A2/A-level Chemistry: Summary of analytic techniques

• Analytic techniques are important part of chemistry as it helps chemist to conduct qualitative and quantitative analysis on various chemicals
• Classic methods consist of separation like precipitation, extraction, distillation etc...
• Modern methods are mentioned in topic 9 and 17 for Edexcel A-level Chemistry

• Tiny amount of the sample is injected into the chromatography column
• Sample is vaporized and carried through the column by the flow of inert gas
• The column contains a liquid ---> stationary stage and it is adsorbed onto the surface of an inert solid
• the components of the sample separate due to them traveling at different speeds ---> reaching the detector
• A flame ionizer burns the separated gases and measures the electrical conductivity of ions present

Normal Phase HPLC

• The column is filled with tiny polar silica particles
• non-polar solvent
• Components in sample are separated according to their differences in polarity/retention time
• Retention time: time needed for the components to be eluted out from the column
• Polar molecules in sample will stick to the polar silica, thus longer time retention time
• Non-polar molecules will run through column more quickly, shorter retention time

Reverse phase HPLC

• The column is filled with tiny non-polar silica particles
• polar solvent
• In reverse, polar molecules in sample have strong attraction to the polar solvent, thus shorter time retention time
• Non-polar molecules will run through column more slower as it is attracted by the non-polar silica, longer retention time

• A radiation with lower energy and shorter wavelength (Radio waves < Microwave < Infrared radiation)
• Microwave radiation creates an electric field which causes polar molecules to line up with the field
• The field switches to the opposite direction so the molecules swivel round to line up again
• Energy in the microwaves is converted to thermal energy
• can be used to separate compounds that dissolved in a solvent (dissolution)
• other uses involve hydrolysis of organic/inorganic compounds, complex metal ions formation etc...

Vibrational modes of molecules (>=2 atoms)

• Symmetrical stretching
• Asymmetrical stretching
• Scissoring
• Rocking
• Wagging
• Twisting

Mechanism

• Molecules absorb infrared radiation which causes the bond to vibrate in various modes
• Different bonds require different amounts of energy to cause vibrations
• Not all molecules are IR active – suitable for those molecules that change their dipole moment & heteronuclear (mostly polar molecules)
• e.g. H₂O, CO₂, HCl, CO...
• Mainly used for functional groups analysis in organic molecules

• Uses electromagnetic and radio frequencies and determine the chemical environment that hydrogen are in for target compound
• Every nuclei in atom has their own spin and are electrically charged
• The energy requires to change the alignment of magnetic nuclei will be measured in external magnetic field
• The energy involved vary as the chemical environment differs for each hydrogen atom
• position of hydrogen can be deduced by studying the signal intensity, couplings and chemical shifts
• mostly used to determine exact structure and connection of atoms in organic compounds

High Resolution NMR

• This splits the peaks into groups of signal ---> multiplets
• This is due to the interaction between protons on neighboring atoms
• n+1 rule: The number of lines on the split pattern for the peak is one more than the number of hydrogen on the adjacent atoms

Low Resolution NMR

• Cannot distinguish individual signals in peaks
• The peaks indicate the different chemical environments of hydrogen
• The area under the peaks correspond to the number of hydrogen in each environment

This is the end of the topic.

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