Gas/Liquid Chromatography
Specification Reference Organic Chemistry, Analytical techniques 37.2
Quick Notes
- Gas/Liquid Chromatography is a technique used to separate and identify compounds in a mixture.
- Stationary phase: high-boiling point, non-polar liquid coated on a solid support inside a column.
- Mobile phase: an unreactive carrier gas (e.g. helium or nitrogen).
- Retention time: the time taken for a compound to pass through the column to the detector.
- Retention time depends on boiling point and interaction with the stationary phase.
- Each peak on a chromatogram corresponds to a different compound.
- Peak area is proportional to the amount of that compound in the sample.
Full Notes
Chromatography has been covered with more background theory and detail
here.
This page is just what you need to know for GL-C for CIE A-Level Chemistry :)
What Is Gas/Liquid Chromatography?
Gas/Liquid Chromatography (GLC) is used to separate and identify compounds in a mixture.
- A small sample is injected into the GLC instrument and vaporised.
- The carrier gas (mobile phase) carries the sample through the column.
- As each component moves through, it interacts with the stationary phase:
- The more strongly a compound interacts with the stationary phase, the slower it moves, giving a longer retention time.
- The less interaction, the faster it travels giving a shorter retention time.
- When compounds exit the column, they are detected and recorded as peaks on a chromatogram.
Key Terms
- Stationary phase: A high-boiling point non-polar liquid coated on the surface of a finely powdered solid (such as silica). This liquid is fixed inside a long, coiled tube (the column). The compounds in the sample interact with this liquid to different extents.
- Mobile phase: An unreactive gas, such as helium or nitrogen, that carries the sample through the column under pressure.
- Retention time: The time taken for a compound to pass through the column and reach the detector. It is measured from when the sample is injected.
Interpreting a Chromatogram
The results obtained from Gas/Liquid Chromatography are plotted on a graph called a chromatogram.
- Retention time (x-axis) identifies the compound (compared to known standards).
- Peak area or intensity (y-axis) indicates the amount of the compound present.
- To calculate percentage composition, divide the area under each peak by the total area, then multiply by 100.
Worked Example
For the following chromatogram, there are four peaks, meaning four compounds were detected. The relative peak areas show that C made up 20% of the sample mixture.
- Given: Area under peak C = 40; total area of all peaks = 200
- Calculation: (40 ÷ 200) × 100 = 20%
The percentage of compound C in the mixture = 20%.
Explaining Retention Time
Two main factors affect how long a compound stays in the column:
- Boiling point:
Higher boiling point = less volatile = more time in liquid phase = longer retention time. - Interaction with the stationary phase:
Compounds that are more soluble in the liquid phase will move more slowly through the column.
If the stationary phase is non-polar:- Non-polar compounds interact more and stay longer.
- Polar compounds move through faster and have shorter retention times.
Summary
- Gas/Liquid Chromatography separates compounds in a mixture using a stationary liquid and a mobile gas phase.
- Each compound has a unique retention time depending on boiling point and interactions with the stationary phase.
- Chromatograms show peaks, where retention time identifies the compound and peak area indicates its amount.
- Non-polar compounds generally have longer retention times with non-polar stationary phases.