The technique of differential thermal analysis (DTA) was first proposed, in a rather crude manner, by LE CHATELILZR in 18871. Since its conception, DTA has been a useful tool in the fields of metallurgy, ceramics, geology, and chemistry. However, only in recent years has this technique been seriously applied to chemical problems. The reasons for this late acceptance in the field of chemistry are many. Perhaps the main reason for this has been the irreproducibility of the thermograms obtnincd from a known sample by various instruments. The curves obtained vary rather widely and are affected by such factors as furnace heating rate, sample particle size, furnace atmosphere, type of sample holder, etc. Another problem has been the interpretation of the thermogram peaks. Generally, additional information is needed to supplement the DTA curve such as thermogravimetric analysis (TGA), X-ray diffraction, infrared absorption spectroscopy, visual phase examination, etc. With this supplemental data it is possible, in most cases, to interpret correctly the nature of the reaction or reactions which are responsible for the peak or peaks. However, due to environmental and instrumental effects, there are discrepancies between the DTA curve and, say, a TGA curve2 which could lead to a completely false interpretation.
This problem has been attacked from the standpoint of analysis of the dccomposition products, and a new apparatus is described which will lend itself to simplifying the interpretation of the DTA curve. An apparatus similar to the one discussed here has previously been described a, but this apparatus was limited in use because of the use of a glass sample holder and also the presence of a large amount of a glass bead diluent in the sample chamber. The apparatus described herein will be designated as a DTA-GE (GE = gas evolution analysis) apparatus. Basically, the instrument consists of a conventional DTA unit with provision for the detection ,and analysis of the evolved gaseous decomposition products. From, 25 to 50 mg of sample, without the use of a supporting diluent for heat transfer purposes, are pyrolyzed in a dynamic helium gas atmosphere, under controlled furnace heating rates. ,The composition of the furnace gas stream is monitored by a thermistor thermal conductivity cell with provision for the collection of the gas samples in glass bulbs prior to their analysis. It is thus possible to record both the DTA and) GE curves as a function of the, furnace temperature.The composition of the gas samples can then be determined by conventional techniques such asrn,@ spectrometry, gas ehronia;