column and gas chromatography

what is the purpose of column chromatography?

• purification technique

• allows us to purify organic compounds

summarise how column chromatography works:

• large glass column packed w/ solid powder (stationary phase)

• sample to be purified added on top

• solvent (mobile phase) flushed through column

• this separates out of the components in the mixture (based on their affinity for the stationary phase or how well they dissolve in the mobile phase)

give 2 examples of what the stationary phase might be in column chromatography:

• silica

• aluminium oxide

what is retention and how is it affected by polarity?

• how strongly a compound is absorbed onto the stationary phase

• like R_f values, they allow certain substances to be identified

• more polar groups will be retained longer through the column (they will come out later)

what is the stationary phase in gas chromatography?

powder packed inside a long capillary tube by an oil

what is the mobile phase in gas chromatography?

unreactive gas e.g. N/Ar that carries the sample along

summarise the process of gas chromatography:

• sample injected - when gas begins to flow, some components flow along w/ gas

• this is when mixture begins to separate - some is retained by oil

• components leave the column (usually a coiled tube) at different times depending on their retention - retention time

in gas chromatography, what does the retention time depend on?

how much time the component spent moving along w/ carrier gas

what does a GC-trace show?

• how long component took to move through the gas column

• area under peak = relative amount in mixture

  

give some uses of gas chromatography:

• often coupled w/ a mass/NMR spectrometer to find mass/charge ratio / structure of components in mixture

• extremely sensitive so can be used in blood/urine samples for testing drug-taking in athletes

• can be used to match compounds by comparing retention times - if same mobile/stationary phases used

what is a GC-MS?

• gas chromatography set up couples w/ a mass spectrometer (hence GC-MS)

• separates out substances then produces mass spectrum for each component)