A Level Chemistry: Chromatography PDF

A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY Chromatography can be used to separate and identify the components of mixture. There are 3 types of chromatography that you need two know about: 1. Thin-layer Chromatography (TLC) 2. Column Chromatogra...

A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY Chromatography can be used to separate and identify the components of mixture. There are 3 types of chromatography that you need two know about: 1. Thin-layer Chromatography (TLC) 2. Column Chromatography (CC) 3. Gas Chromatography (GC) Each is slightly different, but they all work using the same basic principle. Each has a stationary phase (a phase that remain static / in place) and a mobile phase (a phase that travel along / over the stationary phase). Separation of the mixture depends on a balance between its: Solubility in / Attraction to the mobile phase Attraction (Retention) to the stationary phase. Substances in the mixture Substances in the mixture that are more attracted to that are more attracted to the mobile phase travel the stationary phase travel further / faster less / slower This can be achieved as the two phases have different polarities. In other words, one will be more polar than the other. The polar phase attracts polar substances more. The Non-Polar phase attracts non-polar substance more. We can use the results to either: a) Deduce the number of components in a mixture b) Identify the components in a mixture by comparing to a “known” result. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY THIN-LAYER CHROMATOGRAPHY TLC is very similar to paper chromatography that you used at GCSE to separate different inks into their component colours. e.g. Chromatography Paper Solvent Front (Stationary Phase) Pencil Line Solvent (Mobile Phase) Ink Spots TLC works in the same way but is more sophisticated, in that the stationary phase is a silica coated plate. The most common example of TLC that you will find is its use when identifying amino acids. As they are colourless, U.V. light or a developing agent such as Ninhydrin can be used to observe them. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY In this example we are using TLC to identify an amino acid. The plate (stationary phase) is non-polar and the solvent (mobile phase) is polar. A = Unknown B = alanine C = aspartic acid D = cysteine –O SH CH3 O– C O CH2 O– + H 3N C C CH2 O– + H 3N C C O + H 3N C C H H O H O KEY POINTS A must be cysteine as it has travelled the same distance as D. Notice how the most polar amino acid, aspartic acid, has travelled the furthest. This is because it is more attracted to the polar mobile phase. The least polar amino acid, alanine, has travelled the least distance. This A B C D is because it is more attracted to the non-polar stationary phase. We are able to confirm the identity of an unknown by calculating the Rf values. These quantify the distance traveled by each substance to allow us to make more accurate comparisons. Distance travelled by Sample Rf = Distance travelled by Solvent Front The calculated Rf should always be a value between 0 and 1.0 as the solvent will always travel further than the sample. Even without a direct comparison, Rf values can be compared to a reliable data set in order to identify an unknown amino acid, so long as the same condition were used. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY COLUMN CHROMATOGRAPHY Column chromatography uses gravity to cause the mobile phase to move. The stationary phase is fixed to silica beads packed into a column. This increases the surface area of the stationary phase. The mobile phase is the solvent that is used. The sample, dissolved in the solvent, is poured into the top of the column The time taken to reach the bottom of the column is known as the retention time Don’t Forget! Substances in the mixture Substances in the mixture that are more attracted to that are more attracted to the mobile phase travel the stationary phase travel further / faster less / slower AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.16 CHROMATOGRAPHY THIN-LAYER CHROMATOGRAPHY Gas chromatography uses high pressure to cause the mobile phase to move. The stationary phase is actually a liquid that is fixed to the inside of a long, coiled tube. The longer the tube / coil, the greater the surface are of the stationary phase and the more the components are separated. The mobile phase is an inert gas. The sample is vaporised and mixed with it and injected at high pressure. Sample mixed with insert gas and injected under high The time taken for the sample to pass around the coil is recorded. Sample travels around Retention Time the coil containing the stationary phase. Once Again! Substances in the mixture Substances in the mixture that are more attracted to that are more attracted to the mobile phase travel the stationary phase travel further / faster less / slower Gas chromatography is often coupled with Mass Spectroscopy. As the separated samples appear out of the coil, they are directly injected into a mass spectrometer. That way, a mixture can be separated by the GC, then be analysed by mass spectroscopy to measure the abundance of each component and help identify each component. AQA www.chemistrycoach.co.uk © scidekick ltd 2024