Part 2: Column chromatography, GC and GC–MS

Compare column and gas chromatography, interpret retention times and explain how GC–MS and calibration strengthen chemical analysis.

Column chromatography collects separated fractions

A column is packed with a suitable stationary phase such as silica. Dissolve the mixture in a small amount of compatible solvent and apply it as a narrow band. Pass a liquid mobile phase through the column and collect the emerging liquid in successive fractions.

Components with a stronger preference for the mobile phase relative to the stationary phase travel through faster. More strongly retained components emerge later. TLC can help check which collected fractions contain the desired component. Poor packing, channels or an overloaded band can reduce separation.

The time taken to emerge is affected by the stationary phase, solvent composition, column dimensions and flow. Match conditions when comparing retention behaviour. High-performance liquid chromatography, HPLC, is an extension using controlled high-pressure liquid flow and specialised columns; it is not a gas technique.

Gas chromatography uses a gaseous mobile phase

GC separates suitable volatile, thermally stable components. The sample is vaporised and carried through a column by an inert gas such as helium or nitrogen. The stationary phase can be a solid or a high-boiling liquid film supported on a solid or column surface, depending on the system.

In gas–liquid chromatography, components partition between the moving gas and the stationary liquid. Both volatility and interaction with the stationary phase influence retention. A high boiling point often increases retention in comparable conditions, but boiling point alone cannot predict the full order when stationary-phase interactions differ.

The column is held in a controlled-temperature oven and carrier gas is supplied under controlled pressure or flow. A detector produces a signal as components leave the column. The sample needs to be vapour-phase without unacceptable decomposition; not every organic substance is directly suitable for GC.

Use retention times under matched conditions

Retention time is the elapsed time between injection and detection of a component, conventionally using its peak maximum. Compare an unknown with standards using the same column, stationary phase, temperature programme and carrier-gas flow.

Increasing oven temperature generally shortens retention for a given analyte under otherwise comparable conditions because it favours the gas phase. Increasing carrier-gas flow usually shortens transit times. A longer column generally increases retention at the same linear flow velocity, but pressure and flow conditions must be stated for a meaningful comparison.

Two compounds can have similar or identical retention times. A matching time supports an identification; it is not unique proof. Temperature changes can alter selectivity and peak resolution as well as timing, so “hotter is always better” is not a sensible optimisation rule.

Peak count and area need interpretation

Each resolved analyte peak suggests a detected component. A solvent peak, reference substance or contaminant should not be counted as an unknown sample component. Co-elution and undetected substances mean peak count can underestimate the number of components.

For a given compound and calibrated detector within its linear working range, peak area can be related to amount or concentration. Different compounds can have different response factors, so 40% of the total raw peak area does not automatically mean 40% by mass. Use calibration and the units stated in the question.

Original example: a 2.00 mg cm−3 standard of compound X gives peak area 3000 units. An unknown gives 2400 units under the same injection and detector conditions. Assuming a proportional calibration through the origin, concentration = 2.00 × 2400/3000 = 1.60 mg cm−3. If the analysed solution was a tenfold dilution, the original concentration was 16.0 mg cm−3.

If the calibration has a non-zero intercept, use its equation or graph instead of this simple ratio. This quantitative example is an application of calibration, not a universal law about every detector or mixture.

GC separates; mass spectrometry helps identify

Coupling GC to a mass spectrometer allows the mass spectrum of material emerging at a particular time to be recorded. The molecular ion, where observable, and fragmentation pattern provide much more information than retention time alone. A computer can compare spectra with a reference library.

A good library match can support a positive identification, especially alongside retention data and an authentic standard. Co-elution, weak molecular ions, similar spectra or incomplete library coverage can still limit confidence. The instrument does not make ambiguous data infallible.

GC–MS is used in contexts such as forensic, environmental and food analysis. Explain the relevant analytical benefit: separating a mixture before measuring spectra makes it easier to associate spectral evidence with its components. Do not say that the chromatographic column itself measures molecular mass.

Choose the technique for the question

For identification, combine independent evidence wherever the question permits. For obtaining a product, consider whether the technique collects material in the needed quantity. Analytical detection and preparative purification are related tasks but not identical.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

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