Gas/Liquid Chromatography (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define retention time.
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Define retention time.
A retention time is the time taken for a component to travel from the injection point through the GLC column to the detector, used to identify unknown compounds by comparison with data book values.
What is used as the stationary phase in gas-liquid chromatography?
The stationary phase is a non-polar, long-chain, non-volatile hydrocarbon with a high boiling point, mounted onto a solid support (e.g. small silica particles packed in a glass column).
True or False?
In GLC, the mobile phase is a liquid solvent that carries sample molecules through the column.
False. In GLC, the mobile phase is an inert carrier gas (e.g. helium or nitrogen), not a liquid solvent.
GLC is used to analyse .........., volatile liquids and solids in their .......... form.
GLC is used to analyse gases, volatile liquids and solids in their vapour form.
How are the retention times from a GLC chromatogram used to identify unknown compounds?
The retention times of sample peaks are compared with data book values for known compounds under the same conditions. A match identifies the unknown component.
Define gas-liquid chromatography.
Gas-liquid chromatography is an analytical technique that separates volatile components of a mixture by their different interactions with a non-polar liquid stationary phase, using an inert carrier gas as the mobile phase.
True or False?
Each peak on a GLC chromatogram represents a different volatile compound in the analysed sample.
True. Each peak on a GLC chromatogram corresponds to a different volatile component of the sample, with its position (retention time) used for identification.
Why do silica particles in a GLC column improve the separation of components?
Small silica particles provide a large surface area, which increases the contact between sample gas molecules and the stationary phase, improving the separation of components with similar properties.
In GLC, the carrier gas must be .......... so that it does not react with the sample molecules as they travel through the ..........
In GLC, the carrier gas must be inert so that it does not react with the sample molecules as they travel through the column.
Define peak integration value.
A peak integration value is the area under a peak on a GLC chromatogram, representing the relative concentration of that component in the sample mixture.
How is the approximate percentage composition of one component calculated from a GLC chromatogram?
The percentage of a component = (peak area or height of that component / sum of all peak areas or heights) × 100. This gives its approximate proportion in the mixture.
The area under a GLC peak is approximated as .......... × base × height, treating each peak as a ..........
The area under a GLC peak is approximated as ½ × base × height, treating each peak as a triangle.
True or False?
Non-polar components in a GLC mixture have shorter retention times because they interact more strongly with the non-polar stationary phase.
False. Non-polar components have longer retention times because they are more attracted to the non-polar stationary phase and travel more slowly through the column.
Why do polar components tend to have shorter retention times in GLC?
Polar components have weaker interactions with the non-polar stationary phase than non-polar components do. They spend less time in the stationary phase and more time in the mobile phase (carrier gas), travelling faster through the column and giving shorter retention times.
Define chromatogram.
A chromatogram is the output from a chromatography experiment showing peaks at different retention times, where each peak represents a separate component of the analysed mixture.
True or False?
If a peak in a GLC chromatogram is too small to measure its area accurately, the peak height can be used instead.
True. When peak areas are too small or difficult to determine, the peak height is used as an alternative measure of relative concentration.
Explain why compounds with lower boiling points tend to have shorter retention times in GLC.
Compounds with lower boiling points are more easily vaporised and spend more time in the gaseous mobile phase, interacting less with the stationary phase and so eluting faster.
In GLC, molecules with longer retention times are more attracted to the .......... stationary phase, while molecules with shorter retention times prefer to interact with the .......... carrier gas.
In GLC, molecules with longer retention times are more attracted to the non-polar stationary phase, while molecules with shorter retention times prefer to interact with the inert carrier gas.
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