Differential scanning calorimetry reference standards

Provenzano, M. R., and Senesi, N. (1999). Thermal properties of standard and reference humic substances by differential scanning calorimetry. J. Therm. Anal. Calorim. 57(2), 517-526. 

In most cases, the purity of a pharmaceutical substance is determined by comparison of the reference standard with the known purity assigned. On the other hand, when no reference standard sample is available, the purity is determined by an absolute method in which the calculated result is based on theory and not by a comparative method. Purity established by analytical methods such as phase-solubility analysis or differential scanning calorimetry (DSC) is known as absolute purity (Figs. 3 and 4). 

Thermal analysis is not really one subject, because the information gained and the purposes for which it can be used are quite varied. The main truly thermal technique is differential scanning calorimetry (DSC). The heat input and temperature rise for the material under test are compared with those for a standard material, both subjected to a controlled temperature programme. In power compensation DSC the difference in heat input to maintain both test pieces at the same temperature is recorded. In heat flux DSC the difference in heat input is derived from the difference in temperature between the sample and the reference material. Heat losses to the surroundings are allowed but assumed to depend on temperature only. 

The thermal properties were investigated by differential scanning calorimetry on a Perkin-Elmer DSC-2 calorimeter. The melting temperatures and the apparent melting enthalpies were determined from the maxima and the area, respectively, of the melting endotherm by comparison with those of pure reference standards. 

High-purity materials are available as pure solids for chemical uses such as metals used as reference substances for metallurgical analysis, and compounds used as primary standards for many types of titrimetry, and as standards for elemental microanalysis. Other available high-purity substances intended for diverse physical properties include ion activity standards for the calibration of pH and ion-selective electrodes standards for various thermodynamic uses including melting point determinations, differential scanning calorimetry and bomb calorimetry and standards for the calibration of spectrophotometers. 

Thermal methods of measuring Tg are based on differential scanning calorimetry, commonly called DSC. In DSC, the thermal properties of a sample are compared against a standard reference material, typically inorganic, which has no transitions, such as a melting point, in the temperature range of... 

For many years, the thermodynamic description of macromolecules lagged behind other materials because of the unique tendency of pol5nneric systems to assume nonequilibrium states. Most standard sources of thermodynamic data are, thus, almost devoid of polymer information (1-7). Much of the aversion to include polymer data in standard reference sources can be traced to their nonequilibrium nature. In the meantime, polymer scientists have learned to recognize equilibrium states and utilize nonequilibrium states to explore the history of samples. For a nonequilibrium sample it is possible, for example, to thermally establish how it was transferred into the solid state (determination of the thermal and mechanical history). More recently, it was discovered with the use of temperature-modulated differential scanning calorimetry (TMDSC) that within the global, nonequilibrium structure of semicrystalline polymers, locally reversible melting and crystallization processes are possible on a nanophase level (8). 

Thermoanalytical methods comprise calorimetry (Differential Scanning Calorimetry, DSC) and thermogravimetry (TG) [3]. Mostly, the temperature of the sample is increased linearly and peaks of the sample temperature in comparison to a reference sample are recorded (Differential Thermal Analysis, DTA). In thermogravimetry the mass loss is observed. In this way the content of adsorbed liquids can be determined and the desorption kinetics can be observed. For special tasks also a quasi-isothermal treatment may be applied [4]. The most important standards on such methods are summarized in Table 2. 

Differential scanning calorimetry (DSC) is the most popular thermal analysis technique, the workhorse of thermal analysis. This is a relatively new technique its name has existed since 1963, when Perkin-Elmer marketed their DSC-1, the first DSC. The term DSC simply implies that during a linear temperature ramp, quantitative calorimetric information can be obtained on the sample. According to the ASTM standard E473, DSC is a technique in which the heat flow rate difference into a substance and a reference is measured as a function of temperature, while the sample is subjected to a controlled temperature program. As will be seen from this chapter, the expression DSC ... 

Standards for Differential Scanning Calorimetry issued under the jurisdiction of ASTM International Committee E37 on Thermal Measurements are listed below for reference. For more information, see website www.astm.org. 

Calorimetry of liquids and solutes has been revolutionized in recent years by the combination of the differential scanning technique, in which some difference between a sample and a standard is observed, with the continuous flow of fluids through the calorimeter. Instead of having two mineral samples ( 5.6.2), two columns or tubes are used, through which a reference solution and a sample solution flow at a controlled rate (Figure 5.12). As before, the difference in the power required to keep the columns at the same temperature is directly related to the difference in the heat capacities of the two fluids. See Wood (1989) for a history of the development of these methods and then-advantages. 

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