Gas chromatography
Gas chromatography (GC) or gas-liquid chromatography is an analytical method for the quantitative analysis of complex compounds based on the separation of the compounds in a liquid-coated separation column.
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What is gas chromatography?
Gas chromatography (GC) is a variant of adsorption and partition chromatography. In this type of chromatography, the stationary phase (separation column) is a liquid that, however, does not move because it is bound to a fixed carrier material. The mobile phase is gaseous. GC can only be used for components that are gaseous or that can be vaporised without decomposition (boiling range up to 400 °C).
During gas chromatography, the substance to be analysed is introduced into the separation column using an injector and transported to the detector using the carrier gas. On its way through the separation column the analyte interacts with the stationary phase. Depending on the structure and functional groups, the interactions are either only weak or are powerful. The latter cause the separation of the mixture.
The assessment of the individual substances in gas chromatography is based on the varying partial vapour pressures and the varying polarity of the substances analysed. A higher partial vapour pressure means that the substance remains in the gas phase longer, which in turn signifies a shorter retention time. Because the retention time is specific for many substances, the substances can be identified based on their peaks.
Gas chromatography at Quality Analysis
Gas chromatography is in many applications a standard method for the qualitative and quantitative analysis of complex mixtures. However, it involves high equipment costs because, depending on the specific task, completely different variants of the components (separation columns, detectors, carrier gases) must be used. Extensive user experience is also necessary for the reliable evaluation of the results.
At Quality Analysis, we have not only a comprehensive range of technical equipment and experienced experts to be able to answer your questions, we also have a deep understanding of the economic concerns of our customers. As such, our analysis reports are designed for practical relevance. We answer your questions and provide you with specific assistance with which you can rectify sources of defects and design work processes more efficiently. You can always rely on the consistently high quality of our work because we are a test laboratory accredited in accordance with DIN EN ISO/IEC 17025.
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How is a Gas Chromatograph constructed?
A gas chromatograph, in principle, always consists of the same components: the injector, the separation column with a thin liquid layer as the stationary phase and the related detector connected to a PC. There is also the carrier gas as the mobile phase that transports the analyte. There are numerous variants of these basic components, depending on which substance is to be analysed. Therefore only a few particularly important or common types are described in the following.
Carrier Gas / Mobile Phase
The carrier gas transports the sample through the gas chromatograph and forms the mobile phase. The choice of gas depends on the analytical task, the separation column, and the detector. Helium, hydrogen, or nitrogen are frequently used. High purity is crucial: Contaminants such as oxygen, water, or hydrocarbons can impair the stationary phase or affect the detector signal. The flow rate must also be carefully selected. It affects the separation efficiency, the width of the peaks, and the duration of the analysis. The goal is to strike a suitable balance between reliable separation and the most efficient analysis possible.
Injector / Sample Feeder
The injector is used to introduce the analyte into the separation column. This injection is without temperature control and is undertaken via direct feed systems or on-column injectors; the latter in particular have the advantage that they provide very sharp peaks, however they can only be used with samples mixed with a solvent with a low boiling point. On the other hand, if it is necessary to vaporise actively a sample, mostly split/splitless injectors are used that either first completely vaporise the sample and then introduce it into the separation column (splitless injection) or inject it already during vaporisation (split injectors). Temperature-sensitive samples can only be introduced into the separation column using a cold feed system with cooling.
Headspace Analysis (Sample Introduction Method)
A further option for the identification and quantification of highly volatile organic compounds is so-called headspace analysis. For this purpose only the headspace over a sample is analysed, not the sample as a whole. In the headspace above the sample an equilibrium of the volatile components between the headspace and sample becomes established at elevated temperatures, this equilibrium is dependent on the nature and concentration of the analytes. Headspace analysis is automated with the aid of heated autosamplers. The substances in the vapour are analysed with the aid of a gas chromatograph (GC-MS/GC-FID).
Separation columns and the Stationary phase
Separation columns are differentiated between packed columns and capillary columns, where the latter are standard for most analyses these days because they permit significantly better separation and therefore shorter analysis times. As a rule, the length of the column is between 10 and 60 metres, but can also be more than 100 metres for specific applications. The stationary phase coats the inside of the column with a thin film of a viscous liquid of low volatility.
The separation column is in an oven (GC oven) that either ensures a constant temperature in the separation column during the entire analysis, or (less frequently) an increasing temperature. If the carrier gas with the vaporised analyte now flows through the separation column containing the stationary phase, the actual separation as described above occurs.
Detector
The detector detects the components after they exit the separation column. The type of detector used depends on the analytical task. The Flame Ionization Detector (FID) is suitable for quantifying organic substances. For permanent gases, the thermal conductivity detector (WLD or TCD) is used, among others. Coupling with a mass spectrometer is also possible; this method is referred to as GC-MS.
A chromatography data system displays the detector signals as a chromatogram. The peaks are evaluated based on their retention times and signals. Mass spectrometry provides additional information for the identification and structural elucidation of the substances being analyzed.
Applications of Gas Chromatography
Gas chromatography is used in many fields, not only because of the straightforward identification of many substances based solely on their retention time, but also because of its ability to reliably separate very complex mixtures of substances and its high sensitivity, which allows even the smallest amounts of a substance to be detected.
Filmic Contamination
Organic residues on component surfaces can significantly impair function, adhesion, or further processing. Gas chromatography enables the quantification of soluble contaminants (total peak) as well as the analysis of volatile substances using headspace analysis. Typical applications include the determination of siloxanes and solvent residues, and the assessment of the purity of technical components.
Plastics Analytics
Whether for raw material testing, quality control, or failure analysis, gas chromatography provides valuable information about the composition of polymeric materials. It is used to identify and quantify solvents, plasticizers, additives, and other volatile components. The assessment of PAH compounds and material-related emissions is also among its typical applications.
Emissions and Outgassing Analysis
Volatile organic compounds can affect a component’s product quality, odor, or serviceability. Gas chromatography can be used to reliably detect outgassing and emissions from plastics, coatings, adhesives, or packaging. This method helps identify emission sources and evaluate material or process changes in a targeted manner.
Food Chemistry
In food chemistry and analysis, gas chromatography is used to analyze complex mixtures of substances and trace amounts of residues. Among other things, it is used to determine flavorings and ingredients, to verify the authenticity of fats and oils, and to detect residues such as pesticides or other contaminants.
Pharmaceutics and Pharmacology
In pharmaceutics and pharmacology, GC-MS is used both for the analysis of chemical substances in the laboratory and for in vivo research. For example, it is used for the quantification and structural characterization of substances in drug production, as well as for the detection of drug metabolites in samples from the human body.
Summary: Gas Chromatography
Gas chromatography (GC) or gas-liquid chromatography is a chemical analytical method used to separate complex mixtures of substances by inducing strong interactions between them and the stationary phase. Based on the typical retention times, it is possible to identify the individual compounds in the separated mixture.