Gas Chromatography: How It Works

A drop of perfume contains dozens of different molecules. A blood sample from a driver might contain alcohol, drugs or neither. A soil sample near a factory might hold traces of solvents. Gas chromatography (GC) can separate every volatile component of mixtures like these, measure how much of each is present, and, combined with a mass spectrometer, identify them. It’s one of the most important analytical techniques in modern chemistry.

The basic idea

Gas chromatography works on the same principle as all chromatography: components of a mixture distribute between a stationary phase and a mobile phase, and travel at different speeds. See chromatography.

In GC:

• The mobile phase is an inert carrier gas, usually helium, nitrogen or hydrogen.
• The stationary phase is a thin film of high-boiling liquid or polymer coating the inside of a long, narrow column.
• The sample must be volatile: it must turn into a gas at the operating temperature without decomposing.

The instrument

A gas chromatograph has five main parts:

1. Carrier gas supply

A cylinder of pure, dry, inert gas flows at a steady rate through the whole system. It carries the sample through the column but doesn’t react with it.

2. Injection port

A tiny volume of sample, often about 1 μL (a millionth of a litre), is injected through a rubber septum with a microsyringe. The injector is hot (typically 200–300 °C), so the sample vaporises instantly and is swept onto the column. Autosamplers do this automatically for hundreds of samples.

3. Column

Modern capillary columns are made of fused silica, typically 10–60 m long but only about 0.1–0.5 mm wide, coiled up to fit inside an oven. The stationary phase is a very thin film (a fraction of a micrometre to a few micrometres) of a polymer coating the inside wall. Different coatings suit different compounds: non-polar polysiloxanes for hydrocarbons, more polar polyethylene glycol coatings for alcohols and other polar compounds.

4. Oven

The column sits in a precisely controlled oven. Temperature strongly affects how fast compounds move, so it must be accurately controlled. Often a temperature programme is used: the oven starts cool, to separate the most volatile components, then heats up steadily to push the less volatile ones through.

5. Detector

At the end of the column, a detector responds to each compound as it emerges, producing a signal that’s plotted against time. Common detectors:

• Flame ionisation detector (FID): burns compounds in a hydrogen flame; organic compounds produce ions, creating a current. Very sensitive to hydrocarbons and most organic compounds.
• Thermal conductivity detector (TCD): responds to changes in the thermal conductivity of the gas. Works for almost anything, including permanent gases, but is less sensitive.
• Electron capture detector (ECD): extremely sensitive to halogenated compounds, such as some pesticides.
• Mass spectrometer (MS): identifies compounds by their mass and fragmentation pattern (GC–MS).

Why compounds separate

As the carrier gas pushes the sample along the column, each compound repeatedly dissolves into the stationary film and evaporates back into the gas. The time a compound spends in the stationary phase depends on:

• Boiling point / volatility: more volatile compounds (lower boiling points) spend more time in the gas and come out first.
• Interaction with the stationary phase: compounds with similar polarity to the coating dissolve in it more and come out later (“like dissolves like”).
• Column temperature: higher temperatures make all compounds move faster.
• Carrier gas flow rate and column length.

Retention time

The time from injection until a compound reaches the detector is its retention time. Under fixed conditions (same column, temperature programme and flow rate), each compound has a characteristic retention time. It plays the same role as the Rf value in paper and thin-layer chromatography. See calculating Rf values.

To identify a compound, you compare its retention time with that of a known pure standard run under the same conditions. But two different compounds can have very similar retention times, so retention time alone isn’t absolute proof of identity. That’s why GC is often paired with mass spectrometry.

Reading a gas chromatogram

A chromatogram is a graph of detector signal (y-axis) against time (x-axis). Each compound appears as a peak.

• The position of a peak (its retention time) helps identify the compound.
• The area under a peak is proportional to the amount of that compound.

Example. A mixture of ethanol, propan-1-ol and butan-1-ol on a typical column might give three peaks, in order of increasing boiling point: ethanol (78 °C) first, propan-1-ol (97 °C) second, butan-1-ol (118 °C) third.

Measuring amounts (quantitative GC)

Because peak area is proportional to amount, GC can measure concentrations accurately:

1. Relative amounts. If the detector responds equally to each compound, the percentage of each component is its peak area as a percentage of the total area. For example, peak areas of 30, 50 and 20 (arbitrary units) suggest a composition of about 30%, 50% and 20%.
2. Calibration curves. For accurate work, a series of standard solutions of known concentration is run, and a graph of peak area against concentration is plotted. The unknown’s concentration is read from the graph. See calibration curves.
3. Internal standards. A known amount of a different compound is added to every sample and standard. Comparing each peak to the internal standard corrects for variations in injection volume.

In practice, detectors respond differently to different compounds, so response factors are determined with standards.

GC–MS: separation plus identification

In gas chromatography–mass spectrometry, each compound emerging from the GC column goes straight into a mass spectrometer. The mass spectrometer ionises and fragments the molecules and measures the masses of the fragments, producing a mass spectrum that acts like a fingerprint. Software compares it with libraries containing hundreds of thousands of reference spectra.

GC–MS combines the separating power of GC with the identifying power of MS, and is considered a gold-standard technique in many fields.

Where GC is used

• Forensic toxicology: detecting drugs, alcohol and poisons in blood and urine. Blood alcohol is often measured by headspace GC, which analyses the vapour above a heated sample.
• Sports anti-doping: detecting banned substances and their metabolites.
• Arson investigation: identifying accelerants such as petrol in fire debris.
• Environmental monitoring: measuring volatile organic compounds, pesticides and solvents in air, water and soil.
• Petroleum industry: analysing the composition of fuels and crude oil.
• Food and flavour: identifying aroma compounds in coffee, wine and perfumes; detecting contaminants.
• Pharmaceuticals: checking residual solvents in medicines.

Limitations

• Samples must be volatile and thermally stable. Large molecules (proteins, sugars, most polymers) and compounds that decompose on heating need HPLC instead, or chemical modification (“derivatisation”) to make them volatile. See HPLC.
• Retention time alone doesn’t prove identity.
• Equipment is expensive and needs skilled operators.

Key takeaways

• GC separates volatile compounds using an inert carrier gas (mobile phase) and a liquid or polymer film in a long column (stationary phase).
• Compounds separate by volatility and by interaction with the stationary phase; each has a characteristic retention time.
• Peak area is proportional to amount, so GC measures concentrations with calibration standards.
• GC–MS adds identification by mass spectrum and is widely used in forensics, doping control and environmental testing.
• Non-volatile or heat-sensitive substances need liquid chromatography instead.