chromatography and spectroscopy

Question 1

what is chromatography

Answer 1

chromatography is an analytical technique used to seperate and identify components of a mixture

Question 2

what are the different methods of chromatography

Answer 2

• thin layer chromatography (TLC)
• gas chromatography (GC)

Question 3

what is the stationary phase

Answer 3

the stationary phase does not move and is normally a solid or a liquid supported on a solid

Question 4

what is the mobile phase

Answer 4

the mobile phase does move and is usually a gas or a liquid

Question 5

what is the meaning of phase

Answer 5

a phase is a physically distinctive form of a substance , such as the solid , liquid and gaseous states of ordinary matter

Question 6

what is the TLC plate in chromatography

Answer 6

a flat plate either plastic or glass coated with a thin layer of a solid adsorbent substance - usually silica this is the stationary phase

Question 7

why is the jar sealed in TLC chromatography

Answer 7

to prevent evaporation

Question 8

how do you work out the Rf value

Answer 8

distance moved by spot / distance moved by solvent

Question 9

how do you carry out TLC

Answer 9

• 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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Question 10

how do you analyse the results from a TLC plate when they are colourless

Answer 10

• 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

Question 11

what are the differences between TLC and paper chromatography

Answer 11

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

Question 12

what are the benefits of TLC chromatography vs paper chromatography

Answer 12

• TLC chromatography mobile phase moves more quickly and more evenly
• there is often better separation between components and so is more easily analysed

Question 13

how does seperatation of a mixture occur in TLC chromatography

Answer 13

• 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

Question 14

what is gas chromatography

Answer 14

gas chromatography is useful for separating and itentifying volatile organic compounds present in a mixture

Question 15

what is the stationary phase in gas chromatograpy

Answer 15

the stationary phase is a high boiling liquid adsorbed onto an inert solid support.

Question 16

what is the mobile phase in gas chromatography

Answer 16

an inert carrier gas

Question 17

what are the differences between gas chromatography and paper chromatography

Answer 17

• 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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Question 18

what are the advantages of gas chromatography over paper chromatography

Answer 18

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

Question 19

what are the applications of chromatography

Answer 19

-chemical analysis
-catching drug cheats
-forensic science
-enviromental monitoring
-food additive monitoring

Question 20

what are the limitations of chromatography

Answer 20

• 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