Master Thermal Analysis in Just 30 Minutes!

Webinar summary

Differential scanning calorimetry, or DSC, is a thermal analysis technique where the difference between the heat-flow rates into a sample and a reference material is measured during controlled heating or cooling. It can detect and quantify thermal events such as melting, crystallization, glass transition, decomposition, oxidation, and other reactions. A stable region with no heat-flow rate difference is observed between sample and reference forms the baseline. In the described convention, downward peaks represent endothermic events, while upward peaks represent exothermic events. Unlike DTA, DSC can measure reaction enthalpy, making it useful for quantitative analysis of heat flow, enthalpy change, heat capacity, and reaction temperatures.

DSC instruments are generally divided into power-compensation and heat flux types. Heat-flux DSC places the sample and reference material within symmetrically an electric furnace, with thermocouples beneath the sensor plate detecting the temperature difference between them. During typical heating, both the sample and reference material follow the furnace temperature with a slight delay. When the sample undergoes an endothermic process, the sample temperature absorbs heat and temporarily lags behind the reference temperature, producing an endothermic peak. During an exothermic process, heat from the reaction increases the sample temperature relative to the reference, producing an exothermic peak. Heat flux DSC is described as offering high sensitivity and a stable baseline, while power-compensation DSC is described as more susceptible to baseline drift and noise.

Obtaining reliable DSC data requires proper calibration, careful sample preparation, and appropriate measurement conditions. Instrument response characteristics may change over operational time because of transducer aging, oxide accumulation, electronic drift, or contamination from sample spills. Regular baseline, temperature, and energy checks are therefore indispensable. Calibration using multiple temperature or energy standards is recommended, and indium is used as an example to verify temperature accuracy and enthalpy reproducibility.

Sample pan or crucible selection has a major effect on data quality. The sample pan must be compatible with the sample and the temperature range. Aluminum pans are suitable up to about 500°C, while alumina pans can be used up to about 1500°C. Quartz pans are useful for measuring liquid in open pans up to about 1000°C, stainless-steel pans can be used for high-pressure, sealed measurements up to 5 MPa, and platinum pans offer excellent thermal conductivity up to about 1500°C but may cause catalytic reactions with some samples. For example, indium measured in an aluminum pan showed higher intensity and sensitivity than measurements performed in an alumina pan due to aluminum’s higher thermal conductivity.

Sample preparation should maximize thermal contact with the sample pan and avoid contamination. Powders should be spread thinly and uniformly and gently pressed against the bottom of the pan. Blocks or pellets should be cut to expose a large flat, smooth surface as possible. Liquids should be dispensed into the bottom center of the pan without spilling into the sealing area. Sheet or film samples should be cut to fit the inner diameter of the pan, and several layers may be stacked if more mass is needed. The outside surface of the pan should be checked for contamination before measurement because any residue could contaminate the measuring instrument and cause measurement errors.

Sample mass affects both data sensitivity and peak resolution. Larger samples increase sensitivity and produce intense signals, but the temperature distribution within the sample becomes more pronounced, which can lead to broader peak widths. Additionally, because the reaction process takes longer, the apparent reaction temperature may shift toward higher temperatures. Smaller samples improve peak resolution by narrowing transition ranges, reducing the overlap of nearby thermal events. The recommended approach is to use the smallest sample mass while still achieving sufficient sensitivity. Although the reference mass may shift the baseline due to heat-capacity differences compared to the sample, but it does not change the sample’s actual thermal behavior as long as the reference remains thermally inert across the full temperature range.

Measurement conditions also significantly influence DSC results. The temperature range should begin at least 20–30°C below the expected transition temperatures to establish a stable baseline, especially for relatively weak signals such as glass transitions. The upper temperature limit should be set high enough to capture the thermal event of interest but low enough to avoid unwanted decomposition. For unknown materials, performing preliminary STA measurement before selecting a DSC measurement temperature range can help identify the temperature at which mass loss begins. Heating rate involves a trade-off: higher heating increases sensitivity and shortens the measurement time, but shifts the onset, peak, and offset temperatures higher value; lower heating improves peak resolution and makes it easier separate overlapping thermal phenomena. Measurement atmosphere selection is equally important. Air atmosphere promotes oxidation and combustion, while using inert gas such as nitrogen allows for the observation of thermal behavior without oxidation. Dedicated DSC models cannot be used at temperatures where the sample decomposes because the sensors are easily contaminated and damaged. For example, with Polyethylene, , STA reveals that while exothermic combustion resulting in mass loss occurs at approximately 200°C in air, endothermic decomposition resulting in mass loss occurs at temperatures above 400°C in nitrogen.

Several application examples are presented to illustrate what DSC can reveal. Anhydrous caffeine showed a phase transition near 160°C and a sharp melting peak around 237°C. Grinding caused the transition temperature to decrease from Form II to Form I; since the transition temperature dropped to approximately 150°C after just 3 minutes of grinding, it was demonstrated that mechanical processing can alter the thermal properties of solids. This is known as the mechanochemical effect. Amorphous indomethacin showed changes during storage, including shifts in glass transition temperature and the appearance of crystallization peaks. These results show how DSC can monitor issues related to physical stability, recrystallization, and potential shelf-life in pharmaceutical materials. Carbamazepine displayed melting, recrystallization, and subsequent melting of a more stable phase or another polymorph, demonstrating the value of combining DSC data with visual sample observation capabilities. A lithium-ion battery separator showed two melting peaks, at 132°C and 166°C, corresponding to different polymer layers; the lower-melting PE layer supports thermal shutdown behavior, an important battery safety function.

The latest DSC systems may feature high sensitivity, wide temperature range, self-diagnostic capabilities, sample observation cameras, cooling units, and automatic sample changers. Sample observation capabilities allow for real-time visual monitoring of shape, color, and physical state, enabling the interpretation of both visible sample changes and thermal data. Automated sample handling supports high-throughput workflows involving numerous samples and reference materials.