OCR - Analysis

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NMR spectroscopy

· A substance is put into a strong magnetic field and low-energy radio waves are shot at it.

· Nuclei in different environments absorb different amounts of energy.

· We anaylse the results to find out what it is!

TMS is used as the standard to compare other results agaisnt.

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· This tells you about the different environments the carbons in the molecule are in.

· The number of peaks is the different number of carbon environments.

· You can look up the chemical shifts on your data sheet.

· The numbers above the peaks give the ratio.

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· This tells you the number of H+ environments.

· You have to use a deutrerated solvent like CDCl3.

· You can look up the different chemical shifts on your data sheet.

· The area under each peak gives the ratio.

· The spin-spin coupling creates split peaks; these can be singlets, doublets, triplets, quartets...

· You take one away from the number of splits to get the number of neighbouring protons (n+1 rule).

· If the mixture is then shaken with D2O and a singlet disappears, this is an OH group or a NH group.

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· This always consists of a mobile phase and a stationary phase.

· The attraction between the substance and the surface of the stationary phase is called adsorption.

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Thin-layer chromotography

· The stationary phase is silica gel or alumina powder on a glass or plastic plate.

· The mobile phase is a solvent e.g. ethanol.

· Rf= distance moved by spot/distance moved by solvent.

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Gas chromotography

· The mobile phase is an unreactive carrier gas such as helium.

· The stationary phase is a viscous liquid or solid coating the tube.

· The area under the peak is the volume of the substances.

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Gas chromotography - mass spectroscopy

· Gas chromatography is good for separating a mixture into its individual components, but not well at identifying the components.

· Mass spectroscopy is very good for identifying unknown substances, but not good when there’s more than one.

· So some bright spark decided to combine them!

· The mixture is passed through the gas chromatographer to separate the mixture then automatically into the mass spectrometer; this is called a CG-MS machine.

· These are used in forensics and at security in airports.

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You may want to go back over mass spectroscopy and infared spectroscopy from AS, they may well ask about them!

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