Chromatography Flashcards
chromatography is used to separate ___ substances
coloured
absorption refers to a substance in the mobile ____ phase binding to the surface of the stationary ____ phase
gas/liquid; liquid/solid
the solid phase should bind ______ to the substances to be separated
reversibly
why should the solid phase bind reversibly to the substances to be separated?
so the compound can actually separate and move
the surface area of a solid should be
large
what are the advantages of chromatography? (4)
- highly selective
- simple apparatus
- can be used for small-medium samples (mg-10xg)
- can be used for large samples (100xg)
why is it important to shut the cover of the developing chamber?
- the chamber must be saturated with solvent vapour
2. if the vapours escape, it will mess up the Rf
the level of liquid in the developing chamber should be ___ the applied spots of the sample
beneath
why must the solvent not be allowed to travel to the top of the chromatography plate?
the solvent front is used as reference in calculating Rf
how can 2 plates be developed at once?
back to back in the chamber
what happens if a solvent is too polar in TLC?
all components move to the top of the plate
If using a polar solid phase, a small Rf value means the substance is (more/less polar)
more polar (attracted to the polar solid)
list the 7 requirements for an absorbant/ solid phase
- large surface area
- polar functional groups
- stable/inert to maintain nature over time
- react reversibly with components
- preferably white
- cheap, readily available
- highly selective
why is a pencil not used to mark the chromatograph?
ink may run and interfere
why should air bubbles not be made in the absorbant of the column?
they restrict flow by limiting contact with sand, this increases band size
why must elution in the column start with the least polar component?
because they will travel farthest and fastest. Banding will be affected
would you use TLC to separate 5ml of sample?
no, too much, use column or partition
why must a pure sample be spotted on the same plate as an unknown?
to compare colour and Rf in order to identify unknown
what 3 important characteristics of the alumina in the column that are used for chromatography?
- white
- highly selective
- large surface area
state the 5 differences between TLC and column
- TLC faster
- TLC = spots, column = bands
- TLC = simpler, cheaper
- TLC = cant separate large volume
- TLC goes against gravity, column goes with gravity
how is Rf calculated?
distance from center of spot from origin / distance of solvent front from origin
