A2 - Unit 1 - Analysis Flashcards
What is a phase?
A physically distinctive form of a substance, such as the solid, liquid or gaseous states of ordinary matter
What is a mobile phase?
The phase that moves in chromatography
What is a stationary phase?
The phase that does not move in chromatography
What is adsorption?
The process by which a solid holds molecules of a gas or liquid or solute as a thin film on the surface of a solid or, more rarely, a liquid
What are the two different types of chromatography studied in A2?
Thin-layer chromatography TLC
Gas chromatography GC
What happens during chromatography?
A stationary phase is fixed in place, a mobile phase passes over the stationary phase, the stationary phase interacts with the components in the mixture, slowing them down.
Greater the interaction, the more the components are slowed down
This allows different components to flow over the stationary phase at different speeds, separating the components
What is the stationary and mobile phase in thin-layer chromatography?
Stationary: Solid
Mobile: Liquid
What is the stationary and mobile phase in gas chromatography?
Stationary: Liquid or solid on a solid support
Mobile: Gas
What is a chromatogram?
A visible record showing the result of separation of the components of a mixture by chromatography
Give an example of a stationary phase in TLC chromatography
Thin layer of adsorbent silica gel (SiO2) or alumina (Al2O3) coated on a flat inert surface
Usually on a sheet of glass or plastic, called a TLC plate
Give an example of a mobile phase in TLC chromatography
A liquid solvent, which moves vertically up the TLC plate
How do you produce a chromatogram for TLC?
Small sample of mixture is dissolved
Small spot added on one end of TLC plate
Sample spot is allowed to dry (sometimes repeat the last few steps for a better result)
TLC plate placed into jar containing shallow layer of solvent
Jar is sealed
As solvent rises on TLC plate it meets sample, sweeping the components in the mixture upwards with solvent
Separation occurs by the adsorption of the components in the mixture binding to the surface of the solid stationary phase
Different things adsorb more easily, allowing separation
Solvent allowed to reach top of TLC plate and solvent front is marked with a pencil line
Solvent allowed to evaporate
If the components on a TLC plate are colourless how can you see them?
A locating agent to ‘develop’ them
Alternatively, UV radiation can be used to show up each component by florescence
How do you calculate how far a component has moved compared with the solvent front for chromatography (Rf)?
distance moved by solvent front
Give three limitations of thin-layer chromatography
Similar compounds often have similar Rf values
Unknown compounds have no reference Rf for comparison
It may be difficult to find a solvent that separates all of the components in the mixture
What is gas chromatography?
A technique used to separate volatile components in a mixture
Used for many organic compounds that have a low boiling point and evaporate easily
What is the stationary phase in gas chromatography?
Thin layer of liquid or solid, coated on the inside of some capillary tubing
How do you produce a chromatogram for gas chromatography?
Mixture injected into the gas chromatogram, it is vaporised
Mobile carrier gas flushes the mixture through the column
Components slow as they interact with the stationary phase lining in the column
carrier gas flushes components further along tube
different components are slowed down by different amounts, so they separate
Each component leaves the column at a different time and is detected as it leaves the column
A computer processes the results to display a gas chromatogram
What is retention time in gas chromatography?
The time for a component to pass from the column inlet to the detector
Give three limitations of gas chromatography
Potentially thousands of chemicals may have the same retention time, peak shape and detector response
Not all substances will be separated
Unknown compounds have no reference retention times for comparison
To improve the reliability of gas chromatography, what must happen?
Results must be checked against a GC analysis of a reference sample containing only the suspected substance
It is also often combined with mass spectrometry to ensure the results are reliable
Give a strength and a weakness of GC
It can separate components in a mixture, but cannot identify them conclusively
Give a strength and a weakness of MS
Can provide detailed structural information on most compounds for exact identification
Cannot separate them
How does GC-MS work?
Components in mixture separated with GC
Retention time providing a preliminary identification
Separated components directed into a mass spectrometer, where they are detected
Generated mass spectrum can be analysed or compared with a spectral database by computer
Mass spectrum is unique and definitive for a single compound
So MS allows positive identification of each component in the original mixture
Give 4 uses for GC-MS
Forensics
Environmental analysis
Airport security
Space probes
How is GC-MS used in forensics?
Analyses minute particles found at crime scenes
Evidence often provides proof during a trial in the law courts
How is GC-MS used in environmental analysis?
Monitoring and analysing organic pollutants in the environment
Tests: quality of waste water and drinking water, detecting presence of pesticides in food
How is GC-MS used in airport security?
Used to detect explosives in luggage and on human beings
How is GC-MS used in space probes?
Sent to Mars to collect and analyse material
Another sent to Venus to analyse the planet’s atmosphere
Saturn’s largest moon - found evidence for liquid methane on the moon’s surface
What is chemical shift?
A scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS
What are the two different types of NMR studied in A level?
Carbon-13 NMR
Proton NMR
What are the two requirements for NMR spectroscopy?
A strong magnetic field applied using an electromagnet
Low-energy radio-frequency radiation
What is nuclear spin?
Protons and neutrons have a property called spin
which can be in one of two directions
In the nucleus opposite spins pair up
Nuclei of some isotopes there is an uneven number of nucleons, these isotopes have an unpaired nucleon which produces a small residual nuclear spin
This generates a magnetic field
In a strong magnetic field how do the nuclei line up?
With the field
Opposed to the field
What is Deuterium?
An isotope of hydrogen with one proton and one neutron in the nucleus
Why does Deuterium produce no signal in an NMR spectrum?
Because it has an even number of nucleons
What solvent is used in Carbon-13 and proton NMR?
CDCl3
What two pieces of information are taken from Carbon-13 NMR?
The number of different carbon environments - from the number of peaks
The types of carbon environment - from the chemical shifts
What information can you get from proton NMR?
The number of peaks gives the number of proton environments
Chemical shifts give the type of proton environment of each peak
Relative peak areas give the proportions of protons in each environment
Spin-spin coupling gives information about adjacent protons
Why is it difficult to identify OH and NH peaks?
Peaks can appear over a wide range of chemical shift values depending on the solvent used and the concentration of the sample
Signals are often broad
Usually no splitting pattern
What can mass spectrometry show you?
Percentage by mass of each element
Empirical formula
Relative molecular mass
Clues about the carbon skeleton in the molecules of a compound
What can infrared spectroscopy show you?
Information about the bonds present in a molecule
Functional groups present
What can NMR spectroscopy show you?
Carbon-13 :
Information about the number and types of carbon environments in a molecule
Proton:
The number of each type of proton environment
Number of protons on adjacent carbon atoms
