These instruments are available from several manufacturers in a wide range of price and applications. The basic difference between the two techniques is that DTA measures a difference in temperature whereas DSC, in theory allows for the measurement of a change in enthalpy. Classical DTA measures the difference in temperature between the sample and reference which undergoes no phase changes as the furnace goes through a computer controlled temperature program.
The temperatures of both the sample and reference material increase or decrease uniformly until the sample reaches a point that it undergoes a phase change, which is either endothermic or exothermic and the difference in temperature is recorded.
Thermal Analysis of Pharmaceuticals
Although useful information is gained using DTA, there are several drawbacks to the technique which result in making it difficult to quantify the results and obtain information regarding the enthalpy of the sample transitions . These difficulties were overcome with the development of DSC.
Two basic types of DSC instruments are commercially available today. While employing different techniques to obtain the measurement, both types of instruments measure heat flow and this seem to qualify as DSC under the ICTAC definition. Another important difference that should be pointed out is that at this time there is no common convention as to how DSC results should be presented.
Depending on the instrument used, the default settings may be to report endothermic events in the upward direction or downward with exothermic events in the opposite direction. Today it is commonly referred to as a DSC method. This development was an improvement over DTA in that it allowed for a measurement in the changes in heat flow as opposed to temperature only.
This was accomplished by the addition of a second series of thermocouples to measure temperatures of the furnace and a heat sensitive plate. During a phase change, the heat emitted or absorbed by the sample would alter the heat flux through the heat sensitive plate. By measuring the heat capacity of the heat sensitive plate as a function of temperature during the manufacturing process, an estimate of the enthalpy of transition can be made from the incremental temperature fluctuations .
As the name implies, pc-DSC measures the change in energy or power necessary to maintain the sample and references material at the same temperature throughout the heating or cooling cycle. This is accomplished through a different instrument design than that commonly found in hf-DSC instruments. With pc-DSC, two individual heaters are used to control the heat flow to the sample and reference holders.
Individual resistance sensors are placed within each holder and measure the temperature at the base of each. When a phase change occurs and a temperature difference is detected between the sample and reference, energy is supplied or removed until the temperature difference is below the threshold previously mentioned. Energy input as a function of temperature or time is then recorded which is proportional to the heat capacity of the sample .
From Reference 4. Regardless of the type of instrument used researchers will find that DSC offers a plethora of information regarding phase changes of materials. All DSC instruments operate over a wide temperature range both isothermally and dynamically and can be used for solids, semi-solids and liquids. A typical DSC thermogram is shown in figure 1.
Although DSC results reveal temperatures or temperature ranges, at which endothermic or exothermic events occur, interpretation of the results can be challenging. Figure 2 presents several of the events which are typically measured with DSC. As noted in the figure, several events can be endothermic or exothermic but with a basic understanding of thermodynamics the researcher can begin to deduce which event is most likely.
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For example, both melting and evaporation are endothermic events. However for most crystals especially when pure melting peaks are usually much sharper indicating a very small temperature range over which the transition occurs. Likewise, decomposition and oxidation are both broad exothermic peaks, but with the ability to control the atmosphere in which the experiment is conducted the difference can be determine. Crystallization and decomposition may both produce fairly sharp exothermic peaks on DSC but only the decomposition will be accompanied by decrease in mass on TGA in the same temperature range.
As shown in figure 2, most transitions detected by DSC will appear as peaks, where a change exothermic or endothermic is detected and then there is a return of the heat flow to a baseline. These results are typical of first- or second-order thermodynamic phase transitions, which are in an equilibrium state. Glass transitions, on the other hand, are neither first- nor second-order transitions since neither the glassy state nor the viscous state is an equilibrium state .
Typical DSC thermograms will reveal glass transitions as step-wise increases in the heat capacity Cp of the sample. This is due primarily to the increase in molecular motion of the sample above the Tg.
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In some cases, the determination of Tg is relatively straightforward but this work can be some of the most challenging done with DSC. Frequently a sample in which glass transitions need to be studied will contain material that is only partially amorphous or may be in complex mixtures of materials that are crystalline or amorphous. Contaminant in the experimental reaction might lead into wasted weeks, if not months, of irreplaceable research time, effort and expense more. Weighing Solutions without Compromise. Save time, money and materials, and invest in a balance that consistently delivers results you can trust.
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My watch list my. My watch list My saved searches My saved topics My newsletter Register free of charge. Key data can be obtained concerning the study of: — salt choice, — polymorphism screening, — crystallinity, — determination of solubility and its kinetics, — stability as a function of temperature and humidity, — amorphous content.
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Key data can be obtained concerning the study of: — salt choice, — polymorphism screening, — crystallinity, — determination of solubility and its kinetics, — stability as a function of temperature and humidity, — amorphous content Some International References Sanofi-Aventis — France IPSEN — France Biocodex — France GlaxoSmithKline — UK. The one-sided, interfacial heat reflectance sensor applies a momentary constant heat source to the sample.
Thermal conductivity and effusivity are measured directly, providing a detailed overview of the