3.16 Chromatography Flashcards
What are the different types of chromatography
(TLC) - Thin layer chromatography
(CC) - column chromatography
(GC) - gas chromatography
Describe the three different types of chromatography
(TLC) - a plate is coated with a solid and moves up the plate
(CC) - a column is packed with a solid and a solvent moves down the column
(GC) - column is packed with a solid or a solid coated by a liquid, and a gas is passed through the column under pressure at a high temperature
Though chromatography takes a number of different forms, what are the similar Principles they all contain
- there is a mixture of two or more substances which need to be separated
- There is a solid substance called the stationary phase which the mixture passes over in order to be separated
- mixture is carried over the material by a mobile phase, this is either a liquid or a gas
How does an affinity of a substance link with its speed
- If a component of the mixture has a high affinity to the stationary phase it will travel slowly.
- If a component of the mixture has a low affinity to the stationary phase it will travel quickly.
How does the solubility of a substance link with its speed
- If a component of the mixture is highly soluble in the mobile phase it will travel along quickly with the solvent.
- If a component of the mixture has low solubility in the mobile phase it will travel slowly with the solvent.
Process of using a TLC
A thin piece of aluminium or glass is coated in silica gel or aluminia which acts as the stationary phase. This is known as the TLC plate. Samples of the mixture to be separated are dropped on a line on the bottom of the TLC plate. The TLC plate is suspended in a small amount of solvent (also called the eluant) which soaks up the plate. The solvent is the mobile phase. Once dry, the plate is examined under ultra-violet light, or developed using chemical reagents to make the spots visible. The number of spots on the finished chromatogram gives then umber of components present in the mixture,
How to calculate Rf
distance moved by component / distance moved by solvent
What are the advantages of TLC
- Faster than paper chromatography
- Will work on very small samples
- Can be used to determine when a chemical reaction is complete.
What are the limitations of TLC
- Similar compounds may have similar Rf values.
- Conditions must be kept the came in order to fairly compare Rfvalues.
- New and unknown compounds have no reference Rf values.
- It can be difficult to find a solvent which separates all components in a mixture.
- Cannot be used to separate large quantities.
Process of CC
Column chromatography uses a narrow glass tube with a spout and tap (like a burette).
This is called the column.
The column is packed with a powder such as silica or aluminia to act as a stationary phase.
The mixture to be separated is loaded onto the top of the column. A solvent (eluant) is poured onto the top of the column and soaks down through the stationary phase. This solvent is the mobile phase. The different components of the mixture travel down through the solid phase at different rates. One will reach the bottom and can be collected into a beaker first, and the next can then be collected into a separate beaker some time later.
The advantage of column chromatography is that larger quantitiescan be separated.
If one molecule has more charges than the other, which is more polar
Molecule with higher number of charges
Describe process of GC
This separation technique is used to separate complex mixtures of volatile components. It is widely used in organic chemistry. A simplified diagram of a gas chromatograph is shown below:The mobile phase in GC is a gas, known as a carrier gas. This gas is typically an unreactive gas such as nitrogen or helium. This carrier gas flows through a long and very thin tube (around0.5mm in diameter). This is known as a capillary column which can be as long as 100m coiled up. The temp can be varied.
The components move into the detector at different times, and a retention time is recorded for each component. How can you find the compounds from here
Components can be identified by comparing their retention times with known compounds. This area of a peak in a gas chromatogram is proportional to the amount of a component in a mixture
This then enables the relative amounts of components in a mixture to be determined by comparing peak areas.
Uses of gas chromatography - mass spectrometer
GC-MS is used in analysis, in forensics, environmental analysis, airport security and space
probes.
