Chromatography Flashcards
What is chromatography used for?
separate components in a mixture
What does the mobile phase do?
flows past the stationary phase carrying the components of the mixture with it
What does the stationary phase do?
slows components down to a varying extent allowing them to separate
What is thin layer chromotography used for?
to assess the progress of reactions and purify products
What is the stationary phase of a TLC?
a thin layer of solid eg silica gel or SiO2 mounted on a piece of glass, metal or plastic plate
What is the mobile phase of a TLC?
an organic solvent
How does the solvent move up the TLC plate?
by capillary action
How do the components of a mixture move up the TLC plate?
they dissolve in the solvent (mobile phase) and move up the plate with the solvent
but they are held back to varying degrees by adsorption to the plate
What does the speed of the component moving up the TLC plate depend on?
how strongly the component is adsorbed to the stationary phase
stronger adsorbtion = slower migration up
What are the 2 things the strength of adsorption depend on?
the attractive forces b/w the component and stationary phase
the solubility of the component in the mobile phase
How does the strength of adsorption depend on the attractive forces b/w the components and the stationary phase?
the stronger these attractions, the more difficult it will be for the solvent to dissolve the component and carry it up the plate = slower migration
How does the strength of adsorption depend on the solubility of the component in the mobile phase?
the more soluble the component is, the easier it is for the solvent to overcome the adsorption and free the component from the stationary phase = migrates faster up the plate
What does the components solubility in the mobile phase depend on?
solubility depends on the attractive intermolecular forces b/w each component and the solvent
What is the Rf value definition?
Rf is a measure of the distance migrated by an individual substance, relative to the distance moved by the solvent front
if the stationary phase is highly polar, what will the Rf value be like for polar substances?
lower Rf value = polar substances will adsorb strongly to it
What are the advantages of TLC?
quick and equipment is inexpensive
What are the disadvantages to TLC?
limited resolution - compounds with similar Rf values will not be clearly separated
What types of molecules may be difficult to separate?
molecules with similar sizes
molecules with similar functional groups
What is the stationary phase in gas chromotography (GC)?
a liquid adsorbed onto an inert solid support
What is the mobile phase in GC?
a gas that is unreactive and mostly helium
What is retention time?
the time taken for each component of the sample to travel through the capillary tube in GC
What causes a shorter retention time?
the more time a component spends moving with the gas rather than stuck in the stationary phase = shorter retention time
What increases retention time in the stationary phase?
stronger intermolecular forces b/w substances and stationary phase = greater solubility - will take longer to get through column
What causes polar substances to have a long retention time in polar stationary phases?
they can form permenant d-d forces whuch are stronger than induced d-d
What causes non-polar substances to have a long retention time in a non-polar stationary phase?
the molecules with a bigger surface area for contact with other substances will have longer retention times = they have more induced d-d to be broken to vapourise
What molecules will have similar retention times?
compounds with similar functional groups will tend to have similar interactions in the column = similar retention times
What does the area under the peak on a gas chromatogram show?
it is proportional to the amount of each compound
How to you find the actual concentration of a substance from gas chromatography?
using the peak area from a gas chromatogram, you need to use a calibration curve that is proportional to different concentrations
What other spectrum is GC combined with sometimes?
mass spectrums
What are the advantages of GC-MS?
- very good resolution - resolve complex mixtures
- use tiny amounts of sample mixture
- large database of mass spectra available = positive identification
What are the limitations of GC-MS?
- similar compounds have similar retention times = difficult to separate and identify
- unknown compounds will have no reference for mass spectra
- equipment is sophisticated and expensive
What are the uses of GC-MS?
- forensic science - traces from crime scenes and illegal drugs
- airport security - detecting explosives
- space missions - substances that could indicate life traces
What is the purpose of a cover on TLC?
prevent evapouration of the solvent
Why is the GC column inside an oven?
the sample needs to be in gas phases
Why is helium used instead of air in GC?
helium is unreactive
air contains O2 which can oxidise the sample reactants
How can a calibration curve be made to give a more accurate conc of sample?
use a molecule that is similar in shape to the sample to form calibration - it will have a similar retention time
How can you make colourless compounds visible on the TLC plate?
expose the plate to Iodine vapour = iodine adsorbs to the compound spots and it will appear brown
