Evolved Gas Analysis
Couplings / Evolved Gas Analysis (EGA)
With the coupling of a Thermal Analyzer (TGA – Thermobalance, STA (TG-DSC) – Simultaneous Thermal Analysis, DIL – Dilatometer) and a thermal and evolved gas analyzer like a FTIR (Fourier-transform infrared spectroscopy) or a QMS, (Quadrupole -Mass-Spectrometer) a very powerful coupling is generated which gives simultaneous (correlated) information from both gas analysis instruments.
Typical couplings for simultaneous measurements are:
- TG-DSC-MS ( Thermogravimetry , Differential Scanning Calorimetry , Mass Spectrometer)
- TGA-MS (Thermal Balance coupled with Mass Spectrometer)
- TG-DSC-GC/MS (Thermogravity, Differential Scanning Calorimetry, Gas Chromatography / Mass Spectrometry)
Analytical techniques used for coupling with thermal analyzers Couplings can be done with different gas analyzing methods:
- FT-IR spectroscopy
- Quadrupole mass spectrometry (QMS)
- ELIF spectroscopy (Excimer Laser Induced Fragmentation Fluorescence)
- Gas chromatography
The coupling of the thermal analyzer with the spectrometer/chromatograph can be done by different means:
- Heated transfer capillaries (FTIR, GCMS, GC, MS)
- Sniffer coupling (GCMS, GC, MS)
- Optical in situ observation (ELIF)
Heated transfer capillary
The simplest way to do a coupling is by heated capillary. In this case, a heated capillary feeds the evolved gazes from the thermobalance to the spectrometer or chromatograph. The internal diameter of a capillary is < 0,1 mm in case of an EGA MS coupling. The capillary is heated to 200-300°C which results in the risk of condensation of outgassing during transfer and clogging of the capillary.
This technique is used for mass spectrometer coupling. Gases pass through a very small EGA port close to the sample inside the furnace and are transferred in the vacuum line to the mass spectrometer. In this way, gases are sampled at high concentration very close to the sample at high temperature and pass directly to ultra-high vacuum. This technique avoids any risk of condensation during transfer between the thermobalance and the mass spectrometer.
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 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 evolved gas analysis of organic compounds (polymers etc.).
Mass spectroscopy sorts molecules by their molecular weight divided by their electrical charge (m/e). In quadrupol mass spectroscopy (QMS) molecules enter a magnetic quadrupol field 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 EGA 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.
ELIF (Excimer Laser Induced Fragmentation Fluorescence) is a technique used for analysis of alkali metal compounds. Its measuring principle is based on a simultaneous cleavage of molecules, and excitation of the respective alkali atom by a VUV-laser. After the return of the agitated atom to its original state, a photon of a characteristic wavelength is emitted. The intensity of this “fluorescent signal” is a measure of the concentration of the compound in question. This technique is a valuable tool for characterization of alkali metal compounds (NaCl, KCl, NaOH, etc.). ELIF spectroscopy can be used only by optical in-situ coupling (see below).
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).