What is chromatography?

The principle of gas chromatography

In gas chromatography (GC), the mobile phase is a carrier gas. It transports the injected sample through a column containing a stationary phase. The components of the mixture are retained differently and leave the column at different times.

The stationary phase may be a solid adsorbent or a liquid film applied to a support or the inner surface of the column. These approaches are known as gas–solid chromatography and gas–liquid chromatography, respectively.

Separation depends on the properties of the substances, the stationary phase, the column temperature and the carrier-gas flow conditions. The elution order and retention times therefore relate to a particular set of analytical conditions.

GC is suitable for gases and compounds that can be vaporized without decomposing under the analytical conditions. The original sample does not have to be gaseous: the technique is also used for liquids and other sample types after suitable preparation. Some compounds are first converted into more volatile, thermally stable derivatives. This preparation step is called derivatization.

How a gas chromatograph works

A gas chromatograph comprises a carrier-gas supply system, a sample inlet, a column housed in an oven, a detector and a data acquisition system.

The analysis involves several stages.

1. Sample introduction. The sample enters the instrument manually, through an autosampler or through a specialized introduction system. The approach depends on the sample composition and physical state.
2. Component separation. The carrier gas transports the sample through the column. The oven temperature can remain constant or follow a programmed temperature profile.
3. Detection. The detector registers substances leaving the column and converts its response into an electrical signal.
4. Data processing. Software records the chromatogram, identifies peak boundaries and calculates results according to the specified analytical method.

The column separates the components, while the detector registers them. Choosing a sensitive detector does not, by itself, resolve inadequate chromatographic separation.

What a chromatogram shows

A chromatogram plots the detector signal against time. A peak usually appears as a component leaves the column. Its position and size are used for qualitative and quantitative analysis.

Retention time: which substance is present?

Retention time is the interval between sample introduction and the maximum of the corresponding peak. It is compared with the retention time of a reference standard analyzed under the same conditions.

A retention-time match helps identify a component, but does not by itself establish its chemical identity. Different substances can elute at the same time. Additional evidence, a different column or mass-spectrometric detection may be needed.

Peak area: how much substance is in the sample?

Peak area or height provides the analytical signal for quantification. The amount or concentration of a substance is calculated using a calibration relationship between detector response and a known amount or concentration of the component.

The calculation accounts for sample preparation, dilution, sample introduction and the requirements of the method. A peak-area percentage cannot automatically be treated as a mass or volume fraction: different components may produce different responses.

Limits of interpretation

A single visible peak may contain several unresolved components. The number of peaks therefore does not always equal the number of substances in a sample.

The absence of a peak does not prove that a substance is absent. Its concentration may be below the detection limit, the selected detector may not respond to it, or sample preparation may fail to extract or preserve it.

Detectors used in gas chromatography

A detector is selected according to the target substances, their expected concentrations, the sample matrix and the requirements of the analytical method.

Flame ionization detector — FID

An FID measures ions formed when organic substances enter a hydrogen–air flame. It is widely used to analyze hydrocarbons, solvents and many other organic compounds.

The response depends on the chemical structure of the substance. Quantitative analysis therefore requires appropriate calibration; equal amounts of different compounds do not necessarily produce equal signals.

Thermal conductivity detector — TCD

A TCD measures changes in the thermal conductivity of the column effluent relative to the carrier gas. Applications include the analysis of permanent and process gases.

The response depends on the difference in thermal conductivity between the target component and the carrier gas. Carrier-gas selection is therefore part of method development. The detector does not destroy the sample.

Electron capture detector — ECD

An ECD detects compounds capable of capturing electrons. It is used to determine a range of halogen-containing substances, including some pesticides and organochlorine impurities.

High sensitivity to particular groups of compounds does not make the detector universal: its suitability must be assessed for the specific substances and analytical conditions.

Mass-spectrometric detector — MS

In gas chromatography–mass spectrometry (GC–MS), the separated components enter a mass spectrometer. It measures ions according to their mass-to-charge ratio and produces mass spectra.

Mass spectra are used to identify substances, including by comparison with spectral libraries and reference standards. A library match is assessed alongside other analytical evidence rather than treated as conclusive identification.

GC–MS is also used for quantitative analysis. Targeted measurements may use selected ion monitoring (SIM). The suitability of this mode is assessed against the requirements of the particular application.

Applications of gas chromatography

Gas chromatography is used to analyze gases, fuels and petroleum products, solvents, food products and environmental samples. GC–MS is also used in pharmaceutical, toxicological and scientific research.

Depending on the method, analysis can determine the main composition of a mixture, monitor impurities, compare product batches or investigate unknown components.

Suitability is established for a particular sample and list of target substances. One instrument configuration does not provide the same capabilities for every application listed here.

Choosing a configuration for your laboratory

Equipment selection starts with a description of the analysis. To discuss a configuration, prepare:

• a list of target substances;
• a description of the sample and its main constituents;
• the expected concentration range;
• the analytical method or technical specification;
• the required limits of detection and quantification;
• the expected number of analyzes and the required level of automation.

This information guides the choice of sample introduction system, column, detector and sample preparation approach. Gas supply, workspace and staff training requirements are assessed at the same time.

From instrument selection to routine analysis

Reliable results depend on the entire analytical procedure, from sampling and sample preparation to calibration, data processing and measurement quality control.

Instrument configuration should therefore be discussed alongside method implementation, staff training, maintenance and ongoing support.

To discuss equipment bearing the international NeoCHROM trademark, send Neochrom your analytical method or a description of your application.

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