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.
What is used to make the magnetic field in the NMR spectrometer?
A large electromagnet is used and it is cooled to 4K by liquid helium
Chemical shift (sigma)
This is the shift in frequency compared to TMS which is required for a nucleus to undergo nuclear magnetic resonance. It depends on the chemical environment
What is the standard chemical used as the standard reference for chemical shift?
TMS (tetramethylsilane)
How do you run an NMR spectrum?
You dissolve the sample in a solvent and place it in an NMR tube with some TMS.
You then place the tube inside the NMR spectrometer where it is spun to even out imperfections in the magnetic field within the sample. The spectrometer is then calibrated with the TMS standard and the sample is given a pulse of radiation whilst maintaining the magnetic field. Any absorptions of energy are detected by a computer and displayed.
What are the deuterated solvents that are used and why?
You have to use solvents that won’t produce an NMR signal so usually the H1 will be replaced with H2 atoms. Deuterated CDCl3 can be used which can still produce a peak during 13C NMR but will usually be filtered out by the computer.
What does 13C NMR tell you?
- The number of different carbon environments
- The types of carbon environment present
How do you figure out the number of different carbon environments for 13C NMR?
Look at the number of peaks
How do you figure out the types of carbon environment present in 13C NMR?
The chemical shift which is given to you
What are the 4 different types of carbon atom?
- Carbon bonded to carbon
- Carbon bonded to an electronegative atom
- Carbon as part of aromatic ring or C=C
- Carbon as part of a C=O
This is in ascending chemical shift
What happens to the environments of carbons bonded to different groups of atoms?
They will absorb many different chemical shifts
What happens to the environments of carbons that are symmetrical?
They will absorb the same chemical shift and will be part of the same environment and be in the same peak
