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. 

Tabulate the extrapolated onset and peak or inflection temperatures for all transitions, (see Fig. 16.3). 

Figure 1 A hypothetical differential scanning calorimetric profile for an endothermic transition, where Tm is the transition midpoint, and Ti and T2 are the extrapolated onset and completion phase transition temperatures, respectively.

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 oxidation induction time can also be used as an indication of oxidation stability. No correlation has been determined between the results of this test and service performance. In the test (ASTM D-5483), a small quantity of grease in a sample pan is placed in a test cell. The cell is heated to a specified temperature and then pressurized with oxygen and is held at a regulated temperature and pressure until an exothermic reaction occurs. The extrapolated onset time is measured and reported as the oxidation... 

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... 

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