Thermogram analysis

Figure 16-27. DSC thermograms of Oocl-OPV5 (left) and Oocl-OPV5-CN (right), recorded on a Perkin Elmer 7 scries (hernial analysis system at healing and cooling rales of 10 C mill"1.

Analysis of the Thermogram of Melamine Fiber in Nitrogen Atmosphere, at a Heating Rate of 10°C/min ... 

Differential Scanning Calorimeter (DSC) thermograms were obtained on a Perkin Elmer DSC-2 run at 10°C per minutes. Dynamic Mechanical Thermal Analysis (DMTA) spectra were obtained on a Polymer Labs DMTA at a frequency of 1Hz with a temperature range from -150°C to +150°C at a scan rate of 5°C per minute. 

In the broadest sense, thermal analysis (TA) measures physical changes in a material as a function of temperature. TA instruments measure variables in a sample such as heat flow, weight, dimensions, etc. A typical fingerprint of a compound might be the endothermic peak on a thermogram indicating a sample s crystalline melt. 

In the range of linearity, Eq. (29) correctly represents the heat transfer within the calorimeter. It should be possible, then, by means of this equation to achieve the deconvolution of the thermogram, i.e., knowing g(l) (the thermogram) and the parameters in Eq. (29), to define f(t) (the input). This is evidently the final objective of the analysis of the calorimeter data, since the determination of the input f(t) not only yields the total amount of heat produced, but also defines completely the kinetics of the thermal phenomenon under investigation. 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

Fig. 21. Qualitative analysis of a thermogram recorded during the reduction of nickel oxide by a dose of carbon monoxide at 200°C. The thermogram (1) may be considered as the sum of curves (2), (4), and (5). Curve (3) is the difference between curves (1) and (2). Reprinted from (55) with permission. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

The analysis of the thermograms recorded during the interaction of the successive doses of the different reactants in the sequence may also yield very relevant informations. Through the use of different techniques, it has been shown, for instance, that the different steps of the mechanism of the CO oxidation, at room temperature, at the surface of pure [TNJiO (200)3 19, 82) or lithium-doped 54) nickel oxide, may be written ... 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

The calibration of DTA systems is dependent on the use of appropriate reference materials, rather than on the application of electrical heating methods. The temperature calibration is normally accomplished with the thermogram being obtained at the heating rate normally used for analysis [16], and the temperatures... 

Differential thermal analysis proved to be a powerful tool in the study of compound polymorphism, and in the characterization of solvate species of drug compounds. In addition, it can be used to deduce the ability of polymorphs to thermally interconvert, thus establishing the system to be monotropic or enantiotropic in nature. For instance, form I of chloroquine diphosphate melts at 216°C, while form II melts at 196°C [18]. The DTA thermogram of form I consists of a simple endotherm, while the thermogram of form II is complicated (see Fig. 4). The first endotherm at 196°C is associated with the melting of form II, but this is immediately followed by an exotherm corresponding to the crystallization of form I. This species is then observed to melt at 216°C, establishing it as the thermodynamically more stable form at the elevated temperature. 

Probably the main weakness of DTA as a method of analysis remains the difficulty of linking the thermal changes shown on the thermogram, with the actual thermal processes taking place. It should be noted that data obtained by DTA are often similar to those available for differential scanning calorimetry. Indeed the two techniques overlap extensively and may be seen as complementary. A comparison of the two techniques is made at the end of the next section. 

Thermograviraetric Analysis and Differential Thermal Analysis Thermograms of Dobutamine Hydrochloride... 

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