chromatography (3.3.16)

to separate and identify the components in a mixture

• thin-layer chromatography (TLC) • column chromatography (CC) • gas chromatogrpahy (GC)

• mobile phase - gas or liquid • stationary phase - solid, or liquid or solid on solid support

a plate is coated with a solid and a solvent moves up the plate

• wearing gloves, draw a pencil line 1cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along the line • use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry • add the solvent to a chamber or large beaker with a lid so it isn’t more than 1cm in depth • place the TLC plate in the chamber, make sure the level of the solvent is below the pencil line and replace the lid to get a tight seal • when the level of the solvent reaches about 1cm from the top of the plate, remove the plate and mark the solvent front with a pencil • allow the plate to dry in a fume cupboard • place the plate under a UV lamp to see the spots and draw around them lightly in pencil • calculate the Rf values using the observed spots

• gloves - prevent contamination from hands to plate • pencil line - won’t dissolve in solvent • tiny drop - if drops too big spots will merge • depth of solvent - if it is too deep it will dissolve sample spots from plate • lid - prevent evaporation of toxic solvent • fume cupboard - solvent is toxic • UV lamp - used if spots are colourless and not visible

• doing chromatography with two different solvents to separate a complex mixture • some components may have the same Rf value or not separate with the first solvent

a column is packed with a solid and a solvent moves down the column

• glass tube filled with stationary phase (usually silica or alumina in powdered form to increase surface area) • filter or plug used to retain solid in tube • solvent added to cover all the powder • mixture to be analysed dissolved in minimum volume of solvent and added to column • solvent or mixture of solvents is run through column • time for each components to reach end of column is recorded (retention time)

a column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at a high temperature

an inert gas eg. He, N2 (won’t react with components being separated)

the time taken for a compound to travel from the injection of the sample to where it leaves the column to go to the detector

• how many components are in the mixture (number of peaks) • the abundance of each substance (proportional to area under each peak)

• a mass spectrometer can be combined with the column • this generates a mass spectra which can be analysed or compared with a spectral database by computer to identify each component • eg. forensics, airport security, environmental analysis

the balance between solubility in the moving phase and retention by the stationary phase

using retention times and Rf values

by thinking about intermolecular forces: * if the stationary phase is polar and the mobile phase is non-polar, non-polar compounds will pass through the column more quickly than polar compounds * this is because they have a greater attraction to and solubility in the non-polar mobile phase