Temperature extrapolated onset

This comparison suggests that there is little difference in the thermal stability of the three polymers. This is supported by the extrapolated onset temperatures (see table 5) for the degradation obtained from the derivative plots of weight loss versus temperature. 

The most widely recommended calibration method for dynamic DSC operation involves the determination of the extrapolated onset temperature for the fusion of several standard substances, using various heating rates [255,256],... 

Measure the temperatures for the following Tf, Tm,Tc, and Te, where T is the extrapolated onset temperature, Tm is the melting peak temperature, Tc is the crystallization peak temperature, and Te is the extrapolated end temperature. Report two Tm values if observed. 

The thermogram is characterized by a single melting endotherm with an extrapolated onset temperature for melting of 103.4 C. The purity of the sample was found to be 99.34%. 

When the solvent-isolated losartan was subjected to DSC characterization, the DSC curve (Fig. 3-5, curve A) showed a minor endotherm at an extrapolation onset temperature of about 229°C (10°C/min) and a major melting endotherm at an extrapolation onset temperature of about 273°C (10°C/min). When a sample of Form I was heated to 255°C and then cooled back to room temperature, the subsequent DSC curve showed only the high-temperature endotherm (Fig. 3-5, curve B). Chemical analysis (HPLC) and solution NMR showed no change in the material heated to 255°C and cooled back to room temperature. However, XRPD indicated a change in the crystal structure. Therefore, it was concluded that the minor endotherm corresponded to a kinetically irreversible enantiotropic polymorphic transition and that the losartan system was not under complete thermodynamic control. Form I is the low-temperature stable form, up to the transition point, and the high-temperature stable form was... 

A number of parameters can be measured from the various thermal events detected by DSC. For example, for a melting endotherm, the onset, peak temperatures, and enthalpy of fusion can be derived. The onset temperature is obtained by extrapolation from the leading edge of the endotherm to the baseline. The peak temperature is the temperature corresponding to the maximum of the endotherm, and the enthalpy of fusion is derived from the area of the thermogram. It is an accepted custom that the extrapolated onset temperature is taken as the melting point however, some users report the peak temperature in this respect. We tend to report both for completeness. 

The question therefore arises as to where the melting point should be taken from. There are a number of options the temperature at which deviation from the baseline begins (Tb), the extrapolated onset temperature (Tm), the peak temperature and the temperature at which the trace returns to the baseline (Te notation used taken... 

The DTG curve allows the ready determination of the temperature at which the rate of mass-change is a maximum, 7, and this provides additional information to the extrapolated onset temperature Te and the extrapolated final temperature 7. All three temperatures, however, respond to changes in experimental conditions, and Tmax values are no more characteristic of a material than is 7J or 7. ... 

The extrapolated onset temperatures for the NBS-ICTA standard materials are given in Table 11.9 (133). Three sets of data are presented, the 1CTA (134) temperature the DTA (model 673-4 DTA apparatus) temperature and the TS-DTA apparatus temperature. Standard deviations between these onset temperatures and the ICTA mean extrapolated onset values (7iCTA) are —1.5 and — 6-5°C, respectively. The deviation of the DTA 673-4 is negligible... 

CTA Mean Extrapolated onset Temperature (8), Ticta Observed Mean Extrapolated Onset Temperature, DTA 673-4 Observed Mean Extrapolated Onset Temperature, TS-DTA... 

In case the sample mass has not been adjusted, an increase in thermal lag will be found as caused by increasing the heating rate (top symbol h) and the cooling rate (bottom symbol c) as can be seen for an azoxyanisole sample (Figure 5), and by increasing the sample mass, see e.g. indium (Figure 6). Therefore, there is a need for determining the influences of scan rate and sample mass on the temperature calibration of HPer DSCs. In [30], calibration matrices for the extrapolated onset temperatures and for peak temperatures as function of... 

Figure 5 shows a typical mass loss in a decomposition experiment. The obvious definition would seem to be where the mass loss is steepest, which corresponds to the peak temperature T in the DTG plot. However, this is merely the point where reaction is fastest and does not represent the start of reaction, e.g. where bonds in the compound begin to break. The position of T will depend upon the sample size, packing, and heat flow properties. The point Tjis the initial temperature or onset temperature, but is not easy to identify and depends on the sensitivity of the balance and the amount of drift or noise seen. There may be traces of impurities, which decompose or promote some decomposition ahead of the main reaction. A better definition of start of reaction is the extrapolated onset temperature T. This requires drawing of tangents to the curve at the horizontal baseline and the steepest part of the curve and marking their intersection. For a reaction that starts very slowly and only speeds up later, T and Tj will be very different and a more satisfactory point would be shown as temperature where the fraction reacted a is equal to 0.05, i.e. Tq.05- Another definition of reaction temperature, important in kinetic studies, is when the reaction is half over, that is, when the fraction reacted... 

The results from DTA and DSC experiments are displayed as a thermal analysis curve in which the instrument signal is plotted against temperature - usually the sample temperature - or time. Figure 3 shows some of the terminology relating to the results from DSC experiments. The description heat flow is frequently used for the instrument signal. Analysis of the thermal analysis curve is carried out using the instrument software. Of particular importance is the extrapolated onset temperature which is... 

Figure 10 shows the effect of heating rate on the fusion peak of indium displayed against temperature (a) and against time (b). The curves illustrate the efficacy of extrapolated onset temperature compared with the peak temperature as already discussed. 

In principle the method is simple enough - small quantities of the two calibrants in turn are allowed to equilibrate 30 °C below the temperature of the transition as indicated by a constant instrument signal. The calibrants are then heated through the transition and the extrapolated onset temperatures obtained from the endotherms. These temperatures correspond to Texpi and rexp2- The calibration will be affected by heating rate and possibly the nature of the atmosphere and its flow rate. The experimental conditions for the calibration should be chosen to match those for subsequent measurements. Calibration over more than two points may be carried out and the relationship between T and T xp determined statistically. The extent to which the instrument output can be corrected by the software will depend on the detailed design of the computer system. 

Figure 12 Variation of the extrapolated onset temperature with heating rate for the solid-solid transition peak of K2CrO ...

Investigation of the reaction kinetics in an inert atmosphere using DSC confirms the difference between evaporation and cracking processes. However, the extrapolated onset temperatures of the evaporation exceed the T5 % index numbers of thermogravimetry considerably. On the contrary the start temperatures (onset) of the cracking reaction in DSC were found at nearly equal temperatures, to those from thermogravimetry, T ... 

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