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Module 5 - Chapter 29 - Chromatography and spectroscopy I - Coggle Diagram
Module 5 - Chapter 29 - Chromatography and spectroscopy I
Chromatography
basics
Stationary phase
doesn't move (solid or liquid supported on a solid)
Greater affinity of molecule to stationary phase, smaller the retention factor/ time
Mobile phase
Does move (liquid or gas)
Species that are more soluble in mobile phase move further/ faster within mobile phase
Used to separate indicidual components from mixture of substances
TLC
Basics
Absorbent is the stationary phase - different compenents in mixtue have different affinities for absorbent and bind with different strengths to its surface
Adsorption - when solid silica holds different substances in the mixture to its surface
TLC plate is a plastic sheet or glass coated with thin layer of adsorbent substance (silica)
Carrying out TLC
Draw pencip line across teh plate 1cm from one end of the place (base line)
Use capillary tube to spot small amount of solution of sample onto base line
Pour some solvent to a depth of 0.5cm in a small beaker
Place TLC plate in beaker (Solvent musn't cover the beaker). Cover beaker with watch glass
Stop when solvent is 1cm from the top. remove plate and mark solvent front with pencil
Circle any visibile spots - you can hold a UV lamp over the plate and circle any spots you can't see. Alternatively, a locating agent like iodine can be used
Interpretation of a TLC plate
Calculate retention factor (Rf value)
You can identify a component by comparing its Rf value with known values recorded using same solvent system and absorbent
Often you run a TLC alongside pure samples
Gas Chromatography
Basics
Useful for separating and identifying volatile organic compounds present in a mixture
Stationary phase - high boiling point liquid adsorbed onto an inert solid support
Mobile phase - intert carrier gase (helium)
Process
Components slow down as they interact with liquid stationary phase.
Small amount of volatile mixture is injected into apparatus (gas chromatograph) Mobile gase carrier carries component through capillary colum which contains liquid staionary phase
Components are separated depending on solubility and boiling point and they reach the detected at different times
Compound retained in column for the shortest time has lowest retention time and is detectd first
Retention time - time taken for each component to travel through the column
Interpretation
Components are detected as a peak on the gas chromatogram
Retention times can be used to identify components present in the sample of comparing these to known retention times
Peal integrations can be used to determine concentrations
Concentration
Found by comparing its peak integration with values obtained from standard solution of the component
Procedure
Prepare standard solution of known concentration
Obtain gas chromatograms for each solution
Plot calibration curve of peak area against concentration (external calibration and offers method for converting area into concentration
Obtain gas chromatogram of compound being investigated under same conditions
Use calibration curve to measure the concentration of compound
Factors affecting retention time
Lower boiling point (less voltalite) gas has a shorter retention time
Higher solublity means more time is spent in stationary phase, greater affinity to it, and a longer retention time
Higher temperatures increase kinetic energy of the particles, so a lower retention time
Qualitative analysis
Alkene - add bromine water dropwise - bromine water decolourises from orange to colourless
Haloalkane - add silver ntirate + ethanol and warm to 50 degree. White, cream or yellow precip for Cl, Br, I
Carbonyl - add 2,4 dinitrophenylhydrazine - orange precipitate
Aldehyde - add tollen's reagent wan warm - silver mirror
Primary secondary alcohol/ aldehyde - add acidified potassium dichromate and warm in water bath - colour change from organge to green
Carboxyilc acid - add aqueous sodium carbonate - effervescence
Phenol - add bromine - white precipitate forms
NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance
Uses strong magnetic and radio frequency raditation to determine structure of a molecule
Different chemical environments are displayed as different peaks on spectra
Nuclear spin
Nucleus has a nuclear spin
If a magnetic field is applied, spins line up along or against it
Even number of nucleons, the spins with and against are balanced
If there's an odd number of nucleons, radio waves can be absorbed to flip the spin of a nucleon
Resonance
Nucleus can absorb energy and flip between spin states with the right comination of strong magnetic field and radio frequency
Each atom has a resonant frequency, the frequency you need for atom to constantly flip
NMR spectrometer
Radio frequency has much less energy than infrared radition used in IR spectroscopy
Frequency required for resonance is proportional to magnetic field strength
Only in strong uniform magnetic fields can the small quantity of energy be detected
Most spectrometers operate at frequencies of 100,200 or 400 MHz
Chemical shift
All atoms have electrons surrounding the nucleus which shifts the energy and radio frequency needed for nuclar magnetic resonance to take place
Frequency shift is measured on a scaled called chemical shift in units of ppm (parts per million)
If electrons shield the nucleons, less energy is needed to reach the resonant state, as the nucleons aren't affected much by the external field
You can increase energy by a higher frequency
Running the spectrum
Sample is dissolved in solvent and placed in narrow NMR sample tube with small amount of TMS
Tube is placed in side the NMR spectrometer and spun to even out imperfections in magnetic field
Spectrometer is zeroed against TMS standard and sample is given a pulse of radiation containig range of frequencies
Any absorption of energy from resonance is detected and displayed on a computer screen
Deuterated solvents
Deuterated solvent is used in which the hydrogen atoms are deuterium
Deuterum products no NMR signal in frequency ranges used in 1H and 13C NMR spectroscopy
Deuterated trichloromethane is commonly used as a solvent, but still products a peak in carbon-13 NMR spectrum, this is filtered out by the computer
TMS
Tetramethylsilane (TMS) is used as the standard reference chemical against which all chemical shifts are measured.
TMS gives a chemical shift value of 0 ppm.
Produes a sinlge intense peak as all carbons are in the same enviroment
Protons are highly shielded - low amount of energy is required to reach resonant state
Intert and not toxic substance
