Chromatography and Spectroscopy (29) Flashcards
What is chromatography?
It is a method used to separate individual components from a mixture of substances. it has to have a stationary and mobile phase.
Stationary Phase
This is normally a solid or a liquid supported on a solid that doesn’t move.
Mobile Phase
This is a non-polar liquid or a gas that does move. It has to be non-polar or the solute will just dissolve instead of adsorb.
What can you use chromatography for?
Analysing drugs, plastics and is also used a lot in forensic sciences
What is thin layer chromatography (TLC)
This tells you how many components are in a mixture. It uses a plastic sheet or glass with a solid adsorbent substance like silica gel on it as the stationary phase. You get separation because of the relative adsorption of the mobile phase with the stationary phase.
Adsorption
The attraction of an atom to the surface of a substance.
How do you carry out a TLC?
- Decide your stationary and mobile phases. Use alkane, water or alcohol for mobile and silica or aluminium oxide for stationary.
. Draw a baseline in pencil on the stationary phase - Apply a small amount of the mixture on the baseline using a capillary tube
- Pour solvent into the beaker and place the TLC plate in it making sure the solvent doesn’t touch the spot.
- Wait and allow the solvent to rise until it’s near the top of the plate and created a solvent front.
- Record results.
How to interpret the TLC?
Calculate the retardation factor (Rf). The higher the Rf the higher the substance has moved up so the more adsorbent it is to the mobile phase. The smaller the Rf value, the slower it moved up so it is more adsorbent to the stationary phase.
How to work out retardation factor
Rf = Distance moved by the component of the mixture /
Distance moved by the solvent
What happens if you use a non-polar mobile phase?
If a non-polar mobile phase is used and a polar stationary phase. The non-polar solutes will be more soluble in the mobile phase and less soluble in the stationary phase so will travel highest. The polar solutes will be more adsorbent so will move up slower
What happens if you use a polar mobile phase?
The greater the polarity of the solute, the quicker it would move up. The less polar, the slower it would move up.
What should you do if you can’t see the substances?
Put it under UV light
What does it mean if 2 dots are the same height?
They are the same substance
What is gas chromatography?
It separates volatile substances which are substances that are gases at room temperature.
What is the stationary phase?
A liquid covering the surface or a solid material
What is the mobile phase?
An inert (unreactive) gas like helium or nitrogen
How does a gas chromatograph work?
- Your volatile mixture is injected into the heated column where it is vapourised.
- The carrier gas will carry the sample through the heated column which contains the stationary phase.
- The components interact with the stationary phase and the more soluble it is in the liquid of the stationary phase, the slower it will move through.
- The components are then separated by how slowly they move through the column and they reach the detector at different times.
- The retention time is measured.
Retention time
The time taken for a substance to reach the detector in a gas chromatogram depending on how soluble it was on the liquid of the stationary phase.
Peak Integrations
The areas under each peak that can measure the concentration of each component. Plot a calibration curve to find this.
Limitations of a gas chromatogram
- It only works on volatile substances
- Doesn’t work well for large biological molecules and some solids
Benefits of gas chromatography
- It has a higher separating power than any other chromatography
- It’s more accurate as it has a detector
- Only a small amount of the sample is needed
- Can detect and identify all compounds
What is NMR Spectroscopy
It’s nuclear magnetic resonance spectroscopy which is used to analyse organic compounds. It uses strong magnetic fields and radiofrequency
Why is nuclear spin significant in NMR?
The nucleus has a spin and that is significant if there is an odd number of nucleons (protons and neutrons). 1H and 13C are the 2 most common isotopes with odd numbers of nucleons
What is resonance?
An electron has 2 different spin states and so does the nucleus. If you combine magnetic fields and radio frequencies, the nucleus will absorb energy and flip between the 2 spin states. This is resonance.
