Chromatography Flashcards
Phases
The mobile phase may be a liquid or a gas. The stationary phase may be a solid (as in thin- layer chromatography, TLC) or either a liquid or solid on a solid support (as in gas chromatography, GC)
Gas-liquid chromatography phases
In gas-liquid chromatography GC the mobile phase is a inert gas such as nitrogen, helium, argon.
The stationary phase is a liquid on an inert solid.
Thin Layer Chromatography
a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line.
b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry.
c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal.
e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.
Types of stationary phases
A solid stationary phase separates by adsorption,
A liquid stationary phase separates by relative solubility
Two directional chromatography
In order to separate a complex mixture that has components of different solubility in solvents, it may be necessary to do chromatography with two different solvents.
A spot of the mixture on a TLC plate is first separated with one solvent.
Then the TLC plate is rotated 90o and the plate is placed in a second solvent for a second separation to take place
Column chromatography
- A glass tube is filled with the stationary phase usually silica or alumina in powder form to increase the surface area.
- A filter or plug is used to retain the solid in the tube. Solvent is added to cover all the powder.
- The mixture to be analysed is dissolved in a minimum of a solvent and added to the column.
- A solvent or mixture of solvents is then run through the column.
- The time for each component in the mixture to reach the end of the column is recorded (retention time)
HPLC
HPLC stands for high performance liquid chromatography and it type of column chromatography commonly used in industry.
HPLC: stationary phase is a solid silica
HPLC: mobile phase a liquid
Gas liquid chromatography
Gas-liquid chromatography can be used to separate mixtures of volatile liquids
The time taken for a particular compound to travel from the injection of the sample to where it leaves the column to the detector is known as its retention time. This can be used to identify a substance.
phases in gas liquid chromatography
the mobile phase is an inert gas such as helium and the stationary phase is a high boiling point liquid absorbed onto a solid.
Basic gas liquid chromatography
will tell us how many components there are in the mixture by the number of peaks. It will also tell us the abundance of each substance. The area under each peak will be proportional to the abundance of that component.
Inert carrier gas in gas liquid chromatography
It is important to use an inert carrier gas such as helium or nitrogen. These will not react with the components being separated in the GC column.
changing reaction time in gas liquid chromatography
The following factors can be changed to change the retention times of substances being separated: GC column temperature, column length , flow rate.
If the temperature or the flow rate is higher then substance will move more quickly through the column to give shorter retention times.
