The Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is now one of the most widely used laboratory methods for calculating the thermodynamics of various biomacromolecules. Since DSC is used to investigate the thermodynamics of a sample, it is worth noting that the technique can be used to investigate what happens to polymers when heated, also known as the thermal transformation of polymers and other biomacromolecules. DSC is one of the main thermal analysis techniques available in analytical chemistry due to its ability to quantify heat variations in samples (Weber and Salemme 116). Thermal analysis is the study of substances as they change with temperature. Apart from polymers, fats and oils, as well as pharmaceutical components, are some of the substances that can be analyzed using the instrumentation that was developed by E. S. Watson and M. J. O'Neill in 1962. The present paper discusses how DSC instrumentation works with an emphasis on the design, application and the problem that can be solved by the instrument.

Principle of DSC Technique

The DSC instrument is used to measure energy directly as well as allowing the precise measurement of heat capacity. Since the main objective of DSC is to measure heat, heat exchange is the main determinant of how the technique works. The heat exchange leads to heat flow that creates a local temperature difference in a path that enables measurement of the heat flow (Spink 115). Since many chemical processes and physical transitions are associated with the consumption or generation of heat, it can be noted that calorimetry is utilized to measure the exact heat changes. Thermal transitions or phase transitions can, therefore, be noted as the basic principle for application of the DSC. In the event of a physical transformation or phase transition, there will be flow into or from the sample so as to maintain both media at the same temperature.

Melting as an example of transition of a phase entails a solid absorbing heat so as to turn into a liquid. The solid is seen to undergo an endothermic phase in melting. In crystallization, it can be noted that an exothermic phase occurs as less heat is required to raise the temperature. The main principle behind the operation of calorimeters, DSC included, can thus be noted to be embodied in the observance of the differences of temperatures between the sample and the sample hence the ability to measure the heat absorbed or heat released during any transition of a sample.

The result is noted in the temperature of the heat flux versus time that is known as the DSC curves. Lopez and Makhatadze (113) report that DSC is used to measure the heat capacity with the area under the heat capacity profile representing the enthalpy of the sample that is under study. The enthalpy can thus be calculated using the following equation after the integration of the peaks of the curves.

Where is the enthalpy, K is calorimetric constant and A is the area under the curve. The calorimetric constant is seen to vary from one instrument to another and is determined from the analysis of a well characterized sample with known enthalpies of the change that is under consideration.

Differential Scanning Calorimetry Instrumentation

The equipment used for DSC is known as the differential scanning calorimeter and should be properly calibrated to allow accurate and reliable and precise measurement of the sample. The calorimeters measure the temperatures and heat flow that are associated with the transition of the samples. The common uses of the DSC equipment include selecting, comparing and investigating materials in research or for quality analysis of products in manufacturing industries. Some of the properties measured by the DSC instruments include phase changes, crystallization, melting, product stability, cure kinetics and oxidative stability among others. The heat of fusion, reaction energy, and specific heat or heat capacity are examples of energy that can be accurately be measured by DSC instruments in a laboratory. The image below is a basic differential scanning calorimeter. The image was taken from http://pslc.ws/macrog/dsc.htm

The main components of the instruments include the sample and reference pans, heater and a computer that acts to as a control point. The sample to be analyzed is placed on the sample pan while the reference pan is left empty. Each of the pans is heated individually by a heater (chromel-alumel thermocouple) that is controlled by the heater. The heat flow is thus measured by comparing the temperature difference between the sample placed and the reference pan. The instrument can operate within a temperature range of -120°C and 725°C. It can, however, be noted that an inert atmosphere is required for temperatures above 600°C. To avoid reactions with the samples pans are selected are inert in nature and may be chosen from materials such as platinum, copper, gold and graphite. Sample sizes may range from 0.5mg to 100mg.

DSC Instrument Classification

The differential scanning calorimeters are classified into two types based on their mechanism of operation. Power compensated DSCs and heat flux DCS are thus the two main types of the instruments that are available in most research, academic, and quality analysis laboratories. The operation of heat flux DSC includes the sample material enclosed in a pan with the reference pan placed on a thermoelectric disc surrounded by a furnace. Since the furnace is heated at a linear rate, the heat is transferred to the sample and the reference pan through the thermoelectric disc. A temperature difference is created due to the heat capacity of the sample. The areas of the thermocouples are used to calculate the temperature with the heat flow calculated using the Ohms law as shown in the following equation

Where q is the heat flow of the sample is the temperature difference and R is the resistance of the thermoelectric disc (Gill, Moghadam and Ranjbar 168).

The power compensated DSC is noted in the fact that the sample and reference pans are put in different furnaces and are heated using different heaters with the sample and the reference being maintained at the same temperature. The diagram presented above is an example of a power compensated DSC.

Experimental Parameters

Some of the experimental parameters that have been mentioned in the present essay as important components of the DSC technique are the thermodynamic parameters that determine the analytical process. The molar heat capacity Cp is noted as the main parameter that is associated with the investigation of a macro-molecule. The Cp is seen to subsequently yield other parameters such as the partial Cp, enthalpy, and change in entropy (ΔS) that can be measured by a DSC instrument.

Application

According to Gill, Moghadam, and Ranjbar (167), the calorimetry technique is applied in biotechnology and nanoscience to establish a relationship between specific physical properties and temperature. Some of the fields in which the technique is applied include chemistry, cell biology, pharmacology, and biochemistry. It is noted that the DSC equipment are highly sensitive and can only measure materials that are not corrosive. Some of the samples whose thermal properties can be analyzed by the DSC instruments include adhesives, sealants, pharmaceutical materials, fertilizers, metal alloys, plastics, oils, waxes and food and many others.

Application in the Polymer Industry

DSC is extensively applied in the polymer industry to determine the thermal transitions of polymers such as plastics and others. Even though the thermal transitions cannot be used to determine the distinct compositions of various polymers, it can be noted that the technique can be used to compare the materials. The glass transition and melting point of polymers are examples of properties of polymers that can be determined by using the DSC.

Application in the Pharmaceutical Industry

Weber and Salemme (116) report that calorimetric methods including the DSC are increasingly finding use in the drug design where they are used to study protein-ligand interactions. Thermodynamic binding aspects of some drugs have also been studied using the method and similar techniques. Apart from drug design, other pharmaceutical applications include proper characterization of the drug parameters so as to meet the manufacturing and market specifications and standards. The process parameters mainly temperatures can also be determined by the use a DSC instrument. If for example, a drug is to be delivered in crystal form then it is necessary to set the operating temperatures that allow crystallization of the drug formulations to take place optimally. Stability and accelerated tests that are done on the drugs to determine their shelf lives and therefore expire dates depends largely on the thermal analysis through the use of DSC.

The changes in the heat capacity are noted to come from the disruptive forces in the structure of the protein. Some examples of the forces that stabilize the protein structure include the hydrogen bonds, van der Waals, electrostatic interactions and the exposed residues of the proteins. Other factors that influence the analytical determination of the pharmaceutical components are embodied in the physical environment and may thus entail buffer, pH, ionic strength and the excipients that are used in compounding or formulating the drug under investigation.

Advantages and Disadvantages of DSC

Some of the advantages of the DSC as a thermal analysis technique include being accurate, minimal sample preparation, reliable data, easy to use and relatively cheap as compared to some analytical techniques (Grossel 59). On the other hand, it can be noted that the technique has several disadvantages that may include the possibility of sample destruction through excessive heating, usually not qualitative, time-consuming and the limited range of samples that can be used.

Conclusion

Differential scanning calorimetry (DSC) can be noted as an essential thermal analysis technique due to its ability to be applied in various disciplines and industries such as pharmaceutical, food, and polymer. The main principle of operation of the DSC technique and instrumentation entails measuring the temperature difference and hence heat flow between the sample and the reference.

Works Cited

Gill, Pooria, Tahereh Tohidi Moghadam, and Bijan Ranjbar. "Differential scanning calorimetry techniques: applications in biology and nanoscience." J Biomol Tech 21.4 (2010): 167-193.

Grossel, S. S. "Guidelines for Chemical Reactivity Evaluation and Application to Process Design." Journal of Loss Prevention in the Process Industries 4.9 (1996): 295.

Lopez, Maria M., and George I. Makhatadze. "Differential scanning calorimetry." Calcium-Binding Protein Protocols: Volume 2: Methods and Techniques (2002): 113-119.

Spink, Charles H. "Differential scanning calorimetry." Methods in cell biology 84 (2008): 115-141.

Weber, Patricia C., and F. Raymond Salemme. "Applications of calorimetric methods to drug discovery and the study of protein interactions." Current opinion in structural biology 13.1 (2003): 115-121.