3.3.16 - Chromatography

What is chromatography?

An analytical method used to separate the substances in a mixture

What are the three types of chromatography in this chapter?

Thin layer chromatography

Column chromatography

Gas chromatography.

What do all of these techniques require?

All of these techniques require a mobile and a stationary phase.

What is the mobile phase?

Mobile phase- contains the mixture, referred to as solvent or eluent. It is usually a liquid or gas.

What is the stationary phase?

Stationary phase- separates the sample, it is a solid or a liquid.

What is the mobile and stationary phase in TLC?

Mobile phase - liquid solvent

Stationary phase- solid silica/alumina

What is the mobile and stationary phase in CC?

Mobile- liquid solvent

stationary- solid silica/alumina

What is the mobile and stationary phase in GC?

Mobile phase - inert carrier gas

Stationary phase - microscopic liquid film on solid support.

What does the rate of seperation depend on?

Rate of separation depends on the solubility of the components, the retention of the compounds.

What is the retention factor?

Retention factor- used as a measure of solubility. Known as the Rf value.

How do you calculate the Rf value?

Rf= distance travelled by component/distance travelled by solvent.

Is Rf value unique? Can it be greater than 1

Each component has a unique Rf value, and Rf value can't be greater than 1

What does it mean if a compound has a higher RF value?

More soluble components have a higher Rf value.

What is retention time?

Retention time- used as a measure of solubility. This is the time that elapses from the moment of injection to when a component exits the chromatography tube.

What does it mean if a compound has a lower retention time?

More soluble = lower retention time.

What is thin layer chromatography used for?

Thin layer chromatography- used to analyse small samples.

Is the mobile phase polar or non polar?

Mobile phase flows over the stationary phase. It can be polar or non-polar.

What is the stationary phase for TLC?

Stationary phase- commonly a thin metal sheet coated in alumina or silica.

What does the movement of substances depend on?

Seperated components will travel particular distances along the plate. The movement depends on their solubility and retention.

What does it mean if a substance travelled further?

More soluble = weaker interactions with stationary phase, so they travel further.

What happens if the compounds on the chromatogram are not coloured?

If they are not coloured, then you can locate the spots by using UV light, ninhydrin or iodine vapour.

How do you conduct a TLC?

To conduct a TLC analysis, prepare a beaker with a small quantity of solvent, draw the baseline on a TLC plate in pencil. Place a spot of any required pure reference compounds on the baseline as well as a spot of the sample. Allow the spots to dry, then place the TLC plate in a beaker and cover with a lid. As the solvent reaches the top, draw the solvent front in pencil.

What are the two ways to intepret a chromatogram?

Quantitative and Qualitative

What is the quantitiative method?

Quantitative- calculate RF value.

What is the qualitative method?

Qualitative- compare with other reference substances

What is column chromatography used for?

Column chromatography is used to separate larger samples.

What is the stationary phase in CC?

The column has an inert solid as the stationary phase (silica or alumina)

What is the mobile phase in CC?

The mobile phase is a liquid solvent.

How is column chromatography carried out?

The sample is dissolved in the solvent and introduced at the top of the column. More solvent is added to the top, fresh solvent is continuously added so the sample doesn't dry out. The sample moves down the colum via gravity.

What does it mean if a component takes a long time to flow through the column?

The component with the greatest attraction/affinity to the stationary phase takes the longest time to flow through the column.

What do you do when the components aren't coloured.

If the component is colourless then use locationg indications like UV light to show their position. Components can be collected in a beaker as they exit the column.

What is gas chromatography used to analyse?

Gasses, volatile liquids or solids in their vapour form.

What is the stationary phase in GC?

Stationary phase- long coiled column for the stationary phase, packed with a solid.

What is the mobile phase in GC?

Mobile phase- inert carrier gas moves sample molecules through the stationary phase.

How is GC carried out?

Sample is injected into the column through a self-sealing disc and the vapour formed is carried through the stationary phase using the inert-gas mobile phase.

When is the retention time recorded?

Once the molecules reach the detector, the retention time (time taken for component to move through the column) is recorded.

Where are the retention times recorded?

The retention times are recorded on a chromatogram where each peak represents a volatile compound in the analysed peak.

What does it mean if a sample travels through the column the quickest?

Quickest to travel through the column= weakest interaction with stationary phase.

Why is gas chromatography not useful for identifying specific compounds?

Gas chromatography is not useful at identifying specific compounds because different compounds may have the same retention time or it could be difficult to manage the conditions or some unknown compound may not have a reference for comparison in the databases.

What is done to combat this?

This is why gas chromatography is usually paired with mass spectrometry.