done Use the molecular ion peak in a mass spectrum of an organic molecule to determine its molecular mass. Explain that a mass spectrum is essentially a fingerprint for the molecule that can be identified by computer using a spectral database. Key areas to concentrate on You need to be able to: identify molecular ion peaks and fragment peaks in mass spectra (remember, you do not have to identify every peak in order to discern a compound). In the mass spectrum, you may see a small peak, one unit beyond the molecular ion, M +, peak. This is often referred to as the M+1 peak. The M+1 peak arises from the presence of the isotope carbon-13, which makes up 1.11% of all carbon atoms. Don t be fooled into thinking that this is the M + ion. 4 P a g e

5 Past paper questions Rubidium, atomic number 37, was discovered in 1861 by Bunsen and Kirchoff. Rubidium is in Group 1 of the Periodic Table and the element has two natural isotopes, 85 Rb and 87 Rb. (a) Explain the term isotopes. [1] (b) A sample of rubidium was analysed in a mass spectrometer to produce the mass spectrum below percentage relative isotopic mass (i) Use this mass spectrum to help you complete the table below. isotope percentage number of 85 Rb protons neutrons electrons 87 Rb [3] (ii) Calculate the relative atomic mass of this rubidium sample. Give your answer to three significant figures. A r =... [2] [Turn over] 5 P a g e

6 (c) Which isotope is used as the standard against which the masses of the two rubidium isotopes are measured? [1] [Total 7 marks] Examiner s comments (a) (b) (c) Only the weakest candidates failed to score this mark. The commonest incorrect response was that the number of electrons are the same in isotopes ; this is incorrect for ions of isotopes. The majority of candidates correctly calculated the A r value. Fewer candidates than in previous years made an error in significant figures. A common error was the use of the number of neutrons in place of the mass number of each isotope. A surprisingly large number of candidates, including the very able, gave their answer as Rb: P a g e

7 3. Mass spectrometry: fragmentation patterns Module 2 - Alcohols, halogenoalkanes and analysis Modern analytical techniques Done Suggest the identity of the major fragment ions, i.e. m/z = 29 as CH 3 CH 2 +, in a given mass spectrum (limited to alkanes, alkenes and alcohols). Use molecular ion peaks and fragmentation peaks to identify structures (limited to unipositive ions). Key areas to concentrate on You need to be able to: identify molecular ion peaks and fragment peaks in mass spectra (remember, you do not have to identify every peak in order to discern a compound). In examinations feel free to draw or write notes on the spectra provided for analysis. This is good technique and will help you solve problems. 7 P a g e

8 Past paper questions Bromobutane, CH 3 CH 2 CH 2 CH 2 Br, can be reacted with hot aqueous sodium hydroxide to prepare butan-1-ol. CH 3 CH 2 CH 2 CH 2 Br + OH CH 3 CH 2 CH 2 CH 2 OH + Br The butan-1-ol produced can be analysed by mass spectrometry. (i) Predict two fragment ions that you would expect to see in the mass spectrum of butan-1-ol and state the m/z value of each ion. [2] (ii) State a use of mass spectrometry outside of the laboratory. [1] [Total 3 marks] 8 P a g e

9 Revision notes 9 P a g e