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1 of 30© Boardworks Ltd 2009. 2 of 30© Boardworks Ltd 2007.

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Presentation on theme: "1 of 30© Boardworks Ltd 2009. 2 of 30© Boardworks Ltd 2007."— Presentation transcript:

1 1 of 30© Boardworks Ltd 2009

2 2 of 30© Boardworks Ltd 2007

3 3 of 30© Boardworks Ltd 2009 Detecting elements and compounds Mass spectrometry is a highly sensitive technique that can be used to help identify compounds. Samples of only a few milligrams are required. 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. 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.

4 4 of 30© Boardworks Ltd 2009 Mass spectroscopy in space Mass spectrometers have been included onboard several space probes. 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. In 2005, the Huygens probe landed on the surface of Titan, the largest moon of Saturn.

5 5 of 30© Boardworks Ltd 2009 Molecular ions When a compound is analyzed in a mass spectrometer, gaseous molecules are bombarded with high-speed electrons from an electron gun. M (g) + e -  M + (g) + 2e - 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. These knock out an electron from some of the molecules, creating molecular ions, which travel to the detector plates:

6 6 of 30© Boardworks Ltd 2009 Molecular ions and peaks 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. molecular ion peak mass spectrum of paracetamol abundance (%) m/z

7 7 of 30© Boardworks Ltd 2009 High resolution mass spectroscopy 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 (M r ) but different empirical formulae. To integer values, hexane (C 6 H 14 ) and pentanal (C 5 H 10 O) both have a M r of 86. However, using atomic masses to four decimal places, the M r of hexane is , while the M r of pentanal is With a high enough resolution, it is therefore possible to distinguish between hexane and pentanal, which are both colourless liquids.

8 8 of 30© Boardworks Ltd 2009 Determining molecular formulae

9 9 of 30© Boardworks Ltd 2009 What is fragmentation?

10 10 of 30© Boardworks Ltd 2009 Interpreting mass spectra

11 11 of 30© Boardworks Ltd 2009 Mass spectroscopy

12 12 of 30© Boardworks Ltd 2007

13 13 of 30© Boardworks Ltd 2009 IR energy and molecular vibrations

14 14 of 30© Boardworks Ltd 2009 IR energy and dipoles

15 15 of 30© Boardworks Ltd 2009 The infrared spectrometer

16 16 of 30© Boardworks Ltd 2009 Interpreting infrared spectra 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 wavenumber (cm -1 ) transmission (%) 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.

17 17 of 30© Boardworks Ltd 2009 IR radiation and greenhouse gases 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. These gases absorb infrared radiation in the atmosphere, stopping it escaping into space: they are greenhouse gases. However, methane, water and carbon dioxide do absorb strongly in the infrared region.

18 18 of 30© Boardworks Ltd 2009 Greenhouse gases

19 19 of 30© Boardworks Ltd 2009 Using infrared spectroscopy 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.

20 20 of 30© Boardworks Ltd 2009 Infrared spectroscopy: summary

21 21 of 30© Boardworks Ltd 2007

22 22 of 30© Boardworks Ltd 2009 Identifying C–H and C–C bonds Most organic compounds contain C–H and C–C bonds. These are therefore often visible in an infrared spectrum. C–H wavenumber (cm -1 ) transmission (%) C–C 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.

23 23 of 30© Boardworks Ltd 2009 Using the fingerprint region 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. A substance may be identified by comparing the IR spectrum to a database of reference spectra. An exact match in the fingerprint region identifies a compound wavenumber (cm -1 ) transmission (%) fingerprint region

24 24 of 30© Boardworks Ltd 2009 Identifying chemical groups

25 25 of 30© Boardworks Ltd 2009 Spotting characteristic bonds

26 26 of 30© Boardworks Ltd 2009 Interpreting IR spectra

27 27 of 30© Boardworks Ltd 2007

28 28 of 30© Boardworks Ltd 2009 Glossary

29 29 of 30© Boardworks Ltd 2009 What’s the keyword?

30 30 of 30© Boardworks Ltd 2009 Multiple-choice quiz


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