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Evolved Gas Analyzer ( EGA )

Couplings / Evolved Gas Analysis (EGA)

With the coupling of a thermal balance and a gas analyzer like a FTIR spectrometer (Fourier Transform Infrared) or a Quadrupole-Mass-Spectrometer a very powerful analytical instrumentation is used which gives information from the thermal balance (TGA) or simultaneous thermal analyzer (STA) as well as from the spectrometer simultaneously.

The optional Pulse-Analysis injects an exactly predetermined amount of gas into the Thermobalance (TGA) or Simultaneous Thermal Analyzer (STA). This enhances the measurement possibilities significantly.

Type of couplings The coupling of the thermal analyser with the spectrometer/chromatograph can be done by different means:

Optical in-situ observation

In this case, optical windows are integrated in the thermobalance’s During heating samples often undergo phase transitions and/or weight change due to evaporation of solvents and/or chemical reactions. These changes can be detected by thermal analysis: calorimetric techniques (DTA and DSC) give information about the heat involved in these processes and thermogravimetry (TG) shows the weight change. Weight change can be either weight increase due to oxidation reactions or weight loss due to decomposition by liberation of volatile compounds. Analysis of these evolved gases can give valuable information about the sample composition and reaction pathways for decomposition. As thermal analysis gives no information about the nature of the evolved gases, coupling with spectrometers or chromatographs is a valuable tool for evolved gas analysis (EGA).

Infrared spectroscopy:

Infrared light can excite molecular vibrations in molecules. In order to be active in respect to IR-spectroscopy, the molecule has to change its dipolar momentum during excitation. Gases like CO2, CO, hydrocarbons, water vapour etc. have IR-active vibration modes while N2, O2 etc. cannot be detected. The obtained IR-spectra allow identification of the components by characteristic vibrations which are either typical for a certain functional group (CO, COOR etc.) or for a particular compound (so called “fingerprint- region” of the spectra from 1500 – 500cm-1). Spectra libraries are helpful during spectra interpretation. Coupling to TGA and STA is a valuable tool especially in analysis of organic compounds (polymers etc.).

Mass spectroscopy:

Mass spectroscopy sorts molecules by their molecular weight divided by their electrical charge (m/e). In quadrupol mass spectroscopy (QMS) molecules enter a magnetic quadrupolfield after having been accelerated in a static electric field. Molecules and their fragments are sorted by their masses and can be identified. Mass spectroscopy is very useful in order to find the molecular weight of the outgassing as well as to analyse gases which are not active in IR-spectroscopy (N2,O2, CO etc.). Using mass spectroscopy, nearly all molecules can be detected. Also the resulting fragments of bigger molecules are often characteristic for several compounds or functional groups. This method is a common used analytical method that can be found in polymer or organic analysis as well as in forensic, medicinal, biological or inorganic areas like material science. Mass spectrometry can be also combined with a GC method that is used to get information about the purity of the substances that are investigated by the mass spectrometer. So the resulting method called GC-MS gives both, purity and molecular weight of the substrate.

Gas chromatography

The evolved gases can be a complex mixture of compounds. Column Chromatography separates these compounds before analysing them by different techniques. The chromatographic separation column has to be chosen according to the type of molecules to be separated (polar or unpolar). The most frequently used detection techniques are flame ionization detectors (FID) and thermal conductivity detectors (TCD).

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