- A mobile phase sweeps a mixture over a stationary phase:
-Stationary phase is fixed.
-Mobile phase moves in one direction.
- Chromatography works on the basis that different components have different affinities for both the stationary and mobile phase.
-The stationary phase interacts and slows components down.
-Allowing different components to move at different speeds and thus seperate.
- In TLC the stationary phase is a solid, the mobile a liquid.
- In GC the stationary phase is a liquid or solid, and the mobile a gas.
- A solid stationary phase seperates by adsorption, where the component molecules bind with the stationary phase on the surface.
- A liquid stationary phase seperates by relative solubility, some dissolve more in the phase and are thus slowed more than those which are less soluble.
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- The stationary phase is a thin layer of an adsorbent such as silica gel on a TLC plate.
- The mobile phase is whatever solvent is suitable for the components.
- Producing the chromatogram:
-A small sample is dissolved.
-The solvent rises up the TLC plate and they seperate by adsorption.
-This it the chromatogram, from which Rf values can be calculated.
(The spots of components may be coloured, or require a chemical or UV revealing agent)
- Rf values are calculated:
Rf = Distance moved by component / Distance moved by solvent front
-Similar components may have similar Rf values.
-Unknown compounds have no Rf value to reference.
-It's difficult to find a univeral solvent.
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- The stationary phase is a thin layer of solid or liquid coated on an inert solid support, this tubing is 30m long and is called the chromatography column.
- The mobile carrier gas is inert such as He or N is used.
- Producing the chromatogram:
-The mixture is vaporised and inserted.
-Some components when they move through the tubing slow down more than others due to greater solubility or adsorption.
-Each component leaves at a different time and is detected and timed.
- A graph of retention time is produced:
-Different compounds have different retention times.
-Area under each peak represents the amount of the substance.
-Potentially many compounds have similar retention times.
-Not all substances may be detected.
-Unknown compounds have no reference.
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Gas Chromatography - Mass Spectrometry
- GC can seperate compounds but not identify solidly.
- MS can identify but not seperate.
- GC-MS is used by the GC seperating the compound and moving it to the MS, where it detects and produces a mass spectrum for analysis.
- GC-MS is used in:
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Nuclear Magnetic Resonance
- Chemical shift is the place in an NMR spectrum where the nucleus absorbs energy.
- Chemical shift is measured relative to TMS, as it produces one, sharp signal which is set at 0ppm.
- It is chemically unreactive and volatile for easy removal.
- The use of CDCl3 is for a solvent for NMR spectroscopy, when running both C13 and proton spectroscopy; as it produces no signal.
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- This is a C13 shift to show the different delta shifts of C's in differing environments.
- The most important things when analysing a C13 NMR spectra are:
-The number of C environments, shown by number of peaks.
-The types of C environments, shown by delta shifts.
- This can only be practiced through viewing NMR spectra and deducing which peaks represent which environment by analysing delta shift...and then deducing the molecule.
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- Only half decent diagram of C13 spectra I could find, just an example.
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- A proton NMR chemical shift diagram, you will recieve this in an exam.
- NMR spectra read the same way as before but also:
-The relative peak areas gives proportion of H's.
-Spin-spin coupling gives information about adjacent H's.
- Integration traces on a NMR spectra can be used to work out ratio of H's (1:1 etc.)
- The splitting on an NMR spectra is due to spin-spin coupling, used to find the number of protons on adjacent C's.
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Proton NMR (Cont.)
- On a proton NMR spectra there is something called the +1 rule, which means that for every peak there is one more peak than there is a H on adjacent H's.
-Singlet = 1 peak due to no adjacent H's.
-Doublet = 2 peaks due to one adjacent H.
-Triplet = 3 peaks due to 2 adjacent H's
-Quartet = 4 peaks due to 3 adjacent H's.
- Again this requires practice to wholly understand.
- -OH/-NH do not split, they are a singlet and C=O shield any H's from the spectra.
- A D2O shake is used to remove any peaks caused by -OH and -NH, because the H is replaced by D.
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NMR In Medicine
- Is used to relieve pain and combat disease.
- It is used in MRI scanners to produce a view of a patient's insides.
- They are less harmful than x-rays but can't be used by someone with a pacemaker etc.
- It is worthwhile to read up on this a little bit.
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- All I can stress is to practice as much as you can with past paper questions and learn all the techniques to make analysis easier.
- Some of the end questions in papers include analysis that involves GC-MS, infrared and NMR.
- Good luck with your studies.
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