Thermal Analysis – Differential Scanning Calorimetry

Differential Scanning Calorimetry, known as DSC is an instrument used to measure thermal parameters (viz., latent heat, heat flow, and phase transition temperature and enthalpy value) of a given sample.
DSC gives thermal information of the sample. Various types (aluminium, glass, etc.,) and different size of cells are used. The sample can be either liquid or solid. Only a few milligrams/litre of the sample is required. Nitrogen gas is purged into the DSC to create an inert environment. The heating and codings can be programmed. The data in digital format is recorded and stored in the desired place. Retrieval and analysis of the data can be done at any desired time.

Prerequisites for DSC
There are a few prerequisites for DSC.
1) The sample should be either liquid or solid. Only a small quantity (mg/mL) of the sample is required.
2) The melting temperature of the sample should be within the temperature range of DSC
3) A clean dust-free room with A/C is required.
4) A single pan balance of 3 decimal accuracy is required.
5) A crimper to Crip the DSC cell is a mandatory requirement.

Principle of DSC
Heat, a form of energy, is supplied to the sample and a reference. The difference in the heat is plotted with temperature/time. The area under the curve in the DSC thermograms ae calculated using the trapezoidal method by the DSC machine.

Sample preparation

The sample is weighed in a single pan balance to the second digit accuracy and is transferred to a DSC aluminium pan. In the calculation of enthalpy, the weight of the sample plays an important role. This aluminium pan is crimped using a crimping machine. Crimping is done to avoid oozing of the sample when it is heated.

DSC accessories and requirements
DSC instrument requires many accessories. They are listed below:
i. Nitrogen gas cylinder with regulator
ii. Voltage stabilizer with spike suppresser
iii. Gas regulation electronic controller
iv. Analogue to digital converter
v. A single pan balance of third digit accuracy
vi. A computer with 10 Gb RAM and compatible with 95 windows
vii. Clean dust-free room
viii. AC 100,120V, 800VA, 50/60 Hz .
DSC programming
The crimped cell with the sample is placed in the heating oven of the DSC instrument. The instrument is programmed for heating and cooling runs. The scan rate can be set from a minimum of 1oC/min to a maximum of 25oC/min. Even the starting temperature can be fixed to any desired value. After setting the scan rate, the weight of the sample with pan should be entered. The instrument requires the name and location of the data file to be stored.

DSC operation
Before operating the DSC, the nitrogen gas be purged, to create an inert environment. After placing the sample and fixing the scan rate, the instrument requires the location and name of the file to be stored. When the run operation is given, the heating of the sample at the desired scan rate begins and simultaneously the graph is plotted on the computer monitor. The time will be on X-axis and heat flow will be on the Y-axis. A linear line, referred to as the baseline indicates no transition in the sample. When the sample undergoes a transition, a huge peak is observed. The onset along with end set of the peak can be noticed. The area under the curve manifests the enthalpy value. After heating, the machine holds it for a predetermined set time, so as to achieve thermal stability. Then the cooling run prompts again at transition temperatures and peaks are observed.

Analysis of DSC thermograms
Once the DSC thermograms are recorded, the respective file will be stored in a location with .tad extension. Upon opening the file, the data can be converted to excel format or note pad format. In excel format, three columns are observed the first being time, the second is temperature and the last is enthalpy value.  While plotting, the second and the third columns gives a plot with temperature and variation of enthalpy. Using the software embedded in DSC, the peaks can be selected and the area under the curve can be analyzed. The peak value in degrees centigrade temperature along with the enthalpy values in both J/g and Jm/g will be displayed. Thus the transition temperature and corresponding enthalpy values can be analyzed.

Conclusion

DSC and its operation are discussed. Types of DSC cells, measure techniques along with analysis is deliberated