Chromatography Flashcards
(15 cards)
What is TLC?
Thin Layer Chromatography
Used to separate mixtures of organic molecules
Separation works by adsorbtion
What is the stationary and mobile phase in TLC?
STATIONARY PHASE: solid, doesn’t move, polar
MOBILE PHASE: organic liquid (solvent), carries the compounds up the TLC plate
What makes an Rf value low?
More polar the compound, the stronger the adsorption, the lower the Rf value
How is the Rf value measured?
- measure the distance travelled by the solvent (mobile phase) and the distance travelled by the individual spots
- individual spots normally represent single compounds
- distance travelled by compound
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distance travelled by solvent - will always be the same if conditions remain the same (temp/solvent stays the same)
What are limitations of TLC?
Two different compounds may have exactly the same Rf value due to similar polarities (same or similar functional groups)
Why is TLC a very powerful monitoring tool?
Only have to remove a very small amount of the reaction mixture and it can be monitored in real time
Monitor progress of reactions which do not have a fixed time yet so starting mixture (SM), reacting mixture (RXN), and product (P) are all on the same plate
What is gas chromatography?
Separation technique based on solubility in the mobile phase vs solubility in the stationary phase
What is the mobile and stationary phase in gas chromatography?
MOBILE PHASE: inert gas like helium
STATIONARY PHASE: high boiling point liquid adsorbed onto an inert solid
Volatile organic mixtures are vapourised and injected into the spectrometer for separation
What is retention time?
The time taken for a compound to travel through the column to the detector from the point of injection
Different compounds have different retention times - varies with solubility in the stationary phase and temperature of the column
Why does retention time wary?
- solubility in the stationary phase
- temperature of the column
What will more soluble compounds in the stationary phase do in GC?
- more soluble compound in stationary phase = longer the retention time
What will more soluble compounds in the mobile phase do in GC?
More soluble in mobile phase = faster it will travel = shorter retention time
What is considered when looking at GC?
- retention time = help identify the compounds by comparing them with retention times for pure known samples
- area under the peaks = determine relative amount of each compound in the sample (area under sample is proportionate to quantity)
What are the limitation of GC?
- similar retention times due to same/similar functional group
- reference retention times are impossible when dealing with unknown substances
- retention times are dependent on temperature, pressure, choice of gas, individual machine (can be controlled to change relative retention)
What are the other uses of chromatography?
- establish the concentrations of a substance in solutions of unknown compounds
- be combined with mass spectroscopy to identify a compound
