Chapter 29
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What is chromatography used for?
to separate individual components from a mixture of substances
What do all forms of chromatography have?
1) stationary phase 2) mobile phase
What state is the stationary phase?
solid/liquid supported on a solid
What state is the mobile phase?
liquid/gas
What is chromatography used for?
1) analysis of: drugs/plastics/flavouring/air samples/air samples 2) applications in forensic science
What is a TLC plate?
plastic sheet/glass coated with a thin layer of a solid adsorbent substance - usually silica
In TLC, what phase is the adsorbent?
stationary phase
What is adsorption?
the process by which the solid silica holds different substances in the mixture to its surface
How do you carry out TLC?
1) draw a pencil line 1cm from the end of a TLC plate 2) use a capillary tube to spot a small amount of the sample solution on the base line 3) prepare a chromatography tank from a small beaker with a watch glass on top. pour solvent into the beaker to a depth of 0.5cm 4) place the TLC plate in the beaker, making sure the solvent does not cover the spot, and cover with the watch glass 5) allow the solvent to rise until it is 1cm below the top of the plate. remove the plate and mark the solvent front in pencil 6) circle any visible spots/hold a UV lamp over the plate to locate spots/spray with a locating agent (iodine) to locate spots
How are thin layer chromatograms analysed?
by calculating the value for the retention factor, Rf
Rf =
distance moved by the component / distance moved by the solvent front
Why would you run a TLC alongside pure samples of compounds that may be present?
it is easier to identify the amino acids in the unknown sample visually, without needing to calculate any Rf values
What is gas chromatography used for?
separating + identifying volatile organic compounds present in a mixture
What is the stationary phase in gas chromatography?
a high boiling liquid adsorbed onto an inert solid support
What is the mobile phase in gas chromatography?
an inert carrier gas (helium/neon)
How do you carry out gas chromatography?
1) small amount of volatile mixture injected into apparatus - gas chromatograph 2) mobile carrier gas carries components of the sample through capillary column (contains liquid stationary phase) 3) components slow down as they interact with stationary phase inside column 4) components are separated depending on solubility in stationary phase
The more soluble the component is in the liquid stationary phase, the ___ it moves through the capillary column.
slower
What is retention time?
the time taken for each component to travel through the capillary column
What can retention times be used to identify?
the components present in the sample by comparing these to retention times for known components
What is a peak integration?
the area under a peak
What can peak integrations be used to determine?
the concentrations of components in the sample
How do you determine the concentration of a component in a sample?
1) prepare standard solutions of known concentrations of the compound being investigated 2) obtain gas chromatograms for each standard solution 3) plot calibration curve of peak area against concentration 4) obtain gas chromatogram of compound being investigated under same conditions 5) use calibration curve to measure concentration of compound
How do you test for an alkene?
add bromine water drop-wise - bromine water decolourises from orange to colourless
How do you test for a haloalkane?
add silver nitrate + ethanol + warm to 50C in water bath: 1) chloroalkane - white precipitate 2) bromoalkane - cream precipitate 3) iodoalkane - yellow precipitate