EGD-EGA Techniques

There are numerous other types of special identification detectors that can be used for EGD or EGA. These detectors, which are used widely in gas 

Differential thermal gas analysis EGA (38) Concentration change of gas in a reactor gas stream 

Elemental analysis (Pyrochrom) EGA 149) Functional group and C. H, N, 0 analysis 

Flame ionization detection EGD (55) Sometimes called thermal evolution analysis 

Gas chromatography EGA Many different columns and detectors are used 

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The techniques of EGD and EGA are almost always used in conjunction with other thermal analysis techniques or multiple techniques. In multiple techniques, two options are possible (1) One sample may be employed for all the measurements or (2) two or more samples, one for each techniques, may be employed. To distinguish between the two modes, the terms simultaneous will be used for the application of two or more techniques to the same sample at the same time. The term combined will indicate the use of separate samples for each technique. Multiple techniques are indicated by the acceptable abbreviation for each technique such as TG-EGD, TG-DTA-EGD-MS, EGA-MS, and so on. Other terms that may be employed by the EGD-EGA techniques are ... 

The usefulness of the EGD-EGA techniques can be extended by combining the various types of detectors, as given in Figure 8.2, with other thermal analysis methods. These multiple techniques offer a savings in time and effort, and since data are taken at the same time on the same sample, the results are more likely to be comparable than if they are taken separately on two or more different samples. Examples of the more common multiple techniques are given in Figure 8.5 and Table 8.4. 

Besides the multiple techniques given here, a few of the EGD-EGA techniques listed in Table 8.2 are used by themselves and have not been coupled to other thermal analysis techniques. Some of them will no doubt be coupled to TG and DSC techniques in the future. Several of the EGD-EGA techniques will probably never be coupled to other thermal analysis techniques due to the uniqueness of the experimental parameters involved such as thin-layer chromatography. 

The principal role of EGD and EGA is mainly as complementary techniques for other thermal analysis data. Samples are studied by TG, DTA, DSC and other thermal analysis techniques first and if the decomposition reactions are unknown, EGA is usually called on to determine the composition of the reaction products. With these known, as well as the other physicochemical data, the chemical pathway of the reaction can usually be elucidated. As mentioned earlier, the EGD-EGA data can often be obtained simultaneously with the other thermal data using multiple techniques with a substantial saving of time and effort. 

The development of EGD-EGA closely paralleled the introduction of controlled furnace atmosphere DTA and other thermal analysis techniques. In 1927, Orcel and Caillere (23) pointed out the importance of controlling the furnace atmosphere in DTA experiments on metallic chlorites. Some 20 years later, Berg (24) described perhaps the first EGD apparatus in which he... 

Just as it is almost impossible to represent a typical EGD-EGA apparatus, it is equally difficult to describe the wide variety of detectors that have been employed- Using the techniques listed in Table 8.2, the type of detector employed for each method is listed in Table 8.5. As can be seen, there are a number of different types of detectors used, from simple thermal conductivity detectors to more sophisticated ion current detectors in mass spectrometry. It is, of course, impossible to discuss each one in detail here, although the complete apparatus is described in certain cases. 

The main changes in this edition are as follows (1) Numerous new applications of thermal analysis techniques have been added to the chapters on TG, DTA, DSC, EGD/EGA, and others. (2) Other techniques, not used as often, are described in greater detail, such as EGD/EGA, TMA, DMA. thermoptometry, thermoelectrometry, thermosonimetry, and others. (3) The chapter on EGD/EGA has been rewritten, as has the chapter on miscellaneous techniques. (4) The determination of purity by DSC has been rewritten. (5) Commercially available instruments have been briefly described for each technique, including the application of microcomputers to many of these instruments. 

The reaction processes of substances cannot be analyzed by simple DTA/DSC or TG when thermal transition and the mass change due to reaction overlap. If DTA/DSC or TG is coupled with an evolved gas detector (EGD) and/or evolved gas analyzer (EGA), the reaction process can clearly be detected. Among various thermal analysis coupled simultaneous techniques [56], DTA/DSC or TG coupled with EGD and/or EGA is extensively used. TA-EGD-EGA coupled... 

An analytical scheme for the TA-EGD-EGA coupled simultaneous technique is shown in Figure 2.32. The evolved gas from TA (DTA/DSC or TG) is introduced into the EGA system after passing through the EGD system. The evolved gas is directly introduced to a thermal conductivity detector (TCD), avoiding second reactions. Two methods are used for the TA and EGA coupled simultaneous technique (1) on-line coupled simultaneous technique in series and (2) off-line combined method. In technique (1), TA is connected serially with a Gas chromatograph (GC) [61-65], mass spectrometer (MS) [57, 64, 65], Fourier transform infrared (FTIR) spectrometer [66, 67], non-dispersive IR (NDIR) spectrometer [68, 69] or thermo-gas-titrimetric... 

The physical property measured and the corresponding thermal analysis technique are tabulated in Table 1.1 (3) and further elaborated on in Chapter 13. Notice that under the physical property of mass, thermogravimetry (TG), evolved gas detection (EGD), evolved gas analysis (EGA), emanation thermal analysis (ETA), thermoparticulate analysis, and others are included. Similar considerations can be included in the physical proparties of optical characteristics, electrical characteristics, magnetic characteristics, and so on. The definitions of each individual technique are given in the chapter in which they are discussed. A select number of the thermal analysis techniques are summarized in Table 1.2. Each technique is tabulated in terms of the parameter measured, a typical recorded data curve, the instrumentation needed, and the chapter in which it is described. 

The evolution of gas from a thermal analyzer such as a TGA, DTA, or DSC may be determined using evolved gas detection (EGD) or, if qualitative or quantitative analysis of the gas is required, evolved gas analysis (EGA). These techniques are essentially a combination of thermal analysis and MS, tandem mass spectrometry (MS-MS), GC-MS or other... 

Some of the simultaneous instruments available commercially are as follows TGA/DTA/DSC, TGADTA/FTIR, DTA(DSC)/EGA(EGD), and DSC/FTIR. Also, high pressure DSC and photo-DSC instruments are available commercially. In addition, individual researchers have used simultaneous DSC/XRD (X-ray diffraction), DSC/EGA/XRD, and DSC/TRXRD (time-resolved X-ray diffraction). Experiments with parallel, combined, and simultaneous techniques help to affirm the conclusions drawn from a single technique and very often offer definitive clues to the actual mechanisms taking place during thermal analysis. 

Simultaneous or Combined Techniques. Many of the simultaneous or combined techniques that include TGA as a component have been described while discussing DSC/DTA techniques. Others with TGA as a component are TGA-EGA or EGD, TGA-MS, TGA-FTIR, TGA-GC-MS, TGA-MS-MS, and TGA-APCI-MS. APCI stands for atmospheric pressure chemical ionization. More details are... 

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