AQA Chemistry A-Level 3.3.16: Chromatography Detailed Notes PDF

AQA Chemistry A-level 3.3.16: Chromatography Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

AQA Chemistry A-level 3.3.16: Chromatography Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.16.1 - Chromatography Chromatography is an ​analytical technique​ used to separate and identify component molecules of a mixture. It uses a ​mobile phase ​and a ​stationary phase​. Mobile and Stationary Phases The mobile phase is a substance that allows molecules to ​move over or through​ the stationary phase. It can be in the form of a ​liquid or a gas​ depending on the type of chromatography being carried out. ​More soluble​ products ​move further​ with the mobile phase. The stationary phase is a substance that has ​affinity​ to molecules in the mixture being analysed. The ​greater the affinity​ of a molecule to the stationary phase, the ​shorter the distance​ it moves with the mobile phase. Rf Values This is a value that is unique to each different component molecule in the mixture being analysed. It is calculated by comparing the ​distance moved by the mobile phase​ ​and the molecule​. Example: There are four main types of chromatography that use different mobile and stationary phases to obtain Rf values for the molecules present. Thin-Layer Chromatography In this method of chromatography, a ​metal plate​ is coated with a ​thin layer of silica​ and solvent moves up the plate. The plate is then dried in a fume cupboard to reduce toxic fumes. The chemical traces can then be viewed using a ​UV lamp​ and the distances travelled can be measured. Alternatively, a ​developing agent​ can be added, such as ​iodine​, to allow the traces to be seen by the naked eye. Example: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Column Chromatography For this method, a ​vertical column​ is packed with a ​solid, powdered substance​ which acts as the stationary phase. A ​solvent​ containing the mixture being analysed is then added and moves down the column as the mobile phase. Example: The varying affinities of the molecules present means they ​drain out of the column at different times​, allowing them to be collected as ​separate samples​. The time taken to drain out of the column like this is measured as the ​retention time​. Similar to Rf values, retention times allow the individual molecules in the mixture to be identified. Gas Chromatography In this form of chromatography, a ​thin tube ​is packed with a ​solid, powdered substance​ which acts as the stationary phase. Instead of a solvent, a ​high pressure gas​ is passed through this tube as the mobile phase. This method is used to separate mixtures of ​volatile liquids​ which are fed into the gas chromatography machine as vapours. Example: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc The analysis machine ​records a retention time​ for each component molecule in the mixture, allowing them to be identified. GC-MS This is a ​combination​ of analytical techniques, ​gas chromatography and mass spectrometry​, allowing for a more advanced level of molecule analysis. The molecules present are first separated using the gas chromatography technique. Then each molecule present is fed directly into a mass spectrometer so it can be accurately identified. Example: GC-MS is a ​much​ ​faster​ analytical process that produces ​more accurate​ results for molecule analysis and identification. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc