chromatography and spectroscopy Flashcards
what is chromatography
chromatography is an analytical technique used to seperate and identify components of a mixture
what are the different methods of chromatography
- thin layer chromatography (TLC)
- gas chromatography (GC)
what is the stationary phase
the stationary phase does not move and is normally a solid or a liquid supported on a solid
what is the mobile phase
the mobile phase does move and is usually a gas or a liquid
what is the meaning of phase
a phase is a physically distinctive form of a substance , such as the solid , liquid and gaseous states of ordinary matter
what is the TLC plate in chromatography
a flat plate either plastic or glass coated with a thin layer of a solid adsorbent substance - usually silica this is the stationary phase
why is the jar sealed in TLC chromatography
to prevent evaporation
how do you work out the Rf value
distance moved by spot / distance moved by solvent
how do you carry out TLC
- take a TLC plate . using a pencil , draw a line across the plate about 1cm from one end of the plate . this is the base line
- using a capillary tube , spot a small amount of a solution of the sample onto the base line on the plate
- prepare a chromatography tank for the TLC plate . this can be made from a small beaker with a watch glass placed ontop . pour some solvent into the beaker to a depth of about 0.5cm.
- place the prepared TLC plate in the beaker . making sure that the solvent does not cover the spot . cover the beaker with the watch glass placed on the top. cover the beaker with the watch glass and leave it undisturbed on the bench. the solvent will rise up the TLC plate
remove the plate from the beaker and immediately mark the solvent front with a pencil. allow the plate to dry
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how do you analyse the results from a TLC plate when they are colourless
- use UV light - some may show up in UV light , so this is used to temporally reveal the spots. they are circled in pencil so an RF value can be calculated
-other molecules may need to be sprayed with a locating agent . such as ninhydrin . this will stain the molecule so the spot can be easily seen
what are the differences between TLC and paper chromatography
both TLC and paper chromatography have a mobile phase which is a liquid solvent .
the results for both types are also measured using Rf values
- however the stationary phase is a thin layer or silica rather than paper for TLC chromatography
what are the benefits of TLC chromatography vs paper chromatography
- TLC chromatography mobile phase moves more quickly and more evenly
- there is often better separation between components and so is more easily analysed
how does seperatation of a mixture occur in TLC chromatography
- separation is by adsorption
-adsorption is the process by which a solid holds molecules of a gas or liquid or solute as a thin film on the surface of a solid or more rarely a liquid
1) first the particles of the dyes all start off in one place
2) when the solvent arrives , some of them spend more time in solution than others
3) the ones that spend the largest proportion of the time in the solvent move the quickest
what is gas chromatography
gas chromatography is useful for separating and itentifying volatile organic compounds present in a mixture
what is the stationary phase in gas chromatograpy
the stationary phase is a high boiling liquid adsorbed onto an inert solid support.
what is the mobile phase in gas chromatography
an inert carrier gas
what are the differences between gas chromatography and paper chromatography
- both are techniques that can seperate mixtures
- however the mobile phase is an inert gas (nitrogen) in GC and the stationary phase is an organic liquid . the sample injected must be gaseous and the analysis is measured by retention time
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what are the advantages of gas chromatography over paper chromatography
gas chromatography can separate much more complex mixtures and it is quicker. it is more sensitive , analysing not only what is present , but also how much of it there is
what are the applications of chromatography
-chemical analysis
-catching drug cheats
-forensic science
-enviromental monitoring
-food additive monitoring
what are the limitations of chromatography
- similar compounds often have similar Rf values
- unknown compounds have no reference Rf value for comparison
-difficulty in finding a solvent that separates all the compounds in a mixture. if the components are very soluble in the solvent , they will just be washed up the TLC plate with the solvent front. if the components have little solubility they will hardly move . a mixture of solvents is required
