6.3. Analysis: Module 6: Organic Chemistry & Analysis Chemistry OCR A A Level

Chromatography

Chromatography is an analytical technique used to separate and identify component molecules of a mixture. Separation of the mixture depends on distribution between a mobile phase and a stationary phase.

Mobile phase

The mobile phase is a fluid 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. Species that are more soluble in the mobile phase move further and/or faster within the mobile phase.

Stationary 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 and/or the slower it moves within the mobile phase.

Rf Values

An Rf value is a retention factor that is unique to each different component molecule in the mixture being analysed. This is because different molecules within the mixture will have different affinities for the mobile and stationary phase and so will move at different rates within the mobile phase. It is calculated by comparing the distance moved by the component molecule to the distance moved by the mobile phase.

Thin-Layer Chromatography (TLC)

In TLC chromatography, a metal plate is coated with a thin layer of silica and the sample being analysed is dotted on the plate. These initial samples must be placed above the level of the solvent in the container.

Gas Chromatography (GC)

In gas chromatography, a thin coiled tube is packed with a solid, powdered substance that acts as the stationary phase. Instead of a solvent, a high pressure gas is passed through this tube and acts as the mobile phase. This method is used to separate mixtures of volatile liquids which are fed into the gas chromatography machine as vapours. The analysis machine records a retention time for each component in the mixture, allowing them to be identified.

Carbon-12

Carbon-12 is the most common isotope of carbon but is not detected by NMR spectroscopy. This is why carbon-13 is used, however, carbon-13 only has an abundance of roughly 1.1%, so a greater quantity of sample will be needed to obtain a well resolved spectra.

Hydrogen Bonding

The process of hydrogen/deuterium exchange can be used to identify N-H and O-H peaks in an NMR spectra.

Combined Techniques

The analytical techniques covered throughout the course can be used together to predict the structure of unknown compounds.

Mass Spectrometry

This is an analytical technique used to identify different molecules and find the overall relative molecular mass.

High Resolution Mass Spectrometry

High resolution mass spectrometry is a much more sensitive form of mass spectrometry which allows the Mr of a substance to be determined to several decimal places. Precise atomic masses can then be used to calculate the molecular formula of the compound being tested.

Infrared Spectroscopy

Infrared spectroscopy is an analytical technique that uses infrared (IR) radiation to determine the functional groups present in organic compounds.