AC4: Chromatography Flashcards
What is chromatography and what basis does it work upon
an inexpensive technique to separate components in a mixture for analysis or for purification, works on the basis that different component molecules in a mixture have different strengths of adsorption to a solid stationary phase
What does TLC stand for
One - way Thin Layer Chromatography
What does TLC use
a stationary phase coating (often silica) on a glass/plastic plate
Describe and explain the set up for TLC, stating the mobile and stationary phases
TLC plate in a TLC chamber with a lid/watch glass to prevent evaporation/loss of volatile mobile phase
solvent(mobile phase) line at the bottom, pencil(start) line above the solvent line
spot of mixture on pencil line
components migrate upwards, with a line at the top of the TLC plate, this is the solvent front
The stationary phase is the silica coating on the TLC plate
What is adsorption
the process by which a substance binds to a solid (adsorbent) surface
Explain how TLC works in separating components in a mixture
The components are separated by the differences in relative strengths of adsorption to the stationary phase. The stronger the adsorption, the slower the component migrates and the smaller the Rf value
How do you calculate the Rf value of a component in chromatography
Distance moved by component from pencil line
/
distance moved by solvent (pencil line to solvent front)
What does the strength of adsorption of a component depend on
the strength of intermolecular forces between the component and adsorbent (stationary phase - silica coating on TLC plate)
Describe and explain two ways that colourless organic compounds be revealed on a chromatogram
1- U.V. light - fluorescent coating on TLC plate glows, many organic compounds absorb U.V. -> dark spots
2- I2 vapour - I2 (g) forms strong London forces with organic compounds -> stains organic compound
What does GLC / GC stand for
Gas-Liquid Chromatography
What is GLC/GC used for
to separate volatile components in a mixture, often used for low bpt. organic compounds
What is an advantage that GLC/GC has over TLC
GLC/GC gives much higher resolution/better separation than TLC so more complex mixtures can be analysed
Describe the set up for GLC/GC stating the mobile and stationary phases
A carrier gas/mobile phase (inert gas: Ar, He, N2) is fed (with a flow controller) into an inlet in a thermostatic oven. The sample is also injected into the inlet. The thermostatic oven contains the capillary column which is coated with the stationary phase (high bpt liquid, up to 30m long) which goes out of the oven into a detector, then a gas chromatogram is produced by a computer
Describe and explain the process of GLC/GC
1- sample is injected and vapourised
2- The mobile phase (Carrier gas) flushes mixture through capillary column
3- The stationary phase (high bpt liquid) is already coated on inside surface of column
4- The components are separate due to their different solubilities in the liquid stationary phase
5- Retention time = time taken for component to pass from inlet to detector
How are components separated in GLC/GC (2 marks)
The components are separated due to their different solubilities in the liquid stationary phase. The more soluble the component, the longer it is retained in the stationary phase and the longer its retention time
What does the peak area and retention time mean on a gas chromatogram produced by a GLC
Largest peak area = most abundant compound
Longest retention time = most soluble component in stationary phase
What does GC-MS stand for
Gas Chromatography and Mass Spectrometry
How do GC and MS compliment each other as analytical tools for identifying components in complex mixtures
GC is good at separating components, MS is good for helping to identify the structure of a pure sample
State 4 uses of GC-MS
1- Detecting explosives at airport security
2- Drug testing for professional athlete or police work
3- Monitoring air/water pollution
4- Space exploration probes for analysing atmosphere/soil
Explain the role of the mobile phase in TLC
It is an organic solvent that dissolves the components and carries them up the TLC plate
Explain the role of the stationary phase in TLC
It is the solid adsorbent that the components adsorb onto
Explain the role of the lid/cover/watchglass in TLC
This reduces the evaporation of the volatile solvent (mobile phase) from the TLC plate as otherwise the migration of the components will stop
What is a disadvantage that GLC/GC has to TLC
TLC is a simpler technique and quicker to set up and use, it also requires less complex and expensive equipment and materials
Explain why the gas chromatography column is inside an oven
To maintain the test sample as a gas so that its components can flow through the column and be flushed by the carrier gas
The mobile phase in GC is often helium or another inert gas, it would be much cheaper to use compressed air, why is this not done?
Helium is inert and will not react with the component compounds. Oxygen in air may oxidise the heated component compounds. Air is a mixture, with each component gas behaving differently in the column
