Chapter 29 - Chromatography & Spectroscopy

A group of techniques used to separate mixtures

Stationary - does not move and normally a solid or a liquid supported on a solid Mobile - does move and is usually a liquid or gas

Paper: paper, solvent (water) -> depends on solubility of different compounds Thin Layer: thin layer of adsorbent such as silica or alumina gel coated on a glass/plastic sheet, solvent (liquid) -> stronger adsorption to the stationary phase = slower Gas: high boiling point liquid adsorbed onto an inert solid support, inert gas (noble gases) -> solubility in stationary phase

The process by which the solid silica holds the different substances to its surface

Distance travelled by solute (middle of the spot / distance travelled by solvent (bottom of the top line)

Compare Rf values to a database (under same conditions) to identify substances after

Using UV light or a locating agent such as iodine to visualise spots

• to prevent volatile and potentially hazardous solvent from escaping • produces a saturated atmosphere of the solvent to improve the separation

To prevent the baseline from dissolving in solvent

• A small quantity of a volatile mixture is injected into the gas chromatograph • The mobile carrier gas carries the components through a capillary column which contains the liquid stationary phase adsorbed onto a solid support • The components slow down as they interact with the stationary phase. They are separated based on their solubility in the liquid phase • The compound in the mixture reach the detector at different times and retention time is found • Retention times can be compared to a database under same conditions to identify substances after

Time taken for a component to travel through the column

To determine the concentration of components in the sample

• Make and test a number of standard solutions of different concentrations • Measure peak area • Plot a calibration curve (x-axis = concentration, y-axis = peak area) • Run gas chromatography of unknown sample and measure peak area • Use calibration curve to identify the concentration

Using IR/ mass spectroscopy or compare MP/BP

• a strong magnetic field • radio frequency radiation

The protons and neutrons in the nucleus They have spin

When there is an odd number of nucleons so the unpaired nucleon produces a small residual nuclear spin which generates the magnetic field

In 2 directions: - with the field (more stable, lower in energy) - oppose to the field (higher energy, less stable)

A nucleus in its low energy spin state can be promoted to its upper energy spin state by providing energy that is exactly equal to the energy gap This energy is supplied by low-energy radio-frequency radiation. The larger the energy gap, the higher the frequency required The excited nucleus will then drop back to its low-energy ground sate, emitting exactly the same amount of energy With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flips between the two spin states

• the applied strong magnetic field -> stronger external magnetic field = bigger energy change • a weaker magnetic field generated from electrons surrounding the nucleus and in nearby atoms

Electrons shield the nucleus from some of the applied magnetic field depending on the electron density in nearby atoms or groups More electron density = lower energy change

The scale that measures frequency shift in the energy required for resonance compare to TMS, measured in parts per million (ppm)

Depends on the chemical environment and is particularly affected by electronegative atoms and pi-bonds Proximity of electronegative atoms and pi bonds = larger energy change (due to less electron shielding = increase in chemical shift

It is used as the reference against which all chemical shifts are measured and is given the chemical shift value of 0ppm