Moduel 6 Chapter 29
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What is chromatography
It is a technique used to separate components with similar physical properties from a mixture.
How are the different substances separated in chromatography
The mobile phase is passed over a stationary phase and each compound has a different affinity for each phase.
What is a phase
A phase is a physically distinct form of a substance
What does the stationary phase do in chromatography
The stationary phase slows compound movement and the more interactions there are the more it is slowed down —> allowing compounds to move at different rates
What does a solid stationary phase do
Separated molecules by adsorption as molecules bind on to the surface —> stationary adsorption is slower
What does a liquid stationary phase do
Separates by relative solubility as molecules dissolve into the phase —> greater solubility = slower
What is the stationary phase in thin layer chromatography
A stationary phase of silica gel (SiO2) or alumina (Al2O3) is cooled onto a plate of glass or plastic
What is adsorption
The process that occurs when a gas or liquid or solute is held to the surface of a solid.
What is the process of thin layer chromatography being done
A sample is added to a starting point on the plate which is then placed into a liquid solvent which will get drawn up. —> at the end the height of the solvent is marked - the solvent front
How is separation achieved in thin layer chromatography
By the relative adsorptions of substances with the stationary phase
What is an Rf value
It shows how far a component has moved relative the solvent front
What is the formula for calculating the Rf value
Rf = distance moved by component/distance moved by solvent front
How can the Rf value be used to identify components
As it will always have the same Rf value, so by comparing it to values in a database of Rf values of substances using that solvent you can identify the compound.
What happens when compounds have similar rf values
A suitable,e solvent is needed to separate the two separate compounds.
What is the stationary phase in gas chromatography
Solid silicone polymers or a liquid long chain alkane, is coated inside a capillary tube.
What is the mobile phase in gas chromatography
AN inert unreactive gas such as He or N2
How is the sample inserted into the gas chromatogram
It is injected and vaporised before being pushed through the capillary column by a carrier gas.
What does the detector do in gas chromatography
The detector at the end records how long it took for a components to pass through the column which is known as the retention time.
What is the area below a peak indicate in gas chromatography
The area below a peak is proportional to the concentration
What is the retention time used for, however what are problems with this
To help identify the component but, chemicals can have the same retention time, peak shape and detector response, and peaks can be hidden by others with the same retention time but a greater concentration.
What is used at the same time as gas chromatography to improve the results
Mass spectrometry is often run at the same time
Hollow do you work out the concentration of a substance using gas chromatography
Compare its peak integration area with values, these are obtained from a, standard solution of the component.
What are the steps for making a calibration curve to work out the concentration of a substance using gas chromatography
• Prep SS - known conc • Obtain gas chromatograms • Plot calibration curve of peak area against concentration = external calibration, convert area into conc • Obtain chromatogram of compound under same conditions • Use calibration curve to measure the corn of the compound.
What is the chemical shift in HNMR and CNMR
The position at which a nucleus absorbs energy along the NMR spectrum is known as the chemical shift.