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Differential scanning calorimeters, or DSCs, have a wide range of uses and are one of the most prevalent types of calorimeter found in both academic and industrial laboratories.
Principles of DSCs
DSCs are used across a wide-range of scientific fields and industries, and employ a multi-use approach which can be used to measure a variety of properties, including the melting temperature, heat of fusion, latent heat of melting, reaction energy, reaction temperature, glass transition temperature, crystalline phase transition temperature and energy, precipitation energy, precipitation temperature, denaturation temperature, oxidation induction times, the enthalpy of unfolding in biomolecules and the specific heat capacity.
They are also found across a range of industries, and are often used for polymers, rubbers, drug analysis, chemical analysis, foodstuffs, metals, liquid crystals, electronic components, biomolecules, safety screening of compounds and for oxidative stability tests.
DSCs possess some excellent advantages over many other types of calorimeter. Aside from the variety of properties that can be measured, they are also one of the easiest and quickest calorimetry methods in the laboratory to date.
They are exceptional at measuring the change in the endothermic and exothermic energy released from and a sample, and are particularly useful for polymeric materials as they can easily measure a material’s transition.
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DSC, in essence, measures the amount of energy released or absorbed by the sample in question when it is heated and cooled. DSC can only be used with non-corrosive samples, meaning that any material that contains any halogen moieties (F, Cl, Br, I etc) cannot be used.
If corrosive samples are used in a DSC, whether it is by accident or not, the material can damage the cell, which is a costly replacement (around $3000).
When performing a DSC sample run, the sample is placed in a suitable sample holder and subjected to heat. The technique involves heating the sample as a function of temperature, with both the reference material and the analysed material being subjected to the same temperatures.
As the process proceeds, the sample holder temperature increases linearly over time, which gives rise to a defined heat capacity over time. Over time the sample will undergo and will undergo a physical transformation, e.g. phase transformation, which will require more, or less, heat to be transferred to the sample to maintain the constant temperature.
Whether the heat transfer process requires more or less heat to be transferred to the sample is dependent on whether the physical change is an exothermic or an endothermic process. Such examples of endo and exothermic processes determined by a DSC machine are solid-liquid phase transitions and crystallisation processes, respectively.
The resulting output of a DSC machine is a curve, with the amount of heat transferred/heat flow on the y-axis (mW), and the temperature on the x-axis (°C). Alongside this data, which shows peaks and troughs to show the various endothermic and exothermic transitions, respectively, the curve can also be used to calculate the enthalpy of transition using the following formula:
Where ΔH is the enthalpy of transition, K is the calorimetric constant and A is the area underneath the curve. The calorimetric constant does vary from instrument to instrument, but so long as different results are produced on the same DSC machine, it will be relative with no discrepancy.
Heat Flow DSC
Heat flow DSCs are one of the most common types found in a laboratory. In heat flow measurements, the DSC directly measures the heat that enters and leaves the sample. The calorimeter uses a feedback loop to keep the temperature within the material constant and measures the required power to do this against known standards.
The sample holder in these methods is usually an aluminium pan, with the reference run involving the pan only (sometimes an alumina sample can be used as a reference materials instead of the pan). This approach is useful if you want a precise control of the temperature and gives accurate enthalpy and heat capacity measurements.
The general equation for working out a sample using heat flow DSC is as follows:
(dq/dt)p = dH/dt
Where dq/dt is the heating rate and dH/dt is the heat flow measured on the mcal scale.
Using this equation, the difference between the sample being measured and the reference material can be found using the below equation:
ΔdH/dt = (dH/dt)sample – (dH/dt)reference
Heat Flux DSC
Heat flux DSCs work by measuring the temperature changes, or heat flux, between a sample and a reference material, and calculates the heat flow from the calibration data. Throughout the analysis, the temperature of the sample does not change upon melting, but the reference material continues to increase linearly.
The sample in this process does not change in temperature until melting is completed, and the process allows for the two samples to be compared against each other.
The sensor areas are connected to a thermocouple, and can even be integrated as part of the thermocouple itself. This approach is less sensitive to small transitions and generally, produces less accurate results than their heat flow counterparts.
High Pressure DSC (HP-DSC)
High pressure DSC (HP-DSC), is used in four special scenarios. The first is during oxidative stability tests as the atmospheric pressure in a normal DSC machine can be inconvenient and cause the process to take too much time. The second situation is when a reaction forms methanol or water as a by-product and subsequently causes the sample to foam. Using a higher pressure supresses the foam and causes the molecules to resuspend back into the solution.
The third scenario is when the reaction kinetics of a sample are affected by changes (and fluctuations) in the pressure. In these scenarios, using a HP-DSC allows for the pressure to be kept constant and controlled.
The final scenario is when a sample’s transition is sensitive to the pressure. Such examples are the boiling point, which increases under pressure, and the glass transition (Tg) of a material. Running a HP-DSC allows the user to study these transitions and for boiling points, it also allows the user to calculate the vapour pressure of the sample.
Ultra-Violet DSC (UV-DSC)
Ultra-violet DSC (UV-DSC), also known as photo-DSC, is an adapted DSC instrument that allows the sample to be exposed to UV-light. To achieve this, the machine employs a variety of light sources including mercury vapour lamps, or light-emitting diodes (LEDs) to create the UV-light and can be performed using a varying range of frequencies and intensities.
This is a particularly useful technique when the sample in question includes a UV-initiated curing procedure, such as in dental resins, orthopaedic bone cements, hydrogels, paints, coatings and adhesives. This method not only allows the user to study the efficiency of the curing process, but can also provide information on the mechanical strength and kinetic properties of a sample.
Another area in which UV-DSC is often implemented, is in the study of a compound that can be decomposed under UV radiation. This area has a wide-spread use for determining the effects on storing pharmaceutical drugs, antioxidant packaging in polymers, food properties and how dyes are affected by sunlight.
Fast Scan DSC
Fast scan DSC is the general term for the DSC techniques that introduce a very high heating rate onto the sample, in an effort to increase the sensitivity of the DSC analysis. Such methods can also be used to trap kinetic behaviour.
Fast scan DSC heating rates normally vary between 100 °C/min and 300 °C/min. The fast increase of temperature allows the enhancement of the weak transitions in the sample. With the increased heating rate, it also possible to observe low levels of amorphous materials in pharmaceutical products, measure small amounts of natural products, freeze the curing of a thermosetting material, inhibit the recrystallization of polymers and thermally degrade organic compounds.
Modulated Temperature DSC (MT-DSC)
Modulated temperature DSC (MT-DSC), is another term for many DSC techniques in general, and is relevant to describe a technique as such when a non-linear heating or cooling rate is applied to the sample to separate the kinetic and thermodynamic data.
In general, this is done by applying a series of heating, or cooling steps followed by an isothermal hold. This process allows the user to separate the data into an equilibrated heat capacity curve that shows the thermodynamic response and isothermal kinetic baseline of a sample. It is a technique that also removes any kinetic noise from the sample.
DSC with Other Techniques
Unlike other thermal analysis techniques, such as thermogravimetric analysis (TGA), DSC is not widely used with other analytical and spectroscopy techniques, however, there are a few examples.
DSC can be used in conjunction with infra-red spectroscopy (DSC-IR) to look at the evolved solvents in pharmaceuticals, and can be used with mass spectrometry (DSC-MS) to look at the composition of outer-space matter, such as lunar rocks and meteorites. DSC has also been used with Fourier transform infra-red spectroscopy (DSC-FTIR) to observe the changes in a sample during a DSC run.
The most common, and useful, DSC combination is with Raman spectroscopy. This process involves the sample being irradiated with the Raman laser and is commonly used for research involving polymorphic materials, polymeric recrystallization, chain movements during a glass transition, and for hydrogen-bonded polymers. This technique is one of the most useful as the Raman spectrometer doesn’t require the use of a special transmission path cell and no processing of the spectra is required.
Cooling Materials with DSC
Whilst most of the focus of DSCs surrounds heating the sample, they can also be used to cool a sample and define a sample’s heat history from its melt. They are two types of cooling associated with DSC machines- controlled and ballistic.
Controlled cooling, as the name suggests, is when you want the sample to be cooled down to a temperature in a controlled manner. With this type of cooling, a constant temperature change rate is designated which normally lies between 0.1 °C/min and 500 °C/min. For some samples, controlled cooling can be used to implement an 800 °C/min cooling process.
Ballistic cooling, on the other hand, is when the sample is cooled as quickly as possible. This is either achieved by completely cutting the power to the DSC furnace, or by removing the sample and dropping it into liquid nitrogen.
Controlled cooling affords the greatest degree of separation for overlapped peaks and is very sensitive. The process also allows for the determination of isothermal recrystallization and can predict behaviours at rates that haven’t been measured.
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Sources and Further Reading