3.16 Chromotography

Stationary phase

solid

Mobile phase

Allows molecules to move over/through the stationary phase. Either a liquid or a gas

How far do product move up the paper?

More soluble products move further with the mobile phase so they have a higher affinity to the mobile phase

Why do we use pencil lines?

Pencil doesn't dissolve in the solvent so doesn't travel up the plate

Why do we cover the beaker containing a plate with a lid?

To ensure the atmosphere in the beaker is saturated with the solvent

Rf calculation

distance moved by molecule/distance moved by solvent

What is thin layer chromotography?

A metal plate is coated with a thin layer of silica and solvent moves up the plate.

How to view chemical traces in TLC

Chemical traces can be viewed using a UV lamp or adding a developing agent (such as iodine or ninhydrin)

What substances travel up the TLC plate further?

TLC plate has OH bonds so they are polar. This means substances that can form hydrogen bonds have a higher affinity to the stationary phase so don't travel up as far.

How do you determine the number of substances present?

Number of spots obtained

Column chromotography

Column filled with an inert solid of silica (the stationary phase). Silica is polar. A solvent (non polar) containing the mixture is added to the top and runs down, depending on its relative affinity to the stationary phase.

Retention time

Time taken to come out the bottom of the column

Gas-Liquid Chromatography

Thin tube packed with a powdered solid is stationary phase. Sample is vaporised and injected through with a high pressure inter gas as the mobile phase. Usually sample analysed by mass spectrometer after

What is Gas-Liquid chromatography used for?

Separating volatile liquids

Why is Gas Chromatography - Mass Spec good?

It is a very fast analytical method and produces very accurate results for analysis and identification