Flashcards in Module 6.3 Deck (86)
What is chromatography?
A technique used to separate mixtures of compounds
List several types of chromatography
Thin Layer Chromatography (TLC)
High Performance TLC
What is the name for the different phases in chromatography?
Mobile and stationary
If the stationary phase is non- polar, and the molecules are polar, what phase are the molecules more likely to bind to?
Mobile phase (travels further)
If the stationary phase is polar, and the molecules are polar, what phase are the molecules more likely to bind to?
Stationary phase (Doesn't travel as far)
Give the method of TLC
1) Draw a pencil line near the bottom of the plate and put a spot of the mixture to be separated on the line
2) Dip the bottom of the plat (NOT including the spot) into a solvent (usually ethanol)
3) As the solvent spreads on the plate, the different substances in the mixture move with it (but at different distances so they separate out)
4) When the solvent's nearly reached the top of the plate, take the plate out and mark the distances that the solvent has moved (Solvent front) in pencil
What is meant by the solvent front?
The distance that the solvent has moved
Define the term Rf value
What is the formula to work out Rf value?
(Distance travelled by spot/Distance travelled by solvent)
How can we work out what was in the mixture using an Rf value?
Compare it to other Rf values from a table of known values which comes from a TRUSTED source
How does separation work in TLC?
In TLC, if a substance is strongly absorbed will it move slowly or fast?
In TLC, if a substance is weakly absorbed will it move slowly or fast?
During TLC, what external variables must we keep constant and why?
Temperature - Because it can alter how far our mixture moves
How can you confirm your unknown identified compound using TLC?
We can confirm out unknown compound by running a TLC of a pure sample of the known substance ALONGSIDE our unknown compound. If we get identical Rf values, we can confirm our result to be accurate
List the key points of TLC
- Ethanol is the mobile phase
- Silica gel or Aluminium powder is the stationary phase
- Separation via ADSORPTION
- When discussing TLC, we talk about the Rf value.
- Similar chemicals have similar Rf values
What is the method of Gas Chromatography?
1) The sample to be analysed is injected into a stream of gas, which carries it through a coiled tube coated with a viscous liquid (e.g. oil) or a solid
2) We then allow time for the mixture to dissolve in the oil or onto the solid. This then evaporates back into ga and then redissolves as they travel through the tube.
3) We monitor the time taken for the substances to pass through the coiled tube and reach the detector (retention time)
4) We use our retention time and compare it to other retention times from a table of known values, which comes from a TRUSTED source, to identify our product
Define retention time
The time taken for a substance to pass through a coiled tube and reach the detector
How does separation work in GC?
In GC, are the compounds that bind most to the mobile phase are eluted first or last?
Eluted = Given out
On a Gas chromatogram, what does the area of the peak show?
The relative amount of the compound
How can you confirm your unknown identified compound using GC?
We can confirm our unknown compound by running a TLC of a pure sample of the known substance alongside our unknown compound. If we get identical retention times , we can confirm out result to be accurate
What factors affect retention time?
Boiling point of the substance to be tested
Temperature of the gas chromatography instrument
Explain how a highly soluble substance affects retention time
Highly soluble substance = More time spent dissolved = Longer time to go through to tube to the detector = Higher retention time
Explain how a higher boiling point of the substance to be tested affects retention time
If the substance has a higher boiling point, it will spend more time condensed as a liquid in the tube than as a gas. This means that it will take longer to travel through the tube
Higher BP = Higher retention time
Explain how the temperature of the gas chromatography instrument affects retention time
The higher the temperature of the gas chromatography instrument means that the substance will spend more time evaporated in the gas and so it will move along the tube quickly. It shortens the retention time for ALL substances in the tube.
Higher temperature of the GC instrument = Shorter Retention Time.
What do gas chromatograms show?
A series of peaks a the time where the detector senses something other than the carrier gas leaving the tube (another compound)
What can gas chromatograms be used to do?
Identify substances within a sample and their relative proportions
In a gas chromatogram, what does each peak represent?
A substance, with a particular retention time.