Presentation: Gas Chromatography

Gas chromatography (GC) is a powerful analytical method for separating and identifying volatile compounds in a sample. It is based on the principle of distribution and separation of compounds between a stationary phase (usually a coated capillary column) and an inert mobile gas (carrier gas) that transports the sample through the column.

The gas chromatographic process begins with the injection of the sample into the GC system, typically through an injection needle or an autosampler system or, in our case, through a pyrolysis oven. The vaporizable components of the sample evaporate in the injector and are transported to the column by the carrier gas.

As the compounds move through the column, they interact with the stationary phase. These interactions depend on the physico-chemical properties of the compounds as well as the composition and nature of the stationary phase. The separation of compounds is therefore based on their chemical properties such as polarity, molecular size and interaction with the column. The boiling points (or vapour pressures) of the individual compounds also play an important role in the separation of a sample mixture, so that the separation of the individual compounds can be optimized by a temperature program of the heated GC column oven that is specially adapted to the compounds.

The separated compounds finally reach the detector one after the other, which registers their presence and generates a chromatographic signal. Depending on the analysis task, different types of detectors are used in GC. Each detector has its own strengths and applications. The coupling of GC with mass spectrometry (GC/MS) enables the detection of very small amounts of substances and the determination of the structure of the compound using characteristic mass spectra. The most widely used detector in GC is the flame ionization detector (FID), which is ideal for detecting organic compounds. The thermal conductivity detector (WLD/TCD) is used in particular as a very sensitive detector in gas analysis.

The data generated by the detector is displayed in a chromatogram, which shows the intensity of the signal as a function of the retention time of the compounds. The compounds can be identified by comparing the retention times with known standards or reference databases or, in the case of GC/MS coupling, by characteristic mass spectra.

Gas chromatography in combination with various detectors is an extremely versatile and efficient technique for characterizing and quantifying compounds in complex samples. Gas chromatography has established itself as a powerful method in areas such as environmental analysis, materials science, the food and beverage industry, pharmaceutical analysis and forensics.

Pyrolysis (Py)-GC-MS

In pyrolysis (Py), the sample material is strongly heated in an inert gas environment under controlled conditions to induce thermal decomposition. This breaks down high molecular weight compounds into smaller fragments. These fragments are then separated using gas chromatography (GC) and detected using mass spectrometry (MS). Using the resulting mass spectra of the fragments produced during pyrolysis, it is possible to identify them and determine their structure. Conclusions can then be drawn about the composition of the starting material from the characteristic decomposition products.

The advantage of Py-GC/MS over other analytical techniques such as LC-MS and GC-MS lies in the wider range of matrices that can be analyzed, including gases, liquids, solids and cross-linked polymeric materials (solvent-insoluble materials). With this analytical technique, for example, the smallest quantities of complex polymer samples (30 µg - 1 mg) such as plastics, coatings, rubber, resins, cellulose, etc. can often be analyzed directly without prior sample preparation. Py-GC-MS is used, for example, in materials science to determine the composition of polymers by providing information on monomers, copolymers and additives.

Thermal desorption (TD)/Pyrolysis (Py)-GC-MS

Pyrolysis is often preceded by additional thermodesorption (TD) in order to obtain further information. In the so-called “double-shot technique”, volatile and semi-volatile compounds are released from a solid or liquid sample at a selected temperature in a first step, the thermodesorption. The compounds outgassing during the thermodesorption phases are cryofocused with liquid nitrogen and then separated into their individual components by gas chromatography and identified by mass spectrometry. In the following second step, pyrolysis, the sample is completely decomposed at temperatures >550 °C and the decomposition products are analyzed by GC-MS in order to obtain conclusions about the sample material in addition to the outgassing volatile compounds (such as plastic additives).

TD-/Py-GC-MS has the advantage that the (semi-)volatile compounds are selectively extracted from the main matrix at a certain temperature (or several temperatures in the “multi-shot technique”) through thermal desorption, which increases the sensitivity of the analysis. The additional pyrolysis extends the analysis range to non-volatile compounds that may not be detected in a conventional TD-GC-MS analysis.

• For further information see: Organic Analytics

• Contact Person: Tim Hammerschick