Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry

1. Differential Scanning Calorimetry(DSC ) PRESENTED BY – SMRUTI RANJAN MASANTA M.PHARM(1 ST YR) PHARMACOLOGY

2. Content Introduction History Principle DSC curve Type of DSC Application Differential Scanning Calorimeter

3. Introduction Differential scanning calorimetry (DSC) is one of the thermo-analytical techniques in which the amount of heat required to increase the temperature of a sample and reference are measured as function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the process. Only a few mg of material is required to run the analysis. DSC is most often used because of its speed, simplicity and availability.[1]

4. History The technique was developed by E.S. Watson and M.J. O'Neill in 1962, and introduced commercially at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in 1963 .

5. Principle DSC is a technique for measuring the energy necessary to establish a nearly zero temperature difference between a sample and an inert reference, as the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate . When a sample undergoes phase transition, more or less heat will need to flow to it than to the reference (empty pan) to maintain both at the same temp. Whether more of less heat must flow to the sample depends on whether the process is exothermic or endothermic .[1]

6. Exothermic transition If sample release heat during phase transition, then reaction is exothermic. Less energy is needed to maintain sample and reference at same temp.[3] E.g.- Crystallization Degradation Oxidation Curing P olymerization

7. Endothermic transition If a sample absorbs heat during phase transition then reaction is endothermic . More energy is needed to maintain sample and reference at same temp .[3] E.g.- Melting Boiling Sublimation Vaporization

8. DSC Curve The result of a DSC experiment is a curve of heat flux versus temperature or time. This curve can be used to calculate enthalpies of transitions, which is done by integrating the peak corresponding to a given transition. [1]

9. The enthalpy of transition can be expressed using equation: ΔH=KA Where, ΔH = enthalpy of transition, K = calorimetric constant, A = area under the peak Calorimetric constant varies from instrument to instrument and can be determined by analyzing a material of known enthalpies of transition. Area under the peak is directly proportional to heat absorbed or evolved by the reaction. Height of the peak is directly proportional to rate of the reaction.[1]

10. Factors Affecting DSC Curve Instrumental factors Furnace heating rate Recording or chart speed Furnace atmosphere Geometry of sample holder/location of sensors Sensitivity of the recording system Composition of sample containers Sample characteristics Amount of sample Nature of sample Sample packing Solubility of evolved gases in the sample Particle size Heat of reaction Thermal conductivity [1]

11. Types of DSC Two basic types of DSC instruments: power compensation and heat-flux. Heat flux DSC: In heat flux DSC, difference in heat flow into sample and reference is measured with (linear) change in sample temperature. In heat flux DSC, we can write the total heat flow dH/dt as, dH/dt = Cp dT/dt + f (T,t) Where : Dh/Dt - DSC Heat Flow Signal, Cp - Sample Heat Capacity = Sample Specific Heat X Sample Weight, DT/Dt - Heating Rate, F(t,t) - Heat Flow (That Is Function Of Time at an Absolute Temperature).[4]

12. Sample holder - sample and reference holders are connected by a low resistance heat flow path. The material with which the sample holder is made may be aluminium, stainless steel, platinum. Sensors - temperature sensors are thermocouples. Furnace - same block is used for sample and reference. Temperature controller - temperature difference between sample and reference is measured. A metallic disc made of constantan alloy is the primary means of heat transfer. Sample and reference sit on raised constantan discs. Differential heat flow to sample and reference is measured by thermocouples which are connected in series, located at the junction of constantan disc and chromel wafers. With this, it is possible to achieve heating or cooling rates of 100˚c /min to 0˚c.[4] Heat flux DSC

13. Power Compensation DSC : In this, sample and reference heated by separate heaters to keep same temperature, as temp is changed linearly. These heating units are quite small, allowing for rapid rates of heating, cooling. Sample holder - it is made up of aluminium, platinum or stainless steel. Sensors - platinum resistant sensors are generally used. Separate sensors are used for are used for sample and reference cells. Furnace - separate blocks of furnace are used for sample and reference cells. Temperature controller - differential thermal power is supplied to heaters to maintain the temperature of the sample and reference at the programmed value .[4] Power Compensation DSC

14. Application Determination of Heat Capacity- It is used to determine heat capacity, when we start heating two pans, the computer will plot difference in heat output of two heaters against temp. and this is the heat absorbed by substance against temp . The heat flow is heat (q) supplied per unit time (t), whereas, The heating rate is temperature increase (ΔT) per unit time (t ). Heat flow = q/t Heating rate = ∆T / t By dividing heat flow (q/t) by the heating rate ( ΔT/t). ( q/t ) / (∆T / t) = q/ ∆T= Cp = heat capacity.[1]

15. Glass transition temperature: When heating a sample at a certain temperature plot will shift downward suddenly. Which means more heat flow and heat capacity increase because of glass transition. (It is a reversible transition in amorphous material from a hard, brittle state into molten rubber like state ).[1]

16. Crystallization: When polymers fall into these crystalline arrangements, they give off heat. This drop in the heat flow appear as a big peak in the plot of heat flow vs. temperature. The temperature at the highest point in the peak is usually considered to be the polymer's crystallization temperature(Tc). Also , the area of the peak can be measured, which tells us the latent energy of crystallization of the polymer.[1]

17. Melting: If polymer is heated past its Tc, eventually reach another thermal transition, called melting. When polymer's melting temperature is reached(Tm). the polymer crystals begin to fall apart, that is they melt. It comes out of their ordered arrangements and begin to move around freely that can be spotted on a DSC plot. The heat which polymer give off when crystallized is absorbed when reached at Tm .[1]

18. Drug analysis: DSC is widely used in the pharmaceutical and polymer industries. For polymers, DSC is a tool for studying curing processes, which allows the fine tuning of polymer properties .[2] General chemical analysis: Melting-point depression can be used as a purity analysis tool. This is possible because the temperature range over which a mixture of compounds melts is dependent on their relative amounts. Consequently, less pure compounds will exhibit a broadened melting dip that begins at lower temperature than a pure compound .[4 ]

19. Reference: Kodre KV, Attarde SR, Yendhe PR, Patil RY, and Barge VU, Differential Scanning Calorimetry: A Review, Research and Reviews: Journal of Pharmaceutical Analysis , Volume 3, Issue 3, July-September, 2014. Thermal Methods. In: Chatwal GR, Anand SK. Instrumental Methods of Chemical Analysis. Fifth Edition: Himalaya Publication House, 2002. Pg.no.2.701 2.749-2.751 Schick C, Differential Scanning Calorimetry(DSC) of semicrystaline polymers, Analytical and Bioanalytical Chemistry, November 2009, 395:1589-1611. www.sciencedirect.com

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