Chromatography Flashcards
What is chromatography?
A practical technique used to separate and identify the components in a mixture.
What is meant by the stationary phase?
Does not move
What is meant by the mobile phase?
Moves across the stationary phase
What are the 3 types of chromatography?
TLC
Column
Gas
What are the stationary and mobile phases in TLC chromatography?
Stationary - plate coated with silica or alumina gel (polar powder)
Mobile - solvent (non-polar solvent like hexane, or mixture of polar solved like water or alcohols)
What are the stationary and mobile phases in column chromatography?
Stationary - column packed with solid (silica)
Mobile - solvent
What are the stationary and mobile phases in gas chromatography?
Stationary - long coiled tube packed with powder (SiO2 or Al2O3) - coated with liquid?
Mobile - inert gas
Why do components in a mixture travel different distances on a TLC plate?
The component that moves further up the stationary phase is most soluble in the mobile phase. More soluble components are more attracted to the mobile phase so stay in it longer, hence they travel further up the stationary phase. Less soluble components are less attracted to solvent, but more attracted to stationary phase, so travel a shorter distance. *
What is the formula for Rf?
Rf = distance moved by substance/ distance moved by solvent
What are the limitations of TLC?
Very similar molecules may have similar Rf values so difficult to tell them apart.
Some spots do not separate properly, so spots may merge with each other. So it is difficult to tell what spot belongs to a component.
Components with no known Rf values are difficult to compare.
What is two-directional chromatography?
Involves using two different solvents to separate two or more components that will not separate in one solvent (due to same/similar Rf value).
Why is chromatography useful over mass spectrometry?
If a substance has the same Mr, they will have the same m/z values. With chromatography, they may have different Rf values or retention times.
How does column chromatography work?
A glass tube is filled with the stationary phase (silica or alumina powder (powder to increase surface area)).
A filter or plug is used to retain the solid in the tube. Solvent is added to cover all the powder.
The mixture to be analysed is dissolved in a minimum of solvent and added to the column.
The mixture runs through the column.
What is retention time?
The time taken for each component to travel through the column.
What can be deduced from retention time?
Shorter retention time means the component is more attracted to the mobile phase.
Longer retention time means the component is more attracted to the stationary phase.
