Identify the phases and terms in gas-liquid chromatography
Describe the stationary liquid, inert mobile gas and retention time in gas-liquid chromatography.
Section 37 · Analytical techniques
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the mobile phase in gas-liquid chromatography?
What you will learn
- Define the gas-liquid stationary phase
- Define the gas mobile phase
- Read retention time from injection to elution
The sample partitions between two phases
In gas-liquid chromatography, the stationary phase is a high-boiling-point, non-polar liquid held on a solid support. The mobile phase is an unreactive gas that carries vaporised sample through the column.
Each component distributes between the moving gas and stationary liquid to a different extent. This difference helps separate the components as they travel through the column.
Measure retention time from injection
Retention time is the time between introducing the sample and the component reaching the detector, shown by its chromatogram peak. It is read from the time axis relative to the injection point.
Retention time depends on the stationary phase and operating conditions. A time value is useful for comparisons only when those conditions are controlled or matched.
Pause and explain
From what event is retention time measured?
Worked example
A chromatogram has peaks at 1.2 min and 3.8 min after injection. Name the phases and state the retention time of the second peak.
Show the worked solution
- The liquid coating is stationary.
- The unreactive gas moves the sample.
- Read the second peak's time from injection.
Answer The stationary phase is a high-boiling-point non-polar liquid on a solid support; the mobile phase is an unreactive gas. The second component has retention time 3.8 min.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the stationary and mobile phases for the syllabus gas-liquid chromatographic system.
Give me a hint
Describe the phase states and the stationary liquid's properties.
Compare my reasoning
- The stationary phase is a high-boiling non-polar liquid on a solid support.
- The mobile phase is an unreactive gas.
- Keep their roles distinct.
Stationary: high-boiling-point non-polar liquid on a solid support. Mobile: unreactive gas.
Look for these in your work
- Stationary phase defined
- Mobile phase defined
- Roles distinguished
A component enters the detector 2.6 minutes after the sample injection. What is its retention time?
Give me a hint
Use the injection time as the zero reference.
Compare my reasoning
- Locate the peak on the chromatogram time axis.
- Measure from injection.
- Include the time unit.
The retention time is 2.6 min.
Look for these in your work
- Reference event correct
- Value correct
- Unit included
Why should two retention times only be compared when the chromatographic conditions are the same?
Give me a hint
The stationary/mobile phases and operating conditions affect the partitioning and travel time.
Compare my reasoning
- Retention depends on interactions with the stationary phase.
- Changing conditions can shift peak times.
- Matched conditions make the comparison meaningful.
Different phase or operating conditions can change how long a component is retained, so retention times are comparable only under matching conditions.
Look for these in your work
- Interaction linked
- Condition dependence noted
- Comparison qualified
Common mistakes
- Calling the gas stationary
- Describing the stationary liquid as low boiling
- Measuring retention time from the previous peak
- Treating retention time as a universal property independent of conditions
What can you explain now?
What is the mobile phase in gas-liquid chromatography?
Compare with the explanation
An unreactive gas
It carries vaporised sample through the column.
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Make it stick
Which phase moves the sample in gas-liquid chromatography, and how is retention time defined?
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