Chromatography Flashcards
What is chromatography?
A wide range of techniques used to separate a mixture.
What is the basic principles behind chromatography?
There are two phases in a chromatography system; the mobile phase and the stationary phase. The stationary phase is fixed while the mobile phase is moving in one set direction. Different components have different affinities for the two phases and will move at different speeds through the chromatography because of this. Components with a higher affinity for the stationary phase will move slower than components with a higher affinity for the mobile phase.
What are the two types of chromatography?
Partition and adsorption chromatography.
What is partition chromatography?
Mobile phase is a liquid or gas. Stationary phase is non-volatile liquid. Separation depends on the relative solubilities different components of a mixture have for the different phases (how well they dissolve in them). Components more soluble in stationary phase will move slower than components more soluble in mobile phase.
What is adsorption chromatography?
Mobile phase is a liquid or gas. Stationary phase is a solid. Separation depends on how well different components of the mixture adsorb onto the solid surface. The better the components adsorbs onto the solid surface the slower it will move through the chromatography system.
What does TLC stand for?
Thin-layer chromatography.
What type of chromatography is TLC?
In TLC, the stationary phase is the thin layer of adsorbant on the surface of the chromatography plate (usually silica gel or aluminium oxide) while the mobile phase is the liquid solvent, which moves vertically up the plate. Separation depends on how well components of a mixture adsorb onto the adsorbant, therefore TLC is an adsorption chromatography technique.
What are the limitations of chromatography?
- Similar compounds will have similar Rf values.
- No reference Rf values for unidentified compounds.
- Trial and error may be required to find a solvent that produces adequate chromatogram.
- It may be difficult to find a solvent which will dissolve all components of a mixture.
What type of chromatography is gas chromatography?
The stationary phase is a layer of non-volatile liquid or solid that lines the inside of the capillary tubing in a GC machine. The mobile phase is the inert carrier gas (helium or nitrogen). Depending on whether a solid or liquid is used as the stationary phase, GC can be either partition or adsorption chromatography.
What are the principles behind GC?
Different components of a mixture have either different solubilities in the stationary phase (GLC) or different adsorption strengths (GSC). This means that they will move at different speeds through the capillary column in the GC machine, with the components having greater solubilities/ adsorption strengths moving slower. This means that they will move through and leave the column at different times (they have different retention times), so will be detected by the detector at different times.
What is retention time in GC?
The time for a component to pass from the column inlet to the detector.
How is a gas chromatogram interpreted?
- The position of a peak along the x-axis is the retention time of the component.
- The area under each peak is proportional to the amount of the component present in the mixture.
What are the limitations of GC?
- Different compounds may have the same retention times and peak shape due to differing environments, speeds of carrier gas etc. So GC cannot be an absolute identifier.
- Some components are able to mask other components with similar retention times if the concentration difference is great enough.
- Unknown components have no retention times to compare with.
Why is combining GC with MS a good analytical technique?
- GC is able to separate different component of a mixture but is unable to conclusively identify them.
- MS is able to provide more detailed information about structure but is unable to separate different components of a mixture.
- Combining GC and MS means that GC can first be used to separate different components of a mixture and MS can be used to give further detail in order to identify different components.
- MS can be used for fingerprinting different compounds, which is unique for every compound.
What can GC-MS be used for?
- Identifying presence of particular substances in a crime scene in forensic science.
- Identifying level of pollutants in air/ water or detecting pesticides in food. This is environmental analysis.
- Used to detect explosives in luggages at airports.
- Carried on probes to analyse components of different planets such as mars and venus.
