Sampling and reference materials

In a differential scanning calorimeter, a sample and reference material are heated in separate, but identical, metal heat sinks. The temperatures of the sample and reference material are kept the same by varying the power supplied to the two heaters. The output is the difference in power as a function of heat added. 

In differential thermal analysis, a sample and reference material are placed in the same large metal heat sink. Changes in the heat capacity of the sample are measured by changes in temperature between the sample and the reference materials as they are heated at the same rate. 

Discontinuity between the physical form of the sample and reference material used can lead to error. This is another manifestation of the matrix effect, but one which has to be considered when analyzing biological and environmental samples. There is no easy answer to the relationship between partide size and homogeneity. It is a popular assumption that the smaller the partide size the less the degree of heterogeneity. In some cases this may be true but there are a number of considerations. 

Fig. 1 Schematic diagram of the integrating sphere portion of a diffuse reflectance spectrometer, illustrating the key elements of the optical train. Although the detecto has been placed in the plane of the sample and reference materials, in common practice it would be mounted orthogonal to the plane created by the intersection of the optica beams.

Methodology appropriate for the measuring of DTA profiles has been extensively reviewed [12,13]. A schematic diagram illustrating the essential aspects of the DTA technique is shown in Fig. 3. Both the sample and reference materials are contained within the same furnace, whose temperature program is externally controlled. The outputs of the sensing thermocouples are amplified, electronically subtracted, and finally shown on a suitable display device. 

Two types of DSC measurement are possible, which are usually identified as power-compensation DSC and heat-flux DSC, and the details of each configuration have been fully described [1,14]. In power-compensated DSC, the sample and reference materials are kept at the same temperature by the use of individualized heating elements, and the observable parameter recorded is the difference in power inputs to the two heaters. In heat-flux DSC, one simply monitors the heat differential between the sample and reference materials, with the methodology not being terribly different from that used for DTA. Schematic diagrams of the two modes of DSC measurement are illustrated in Fig. 9. 

Differential thermal analysis (DTA) is a technique in which the temperature difference between a substance and reference material is measured as a function of temperature or time while the substance and reference material are subjected to a controlled increase in temperature. Differential scanning calorimetry (DSC) is a technique in which the difference in energy inputs into the sample and reference material required to keep their temperatures equal is measured as a function of temperature while the substance and reference material are subjected to a controlled increase in temperature [70]. 

Reference samples and reference materials have served a critical role in analytical chemistry since its inception. The reliability of all analytical results is completely dependent on the availability of suitable reference materials, and now nearly all branches of analytical chemistry declare an... 

Analyse samples and reference materials by the candidate and other independent methods. 

The principle behind DTA is measurement of the temperature difference between the sample of interest and a reference sample which is inert during a heating and/or cooling cycle and whose thermal behaviour is accurately known. A typical configuration would be that the sample and reference material are placed in separate chambers within a heating block which is contained within the furnace. A thermocouple is then placed in both chambers directly in contact with the sample and cormected in series. The furnace and sample block are then heated and the temperature of the sample and reference material monitored. There is a difference in temperature between the sample and reference material which is a function of (1)... 

DTA Temperature difference between sample and reference material DTA apparatus... 

Differential Scanning Calorimetry In DSC, a sample and reference material are... 

In any DSC instrument, the sample and reference material are placed in small individual pans or crucibles, which may be open or hermetically sealed, of 10-20 p,L capacity (Harwalkar and Ma, 1990). Sample size is... 

The effects of antioxidants on OT of SME by non-isothermal (conventional) DSC, static mode P-DSC, and dynamic mode P-DSC were investigated by Dunn (2006a), which is summarized in Table 1.15. Results from all three methods consistently showed that treating SME with antioxidants TBHQ and a-tocopherol increased OT with respect to untreated SME. Statistical comparison of P-DSC results with those from isothermal analysis of OSI at 60°C was facilitated by calculation of the corresponding response factors (defined ratios of OT of the sample to that of methyl oleate, and of OSI of the sample to that of methyl oleate). Data for the sample and reference material (methyl oleate) were measured under the same experimental conditions. Results showed the highest degree of correlation (P = 0.79) between dynamic-mode P-DSC and isothermal OSI analyses. 

X-Ray Absorption-Edge Positions, Edge Shift, First-Shell Radii (Corrected), and Related Data for Mn02-Si02 Catalyst Sample and Reference Materials... 

In an applied magnetic field B0 the sample and reference materials, with shield-ings crs and crR, respectively, have resonance frequencies vs and %. It is conventional (and in accord with IUPAC recommendations) for the chemical shift to be designated by the symbol 8 and for the scale to increase to higher frequency from the reference. Thus 8 is defined by... 

Space Administration (NASA). The NMR facility at Caltech was supported by the National Science Foundation (NSF) under Grant Number 9724240 and partially supported by the MRSEC Program of the NSF under Award Number DMR-520565. We thank H. Brinks, B. Hauback, W. Luo, Z. Fang, and S. Jalisatgi for providing samples and reference materials. The support and contributions of C. C. Ahn are appreciated. 

Two principal DSC designs are commercially available—power compensated DSC and heat flux DSC. The two instruments provide the same information but are fundamentally different. Power-compensated DSCs heat the sample and reference material in separate furnaces while their temperatures are kept equal to one another (Fig. IB). The difference in power required to compensate for equal temperature readings in both sample and reference pans are recorded as a function of sample temperature. Heat flux DSCs measure the difference in heat flow into the sample and reference, as the temperature is changed. The differential heat flow to the sample and reference is monitored by chromel/ constantan area thermocouples (Fig. IC). ... 

The original work extends the discussion to more complex reactions and the determination of activation energies and heats of reaction. However, the equations were developed for homogeneous reactions in solution and required twelve assumptions some of which are very difficult to satisfy when applied to dta studies of solid state reactions. These assumptions 2u e (/) the heat transfer coefficients and heat capacities of reactants and products are equal and constant, and (ii) that the temperature is uniform throughout the sample and reference material. Freeman and Carroll and Wendlandt have suggested simplifications in Borchardt and Daniels procedure. 

In the DSC method, the sample and reference materials are maintained at the same temperature, and the heat flow required to keep the equality in temperature is measured. DSC plots are therefore obtained as the differential rate of heating (in units of watts/second calories/second or joules/second) against temperature.39,40 The area under a DSC peak is directly proportional to the heat absorbed or evolved by the thermal event, and integration of these peak areas yields the heat of reaction (in units of calories/second-gram or joules/second-gram). 

DSC, one simply monitors the heat differential between the sample and reference materials, with the methodology not being terribly different from that used for DTA. 

Differential thermal analysis is the simplest and most widely used thermal analysis technique. The difference in temperature between the sample and a reference material is measured while both are subjected to the same heating program. In classical DTA, as represented schematically in Fig. 2A, a single block with symmetrical cavities or inserts for the sample and reference is heated in the surrounding furnace. The block is effectively a heat sink and ensures a measurable differential temperature signal during a thermal event. In conventional DTA, as represented schematically in Fig. 2B, the sample and reference material are contained in separate crucibles and both are subjected to the same temperature program. 

There are two types of DSC—heat-flux DSC and power-compensated DSC. The former is essentially a quantitative DTA. In power-compensated DSC, the aim is to maintain the sample and reference material at the same temperature throughout the controlled temperature program. The difference in the independent... 

In previous chapters, the principles and applications of differential scanning calorimetry (DSC) have been outlined, and it should be clear that the technique is both versatile and extremely sensitive. Using DSC, it is possible to analyze a wide range of systems quickly and cheaply so that thermodynamic parameters may be obtained. These qualities have led to the widespread use of DSC for not only pure research but also for routine thermal analysis. DSC does, however, have some drawbacks. To achieve good thermal contact with a sample, most DSC instruments are equipped with a pair of sample holders into which prepared sample and reference materials are placed. These materials are usually encapsulated in crimped aluminum ampoules, a typical sample mass being 5 to 10 mg. Such a small mass of sample contributes... 

Thermal Analyses. Samples of Na/BaA were weighed into sample tubes and subjected to DTA up to 500°C. Great care was taken to ensure even packing of sample and reference material. Two series of experiments were carried out with alumina and NaA as reference material, respectively. Representative samples were also examined by TGA. Thermal analyses were performed using a DuPont 900 thermal analysis unit with a 950 TGA attachment. 

One unique feature of the balance is the ability to use samples up to 6 g however, the electrical weighing range is only —2--i-2g. Reproducibility of the mass-measuring system is 10 with a linearity of 10 "4. Alumina cups, of 50 pL capacity, are used to contain the sample and reference materials. Calorimetric sensitivity of the DSC measurements is 2.5 pV/mW, with an accuracy of + 5% and a reproducibility of 2%. 

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