TG – FTIR: More than Just the Sum of its Parts

Thermogravimetric analysis (TG) follows changes in mass of the sample as a function of temperature and/or time. TG gives characteristic information about the composition of the measured sample, in particular the amounts of the various components and their thermal behavior.

An identification of gases released from the sample during thermal treatment cannot be performed by thermal analysis. For this purpose, coupling a spectroscopic method such as Fourier-Transform – Infrared (FT-IR) spectroscopy is an excellent solution. Infrared spectroscopy is a classical technique which depends upon the interaction of infrared radiation with the vibrating dipole moments of molecules. In the vapor phase, with the exception of homonuclear diatomics and noble gases, a characteristic spectrum for each substance is obtained. For monitoring continuous processes, the FT technique offers the specific advantages of speed and sensitivity.

FT-IR spectroscopy is a suitable method for the analysis of gas mixtures which usually occur during thermal decomposition. An optimally coupled TG-FT-IR system is necessary in order to analyze in detail complex and in particular overlapping decomposition steps. For complex mixture analysis, subtraction routines allow separating known compounds from the mixture spectrum for further library identifications and even factor analysis for quantitative considerations of overlapping spectra.

In the case of single component release, like softeners in polymers, easy monitoring as a function of time or temperature is given by simply setting a window in the IR spectrum. Choosing a corresponding frequency range enables a quick quantitative evaluation by correlating with the thermogravimetric data.

The vacuum-tight construction of the TG 209 thermal micro balance and the gas cell, allows evacuation of both systems together. The much smaller amount of residual gases minimises their influence on the decomposition process and their reaction with the released gases.

Lowered pressures offers a further parametric variation and therefore a more optimised separation and/or larger fragments for the IR identification leading to a better understanding of material behavior.

The analysis of gases evolved during thermogravimetric analysis yields important information about sample structure and decomposition behavior. FTIR is a technique that is particularly well suited to this type of analysis, being both sensitive and fast. Further, FTIR is well suited for the analysis of complex mixtures of gases. Proper optimization of a TGA-FTIR interface allows the analysis of complex or overlapping decomposition steps.

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