Strategies for Separating Overlapping Effects, Part 1: DSC - METTLER TOLEDO

Strategies for Separating Overlapping Effects, Part 1: DSC

The interpretation and quantitative evaluation of thermal analysis measurement curves is difficult when several effects take place simultaneously. A number of methods are available that can be used to separate overlapping effects and analyze them individually afterward. Using suitable examples, we discuss strategies for DSC curves. A second article to be published in the next UserCom will cover TGA applications.

Introduction

The interpretation and quantitative evaluation of thermal analysis measurement curves is difficult when several effects occur simultaneously. For DSC measurements, there are four main strategies that can be applied to separate overlapping effects:

a) Variation of the temperature program. This includes the use of different heating and cooling rates as well as heating-cooling-heating cycles.
b) Changing the environmental conditions. This includes using different gases, different crucibles (e.g. highpressure crucibles, platinum crucibles) as well as different methods to seal the crucibles (e.g. hermetically sealed crucibles, lids with 50-μm holes, open crucibles).
c) Modulated techniques. If reversing (e.g. a glass transition) and nonreversing effects (e.g. vaporization, crystallization) overlap, they can be separated using temperature-modulated DSC measurements (IsoStep, ADSC, TOPEM®).
d) Combined techniques. If the DSC measurement alone is inconclusive, techniques such as DSC-microscopy or DSC-chemiluminescence can be helpful.

In the following sections, we will discuss these strategies and illustrate them with examples.

Variation of the temperature program

Variation of the heating rate

The temperature at which a thermal effect occurs often depends on the heating or cooling rate used. For example, chemical reactions or glass transitions shift to higher temperature with increasing heating rates whereas crystallization shifts to lower temperatures with increasing cooling rates. Other effects, such as the melting of a pure substance or liquid-liquid transitions take place at the same temperature independently of the heating rate used for the measurement. The different behavior can be employed to separate overlapping effects.

Figure 1 shows DSC curves of enalapril maleate (C20H28N2O5·C4H4O4) samples that were measured at different heating rates [1]. Small organic molecules like enalapril maleate are often thermally unstable. Their decomposition takes place simultaneously with melting or even before.

This can lead to the sample becoming impure during the heating ramp due to the formation of decomposition products. This then affects the melting point: the larger the amount of impurities, the lower the melting point.

The black curve in Figure 1 was measured at 0.5 K/min. During this slow heating ramp, numerous contaminants are formed as a result of decomposition and melting occurs at a relatively low temperature. With increasing heating rates, the decomposition is shifted to higher temperatures. The concentration of the contaminants in the sample decreases and melting occurs at a higher temperature.

Whether one can completely separate the overlapping effects from one another (in this example melting and decomposition) by varying the heating rate depends on the rate dependence of the effects in question. For example, the glass transition temperature typically shifts by 5 K per decade with a change of the heating or cooling rate.

In the case of enalapril maleate, melting and decomposition could not be completely separated with the heating rates used. Figures 2 and 3 show an example in which melting and decomposition were successfully separated - in this case however it needed the use of the Flash DSC. Figure 2 shows the DSC heating curve of a sample of an organic pigment measured in air at 150 K/min. Melting occurs at about 309 °C and decomposition at about 376 °C (peak temperatures). Here again, the two effects could not be separated at the heating rate used (150 K/min) [2]. Additional measurements were therefore performed using the Flash DSC to completely separate the two effects. The resulting measurement curves are displayed in Figure 3.

Conclusions

With the aid of typical examples, we have discussed different strategies that can be used to separate overlapping effects on DSC curves. In particular, the use of different temperature programs (different heating and cooling rates, heatingcooling- heating cycles) has proven to be especially effective.

Strategies for separating overlapping effects, Part 1: DSC | Thermal Analysis Application No. UC 454 | Application published in METTLER TOLEDO Thermal Analysis UserCom 45