8303163-55-Mass-Spectroscopy

8303163-55-Mass-Spectroscopy - Chem Factsheet...

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Organic Analysis II- Mass Spectroscopy Number 55 1 C hem F actsheet www.curriculumpress.co.uk Before reading through this Factsheet you should: Have a good understanding of Atomic Structure (Factsheet 01); Have a basic understanding of the way in which a mass spectrometer works; Have a good knowledge of the Organic Chemistry covered at AS and A2 level; Understand covalent bonding and molecular structure (Factsheets 05 and 06). After working through this Factsheet you will be able to: Interpret simple mass spectra; Recognise mass spectra as useful tools in organic analysis. Before we discuss the role of mass spectra in organic analysis, candidates should remind themselves of the simplified appearance of a mass spectrometer (as shown below) and how the machine works. Fig 1. Mass Spectrometer 5 2345678 12345678 12345678 12345678 12345678 12345678 12345678 12345678 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 123456789012345 + 1. vaporisation 2. ionisation 3. acceleration (by electric field) 4. deflection (by magnetic field) 5. detection 3. To vacuum pump 1. 4. 2. The five stages listed on the above diagram are discussed in more detail in Factsheet 01 – Atomic Structure. Fig 2. Mass spectrometer traces 10 20 30 40 70 80 0 100 0 Mass / Charge Ratio 60 sodium (Na) (100 %) Sodium Iron Mass spectrometer traces (Fig 2) are used to identify atomic masses of any isotopes present and to provide the information to calculate relative atomic mass. Note the axis labels: Relative abundance Mass/Charge ratio (or m/e) The relative abundance is self explanatory the more common a particular mass of particle, the higher the peak. Usually the highest peak ( base peak ) is given a value of 100, with the other peaks scaled accordingly. An understanding of the mass/charge ratio is important. After electron bombardment, the majority of the ions formed have a +1 charge as just one electron is removed. If the ion has a +1 charge then the mass/charge ratio is the same as the mass. e.g. 23 Na + m/e = 23/1 = 23 If a higher number of electrons are removed from a species, a different peak will be generated on the spectrum. e.g. 23 Na 2+ m/e = 23/2 = 11.5 However, it is worth reinforcing that the vast majority of ions formed have a +1 charge. Mass Spectra in Organic Analysis . Mass spectra, like infrared spectra (Factsheet 54 Organic analysis I- Infrared Spectroscopy), are a useful tool in identifying organic molecules. The aim of this Factsheet is to give candidates the necessary experience in interpreting mass spectra to be able to extract the relevant information from them. This information is likely to be used in conjunction with other data to identify chemicals. In exam questions the mass spectrum of an unknown organic compound is often supplied with the IR spectrum and results from chemical tests.
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303163-55-Mass-Spectroscopy - Chem Factsheet...

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