Chapter 33 - Chromatography
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what does the mobile phase do
carries the soluble components of the mixture with it
what does the stationary phase do
holds back components in the mixture that are attracted to it
describe in general terms how chromatography works
different components of a mixture are separated by their affinities for the mobile and stationary phases
describe how to set up a tlc chromatograph
• dissolve the mixture in some solvent to make a saturated solution • draw a faint pencil line on tlc plate 1.5cm from the bottom • place small drops of the mixture on the pencil line using a capillary tube and allow to dry fully • fill a developing tank with solvent so that the depth of the solvent is below 1cm • place the plate in and seal it with a lid
why should gloves be worn when handling a tlc plate
so that subtances on hands do not contaminate the plate
why must a lid be used in tlc chromatography
as the solvent is often volatile, a lid prevents it from escaping it also ensures a saturated atmosphere
why should the depth of the solvent not exceed the penil line
to prevent the mixture from going down into the solvent and ensure it travels up the plate
why should the tlc plate be dried in a fume cupboard
to prevent the volatile solvent from contaminating the atmosphere, as it is often toxic
what is compounds are used as the stationary phase on tlc chromatography
either silicon dioxide or aluminium oxide also called silica gel or alumina
what advantages does tlc chromatography have over paper chromatography
it is faster smaller samples of the mixture can be used spots usually spread out less plates are more robust than paper
how to calculate rf value
the distance traveled by the spot divided by the distance travelled by the solvent
what does a large rf value signify
a large affinity for the mobile phase
if the stationary phase is non-polar and a substance is polar, what will its rf value be
close to one
if the stationary phase is non-polar and a substance is non-polar, what will its rf value be
close to zero
if the stationary phase is non-polar and a substance is non-polar, what will its retention time be
high
if the stationary phase is non-polar and a substance is polar, what will its retention time be
low
how can you identify what amino acids are present in a substance after tlc chromatography
using ninhydrin or placing the plate under uv light
what is the stationary phase in column chromatography
a power packed into a column silica or aluminium oxide or a resin is used
how does column chromatography work
the solvent is added to the column as it runs down the column, the different components in the mixture move at different rates based on their affinities for the stationary phase and mobile phase
what are the advantages of column chromatography over other methods
fairly large amounts of mixtures can be separated and collected
how does gas chromatography work
a mixture of gases is passed through a long capillary tube packed with power and the different components take different times passing through the tube based on their affinities for the powder different compounds leave the column at different times, called retention times
how can the amount of a component in a gas mixture be identified using a graph
the area under the peak is proportional to the amount of the component
what are the benefits of gas chromatography
it is extremely sensitive and is especially useful for detecting trace substances in food or urine which makes it a good drug test
what are the stationary and mobile phases in gas chromatography
the stationary phase is the packed power and oil in the capillary tube the mobile phase is an inert gas such as argon