chromatography and qualitative analysis Flashcards
what are the basic principles of all kinds of chromatography
- a family of separation techniques that depend on the principle that a mixture is separated
- if it is dissolved in a solvent and this mobile phase is passed over solid (the stationary phase)
what is the mobile phase
carries the soluble components of the mixture
what relationship between a sample and the mobile phase makes the sample move faster
more soluble components / components with more affinity to the solvent move faster
what does the stationary phase do?
holds back components of the mixture that are attached to it
what relationship between a sample and the stationary phase makes the sample move slower?
what kind of bonding does this often involve?
- more affinity for the stationary phase means that a component moves slower
- often attracted by hydrogen bonding
how are substances separated by chromatography
- if suitable stationary/mobile phases are chosen
- the balance between affinity for the mobile phase and affinity for the stationary phase is different for each component of the mixture
- they move at different rates and are separated over time
why will different substances show different Rf values
- they are bonded differently and have different polarities
- more polar bonds mean longer retention time or small Rf
- since hydrogen bonding/dipoles are attracted more strongly to the stationary phase
what does TLC stand for?
thin layer chromatography
what is the stationary phase in TLC
plastic/glass/metal sheet or plate coated in silica (SiO2) or alumina (Al2O3)
what are the advantages of TLC over paper chromatography
- runs faster
- smaller amounts of a mixture can be separated
- TLC plates are more robust than paper
how can you observe colourless spots
- shine UV light on them
- or spray with a developing agent
- developing agent for amino acids ninhydrin turns amino acid spots from colourless to purple so they can be seen
how do you calculate the Rf value?
- measure the distance from the initial line (that the mixture was spotted onto) to the solvent front, and the distance from the initial line to the spot
- calculate Rf using Rf = distance moved by spot / distance moved by solvent front
what does Rf value stand for
- retention factor: a measure of the rate of movement of a component through the chromatography apparatus
- a ratio between the rate of movement of the solvent and that component
how could you confirm the identity of a substance from its Rf value
- compare your Rf value to accepted values Rf for that substance run in the same solvent and set-up
- if they match then identity is confirmed
what is the stationary phase in gas-liquid chromatography
- powder, coated with oil
- packed into a long, thin, capillary tube
- coiled and place in an oven, the temperature of which can be varied
what is the mobile phase in gas-liquid chromatography
- carrier gas
- inert N2 or He
what do you measure in gas-liquid chromatography
- retention time
- different components of the mixture take different amounts of time to move through
what are the advantages of GLC (gas-liquid chromatography)
- very sensitive
- GC can detect minute traces of substances in foodstuffs and link oil pollution on beaches to the specific tanker the oil came from
what are GLC’s uses (gas-liquid chromatography)
- tests athletes and horses blood
- urine for drugs
how can you use GC or GCMs to identify substances
- match the gas-chromatograph to that of a known substance under the same conditions
- retention time should exactly match
- substance’s identity can be confirmed by mass spectrometry, NMR or infrared spectroscopy
how does GCMS work
- gas chromatography is run
- retention time is recorded
- then the mixture is run through a mass spectrometer
- fragmentation pattern/molecular ion peak confirms identity
how do you test for alkenes? what is the result
- shake with bromine water
- the result is bromine water is decolourised (orange to colourless)
how do you test for haloalkanes? what is the result?
- add NaOH and warm
- acidify with HNO3
- add AgNO3
- result: precipitate of AgX (Cl = white, Br = cream, I = yellow)
how do you test for alcohol? what is the result?
- add acidified K2CR2O7 and reflux
- result: colour change from orange to green, no change for tertiary alcohols
how do you test for aldehydes? what is the result
- warm with fehlings solution, result: red brick ppt forms from blue solution
- warm with tollens reagent, result: silver mirror (Ag ppt) forms
how do you test for carboxylic acids? what are the results
- add Na2CO3
- result CO2 given off - effervescence
how do you test for phenols?
- weak acidity so a neutralisation reaction
- reacted with NaOH but no reaction with CO3 2-
how do you test for carbonyl compounds
react with 2,4 DNP and an orange precipitate should form
