Differential scanning Colorimetry (DSC).

DSC

1. Differential scanning Calorimetry Mrs. Poonam Sunil Aher ( M.Pharm , PhD) Assistant Professor Sanjivani College of Pharmaceutical Education and Research (Autonomous),  Kopargaon , Ahmednagar-423603 (M.S.), INDIA Mobile: [phone]

2. Thermal Analysis Thermal analysis includes a group of techniques in which specific physical properties of a material are measured as a function of temperature. Thermal Analysis is useful in both quantitative and qualitative analysis. In Qualitative analysis samples are identified and characterized by thermal behavior. In Quantitative analysis results are recorded with changes of thermal reaction in weight and enthalpy

3. Summary Thermal Analysis Techniques Technique Quantity Measured Application Differential scanning Calorimetry (DSC) Heat and temperatures of transitions and reactions Study kinetic reactions Purity analysis Polymer study Polymer curves study Differential thermal Analysis ( DTA) temperatures of transitions and reactions Phase diagram Thermal stability Thermo gravimetric analysis (TGA) Weight changes Thermal stability Compositional analysis Thermomechanical Analysis (TMA) Dimensions and viscosity changes Study of temperatures Expansion coefficients Dynamic mechanical analysis (DMA) Modules , damping and viscoelastic behavior Impact resistance Mechanical stability Evolved gas analysis( EGA) Amount of gaseous products of thermally induced reactions Analysis of volatile organic components of shale

4. Differential scanning Calorimetry

5. Differential scanning calorimetry , or  DSC , is a thermo analytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.

6. History The technique was developed by E. S. Watson and M. J. O'Neill in 1962, and introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. The first adiabatic differential scanning calorimeter that could be used in biochemistry was developed by P. L. Privalov and D. R. Monaselidze in 1964 at Institute of Physics in Tbilisi, Georgia. The term DSC was coined to describe this instrument, which measures energy directly and allows precise measurements of heat capacity.

7. Principle The basic principle underlying this technique is that when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid, it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid.

8. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to observe more suitable physical changes, such as glass transitions. It is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer.

9. DSC, change in enthalpy ∆H of the sample is equal to the difference between the heat flow of sample and reference ∆H= Q s - Q r According to thermal analogs of Ohm’s law, Q = T 2 -T 1 R th Where Q s and Q r is heat flow from sample and reference R th is thermal resistance

10. The heat flow is proportional to the driving force. Driving force is the force required for temperature difference T 1 anT 2 Heat flow is inversely proportional to thermal resistance ∆H= Q s - Q r = T e - T s _ T e – T r R th R th Where T e is external temperature ∆H = _ T s - T r R th The measured signal of voltage from thermocouple is proportional to (T s – T r )

11. Instrumentation A typical DSC cell uses a constantan (Cu-Ni) disk as the primary means of transferring heat to the sample and reference positions and also as one element of temperature sensing thermoelectric junctions. The sample and reference are placed in separate pans that sit on raised platforms on the disk. Heat is transferred to the sample and reference through constantan disk

12. The differential heat flow to the the samples and reference is monitored by the chromel or constantan thermocouples. Which are formed by the junction of the constantan disk and the chromel wafer covering the underside of each platforms. Chromel and aluminium wires are connected to the underside of the wafers from thermocouple, which directly monitor the sample temperature. Constant calorimetric sensitivity is maintained by computer software, which linearizes cell calibration coefficients DSC provides maximum calorimetric accuracy from -170 to 750 ◦ C Sample size ranges from 0.1 to 100 mg It measure enthalpy.

13. DSC curves The result of a DSC experiment is a curve of heat flux versus temperature or versus time. There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak, depending on the kind of technology used in the experiment. This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition. It can be shown that the enthalpy of transition can be expressed using the following equation: ∆H =KA where  ∆H is the enthalpy of transition, K is the calorimetric constant, and  A is the area under the curve.

14. Instrumentation Basics:  Types of DSC Instruments:    1. Power compensated DSC 2. Heat flux DSC 3. Modulated DSC   4. High sensitivity DSC (HS-DSC) 18

15. 1.Power compensated DSC: 19

16. In power compensated DSC, the base of the sample holder unit is in direct contact with a reservoir of coolant. Resistance sensor measures the temperature of the base of the holder and a resistance heater. Phase changes in sample temperature difference is detected electrical power gets supplied to bring temperature difference below threshold value. Temperatures of both are increased or decreased linearly. The power needed to maintain the sample temperature equal to the reference temperature is measured. Advantages over Heat flux DSC: Its response time is more rapid than heat flux DSC. It is also capable of higher resolution. 20

17. 2.Heat flux DSC: 21

18. In heat flux DSC, the difference in the heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate. Both sample and reference are heated by a single heating unit. Heat flows into both the sample and reference material via an electrically heated constant thermoelectric disk is given by where, f (T, t) is the kinetic response of the sample in J mol ˉˡ 22

19. 3.Modulated DSC Instruments (MDSC) :   In MDSC, a sinusoidal function is superimposed on the overall temperature program. And produces a micro heating and cooling cycle as the overall temperature is increased or decreased. The overall signal is mathematically deconvoluted into two parts A reversing heat flow signal- Associated with the heat capacity component of the thermogram . Non- reversing heat flow signal- Associated with the kinetic processes. Step transitions appear only in the reversing heat flow signal. Exothermic and endothermic events may appear in either or in both signals. It can be useful in resolving polymer glass transitions, which are difficult to analyze owing to concomitant solvent evaporation. 23

20. Sample Preparation If possible, clean and dry your sample prior to DSC analysis. Wear gloves or use forceps when handling your DSC sample. DSC samples should be small enough. Powder samples or flat solid samples (less than 2 mm tall) work best. Sample weight should be between 0.5 and 100 mg.   DSC sample preparation procedures : Weigh sample about 0.5 mg with the analytical balance. Record the weight. Use forceps to place sample. Use forceps to place the aluminium pan lid on top of your sample. Use forceps to load the aluminium pan and sample into the sample encapsulating press . Align the sample pan in the encapsulating press, and press down on the handle to seal the aluminium pan. Use the empty pan as a reference sample. Need inert gas like nitrogen to avoid oxidation or decomposition.   24

21. Precaution : Sample Decomposition The DSC is capable of heating samples to 600°C . Many materials may decompose during the heating, which can generate hazardous by-products. WARNING : If you are using samples that may emit harmful gases, vent the gases in an appropriate manner. In general, samples should not be heated above their decomposition temperatures to prevent the release of hazardous materials. 25

22. Instrument Operation Powering up the instrument,   Loading your sample,   Setting your testing conditions,   Running a scan and collecting data   Analyzing your results. 26

23. Applications Quantitative application includes the determination of heat of fusion and the extent of crystallization for crystalline materials. Glass transition temperatures and melting points are useful for qualitative classification of materials. DSC is used to study ‘aging’ and shelf life of pharmaceuticals, as well as other basic research and development. In the Food Industry, DSC has numerous applications to monitor thermal events discussed earlier such as melting, crystallization, etc, as well as decomposition, denaturation , dehydration, polymorphism, oxidation, etc. Evaluation of metal catalysts by pressure DSC . Measurement of heat capacity Measurement of thermal conductivity 27