Chromatography
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When performing TLC chromatography, how does the stationary phrase work?
• This phase uses a thin metal sheet, usually made of Al₂O₃ or SiO₂ • The molecules in the solute adsorb onto the surface • Depending on the strength of said adsorbsion, the components of the substances will travel paticular distances through the plate
What occurs during the mobile phase of TLC chromatography?
• The solvents flow over the stationary phase sheet • A solvent carries the components of the compound being investigated
State an example of a polar solvent.
Water or ethanol
State an example of a non-polar solvent.
Alkanes
How are spots identified on a TLC diagram?
• UV light • Ninhydrin • Iodine vapour
State an example of a non-polar solvent.
Alkanes
How are spots identified on a TLC chromatogram?
• If the sample components are coloured, their spots are obviously identifiable. • If the sample components are not coloured, we can locate them and then draw around them using pencil. The spots can be located by either using…. - UV light - Ninhydrin - Iodine vapour
Why is ninhydrin not used to locate TLC spots in a lab environment?
Carcinogenic
State a full method for setting up a TLC chromatograph.
1) Prepare a beaker with a small amount of solvent at the bottom. 2) On the TLC plate, draw a horizontal line at the bottom edge in pencil. 3) Place a spot of the pure compound at the end of the pencil line as a reference then a spot of the impure sample that is analysed along the baseline and allow to air dry. 4) Place the TLC plate inside the beaker with the solvent. The solvent should not cover the spots. Add a lid to cover the beaker. The solvent will travel up the plate, dissolving the compounds as it goes. 5) As the solvent reaches the top, remove the plate and draw another pencil line indicating where the solvent has reached. This is the solvent front.
What is the equation to calculate the Rf value?
Distance travelled by component/Distance travelled by solvent
What is the link between polarity of molecules and their Rf values?
• Less polar components travel further up the plate and as such have a higher Rf value. • More polar components do not travel as far up the plate.
What types of compound can gas chromatograms analyse?
• Gases • Volatile liquids • Solids in their vapour form
What happens during the stationary phase of a gas chromatogram?
A long coiled column is used with a non-volitile liquid present
What is the link between polarity of molecules and their Rf values?
• Less polar components travel further up the plate and as such have a higher Rf value. • More polar components do not travel as far up the plate.
What happens during the mobile phase of a gas chromatogram?
• The sample is injected into the column using a self-sealing disc. The vapour formed is carried through the stationary phase in the mobile phase. • An inert carrier gas (helium or nitrogen) moves the sample molecules through the stationary phase.
What is meant by a retention time?
• The time taken for a component to travel through the column of gas
What effects the retention time when performing gas chromatography?
• The attraction between the solute and the phases of the chromatography • The volatility of the solute
How can a compound be identified from it’s retention value graph?
Measure the retention time and compare that time to a known value in a data booklet.
How is an organic liquid purified?
• Pour mixture into a separating funnel • Neutralise any excess acid using aqueous sodium carbonate • Allow 2 layers to settle. Use density values to identify the organic layer. • If density values are unknown, add water. The aqueous layer grows in size. • Collect the organic layer. • Add a drying agent to remove small traces of water still left in the organic layer. • Distill the organic product and collect the fraction that boils at the known BP of the pure substance.