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Summary ALevel Chemistry - Gas Chromatography (GC) and GC-MS Chromatrography
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Gas Chromatography (GC) and GC-MS Chromatrography
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Gas Chromatography (GC) and GC-MSChromatrography
If there is a mixture of gas, volatile liquids, or solids in vapour form (which turn into gases) then
GC can be used to separate them out so they can be identified.
The stationary phase is a solid or solid coated by a viscous liquid such as an oil packed into a
long column. The column is coiled to save space and is then built into an oven. The mobile
phase is an unreactive carrier gas such as nitrogen and the sample is vaporised and passes
through the oven as a gas. Each component takes a different amount of time from being
injected into the column to being recorded at the end. This is called the retention time which is
shown on a chromatograph. The retention time depends on how much time the component
spends adsorbed to the viscous liquid. Under standard conditions each substance has a
separate retention time which helps identify the components of the mixture.
The area under each peak tells you the relative amount of each component present in the
mixture.
The peak with the shortest retention time spent the least amount of time in the stationary
phase.
Stationary Phase - long coiled column.
Mobile Phase - inert carrier gas.
Uses of GC:
GC can be used to find out the level of alcohol in blood or urine and the results are
accurate enough to be used in court.
It can also be used to find the proportions of various esters and oils used in plants.
GC-MS
Mass spectroscopy can be used to identify substances from their mass/charge ratio. It is good
at identifying singular substances but gives confusing results for mixtures. Gas chromatography
is good at separating a mixture into individual components but not good at identifying the
components.
1. The sample is separated using gas chromatography but instead of going into a detector
the components are fed into a mass spectrometer.
2. The mass spectrometer produces a mass spectrum for each component which can be
used to identify each one and show what the initial sample consisted of.
3. Computers can be used to match up the spectrum to a data base so the whole process is
automated.
The advantage of this method over normal GC is that the components separated by the
chromatograph can be positively identified which is impossible for a normal chromatograph due
to the similar retention times.
It is much faster and gives more accurate results.
Retention Time
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