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How to Choose a Calorimeter

Differential scanning calorimetry can help meet your needs in characterizing heat-related material properties

Andy Tay, PhD

When materials are exposed to high temperatures, their physical properties and behavior change. Such changes may render the material unsuitable for use in certain environments. As such, it is often critical to measure a material’s heat flow beforehand to ensure it will behave as expected in the environment it will be exposed to. This characterization is performed via calorimetry, which refers to a technique to measure the thermal properties of materials to establish a relation to the materials’ physical properties. A popular way to do so is differential scanning calorimetry (DSC) as it provides a temperature readout by measuring the heat quantity, which is radiated or absorbed by the sample due to temperature difference between the sample and the reference material. DSC is an extremely important tool in the polymer industry to identify the temperature at which material transits from one state to another. It is also highly used by pharmaceutical companies to optimize their drug formulations to improve solubility and bioavailability. Here, we will describe the basic operating principles of DSC and some of the major considerations when purchasing such a tool.

How does DSC work?

In the most basic DSC measurement, energy is introduced both into a sample cell containing solvent and molecules of interest and a reference cell containing only the solvent. The temperatures of both cells are raised linearly as a function of time. The difference in input energy required to match the temperature of the sample to that of the reference would be the amount of excess heat absorbed or released by the molecules of interest. With this capability, DSC can be used to measure how material properties such as pH, buffer type, and ionic strength change under different physical environments.

What to look for in DSC

Temperature range: A standard DSC comes with an aluminum sample pan and cover that can be used for solids and powder that do not decompose or boil in the range of -170oC and 600oC. The pans can also be used for metals and inorganic material like rocks if they do not react with aluminum. Nevertheless, if a temperature higher than 600oC is needed, the aluminum covers can be substituted with material like copper, gold, or alumina. This flexibility allows users to customize their DSC setup for various samples.

Rate of temperature change: Some experiments may be performed at very high heating or cooling rates (>300oC/min). In such cases, pans with smaller mass may be used to improve the rate in which heat can flow or conduct through the pans to the sample and reference cells.

Modern DSC instruments are highly modular to enable users to switch parts like pans and covers to meet the needs of their analyses and characterizations.

Sample properties: Standard pans and covers are poorly suited to measure the thermal property of liquids due to evaporation at high temperature, which can cause huge build-up in pressure while not being compatible for cream-like and greasy samples that can flow out of the pans. However, this problem can be overcome by using a vented cover and a pan that is closed, but not sealed, to allow for the gas to escape. Additionally, if a sample is only available in small quantity, users can use a mini-DSC pan and cover. Such designs are becoming more common as users prefer to screen a larger number of samples at small quantities in the microgram or microliter range.

Sensitivity: There are different grades of aluminum that are used to make the pans. If highly sensitive and reproducible data are needed, it is advisable to use high-purity aluminum pans. This is because the pans can be cleaned with organic solvent and then dried in a high temperature oven to completely remove any organic residue. Some manufacturers are also selling disposable pans and covers that can help ensure data reproducibility or work with robots that are taking measurements automatically. Users still need to be mindful that disposable pans and covers are usually not made from materials of very high quality (to keep costs affordable), which can negatively affect sensitivity. 

High pressure experiments: Some applications may require the pressure to be more than 3 bar, such as the study of protein denaturation in large volumes of water. In this case, users can use an o-ring and thicker steel pan to resist the internal pressure build-up during water evaporation and deform and expand the sealed pan. Such a setup has an internal pressure resistance up to 24 bar, making it suitable to study any thermal processes involving highly diluted samples in water.

Automation: In line with lab automation, companies have also introduced DSC systems with autosamplers for screening lots of samples. Notably, such systems can also be programmed to self-clean and decontaminate for both chemical and biological residues. This is advantageous for high-throughput analysis and is usually integrated with disposable consumables to minimize human errors such as incomplete cleaning of pans.

DSC can be used to measure how much material properties such as pH, buffer type, and ionic strength change under different physical environments.

Portability: With advanced manufacturing, miniaturized, portable DSC is now available. However, these models typically do not provide as high of a sensitivity and temperature range as non-portable ones. They are, nevertheless, great devices for use in field work or when users need to move from different locations within a factory, for instance. 

DSC has a wide range of applications ranging from determining the thermal properties of novel polymers to monitoring batch to batch variability in drug manufacturing. Modern DSC instruments are highly modular to enable users to switch parts like pans and covers to meet the needs of their analyses and characterizations. DSC measurements can also be quite time consuming, which is why devices offering high heating and cooling rate and automation— which offer users a way to spend their time in the lab more productively—are becoming more popular.