3.16 Chromatography
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What is chromatography?
An analytical technique used to separate and identify components of molecules in a mixture
What are the two phases of chromatography?
Mobile phase Stationary phase
What is the mobile phase?
It allows molecules to move over or through the stationary phase The more soluble products move further with the mobile phase (greater affinity for the mobile phase)
What is the stationary phase?
It retains the sample The greater the affinity to the stationary phase the shorter the distance traveled with the mobile phase
What are Rf values?
Retention factor Used for comparison
Calculation of Rf values?
Distance moved by sample Divided by Distance moved by solvent Need to measure from the centre of the spots! 2dp and no units
What are the types of chromatography?
Thin layer chromatography Column chromatography Gas chromatography GC-MS chromatography
What happens in thin layer chromatography?
A metal plate is coated with a thin layer of silica and the solvent moves up the plate The plate is dried in a fume cupboard to remove toxic fumes The plate is viewed under UV light or a developing agent such as iodine or ninhydrin is added so the spots can be seen by the naked eye
What must be specifically done in thin layer chromatography for accuracy?
A pencil line to start that doesn’t run into the solvent or disrupt the silica A capillary tube to apply a small amount of sample Use a lid to seal the chamber - to stop the solvent evaporating Don’t touch the plate as you can contaminate it Solvent shouldn’t reach the top
What is significant about the solvent in TLC?
Different samples require different solvents based on it’s polarity/solubility
What does column chromatography use?
A vertical column is packed with a solid powdered substance - stationary phase A solvent containing the mixture being analysed is added moving down the column as the mobile phase
What do we measure in column chromatography?
The time taken to drain out of the column called Retention Time The molecules will have varying affinities to the phases meaning they have different retention times
What does Gas chromatography use?
A thin tube is packed with a solid, powdered substance - stationary phase A high pressure gas passed through the tube - mobile phase
What is gas chromatography used for?
To separate mixtures of volatile liquids - presented as vapours through the gas chromatography machine
What does gas chromatography record?
Retention time
What is GC-MS?
Gas chromatography and Mass spectrometry The molecules are first separated in chromatography and then accurately identified in mass spectrometry
What is the advantage of GC-MS?
It is faster and more accurate