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Boardworks AS Chemistry Analytical Chemistry

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1 Boardworks AS Chemistry Analytical Chemistry

2 Boardworks AS Chemistry Analytical Chemistry

3 Detecting elements and compounds
Boardworks AS Chemistry Analytical Chemistry Mass spectrometry is a highly sensitive technique that can be used to help identify compounds. Samples of only a few milligrams are required. Mass spectrometry is useful in the identification of unknown materials, because the mass spectrum of an unknown sample can be compared with a database of known spectra. This makes it useful in forensic science. Spectra of samples could be compared, for example, to link a suspect to a weapon that has been fired. Photo credit: James King-Holmes / Science Photo Library Researcher working on an accelerator mass spectrometer (AMS) used for carbon dating. All living material incorporates a radioactive isotope of carbon, carbon-14 (14C), and a stable isotope, carbon-12 (12C), into its tissue at a known ratio. When the tissue dies the amount of 12C remains constant, but 14C decays. Measuring the amount of 14C compared to 12C in a sample indicates how long ago the tissue died. The AMS converts atoms from a sample into a beam of ions. It then measures the mass of the ions by the application of electric and magnetic fields. Photographed at Oxford Radiocarbon Accelerator Unit, University of Oxford, UK. Teacher notes See the ‘Atomic Structure’ presentation for more information about how mass spectrometry works. Mass spectrometry may also be combined with gas chromatography or high performance liquid chromatography, allowing the identification of the different compounds present in a mixture.

4 Mass spectroscopy in space
Boardworks AS Chemistry Analytical Chemistry Mass spectrometers have been included onboard several space probes. In 2005, the Huygens probe landed on the surface of Titan, the largest moon of Saturn. During the descent towards the surface of Titan, the mass spectrometer analyzed the atmospheric composition at various heights. It was found that the atmosphere consists mainly of nitrogen and methane. The presence of argon-40 was also detected. Photo credit: NASA/JPL/Space Science Institute

5 Boardworks AS Chemistry Analytical Chemistry
Molecular ions Boardworks AS Chemistry Analytical Chemistry When a compound is analyzed in a mass spectrometer, gaseous molecules are bombarded with high-speed electrons from an electron gun. These knock out an electron from some of the molecules, creating molecular ions, which travel to the detector plates: M(g) + e-  M+(g) + 2e- Teacher notes See the ‘Atomic Structure’ presentation for more information about how a mass spectrometer works. Molecular ions (M+) are sometimes represented with a dot under the plus sign, indicating that the ion contains a single unpaired electron. The peak with the highest mass-to-charge ratio (m/z) is formed by the molecular ion, and the value of m/z is equal to the relative molecular mass of the compound.

6 Molecular ions and peaks
Boardworks AS Chemistry Analytical Chemistry The peak at the highest m/z on the mass spectrum is formed by the heaviest ion that passes through the spectrometer. Unless all molecules of the original substance break up, this corresponds to the molecular ion of the sample substance. 100 molecular ion peak mass spectrum of paracetamol 80 60 abundance (%) 40 20 m/z 40 80 120 160

7 High resolution mass spectroscopy
Boardworks AS Chemistry Analytical Chemistry High resolution mass spectrometry can determine the m/z of a peak to several decimal places. This can distinguish between compounds that have very similar relative molecular masses (Mr) but different empirical formulae. To integer values, hexane (C6H14) and pentanal (C5H10O) both have a Mr of 86. However, using atomic masses to four decimal places, the Mr of hexane is , while the Mr of pentanal is Photo credit: © 2009 Jupiterimages Corporation Hexane is a component of gasoline. Teacher notes See the ‘Moles and Formulae’ presentation for more information about relative molecular mass and empirical formulae. With a high enough resolution, it is therefore possible to distinguish between hexane and pentanal, which are both colourless liquids.

8 Determining molecular formulae
Boardworks AS Chemistry Analytical Chemistry

9 Boardworks AS Chemistry Analytical Chemistry
What is fragmentation? Boardworks AS Chemistry Analytical Chemistry Teacher notes When fragmentation occurs, one fragment retains the positive charge and the remainder of the molecular ion becomes a radical. For example: [CH3CH2CHO]+●  [CH3]+ + [CH2CHO]● Sometimes on a mass spectrum, very small peals appear at m/z values half of those expected. These are caused by M2+ (m/z = Mr/2 in these cases) ions being picked up by the detector.

10 Interpreting mass spectra
Boardworks AS Chemistry Analytical Chemistry

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Mass spectroscopy Boardworks AS Chemistry Analytical Chemistry

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13 IR energy and molecular vibrations
Boardworks AS Chemistry Analytical Chemistry

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IR energy and dipoles Boardworks AS Chemistry Analytical Chemistry

15 The infrared spectrometer
Boardworks AS Chemistry Analytical Chemistry

16 Interpreting infrared spectra
Boardworks AS Chemistry Analytical Chemistry An infrared spectrum is a plot of transmission of infrared radiation against wavenumber (1 / wavelength). Any wavelength that is absorbed by the sample will transmit less than the others, forming a dip or ‘peak’ in the graph. For instance, C–H bonds absorb radiation at a wavenumber of 2950 cm-1, which produces a peak in an infrared spectrum as shown on the right. 100 80 transmission (%) 60 40 20 3000 2000 1000 wavenumber (cm-1)

17 IR radiation and greenhouse gases
Boardworks AS Chemistry Analytical Chemistry Nitrogen and oxygen, the most abundant gases in the atmosphere, do not absorb infrared energy. This is because the vibrations caused would not change the dipole of the molecules. However, methane, water and carbon dioxide do absorb strongly in the infrared region. These gases absorb infrared radiation in the atmosphere, stopping it escaping into space: they are greenhouse gases. Photo credit: © Shutterstock 2009, thumb Teacher notes See the ‘Green Chemistry’ presentation for more information about greenhouse gases.

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Greenhouse gases Boardworks AS Chemistry Analytical Chemistry

19 Using infrared spectroscopy
Boardworks AS Chemistry Analytical Chemistry When breathalyzed by police, motorists blow into a handheld device, which gives an indication of the amount of alcohol in their breath. However, the result is not accurate enough to be used as evidence. At the police station, the motorist blows into a more accurate breathalyzer, containing an IR spectrometer. The breathalyzer calculates the percentage of alcohol in the breath by looking at the size of the absorption due to the C–H bond stretch in the alcohol. Photo credit: Jim Varney / Science Photo Library

20 Infrared spectroscopy: summary
Boardworks AS Chemistry Analytical Chemistry

21 Boardworks AS Chemistry Analytical Chemistry

22 Identifying C–H and C–C bonds
Boardworks AS Chemistry Analytical Chemistry Most organic compounds contain C–H and C–C bonds. These are therefore often visible in an infrared spectrum. In the spectrum of an alkane, such as butane shown here, there is a peak at 2950 cm-1 (due to C–H) and a peak below 1500 cm-1 (due to C–C). The peak due to C–H is the stronger of the two because there are many more C–H bonds than C–C bonds in the compound. 100 80 C–C 60 transmission (%) 40 C–H 20 3000 2000 1000 wavenumber (cm-1)

23 Using the fingerprint region
Boardworks AS Chemistry Analytical Chemistry The region below 1500 cm-1, usually called the fingerprint region, has many peaks that are difficult to assign. The pattern of these peaks is unique to a particular compound. fingerprint region A substance may be identified by comparing the IR spectrum to a database of reference spectra. 100 80 60 transmission (%) 40 20 An exact match in the fingerprint region identifies a compound. 3000 2000 1000 wavenumber (cm-1)

24 Identifying chemical groups
Boardworks AS Chemistry Analytical Chemistry Teacher notes Students could be asked to name the functional groups of each group of compounds before being shown the information on that group’s IR spectrum. See the ‘Introducing Organic Chemistry’ presentation for more information on the general structure of each of these chemical groups.

25 Spotting characteristic bonds
Boardworks AS Chemistry Analytical Chemistry

26 Interpreting IR spectra
Boardworks AS Chemistry Analytical Chemistry

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Glossary Boardworks AS Chemistry Analytical Chemistry Teacher notes bend – A type of molecular vibration during which two bonds vibrate toward and away from each other, changing the angle between them. fingerprint region – The region below 1500 cm-1 in an infrared spectrum, where there are many peaks which are difficult to assign. An exact match of the fingerprint region with a reference spectrum gives the identity of a compound. fragmentation – A phenomenon whereby some ions disintegrate into smaller fragments during mass spectrometry, leading to multiple peaks in the resulting spectrum. greenhouse gas – An atmospheric gas that absorbs infrared energy, such as carbon dioxide or water. The presence of these gases in the atmosphere is thought to lead to the greenhouse effect. high resolution mass spectrometry – A technique that can be used to determine the mass of a molecule or atom to several decimal places. infrared spectroscopy – A technique that measures the absorption of different frequencies of infrared energy by a sample of a substance. mass spectrometry – An accurate instrumental technique that can be used to determine the relative isotopic mass and the relative abundance for each isotope of an element in a sample. This can be used to identify the sample. mass-to-charge ratio – The mass divided by the charge of an ion. Often called the m/z value. molecular ion – The ion formed when a molecule loses an electron during mass spectrometry but does not fragment. Represented by the symbol M+. molecular ion peak – The peak on the mass spectrum of a compound at the highest mass-to-charge ratio, which corresponds to the molecular ion. molecular vibration – A change in the structure of a molecule caused when one or more bonds in the molecule absorb infrared radiation. Sometimes called bond vibration. Two simple types are stretch and bend. stretch – A type of molecular vibration during which an atom vibrates backwards and forwards along the axis of a bond. wavenumber – The number of cycles of waves in a unit length; the reciprocal of wavelength (1 / wavelength). Usually measured in m-1 or cm-1.

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What’s the keyword? Boardworks AS Chemistry Analytical Chemistry

30 Boardworks AS Chemistry Analytical Chemistry
Multiple-choice quiz Boardworks AS Chemistry Analytical Chemistry


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