Chromatography Flashcards
The two phases
Solid substance- ‘stationary phase’
Mixture passes over to be separated
‘Mobile phase’- either liquid or gas
Carries mixture over solid
How is the mixture separated
Separated due to either differences in affinity to stationary phase
Or
Differences in solubility in the mobile phase
Separation due to affinity
High affinity to the stationary phase- travel slowly
Low affinity to stationary phase- travel quickly
Separation due to solubility
Highly soluble in the mobile phase- travel quickly with solvent
Low solubility- travel slowly with the solvent
Thin layer chromatography
Stationary phase- thin piece of aluminium or glass coated in silica gel or aluminia
Mobile phase- solvent
Samples dropped on line at bottom of plate
Placed in solvent which travels up plate
Once dry plate examined under uv light
Or
developed using chemical reagents to make spots visible
Number of dots= number of components in mixture
Solubility and affinity
(Same in all types of chromatography)
Sample b more soluble to mobile phase as it has travelled further up the plate
Sample a has low solubility to mobile phase as it travelled shorter distance up the plate
Sample b has low affinity to stationary phase as it has travelled further up the plate
Sample a has a greater affinity to the stationary phase as it has travelled a shorter distance up the plate
Rf formula
Distance moved by spot/ distanced travelled by solvent front
Advantages of thin layer chromatography
Faster than paper chromatography
Will work on very small samples
Can be used to determine when a reaction is complete
Limitations of thin layer chromatography
Similar compounds may have similar rf value
Conditions must be kept the same to have fair comparisons of rf value
New and unknown compounds have no reference rf value
Difficult to find solvent which separates all components in mixture
Can not be used to separate late quantities
Column chromatography
Uses a narrow glass tube with a spout and tap called a column
Stationary phase- silica or aluminium powder in column
Mobile phase- solvent
Mixture is loaded on top of column
Solvent poured ontop of column and soaks down stationary phase
Different components of mixtures travel down at different rates
One will reach the bottom and be collected into a beaker then after sometime the next component will be collected
Polarity of stationary and mobile phase
Rule of thumb:
Mobile phase- ‘like dissolves with like’
Stationary phase- ‘like sticks with like’
Eg
Molecule a is more polar than molecule b as it has more charges on it. Two molecules can be separated by different methods of chromatography
So in TLC if a polar solvent is used molecule a will dissolve well and travel further whereas if a polar stationary phase is used molecule a will have a higher affinity to plate and move less distance
Gas liquid chromatography
Used to separate mixtures of volatile components
Mobile phase- a gas known as carrier gas eg hydrogen or nitrogen
Stationary phase- thin coating of either liquid or solid on column
Carrier gas flows through capillary column
Liquid stationary phase- high molecular mass alkane w high melting point
Solid stationary phase- silicon polymers
Sample to be separated is injected into the column
Carrier gas transports the components along the column at different rates causing the to separate
ONLY CAN EXPLAIN SEPARATION DUE TO AFFINITY TO STATIONARY PHASE (bc mobile phase not a solvent so can not explain due to solubility)
Measure retention time not rf value
Retention time- time taken from point of injection for component to reach detector
Once reached electrical signal is produced
How are mixtures identified using gas liquid chromatography
Comparison of retention times with known compounds
Area of peak proportional to amount of component in mixture
Relative amounts of components in a mixture can be identified by comparing peak area
Limitiations of gas liquid chromatography
Lots of chemicals have similar retention times, peak shapes and detector responses so compounds can not be positively identified
Not all substances in sample may be separated and peaks may be hidden behind other peaks
Unknown compounds have no reference retention times
How can gas liquid chromatography limitiations be overcome using mass spectrometry
Gc can separate compounds in a mixture but not identify them
Ms can not separate components in a mixture but can identify them
It is quick
Powerful
Requires small amount of sample
