Chromatography Flashcards
What is chromatography
Used to separate and identify components in a mixture
What does separation depend on
The balance between solubility in the moving phase and affinity by the stationary phase
Affinity to stationary phase
If the component of the mixture has high affinity to the stationary phase it will travel slowly
If the compound of the mixture has low affinity to the stationary phase it will travel fast
Solubility and mobile phase
If component of mixture is highly soluble in the mobile phase it will travel along quickly with the solvent
If the component of the mixture has low solubility in the mobile phase it will travel slowly with the solvent
Thin layer chromatography TLC
Thin piece of aluminium/glass is coated in silica gel/alumina - stationary phase
Sample of mixture are dropped onto a line at the bottom of tlc plate
TLC plate suspended in small amount of solvent which soaks up the plate - mobile phase
Plate is examined under ultra violet light or developed using chemical reagents to make spots visible
Number of spots gives the number of components present in the mixture
What does filter paper on TLC Do
Filter paper in chamber to check if atmosphere is saturated
What does lid do in TLC
Solvents are volatile so could evaporate
How to calculate rf
Rf = distance moved by component / distance moved by solvent front
Advantages of TLC
Faster than paper chromatography
Will work on very small samples
Can be used to determine when a chemical reaction is complete
Limitation of TLC
Similar compounds may have similar rf values
New and unknown compounds have no reference rf values
Cannot be used to separate large quantities of
It can be difficult to find a solvent which separates all components in a mixture
Column chromatography
Uses a narrow glass tube with a spout and tap
Column is packed with a powder such as silica/aluminia - stationary phase
Mixture to be separated loaded on top of the column
Solvent poured on top of column and soaks down stationary phase - mobile phase
Different compounds in mixture travel down at different rates
Collected in beakers at larger rates
Larger quantities can be separated
Mobile and stationary phase
Mobile phase - like dissolves like
Polar molecule will dissolve well in polar solvent and will travel further
Stationary phase - like sticks with like
Polar molecules will have high affinity to polar stationary phase so will travel slowly
Gas-liquid chromatography
Used to separate complex mixtures of volatile components
Mobile phase in gc is a gas, carrier gas - normally unreactive like nitrogen or helium
Carrier gas flows through thin capillary column
Column made of glass/metal and has thin coating of liquid(normally high molecular mass alkane with high bp)/solid (silicone polymers)- stationary phase
Sample to be separated is injected into the column
Carrier gas transports the components of the mixture along the column at different rates to separate them
U don’t calculate rf value u record retention time - time it takes from the point of injection for the component to the detector
When it reaches detector and electrical signal is produced
Chromatogram points
Components can be identified by comparing their retention times with known compounds
Area of a peak is proportional to the amount of a compound in a mixture
Limitations of gc
Maybe 1000s of chemicals have similar retention times / peak shapes
Not all substances in a sample may be separated
