In simple terms
A friendly intro before the formal notes — no formulas yet.
The Great Molecular Race
Gas/Liquid Chromatography separates volatile compounds by passing them through a long, coated tube. How fast each compound travels depends on its boiling point and how much it interacts with the tube's liquid lining versus the moving gas.
Imagine a group of friends racing through a street lined with tempting food stalls. The friends who are easily distracted and stop often at the stalls (interacting with the 'stationary phase') will finish the race much later. The friends who ignore the stalls and just keep running with the crowd (the 'mobile phase') will finish quickly. In GLC, less volatile molecules are like the easily distracted friends, taking longer to get through the column.
- 1
Volatile sample in carrier gas (mobile phase). | Sim hint: Liquid stationary phase on column.
- 2
Retention time identifies components. | Sim hint: Compare with standards.
- 3
More volatile → shorter retention time. | Sim hint: Temperature affects separation.
- 4
Area under peak ∝ amount. | Sim hint: Quantitative GLC.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Principles of Gas/Liquid Chromatography (GLC)
In GLC, the mixture to be separated is first vaporised and then carried by an inert gas (the mobile phase) through a very long, thin tube called a column. This column contains the stationary phase: a non-volatile liquid coated onto either a solid packing material or the inside wall of the column. The entire column is housed within a temperature-controlled oven.
As the components of the mixture travel through the column, they continuously partition between the mobile gas phase and the stationary liquid phase. The extent to which a component dissolves in the stationary liquid depends on its volatility and its intermolecular forces with the liquid. Components that are less volatile or more soluble in the stationary phase spend more time 'stuck' and move through the column more slowly. Components that are more volatile or less soluble move quickly with the gas. This difference in speed results in the separation of the mixture.
Mobile Phase: An inert gas (e.g., , He) that carries the sample.
Stationary Phase: A non-volatile liquid on a solid support inside the column.
Principle: Separation by partition based on relative volatility and solubility.
Requirement: The sample must be volatile or be made volatile (derivatised).
Interpreting a Chromatogram
As each separated component leaves the column, it passes through a detector which generates an electrical signal. A computer plots this signal against time to produce a graph called a chromatogram. Each peak on the chromatogram represents a different component from the original mixture.
Retention Time (): The time from injection to detection for a specific component. It is found on the x-axis and is used for qualitative identification by comparing with known standards.
Peak Area: The area under a peak is directly proportional to the amount (concentration) of that component. This is used for quantitative analysis.
Number of Peaks: Indicates the minimum number of separable components in the mixture.
Factors Affecting Separation
The quality of separation in GLC can be fine-tuned by adjusting several parameters. The goal is usually to achieve good resolution (sharp, well-separated peaks) in a reasonable amount of time.
Column Temperature: Higher temperatures decrease retention times for all components as they spend more time in the vapour phase. This can be useful for eluting high-boiling point compounds faster, but may cause peaks that are close together to merge, reducing resolution.
Polarity of Stationary Phase: A non-polar stationary phase (like a long-chain hydrocarbon) will retain non-polar analytes longer via van der Waals forces. A polar stationary phase (like one with -OH or -CN groups) will retain polar analytes longer via dipole-dipole interactions or hydrogen bonding.
Column Length: A longer column provides more opportunity for separation to occur, generally leading to better resolution but longer analysis times.
Carrier Gas Flow Rate: Increasing the flow rate will decrease retention times, but there is an optimal flow rate for maximum separation efficiency. Too fast or too slow will broaden the peaks.
When asked to predict the order of elution (the order components leave the column), first look at boiling points. The compound with the lowest boiling point is the most volatile and will have the shortest retention time. If boiling points are similar, consider polarity. The compound that is most 'like' the stationary phase in terms of polarity will be retained for longer.
Quantitative Analysis
GLC is not just for identifying substances; it's also a precise quantitative tool. The area under a peak on the chromatogram is directly proportional to the concentration of the component that produced it. To find the absolute concentration of a substance, a calibration curve must be constructed.
Concentration of component ∝ Peak area of component
This involves preparing a series of standard solutions with known concentrations of the analyte. Each standard is run through the GLC, and its peak area is measured. A graph of peak area versus concentration is plotted, which should yield a straight line passing through the origin. The unknown sample is then analysed under the same conditions, and its peak area is used to determine its concentration from the calibration graph.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A mixture containing several straight-chain alkanes is analysed by GLC. The chromatogram shows three major peaks with retention times of 2.8 min, 4.5 min, and 6.9 min. Separate analyses of pure hexane, heptane, and octane under identical conditions gave retention times of 4.5 min, 6.9 min, and 9.8 min, respectively. Identify the components present in the mixture.
- 1
Compare retention times: The retention time of a compound is a characteristic property under specific GLC conditions.
The concentration of caffeine in a soft drink was determined using GLC with an internal standard. A series of standard caffeine solutions were prepared, each containing a constant concentration of an internal standard. The ratio of the caffeine peak area to the internal standard peak area was calculated. The results were:
- 10 mg dm^{-3} caffeine: Peak Area Ratio = 0.52
- 20 mg dm^{-3} caffeine: Peak Area Ratio = 1.05
- 30 mg dm^{-3} caffeine: Peak Area Ratio = 1.56
- 40 mg dm^{-3} caffeine: Peak Area Ratio = 2.10
The soft drink sample, with the same amount of internal standard added, gave a Peak Area Ratio of 1.35. Determine the concentration of caffeine in the drink.
- 1
Plot a calibration curve: Plot Peak Area Ratio (y-axis) against Caffeine Concentration (mg dm^{-3}) (x-axis).
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
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.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Mobile Phase: An inert gas (e.g., , He) that carries the sample.
- ✓
Stationary Phase: A non-volatile liquid on a solid support inside the column.
- ✓
Principle: Separation by partition based on relative volatility and solubility.
- ✓
Requirement: The sample must be volatile or be made volatile (derivatised).
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Practice Questions: Gas/Liquid Chromatography
Practice Questions: Gas/Liquid Chromatography
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Practice Questions: Gas/Liquid Chromatography on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.