37.2 Gas / liquid chromatography
- Syllabus
- 9701–2028–2029
- Topic
- 37.2
- Level
- A2
Learning objectives
37.2.1And understand the terms• Describe and understand the terms: (a) stationary phase; a high boiling point non-polar liquid (on a solid support); (b) mobile phase; an unreactive gas; (c) retention time37.2.2Gas / liquid chromatograms• Interpret gas / liquid chromatograms in terms of the percentage composition of a mixture37.2.3Retention times• Explain retention times in terms of interaction with the stationary phase
GLC uses a high-boiling liquid stationary phase and an unreactive gas
| Term | Meaning in GLC |
|---|---|
| stationary phase | high-boiling-point, non-polar liquid coated on a solid support inside the column |
| mobile phase | unreactive carrier gas such as helium or nitrogen that moves vaporised sample through the column |
| retention time | time from sample injection until that component reaches the detector and produces its peak |
Volatile sample components distribute between travelling with the carrier gas and interacting with/dissolving in the stationary liquid. Their different residence times separate them before detection.
The chromatogram plots detector response against time. Peak position gives retention time; GLC does not use the TLC Rf ratio.
The carrier gas is the mobile phase, not the sample. Retention time is an experimental time under specified column and operating conditions, not the compound's boiling point.
Use relative peak areas to calculate percentage composition
Each resolved peak represents a detected component. Its retention time locates the component, while its integrated area represents the relative amount detected when response is treated as proportional under the stated method.
% component i=∑areas of all relevant peaksarea of peak i×100
If no integration values are supplied and peaks are approximated as triangles, estimate each area as 1/2 x base width x height. Use the areas consistently; peak height alone can mislead when widths differ.
For integrated areas 20, 30 and 50 arbitrary units, the total is 100 and the estimated percentage compositions are 20%, 30% and 50%.
A later retention time does not mean a larger percentage. Retention time concerns peak position/interaction; relative area concerns composition. Real detector response factors may require calibration, but use the supplied or integrated areas as the exam data specify.
Stronger stationary-phase interaction gives a longer retention time
A component with stronger attraction to or greater solubility in the stationary liquid spends a larger fraction of its journey retained in that phase. It moves with the carrier gas for less time and therefore reaches the detector later.
| Relative stationary-phase interaction | Fraction of time moving with gas | Retention time |
|---|---|---|
| weaker | greater | shorter |
| stronger | smaller | longer |
With the specified non-polar stationary liquid, intermolecular compatibility can make non-polar components interact more strongly. Compare compounds under the same column, temperature and carrier-gas flow because those conditions also shift measured times.
Retention time is not a universal identity constant and cannot be explained from polarity or boiling point alone. The required causal statement is the component's relative interaction with the stationary phase in that actual system.