In A2/A-Level Chemistry, chromatography is a technique used to separate components in a mixture and identify them by comparing their R_f/retention time value to a database.

Mobile phase – the phase that moves --> liquid or gas

Stationary phase – the phase that doesn’t move --> solid or liquid

Chromatography works on the basis that different components have different affinities for a stationary and mobile phase. The greater the interaction the component has with the stationary phase the more it is slowed down; the difference in speeds is what creates the separation.

Methods of separation:

• Adsorption --> solid or liquid stationary phase
• Relative solubility --> liquid or gas stationary phase

Adsorption – the process by which a solid holds molecules of a gas or liquid or solute as a thin film on the surface of a solid or, more rarely, a liquid.

In A2/A-Level Chemistry, methods of chromatography can be separated into Thin Layer Chromatography (TLC) and Gas Chromatography (GC).

For TLC:

• Method of separation: Adsorption
• Mobile phase: Liquid solvent
• Stationary phase: Silica gel (SiO₂) or alumina (Al₂O₃) supported on a sheet of glass or plastic

Method/ process:

1. Draw a sample line on the TLC plate and pipette small drops of your sample(s) on this line.
2. Place the plate in a tank with a small volume of solvent that should be higher than the sample line
3. Cover the tank and allow the solvent time to soak up the plate and then take it out before the solvent runs off the edge
4. Mark the solvent line and allow the remaining solvent to evaporate
5. Analyse the plate by circling the spots and calculating the R_f value(s) – if the samples aren’t visible use a UV light

Limitations:

1. Similar compounds often have similar R_f values – components hide behind each other
2. Unknown compounds have no reference R_f value for comparison– the analyst needs to have a vague idea what the component is for the results to be meaningful
3. Can be difficult to find (mixture of) solvent(s) that will cause all the components to separate at an optimum rate

For GC:

• Method of separation: Relative solubility
• Mobile phase: Carrier gas (inert unreactive gas, usually helium or nitrogen)
• Stationary phase: Adsorbent (thin layer of a solid or liquid, such as a long-chained alkane (l) or silicone polymers (s), coated on the inside of coiled capillary tubing in an oven)

Method/ process:

1. Sample mixture is injected into the gas chromatogram and is vaporised
2. Vaporised mixture is forced through the capillary column due to the pressure from the carrier gas
3. Components dissolve into/ adsorb to the stationary phase and depending on the strength of their interactions determines the rate at which each component moves through the capillary column
4. A detector records the retention time (how long it takes to travel through the capillary column) of each component
5. The results are processed by a computer and are then displayed in a gas chromatogram

Limitations:

1. Thousands of chemicals have the same retention time, peak shape and detector response, so it doesn’t positively identify most components
2. Unknown compounds have no reference retention time for comparison – the analyst needs to have a vague idea what the component is for the results to be meaningful
3. Not all components will be separated and detected because some components ‘hide’ behind other components in higher concentrations with the same retention time

In A2/A-Level Chemistry, it is a visible record showing the results of separation of the components of a mixture by chromatography.

R_f value is unit used to compare how far a component has travelled up a TLC plate in relation to the solvent front.

R_f = distance moved by component ÷ distance moved by solvent front

It is the time for a component to pass from the column inlet to the detector. It isIn GC the area under each peak is proportional to the amount of a compound in the sample.

That's the end~

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