9701 · 37.2
Gas/liquid chromatography flashcards
Revision flashcards for Cambridge 9701 Gas/liquid chromatography (syllabus 37.2). Flip, recall, then mark a real past-paper question.
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What is the principle of separation in Gas/Liquid Chromatography (GLC)?
Partition. Components partition (distribute) themselves between the mobile gas phase and the stationary liquid phase based on their relative volatility and solubility in the stationary phase.
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What is the mobile phase in GLC?
An inert carrier gas, such as nitrogen ($N_2$), helium (He), or argon (Ar). It must be unreactive.
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What is the stationary phase in GLC?
A high boiling point, non-volatile liquid coated onto a solid support or the inner wall of a long, thin capillary column.
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Define retention time ($t_R$).
The time taken for a component to travel from the point of injection to the detector in a chromatography column.
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How is a substance identified using its GLC chromatogram?
By comparing its retention time to that of a known standard, run under identical conditions (same column, temperature, flow rate).
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What does the area under a peak on a chromatogram represent?
The relative amount or concentration of that specific component in the mixture. It is directly proportional to the quantity of the substance.
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How does a compound's volatility affect its retention time?
A more volatile compound (lower boiling point) spends more time in the mobile gas phase and has a shorter retention time. A less volatile compound (higher boiling point) spends more time dissolved in the stationary liquid phase and has a longer retention time.
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What is the effect of increasing the column temperature in GLC?
It increases the vapour pressure of all components, causing them to move faster through the column. This decreases all retention times but may reduce the resolution (quality of separation) between peaks.
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How is quantitative analysis performed using GLC?
A calibration curve is created by running standard solutions of known concentrations and plotting their peak areas against concentration. The peak area of the unknown sample is then used to find its concentration from this graph.
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Why must the sample be volatile for GLC analysis?
The sample is injected into a hot port to vaporise it instantly. Only gaseous components can be carried through the column by the mobile gas phase. Non-volatile substances will not elute.
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What is a common pitfall when identifying compounds by retention time?
Assuming a single retention time is definitive proof of identity. Different compounds can have identical retention times under one set of conditions. Coupling GLC with mass spectrometry (GC-MS) provides much more certain identification.
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How does the polarity of the stationary phase affect separation?
Like dissolves like. A polar stationary phase will retain polar components for longer (longer $t_R$), while a non-polar stationary phase will retain non-polar components for longer.