Chromatography Flashcards
what is chromatography?
a series of separation techniques
what is the mobile phase?
the phase that carries the soluble components of the mixture with it
the more soluble the component in the mobile phase = ?
the faster it moves
what is the stationary phase?
the phase over which the mobile phase passes - it does not move with the sample
the more affinity the component has for the stationary phase = ?
the slower it moves with the solvent
what are the 3 types of chromatography?
thin-layer chromatography (TLC)
column chromatography
gas-liquid chromatography (GC)
what is the eluent?
the solvent in the mobile phase
what must all Rf values be less than?
1
Rf = ?
distance moved by spot / distance moved by solvent
what acts as the stationary phase in TLC?
a glass, metal or plastic sheet coated with a thin layer of silica gel or alumina (plates)
what are the advantages of TLC over paper chromatography?
it runs faster
the plates are more robust than paper
smaller amounts of mixtures can be separated
what acts as the stationary phase in column chromatography?
a powder (silica or aluminium oxide) or a resin
how does column chromatography work?
the eluent runs down the column and the components of the mixture move at different rates and so can be collected separately in flasks at the bottom
how can you get a better separation in column chromatography?
use more than one eluent
what acts as the stationary phase in gas-liquid chromatography?
a powder coated with oil, packed into or coated onto the inside of a long capillary tube
what is the mobile phase in gas-liquid chromatography?
an unreactive gas e.g. nitrogen, helium
how does gas-liquid chromatography work?
the sample is injected into the apparatus (under pressure at high temp) and carried along by the gas
the mixture separates because the components have different retention times so they reach the detector at different times
how can colourless spots be seen on a TLC chromatogram?
by shining UV light on the plate or by spraying the plate with a locating agent e.g. iodine (which reacts to give coloured compounds)
what does the number of spots on a TLC plate tell you?
the number of substances in the mixture
what would change the Rf value for a given substance?
if the temperature, solvent or make-up of the TLC plate changes
what is column chromatography often used for?
separating and purifying larger quantities of a mixture (TLC is useful for smaller quantities)
what is gas chromatography useful for?
to separate a mixture of liquids that are volatile
what is GCMS?
gas chromatography mass spectrometry (two techniques in one!)
how does GCMS work?
a mass spectrometer is used as the detector for a gas chromatography system
what does separation depend on?
the balance between solubility in the moving phase and retention by the stationary phase
what is used to identify substances in chromatography?
Rf values and retention times
