Unit 4 - Choromatography Flashcards
What is chromatography
- a method used to seperate soluble substances by their partition between two different phases
(A method to separate mixture of substances )
What phases to all types of chromatography have
Mobile phase
Stationary phase
What is the stationary and mobile phase
Stationary phase - substance stays fixed inside the column
Mobile phase - solvent moving through column
What does TLC use
Instead of chromatography paper (paper chromatography ) TLC uses a thin layer of silica gel or alumina coated on glass/ metal or plastic
Silica gel is a _____ formed from silicic acid that has many _______ groups
What does this do ?
Polymer
Hydroxyl
Makes the surface of the gel very polar
So.. ______ substances tend to be held by the gel , making their movement much slower than that of ___ ______ substances
Polar
Non polar
Remember : chromatography separates substances by making use of differences in their _____ or _______ ______
Polarity
Molecular size
What makes up the stationary phase / mobile phase in TLC
The water held on silica gel makes up stationary phase
The mobile phase is a suitable solvent or mixture of solvents
Different components in the mixture travel across the _________ _____ at different rates
What does this allow
Stationary phase (the silica gel) This allows them to separate as the mobile phase moves along the stationary phase
How is a TLC plate prepared and explain the chromatography happening
- to prepare a TLC plate , a fine pencil line called the base line is drawn about 1.5cm from one end. A pencil cross is put where a sample is to be added.
- the sample solution is spotted on several times to get a concentrated spot in the plays
- the TLC plate is then placed in a suitable solvent (the mobile phase ) with the solvent is below the spot
- the solvent travels up the plate, and the components of the sample separate out according to their relative attractions to the stationary phase on the plate
- once finished a pencil line is drawn to show where the solvent has reached on the chromatogram - this is called the solvent front.
The distance the solvent has moved is measured in _______
Centimetres
How is the Rf value (retardation value) calculated
Rf = distance travelled by the sample (to centre of spot) / distance travelled by solvent (where the solvent front is)
Rf = spot / solvent front
Remember
: molecules that have more polarity will move _____ through the plate than molecules with less polarity
Molecules with a large molecular mass will move more _____ than molecules with a low molecular mass
- slower
- slowly
So… larger and more polar molecules will have _____ rf values
Lower
The uses of TLC
- to identify a compound /substance
- to assess the purity of a compound
Identifying a compound / substance with TLC
The identity of a compound can be confirmed by comparing Rf values with a literature or known value determined under the same conditions. Pure samples can be run alongside the sample being analysed to see whether the same Rf values are obtained
Assessing the purity of a compound using TLC
- another way to determine whether a sample is pure is to use a co spot.
- this is a 50:50 mixture of the sample and a known pure sample and it can be spotted on the same cross. If the sample is pure then only one spot will be present on the chromatogram. If the sample is impure, then more than one spot will be observed
What has an effect on Rf value for an individual component
- type of stationary phase
- composition of mobile phase
- polarity of component
- the solvent used
- Distance solvent front moves has no effect
- distance solvent moves has no effect
- concentration of sample has no effect
- length of TLC plate has no effect
