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Thermogravimetry (TG) is a thermal analysis technique in which the mass of a sample is measured over time, in a controlled atmosphere, as temperature changes. The data from this procedure is examined by thermogravimetric analysis (TGA), one of several methods in which one physical property of a sample is measured relative to temperature change. Similar techniques involve the study of thermal flow, length and elasticity. Thermogravimetry is often used in the pharmaceutical industry to analyze drug stability and agriculture to regulate the dehydration process for crops.
The procedure requires a precision balance, a heat source, and a closed reaction chamber. As the sample is being heated, its weight and temperature are continuously monitored and recorded. A thermocouple, or temperature sensor, is typically placed in direct contact with the sample; thermogravimetry involves absolute changes in mass and is independent of the sample's heating rate. The environment might be a gas or mixture of gasses at any required pressure, or a vacuum.
When plotted, the gathered data forms a curve relating the mass of a sample to its temperature. The correlation between temperature and mass change is examined by comparing points on the graph to the two axis scales. A more informative display is derived from the data by means of differential thermogravimetry, where the rate of change in mass versus temperature is plotted. Individual episodes of change not apparent in the simpler mapping can be easily distinguished, leading to a more complete and meaningful analysis.
Change in a sample's mass, as reflected by its weight, as temperature changes may be due to the breakdown of compounds into constituent elements, the reaction of the sample with oxygen or its loss of water content. At what temperature and under which atmospheric conditions these changes occur provide important information about the sample material. Thermogravimetry is often employed to study how these factors will effect the stability and lifetime of a product. Analysis of a sample's reaction to thermal change can also be used in forensics to help identify unknown materials.
The behavior of related materials at high temperature under a selected atmosphere is an important consideration in product design and development. Thermogravimetry can also be used to establish a material's characteristics for use in later identification or quality control. By careful choice of temperature and atmosphere, sample materials can be selectively decomposed into constituent components. This method is often used in the study of polymers, large molecules composed of repeating parts.
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