1988 TGAFT IR Thermogravimetric Analysis with Fourier

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Mikrochim. Acta [Wien] 1988, I, 179--182 Mikrochimica 
Acta 
�9 by Springer-Verlag 1988 
TGA/FT-IR: Thermogravimetric Analysis with Fourier 
Transform Infrared Detection of Evolved Gases 
Richard C. Wieboldt*, Steven R. Lowry, and Robert J. Rosenthal 
Spectroscopy Research Center, Nicolet Instrument Corporation, 5225-1 Verona Road, 
Madison, WI 53711-0508, USA 
Abstract. Thermogravimetric analysis (TGA) is a widely employed tech- 
nique for measuring the change in weight of a sample as a function of 
temperature or time in a controlled atmosphere. FT-IR has been utilized 
with success in the identification of gases [1]. The combination of these 
two techniques permits a complete characterization of materials in 
terms of thermal stability and decomposition mechanisms [2]. A 
complete integrated system for TGA/FT-IR analysis is described. 
Key words: thermogravimetric analysis, Fourier transform infrared spec- 
troscopy, TGA/FT-IR. 
The types of samples analyzed by TGA are extremely varied. By combining 
the two techniques, TGA and FT-IR provide a complete sample analysis 
with quantitative weight loss data from TGA and identification of evolved 
gases by FT-IR. In many cases, the components of interest are light 
molecular weight compounds such as entrained solvents and plasticizers, 
present in low concentrations. These evolve before the sample reaches 
decomposition temperatures. During the sample decomposition, large 
volumes of materials are expelled. This evolution is a mixture of decompo- 
sition products and is usually accompanied by particulates. Portions of this 
material invariably condense in cooler portions of the flow system. The 
design of an FT-IR interface cell must be capable of handling both the trace 
components as well as the large quantities of material released during 
sample decomposition. 
Experimental 
The TGA/FT-IR interface (patents applied for), Fig. 1, is designed for use 
with NICOLET SXC series FT-IR spectrometers and DuPont Instruments 
951 Thermogravimetric Analyzer module. The inlet sidearm of the FT-IR 
* To whom correspondence should be addressed 
180 R.C . Wieboldt et al. 
flow cell attaches directly to the furnace tube of the DuPont TGA via a 
simple ball and socket ground glass connection. The main cell body is 
contained in an insulated chamber which is maintained at any preselected 
temperature from ambient to 325 ~ C. Gases evolved from the sample during 
the TGA are swept from the furnace by a stream of purge gas. The cell 
contents are efficiently exchanged using the typical TGA purge gas flow 
rates of 100 ml/min or greater. 
A problem often encountered in TGA analysis is fouling of the cell. In 
designing the FT-IR flow cell, the problem of handling the large quantities 
of material released during sample decomposition was kept in mind. Three 
key features of the cell design successfully deal with this situation. First, the 
inlet sidearm attached to the TGA furnace is not actively heated. Thus 
heavier particulates condense prior to fouling the cell optics. Lighter 
components, generally of interest, are carried through this zone without 
condensation. Secondly, the infrared beam does not see the walls of the 
flow cell. Materials which condense on the walls of the flow cell do not 
contribute to the infrared spectrum. Finally, the cell body is a separate 
assembly which quickly snaps in and out of the heated chamber. The cell 
may be easily exchanged between experiments if particularly dirty samples 
are under investigation. 
The data presented here were acquired with a NICOLET 20 SXC FT-IR 
equipped with a DTGS pyroelectric bolometer. The standard NICOLET 
SID software was utilized for the real time data collection and post 
TO BENOH 
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TGA/FTJR INTERFACE 
S INSULATED SAMPLE COMPARTNENT 
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Fig. 1. Schematic of N ICOLET 
TGA/FT- IR interface