Dynamic heating methods

For the DSC tests we use a Dupont 1090 with the pressure DSC measuring cell. In our DSC tests we use a dynamic heating method with a heating rate of 10 C/min. We run two tests, one at atmospheric pressure and one at 500 psi of applied pressure. 

For our DTA tests we use equipment from Adolf Kuhner AG ( 1 ) We run two types of DTA tests. One type is a dynamic heating method with a 2.5 C/min. heating rate, and the other is an isothermal heating method. In the isothermal test we preheat the heating block to a specific temperature, insert the sample, and keep the heating block at the specific temperature for at least eight hours. 

Another reason that isothermal heating methods are used in the initial screen is to identify materials that have time dependent thermal stability. These materials have a thermal decomposition that does not follow a simple Arrhenius relationship in which the reaction rate increases exponentially with an increase in temperature. Instead an extended induction period is required before the decomposition becomes detectable. An example of this behavior is shown in Figure 2. The DTA isothermal test recorder traces of methane sulfonic acid, 3,7-dimethyloctyl ester at different test temperatures are shown. The induction time varies from less than 1 hr. at 180 C to 46 hr. at 130 C. As with this compound, it is not unusual that once decomposition is detected it proceeds very rapidly, releasing all of the heat in a short period of time. Dynamic heating methods do not indicate if this type of thermal instability is present if it is, the initial detection temperature from dynamic tests will be grossly misleading as to the thermal stability of the material. 

The initial screen uses both DSC and DTA dynamic heating method tests to compensate for some of the problems inherent in each test. The DSC test is fast, simple, sensitive, and quantitative. It requires only a small amount of sample. The small sample size, however, can be a problem with some samples, such as distillation residues, because of a lack of sample homogeneity. Also, the only inexpensive sample containers for DSC are aluminum or stainless steel. The containers can sometimes cause problems because of chemical reactions between the sample and pan. The DTA test addresses both of these problems. It uses a 2-5 g sample and the containers are glass. Its disadvantages are that it lacks the sensitivity of the DSC and that it is not quantitative. 

The oxidative induction time (OIT) is determined from data recorded during the isothermal test. The elapsed time between time zero and the extrapolated onset of oxidation is recorded as a measure of oxidative stability. A dynamic heating method is also used. In this method, the sample is heated at a rate of 10 °C min from ambient temperature in an air or oxygen atmosphere. The temperature at the onset of oxidation is taken as a measure of oxidative stability. When... 

Oxygen concentration has also been found to have a pronounced effect on ignition temperature. Mason and Wheeler,78 using an electrically heated quartz tube as an ignition chamber and the dynamic flow method found that the ignition temperature of pentane in oxygen was lowered from 548° C. to 4/6° C. by increasing the pentane concentration from 1.50 per cent to 7.65 per cent. 

Dynamic allele-specific hybridization, a method to detect SNPs, is based on dynamic heating and coincident monitoring of DNA denaturation and avoids the use of additional enzymes or reaction steps. 

In the dynamic headspace method, the sample is put in a thermal desorption unit in order to desorb the RS a continuous flow of a carrier gas pushes the RS into a trapping system which is refrigerated and where they are accumulated prior to analysis. Then the RS are rapidly desorbed by rapid heating and carried onto the column via the carrier gas. There are different ways to apply this technique. The arrangement when purge gas passes through the sample is often called the purge and trap technique (some other equipment uses the acronym DCI (desorption, concentration, injection)). This method is particularly useful for very low concentrations of RS as the total amount of a substance is extracted and can be applied directly to powders without need to dissolve them. The main drawback is that the dynamic headspace methods are not readily automated. ... 

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