Basis of Differential Scanning Calorimetry

The principles behind DSC

Differential scanning calorimetry (DSC) measures the difference in heat flow between a sample and an inert reference as they are heated or cooled. When the sample undergoes physical or chemical transitions – such as melting (endothermic) or crystallization (exothermic) – the heat flow changes, with these events appearing as peaks in the DSC signal.

Types of DSC

There are two types of DSC:

• Heat-flux
• Power compensated

Here we will focus on heat-flux DSC, since this is the technology underpinning the Julia DSC instruments. In a typical heat-flux DSC, sample and reference crucibles sit on separate sensors connected to thermocouples inside a temperature-controlled furnace. The temperature difference between the two sensors is proportional to the heat flow needed to maintain identical heating conditions. The heat flow is directly related to both the enthalpy changes occurring in the material and the heat capacity. By tracking heat flow as a function of temperature, DSC can characterize thermal transitions and, with appropriate calibration, determine quantities such as heat capacity (Cp) and transition enthalpies.

In a power compensated DSC, the sample and reference crucibles are housed in separate furnaces, allowing for independent temperature control. The primary measured value is the electrical heating power needed to keep the sample and reference temperatures as close as possible. Because electrical power is the registered signal, this type of DSC can achieve very high cooling and heating rates while requiring only small sample quantities.

For the correct interpretation of a thermogram, it is important to indicate the direction of exothermic events. Exothermic events typically point up (indicated by an arrow). Below, we illustrate the most common thermal events that can be observed during a thermal analysis, plotted at progressively increasing temperatures. 

Anton Paar offers two heat-flux DSC models: Julia DSC 300 and 500. Coupled with our Julia Suite software, they are suitable for applications ranging from research to quality control across a vast range of materials. Depending on the cooling module, the temperature range spans from as low as -170 °C to 700 °C. The removable trays accommodate 70 samples, with a dedicated space for up to eight different references. Both devices can be equipped with an autosampler for 24/7 unattended operation.

Differences between Julia DSC 300 and DSC 500

Julia DSC 300 supports the Air Cooling Module, covering a temperature range of -35 °C to 700 °C. Julia DSC 500, on the other hand, is compatible with all of our available cooling modules, reaching a minimum temperature of -170 °C. Both models can be equipped with an autosampler for reduced operational workload.

Cooling module selection

Air Cooling Module (ACM)

Our patented Peltier built-in air-cooling module reaches sub-ambient temperatures down to -35 °C. The ACM supports a maximum temperature of 700 °C, with a maximum heating rate of 300 K/min and a maximum cooling rate of 150 K/min.

Refrigerated Cooling Module (RCM)

The RCM reaches temperatures of -90 °C, with a maximum cooling rate of 150 K/min. The maximum temperature is 700 °C, with a maximum heating rate of 300 K/min. Its closed-circuit design eliminates the need for the user to refill the coolant. The RCM is well-suited to exploring lower temperature transitions, such as those typical of polymers. It is compatible with Julia DSC 500 only and requires additional space beside the instrument.

Nitrogen Cooling Module (NCM)

The NCM is designed for analysis across a complete temperature range, from -170 °C to 600 °C. It offers a maximum heating rate of 300 K/min and a maximum cooling rate of 200 K/min. This powerful cooling module is ideal for reaching the lowest temperatures, making it especially valuable for research on specialized materials and low-temperature transitions. The NCM requires additional space beside the instrument for the nitrogen dewar, which needs periodic filling. It is compatible with Julia DSC 500 only.

Calibration

Calibration is fundamental to obtain true values from a measurement. As heat-flux DSC is based on an indirect measurement of the sample temperature – rather than the actual temperature – the values can deviate from the true ones depending on the setup. Measuring known standards establishes a relationship between the displayed values and the real (literature) values of thermal properties. Indium is typically used for this purpose, with a melting point of 156,60 °C and a fusion enthalpy of 28,58 J/g. The scaling factors determined during calibration are used to adjust the measured values accordingly.

For a calibration performed with Julia DSC 300 and 500, at least two calibrants are needed to calculate the correction within the temperature range between them. Indium and zinc (melting point: 419,53 °C) are commonly used to calibrate measurements between 120 °C to 470 °C. It is also useful to perform sub-ambient calibrations. Common calibrants for this task include water (0.0 °C), n-dodecane (-9.65 °C), and n-octane (-56.85 °C). Calibrants should be selected according to the cooling module used and the analysis type. The following table lists the characteristic values and recommended sample masses for the most commonly used calibrants.

Table 2

During calibration, it is also possible to calibrate the temperature modulation for sinusoidal DSC or SDSC. SDSC is used to separate the contributions of reversible and irreversible thermal events during analysis. Melting, crystallization, and glass transition are considered reversible, whereas enthalpy relaxations superimpose on the glass transition are irreversible. Calibration for SDSC should be performed at two different heating rates, typically between 5 K/min and 20 K/min. 

As the compensation terms depend on the crucible type, the crucible’s contribution should also be included. This is done during TruPeak measurement. Due to the intrinsic differences between sensors, asymmetries in the instrument can introduce systematic errors. TruPeak involves a series of calibration measurements using sapphire – chosen for its heat capacity – as well as with empty furnaces and crucibles to determine a good baseline for future analysis.

Preparing and conducting a DSC measurement

Toolbox

Each Julia DSC is equipped with a sample preparation toolbox that includes everything from a blade and syringe to a push rod and funnel. Cleaning tools for the furnace are also provided, such as a glass fiber brush for the sensors and a blowing bell for the furnace cell.

Crucibles
 

In principle, any type of crucible can be used with the Julia DSC instruments after proper calibration. However, Anton Paar provides a range of benchmarked crucibles optimized for everyday analysis and categorized by size and suggested application. We recommend not to fill crucibles beyond two-thirds capacity to prevent contamination of the rims and sample spillage.

1. Aluminum (Al) 20 µl crucible – closed or open (pan only)
   Ideal for small quantities of sample (solids, powders), this crucible is not compatible with the sealing press. The lid, which is not hermetically sealed, is closed using a push rod. This makes the crucible especially well-suited for foils or fibers, as the lid ensures better thermal contact between the samples and the crucible bottom, leading to more accurate results.
2. Aluminum (Al) 40 µl crucible – closed or open (pan only)
   The most commonly used crucible, this is suitable for a wide range of sample types, including liquids when used with the full lid. It can be hermetically sealed using the sealing press. The lid is cold welded to the crucible.
3. Aluminum (Al) 100 µl crucible – closed or open (pan only)
   Used when larger amounts of sample are required, this crucible is compatible with any kind of sample. It can be sealed using the sealing press.
4. Pre-pierced lids – for both 40 and 100 µl crucibles
   Pre-pierced lids feature a laser-cut hole in the same diameter and position. This is the most common lid used in experiments as it prevents the crucible from popping due to the buildup of internal pressure during analysis. Pre-pierced lids are also suitable for solid compounds.
5. Pierceable lids – for 40 and 100 µl crucibles (only for use with Autosampler)
   Pierceable lids are designed to be used exclusively with the Autosampler. They are ideal for samples that need to be treated in an inert or controlled atmosphere until the moment of measurement (e.g., reactive compounds). The Autosampler is equipped with a software-controlled needle, which pierces the lid directly in the furnace just before the measurement begins.

Standard full lids (used with 40 and 100 µl crucibles) can be manually pierced using a needle, such as the one provided in the toolbox. Piercing the lid before performing a DSC measurement allows developed vapors to escape, preventing deformation of the crucible. Crucible pans may also be used without a lid for special applications such as oxidation analysis or when a reaction between the sample and a method gas is required.

Note: Always wear gloves and protective glasses when handling samples!

Samples

Powders

Powdered samples often contain a large amount of air, which is a poor thermal conductor. For optimal thermal contact, it is good practice to compact the powder firmly in the crucible using the push rod, always pressing against a hard surface to avoid deforming the crucible bottom. This improves contact between the sample and the crucible bottom. To prevent contamination of the crucible rim during filling, a funnel is recommended. A pre-pierced lid is typically used for standard analyses. Powders can also be loaded in the crucible pan without a lid for oxidative studies.

Step-by-step guide:

• Weigh the empty crucible and lid (if required)
• Load the powder using a spatula and funnel
• Compact the powder using the push rod
• Seal the crucible (if required) with the appropriate lid using the sealing press
• Weigh the filled crucible and determine the sample mass by difference
  Note: If using an Al 20 µl crucible, close the lid using the push rod

Liquid samples

The wetting behavior and volatility of liquid samples should be carefully considered during sample preparation. Only 40 and 100 µL aluminum crucibles with hermetically sealed lids are recommended for such samples. This prevents sample loss before and during the analysis, as mass losses greater than 1 % can significantly impact DSC results. Low-temperature calibrants, which are often liquid at room temperature, are typical examples of liquid samples that require special care during preparation.

Step-by-step guide:

• Weigh empty crucible and lid
• Load the liquid sample using the syringe from the toolbox
• Deposit a small amount in the crucible paying attention not to spill it on the rim
• Seal the crucible and weigh
• Determine the sample mass by difference

Foils and fibers

Achieving the best possible thermal contact to the bottom of the crucible is crucial for the analysis. Fibers and foils are light and can possibly bend and tilt if the crucible used is too big (40 or 100 µl crucibles). We suggest using the 20 µl crucibles with a lid, which is pressed down onto the sample during the analysis. This will also improve the baseline. 

Step-by-step guide:

• Weigh empty crucible and lid
• Load the sample using the tweezers from the toolbox
• Press the lid onto the crucible using the correct side of the push rod
• Weigh the filled crucible and determine the sample mass by difference

Creating a temperature method

Creating a method is a fundamental step in preparing for a DSC analysis. While knowledge of the sample is helpful, a well-defined method can make the difference between a good measurement and a bad one. With Julia Suite, temperature methods can be created intuitively by selecting from various segments and purging gases. (Note: The temperature limits will depend on the type of cooling module used.) 

Method development involves setting the temperature, heating and cooling rates, method gases, and the type of analysis to be performed. Below are some key considerations to keep in mind when preparing a method.

• It is important to set a starting temperature two to three minutes before the beginning of the first expected transition, taking into account the selected heating rate. This corresponds to around 20 °C before the transition when using a heating rate of 10 K/min.
• A heating rate that allows clear observation of the transition without losing information is preferrable. Standard heating rates are between 5 K/min to 20 K/min. Increasing the heating rate enhances the sensitivity of the instrument (stronger signal). However, this comes at the cost of resolution. To improve resolution, lower heating rates are preferred.
• The end of the analysis should be set two to three minutes after the end of the transition, which corresponds to 20 °C beyond the expected transition temperature at a heating rate of 10 K/min.
• The method gas should be selected according to the type of analysis. For oxidation or decomposition, a reactive gas such as air or oxygen is used. In other cases, inert gases such as nitrogen, argon, are preferred. OIT (oxidation induction time) and OOT (oxidation onset temperature) are typical examples where air or oxygen are required.

Typical segment types available in Julia Suite include:

• Isothermal: Maintains the temperature stable at an arbitrary value over an amount of time defined by the user
• Dynamic: Heats or cools the sample within a temperature range at a specific heating or cooling rate
• Ballistic: Heats the sample to a target temperature using the maximum heating rate
• Equilibrate: Heats the sample to a target temperature at the maximum heating rate and maintains it for 30 seconds
• Loop: Repeats a segment, changing the temperature extremes by a defined amount

The tables below can help users identify the transitions that characterize a sample. While glass transitions, melting, and crystallization points are the most common transitions observed during DSC analysis, there are many more depending on the type of sample and experimental conditions.

Table 3

Table 4

Table 5