29 - Chromatography and Spectroscopy Flashcards
What can chromatography be used in?
Analysis of drugs, plastics, flavourings, air samples and has applications in forensic science
What is thin layer chromatography (TLC) ?
Quick and inexpensive analytical technique that indicates how may components are in a mixture
Uses a TLC plate
What is a TLC plate?
A plastic sheet of glass, coated with a thin layer of adsorbent substance - usually silica
What is the stationary phase in TLC?
The adsorbent
How do the different components act on the TLC plate?
Different components in the mixtures have different affinities for the absorbent and bind with differing strengths to its surface
What is adsorption?
Process by which the solid silica holds the different substances in the mixture to its surface.
How do you carry out TLC?
- Use a pencil to draw a line across the TLC plate about 1cm from the base (baseline)
- Using a capillary tube, spot a small amount of a solution of the sample on to the baseline
- prepare a chromatography tank for the TLC plate - pour solvent into a small beaker to about 0.5 cm depth
- Place prepared TLC plate upright in the beaker, making sure the solvent does not cover the spot.
- Leave to stand and allow solvent to rise up the TLC plate.
- Once solvent has reached about 1cm from top of plate, remove from beaker and immediately mark the end of the solvent with a line (solvent front)
- Circle any visible spots with a pencil
What is a locating agent?
A chemical such as iodine that is sprayed over the TLC plate to show the position of spots that may be colourless
How do you calculate the retention factor, Rf Value?
Rf = distance moved by component/distance moved by solvent front
What is gas chromatography?
Used for separating and identifying volatile organic compounds present in a mixture
What is the stationary phase in gas chromatography?
a high boiling point liquid adsorbed on to an inert solid support
What is the mobile phase in gas chromatography/
An inert carrier gas such as helium or neon
How is gas chromatography carried out?
- Small amount of mixture is injected into the apparatus called a gas chromatograph
- Mobile carrier gas carries the components in the sample through the capilalry column whihc contains the liquid stationary phase adsorbed on to the solid support
- Components of the mixture are separated depending on thier solubility in the liquid bstationary phase
- Compunds in the mixture reachb the detector at different times depending on their intreactions with the stationary phase in the column
Which compound is detected first in gas chromatography?
The compound retained in the column for the shortest amount of time (lowest retention value)
What is the retention time?
Time taken for each component to travel through the column
What two pieces of information can be obtained from a gas chromatogram?
Retention times Peak integrations (areas under each peak)
What can retention times be used for?
Identify the components present in the sample by comparing them to retention times for known components
What can peak integrations be used for?
Determine the concentrations of components in the sample
How are the concentrations of components determined?
By comparing its peak integration (peak area) with values obtained from standard solutions of the components
What is the chemical test for alkenes?
- Bromine water
Decolourised from orange to colourless
What is the chemical test for haloalkanes?
-Silver nitrate and ethanol and warm to 50 degrees in water bath
Cl = white precipitate
Br = cream precipitate
I = yellow precipitate
What is the chemical test for carbonyls?
- 2,4 - DNP
yellow/orange precipitate
What is the chemical test for aldehydes?
- Tollens reagent and warm
silver mirror
What is the chemical test for primary and secondary alcohols and aldehydes?
- Acidified potassium dichromate (VI) and warm in water bath
Colour change from orange to green
What is the chemical test for carboxylic acids?
Add Na2CO3 (aq) effervescence
When is nuclear spin significant?
If there is an odd number of nucleons (protons and neutrons)
For what isotopes of organic compounds is NMR relevant for?
13C and 1H
What can NMR spectroscopy be used for?
Detect isotopes of other elements with odd numbers of neutrons
What are the most common forms of NMR analysis used?
1H and 13C
What is 1H NMR usually referred to as?
- proton NMR
- nucleus consists of just a proton
What is nuclear magnetic resonance?
- Both the nucleus and an electron have two different spin states with different energies
- With the right combination of magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flip between the two spin states
What is the difference between radio frequency radiation and infrared radiation?
- radio freq. radiation has much less energy
- freq. required for resonance is proportional to the magnetic field strength
- small quantity of energy can only be detected in strong and uniform magnetic fields
Where are NMR spectrometers commonly found?
MRI body scanners
What is meant by chemical shift?
fhgfyh
What is the standard reference chemical?
Tetramethylsilane (TMS), (CH3)4Si
What is the chemical shift value given to TMS?
0ppm
How is NMR spectroscopy carried out?
- Sample dissolved in a solvent and placed in a narrow NMR sample tube, with a small amount of TMS
- Place placed inside the NMR spectrometer where it is spun to even out any imperfections in the magnetic field within the sample
- Spectrometer is zeroed against the TMS standard and the sample is given a pulse of radiation containing a range of radio frequencies, whilst maintaining a constant magnetic field
- Any absorptions of energy resulting from resonance are detected and displayed
How is a sample recovered after NMR spectroscopy analysis?
evaporation of the solvent
When is a deuterated solvent used?
When the 1H atoms in 1H NMR spectra have been replaced by 2H atoms (deuterium, D)
- Produces no NMR signal in the frequency in the frequency ranges used in 1H and 13C NMR spectroscopy?
What is a common deuterated solvent?
Deuterated trichloromethane, CDCl3
