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Chemistry Chromatography – Q’s And A’s
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Chemistry Chromatography – Q’s And A’s
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Chemistry Chromatography – Q’s And A’sWhat is chromatography? ✔️Ans - Technique used to separate and
identify the substances present in a mixture.
What is a stationary phase? What is the mobile phase? ✔️Ans - SP:
Remains immobile so that different components of a mixture can adhere or
stick to it.
MP:
Adsorption vs desorption? ✔️Ans - The process of sticking to a surface
The process of detaching from the surface of the stationary phase
What is the stationary phase and mobile phase in paper chromatography?
✔️Ans - Stationary: Special chromatography paper
Mobile: solvent mixture
How does paper chromatography work? ✔️Ans - ♥ Paper has regions of
pos and neg charge on surfacedue to polar cellulose molecules. Molecules of
different components stick to surface by electrostatic attraction.
♥ Electrostatic attraction between surface particles of stationary phase and
molecules in mixture is adhesive force and can involve dispersion, dipole
dipole, hydrogen bonding etc.
♥ components attach, dissolve back into solvent then attach again
♥ components travel at different speeds and so separate out
Rate of movement of components depend on ✔️Ans - ♥ how strongly it
adsorbs to SP
♥ how readily it dissolves in mobile phase
Rf value is ✔️Ans - distance moved from origin by component over
distance moved from origin by solvent
What can we do if sample is uncoloured? ✔️Ans - ♥ UV light: organic
compounds fluoresce and appear blue under UV light
♥ Spray to make coloured: chromatogram can be sprayed to chemical to form
coloured/fluorescent compounds
, ♥ Radioactive labelling and xray: radioactively label your desired components
and then take x ray of TLC plate
Gas chromatography used to: ✔️Ans - identify and determine
concentrations of gas mixtures/liquid mixtures (easily vapourised)
Main differences between GC and TLC ✔️Ans - 1. Gas particles travel
quickly in comparison to liquids thus larger distance for them to have a
chance to separate
2. Gas readily diffuse into space thus gas mixture must be confined to a
narrow tube called a 'Capillary tube'
3. Forces between gases are negligible thus components not attracted to the
mobile phase but are swept along it (adsorption and desorption occurs in the
column)
MP in gas chromatography ✔️Ans - An inert gas that sweeps along the
components and doesn't react with or attract them
GLC vs GSC ✔️Ans - ♥ GLC: uses a liquid SP which is spread over a porous
support. Liquid can be high bpt hydrocarbon or ester
♥ GSC: uses a solid SP (tiny glass beads or ground brick)
Retention time ✔️Ans - ♥ qualitative measure
♥ depends on molecular structure of components being separated,
temperature, length of column, nature & chemical structure of stationary
phase
Peak area ✔️Ans - ♥ quantitative measure
♥ directly related to concentration of substances present
♥ standards of known concentrations can be run and peaks measured and
used to generate a calibration curve (used to determine unknown
concentrations)
Advantages and Disadvantages of gas chromatography ✔️Ans - ♥
Advantages: very sensitive, rapid and reliable
♥ Disadvantages: limited to compounds that can be readily vapourised
without decomposing
Hence usually used for organic compounds that have <300gmol, weak
intermolecular forces but strong intramolecular forces
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