what is chromatography used for
- Used to separate compounds in a mixture, once the mixture is separated out then you can identify the different components
describe how to work thin layer chromatography
- Draw a pencil line near the bottom of the plate and put a spot of the mixture to be separated on the line
- Dip the bottom of the plate into a solvent
- As the solvent spreads up the plate the different substances in the mixture move with it but difference distances so they separate out
- When the solvents nearly reached the top of the plate, take the plate out and mark the distance that the solvent has moved in pencil
what is adsorption
the attraction between a substance and the surface of plate is called adsorption
describe factors that affect how much a spot travels up a chromatography plate
- A substance that is strongly adsorbed will move slowly so it wont travel as far as one that is weakly adsorbed
- Polarity – this affects how strongly adsorbed a particular substance is to the plate,
- Distance depends on the solid coating of the plate , the solvent used and the external variables such as temperature
- Run a pure sample of the known substance alongside the unknown compound on a TLC plate
describe gas chromatography
- The sample is injected into a stream of gas which carries it through a coiled tube coated with viscous liquid or a solid
- Components of the mixture dissolve in the oil or onto the solid, evaporate back into the gas then redissolve as they travel through the tube
- Time taken for the substance to pass through the coiled tube and reach the detector is called the retention time and can be used to identify substances
describe the peaks in gas chromatography
- Each peak corresponds to a retention time
- Retention times are measured from zero to the centre of each peak and can be looked up in a refernce time to identify the substance present
- Area under the peak is proportional to the relative amount of each substance in the original mixture,
describe what the retention times are affected by in gas chromatography
- Solubility – this determines how long each component of the mixture spends dissolved in the oil or on the solid and how long they spend moving along the tube in the gas
- Highly soluble substance will spend more time dissolved so takes longer to travel through the tube to the detector than one with lower solubility
- Boiling point – a substance with a higher boiling point will spend more time condensed as a liquid in the tube than as a gas therefore it takes longer to travel through the tube than one with a lower boiling point
- Temperature of the gas chromatography instrument – a high temperature will mean that the substance will spend more time evaporated in the gas so will move along the tube quickly, it shortens the retention time for all the substances in the tube
what is the retention time
- is the time taken for a component to travel from the inlet and be injected to the detector in a gas chromatograph
whats the stationary phase and mobile phase in GC
stationary - liquid or solid in coiled tube - this is usually a hydrocarbon with a high boiling point
mobile - inert gas - such as helium or nitrogen as they are unreactive
whats the stationary phase and mobile phase in TLC
stationary - solid
mobile - liquid solvent
whats the adsorbent solvent in TLC
- aluminium oxide
what is gas chromatography for
- separate volatile components in a mixture,
What is the mobile phase
The phase that moves
What is the stationary phase
The phase that doesnt move
What are the limitations of gas chromatography
- similar compounds have similar retention rimes so may not be identified
- may not have reference retention time to be compared with
- may go undetected if concentration is low
How is calibration curve calculated
- concentration is added to chromatography
- run
- ratio of peaks areas for the compound with known and unknown concentrations can be calculated
- ratio can be used to calculate the unknown concentration
