Calibration of DTA and DSC

Calibration of DTA and DSC instruments is usually carried out using standards with well-characterized and tested transition temperatures and enthalpies of reaction for example, the melting of indium occurs at 156.6°C and absorbs 28.7 J g while zinc melts at 419.4°C and absorbs 111.2 J g". ... 

Numerous compounds have been proposed to calibrate the DTA and DSC sample holders for quantitative determinations. Most of the standards used are pure metals, although many organic compounds of high purity have also been employed. The heat of fusion is the thermal transition normally used, although dehydration and decomposition reactions have also been recommended by numerous investigators. A list of standard materials used for calibration purposes is given in Table 5.9. 

Because of the large number of applications of the techniques of DTA and DSC, the applications described here will be concerned mainly with analytical chemistry problems. In this area, DTA and DSC can be used as a control or a routine tool for comparing similar but not identical materials. As a control technique, it may be used to distinguish between raw materials quickly and easily in those cases in which the treatment of the material must be modified if slight changes in the material are encountered. As a comparison technique, DTA and DSC may be used in some cases to detect materials that yield anomalous results by other tests. Lastly, by suitable calibration of the instruments, these techniques may be used for the quantitative estimation of a substance or mixture of substances, or for purity determinations (see... 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

The DSC peak area must be calibrated for enthalpy measurements. The same types of high purity metals and salts from NIST discussed for calibration of DTA equipment are also used to calibrate DSC instmments. As an example, NIST SRM 2232 is a 1 g piece of high purity indium metal for calibration of DSC and DTA equipment. The indium SRM is certified to have a temperature of fusion equal to 156.5985°C + 0.00034°C and a certihed enthalpy of fusion equal to 28.51 + 0.19 J/g. NIST offers a range of similar standards. These materials and their certified values can be found on the NIST website at www.nist.gov. Government standards organizations in other countries offer similar reference materials. 

In contrast to calorimetric standards, there exists an official set of temperature standards for DTA and DSC measurements (Table 2). They were developed by the International Confederation for Thermal Analysis (ICTA). The materials can be obtained from this organization. These standards were created several years ago, when accurate T and AH measurements were still not possible to be made simultaneously. Unfortunately some of them are not suitable for calorimetric calibration, indicated by in... 

Many instrument manufacturers produce simultaneous TG-DTA apparatus. The advantage of such apparatus is not only that the sample and experimental conditions are identical but also that standard reference materials for DTA and DSC can be used for temperature calibration (see Section 2.2.1.7). 

Since heat capacity relates to the quantity of heat required to raise the sample temperature by one Kelvin it may be studied by DTA and DSC. Physical changes such as melting and vajxuization as well as crystal structure changes give peaks, and some may be used to calibrate the... 

Accurate temperature calibration using the ASTM temperature standards [131, 132] is common practice for DSC and DTA. Calibration of thermobalances is more cumbersome. The key to proper use of TGA is to recognise that the decomposition temperatures measured are procedural and dependent on both sample and instrument related parameters [30]. Considerable experimental control must be exercised at all stages of the technique to ensure adequate reproducibility on a comparative basis. For (intralaboratory) standardisation purposes it is absolutely required to respect and report a number of measurement variables. ICTA recommendations should be followed [133-135] and should accompany the TG record. During the course of experiments the optimum conditions should be standardised and maintained within a given series of samples. Affolter and coworkers [136] have described interlaboratory tests on thermal analysis of polymers. 

As alluded to in the introduction, thermal analysis instruments must be calibrated using well-characterized materials. The melting of pure metals is the most common calibrant for DTA s and DSC s. Table 3.1 provides the melting temperature and latent heats of transformation of standard materials. The software in more contemporary instruments permit input of peak area values and onset temperatures determined by a test run, as well as values from the literature, into a program. It then automatically applies abscissa and ordinate corrections to all future data collected by the instrument. 

Using differential scanning calorimetry (DSC) (or, less directly, differential thermal analysis (DTA)) (see Section 2.8.5., above) it is possible to measure several of the thermodynamic properties of solids and of solid state reactions. The DSC response is directly proportional to the heat capacity, Cp, of the sample, so that by use of a calibrant it is possible to obtain values of this fundamental thermodynamic property, at a particular temperature, or as an average over a specified temperature range. Other thermodynamic properties are readily derived from such measurements ... 

Thermal events in the sample thus appear as deviations from the baseline, in either an endothermic or exothermic direction, depending upon whether more or less energy is supplied to the sample compared with the reference material. The operational temperature range of DSC is less than that of DTA—being typically subambient to 750°C. Temperature and energy calibration of DSC instruments is achieved using the ICTAC certified reference materials. ... 

There are several different types of instrument covered under the term DSC, which have evolved from differential thermal analysis (DTA) and measure the temperature difference between sample and reference pans located in the same furnace. This is then converted to heat flow using a calibration factor. A detailed analysis of DSC requires consideration of the various sources of heat loss, and these are generally captured in the calibration routine for the instrument. Absolute temperature calibration is achieved through the use of pure indium (156.6 °C) and tin (231.9 °C) melting-point standards. A comprehensive analysis of the theory of DSC contrasted with DTA may be found in several reference works (Richardson, 1989, Gallagher, 1997). 

Once the DTA or DSC cell is calibrated and the calibration coefficient determined in the temperature range of interest the AH of an unknown sample thermal transition can be calculated by use of the simple expression... 

As with many other analytical techniques, the temperature axis used in differential thermal analysis (and DSC) must be calibrated with materials having known transition temperatures. The International Confederation of Thermal Analysis (ICTA) has been very active in developing a set of standard materials for this purpose (19) and has worked with the U.S. National Bureau of Standards to have these materials made commercially available (20). The U.S. National Bureau of Standards GM 754-GM 760 DTA temperature standards are listed in Table 6.2. They cover the temperature range from —83 to 925 C. The results of an ICTA round-robin study with 24 cooperating laboratories have been reported by Menis and Sterling (20). 

As previously discussed in Chapter 5, the DTA or DSC curve consists of a series of peaks in an upward or downward direction on the AT or heat-flow axis. The positions (on the temperature or X axis), shape, and number of peaks are used for purposes of qualitative identification of a substance, while the areas of the peaks, since they are related to the enthalpy of the reaction, are used for quantitative estimation of the reactive substance present or for thermochemica determinations. Because of the various factors which aflect the DTA or DSC curve of a sample, the peak temperatures and the shape of the peak are rather empirical. Generally, however, the curves are reproducible for any given instrument, so that they can be useful in the laboratory. By use of various calibration substances, the areas enclosed by the curve peaks can be related to heats of reaction, transition, polymerization, fusion, and so on. Or, if the heat of the reaction is known, the amount of reacting substance can be determined. 

Unfortunately, K is highly temperature-dependent in the DTA experiment, so it is necessary to calibrate the peak area in the same temperature region as the peak of interest. This may require multiple calibration standards and can be time consuming. As we shall see, the calibration constant K for DSC is not temperature dependent therefore DTA is usually used for qualitative analysis, while DSC is used for quantitative measurements of AH and heat capacity. 

DTA measures the difference in temperature between the sample and a standard for the same rate of heat input. DSC compares the rate of heat inputs for the same rate of temperature rise. The latter is easier to analyze as it gives a direct measure of the rate of heat input. The method is based on the assumption that the samples are so small that thermal equilibrium is obtained almost immediately. For polymers this is not correct, and errors from this source are discussed by Strella and Erhardt [82]. Richadson [83 -86] and Laye [87] have discussed methods of calibrating the DSC to improve the accuracy of the results. The method is also outlined in ASTM El269 [88]. 

A DuPont DSC/DTA 900 thermal analyser was used with a DSC cooling attachment. The DSC was purged with nitrogen and the subambient temperature was attained with liquid nitrogen. The cell constant was determined using a sapphire disc. The temperature scale of the DSC cell was calibrated using indium (mp = 156.6 °C), water (0 "C), cyclohexane (6.5 °C), and the crystallisation temperature of cyclohexane (-87.1 "C). With careful calibration and weighing, precisions of 2% for the enthalpy of transition and 1 °C for the transition temperature were obtained. 

Calibration and Standards. Thermal analysis methods are not absolute and calibration is needed to record the correct abscissa value of temperature T (in Kelvin) and time t (in seconds or minutes). On the ordinate, calibration is necessary for the amplitude of deflection, AT, expressed as the difference in temperature (in Kelvin) for DTA or as heat flux, dQldT (in joules per second or watts) for DSC. Each instrument manufacturer provides methods and standard materials for these calibrations. In addition, ICTAC, in collaboration with the National Institute of Standards and Technology (NIST), has developed a series of materials as calibration standards for DSC/DTA. These reference materials can be used to calibrate both the temperature scale (K, abscissa) and heat flow (J/g, ordinate) on the basis of the integrated area under the curve. Figure 4 shows the heat flow-temperature relationship for various solid-solid and solid-liquid melting standards. Table 3 lists the solidi-to-solid2 transitions, melting points, and Curie temperatures of various pure metals, and also their transition enthalpies (J/g) (11). 

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