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