Organic analysis

• Mass spectrometry is an analytical technique used to identify different isotopes and find the overall relative atomic mass of an element.
• Time of Flight (ToF) mass spectrometry records the time it takes for ions of each isotope to reach a detector.
• The first step of ToF MS is ionisation. Electron impact ionisation is where electrons are fired at the sample and knocks off an electron from the atoms. This is good for small molecules and can lead to fragmentation which can be useful to determine structure. Electrospray ionisation is where the sample is vapourised and passed over a high voltage to remove an electron. This does not typically cause fragmentation and is good for large molecules.
• The second step of ToF MS is acceleration. The positively charged ions are accelerated towards a negatively charged detection plate and all gain the same kinetic energy (but speed will depend on mass).
• The third step of ToF MS is ion drift. The ions are deflected by a magnetic field into a curved path. The radius of their path is dependent on the mass and charge (since 2+ ions can be formed during ionisation) of the ion.
• The fourth step of ToF MS is detection. When the positive ions hit the negatively charged detection plate, they gain an electron producing a flow of charge. The greater the abundance, the greater the current produced.
• The fifth and final step of ToF MS is analysis. Current values and flight times produce a spectra showing the mass to charge ratio (m/z) compared with relative abundance. The Ar can be calculated as (m/z x relative abundance)/total abundance.
• Primary and secondary alcohols are tested for using acidified potassium dichromate. It oxidises the alcohols and turns from orange to green.
• Tollen's reagent can identify aldehydes when warmed gently. A positive result is a silver mirror formed from a colourless solution.
• Fehling's solution can identify aldehydes when heated. A positive result is a brick red precipitate from a blue solution.
• Bromine water is used to test for unsaturated hydrocarbons. A positive result is a colour change from orange - brown to colourless.
• Sodium carbonate is used to test for carboxylic acids as they react as acids to produce carbon dioxide. The gas produced can be bubbled through limewater. A positive result is the limewater turning cloudy.
• High resolution mass spectrometry is a more sensitive form of mass spectroscopy which allows the Mr of a substance to be measured to several decimal places.
• Infrared spectroscopy can be used to determine the functional groups present in organic compounds as different types of bond absorb different amounts of the radiation, which can be measured.
• The fingerprint region of an IR spectrum is unique for every compound and is between 1500 and 500 cm-1.
• NMR spectroscopy is an analytic technique that allows the structure of a molecule to be determined by analysing the bond environment.
• The peaks in NMR are measured against a standard molecule: TMS, which contains four identical carbon and hydrogen environments.
• In carbon NMR, carbon environments near an oxygen are shifted to the right as oxygen is very electronegative.
• In hydrogen NMR, the sample is dissolved in CCl4 as it doesn't contain hydrogen so doesn't cause a peak on the spectrum.
• In hydrogen NMR, a splitting pattern is shows the number of hydrogen on the adjacent carbon atom, where the number of hydrogen atoms is equal to the number of peaks minus one.
• Chromatography is an analytical technique used to separate components of a mixture.
• The mobile phase is a fluid that allows molecules to move over or through the stationary phase.
• More soluble products move further up the stationary phase as they have a greater affinity for the mobile phase than the stationary phase.
• Rf values are unique to each component molecule in a mixture and can be found by dividing the distance moved by the molecule by the distance moved by the solvent.
• Thin layer chromatography uses a plate covered with silica gel, and either a UV lamp or ninhydrin is used to view the traces.
• Column chromatography uses a vertical column filled with a solid, powdered substance as the stationary phase. The different retention times (time taken to drain out) are used to separate the mixture.
• Gas chromatography uses a high pressure gas passed through a powdered solid. It is used to separate mixtures of volatile liquids.
• GC-MS combines gas chromatography and mass spectroscopy to separate volatile liquids based on their retention time and then identify them.