DIFFERENTIAL THERMAL ANALYSIS �& DIFFERENTIAL SCANNING CALORIMETRY

DIFFERENTIAL THERMAL ANALYSIS �&
DIFFERENTIAL SCANNING CALORIMETRY

1. DIFFERENTIAL THERMAL ANALYSIS & DIFFERENTIAL SCANNING CALORIMETRY Prachi Pathak

2. Differential Thermal Analysis (DTA) The temperature difference between a sample and an inert reference material, D T = T S - T R , is measured as both are subjected to identical heat treatments. Differential Scanning Calorimetry (DSC) The sample and reference are maintained at the same temperature, even during a thermal event (in the sample) The energy required to maintain zero temperature differential between the sample and the reference, dD q /d t , is measured

3. DTA Sample and Reference Placed in Heater Constant Heating Rate Initial Temp Final Temp Heating Rate ( ° C/min) Data Temp of Sample vs Time (or Temp) Temp of Reference vs Time (or Temp) Reference should be inert Measures Heat of crystallization Glass Transition Temperature

4. Differential Thermal Analysis DTA involves heating or cooling a test sample and an inert reference under identical conditions , while recording any temperature difference between the sample and reference. This differential temperature is then plotted against time, or against temperature. Changes in the sample which lead to the absorption or evolution of heat can be detected relative to the inert reference.

5. TGA+ DTA

6. <10 mg of sample (s) and inert reference (r) are contained in Al pans each with thermocouple, held in heating block. There is a constant temperature difference T between s and r since they have different heat capacities. But when the sample undergoes an endo ( exo ) thermic changes in T becomes different. If the test sample generates heat , its temperature will be higher than the reference sample . If the sample absorbs heat , its temperature will be lower than the reference sample. DTA

7. DTA

8. Modern instrumentation used for thermal analysis usually consists of four parts: sample/sample holder Thermocouples : sensors to detect/measure a property of the sample and the temperature Furnace : an enclosure within which the experimental parameters may be controlled a computer to control data collection and processing DTA power compensated DTA

9. Instrumentation sample holder sample and reference cells (Al) Sensors Platinum/Rhodium or chromel / alumel thermocouples one for the sample and one for the reference joined to differential temperature controller Furnace alumina block containing sample and reference cells Temperature controller controls for temperature program and furnace atmosphere sample pan inert gas vacuum reference pan heating coil alumina block Pt/Rh or chromel / alumel thermocouples

10. Differential Thermal Analysis Advantages: instruments can be used at very high temperatures instruments are highly sensitive flexibility in crucible volume/form characteristic transition or reaction temperatures can be accurately determined Disadvantages : uncertainty of heat of fusion, transition, or reaction estimations is 20-50% DTA

11. Some uses of DTA 1. To construct phase diagrams and study phase transitions 2. To find H Peak areas depend upon sample mass, m , enthalpy change H of the process, and geometric and conductivity factors such as heating rate  and particle size. peak area  H  m (cm 2 ) (Jg -1 ) ( g)

12.  T 3. To fingerprint substances DTA of (a) butter and (b) margarine temp  a b 4. To determine M.Pt ., B.Pt ., decomposition temperatures of organic compounds DTA of benzoic acid A ambient pressure; B 200 lb in -2 pressure

13. Differential Scanning Calorimetry  

14. Differential Scanning Calorimetry   DSC is a thermal analysis method where differences in heat flow into a substance and a reference are measured as a function of sample temperature, while both are subjected to a controlled temperature program.

15. The basic difference between DTA and DSC DSC - calorimetric method, energy differences measured. DTA - temperature differences measured. The applications of both techniques are similar, but DSC is now more popular. DTA is used for higher temperature and qualitative applications. DSC is used for calorimetric determinations, sample purity determinations and kinetics. Differential Scanning Calorimetry  

16. DSC differs fundamentally from DTA in that the sample and reference are both maintained at the temperature predetermined by the program . During a thermal event in the sample, t he system will transfer heat to or from the sample pan to maintain the same temperature in reference and sample pans. Two basic types of DSC instruments: power compensation and heat-flux Differential Scanning Calorimetry power compensation DSC heat flux DSC

17. Two major types of DSC instruments are available Heat flux device – more popular; more stable baseline and more durable cell. Difference in heat flow into s and r is measured with (linear) change in sample temperature . Power compensation device – better resolution; faster heating and cooling rates. s and r heated by separate heaters to keep same temperature, as T is changed linearly.

18. Power Compensation DSC Sample holder Al or Pt pans Sensors Pt resistance thermocouples separate sensors and heaters for the sample and reference Furnace separate blocks for sample and reference cells Temperature controller differential thermal power is supplied to the heaters to maintain the temperature of the sample and reference at the program value. sample pan D T = 0 inert gas vacuum inert gas vacuum individual heaters controller D P reference pan thermocouple

19. sample holder sample and reference are connected by a low-resistance heat flow path Al or Pt pans placed on constantan disc Sensors chromel ®-constantan area thermocouples (differential heat flow) chromel ®- alumel thermocouples (sample temperature) Furnace one block for both sample and reference cells temperature controller the temperature difference between the sample and reference is converted to differential thermal power, dD q /d t , which is supplied to the heaters to maintain the temperature of the sample and reference at the program value Heat Flux DSC sample pan inert gas vacuum heating coil reference pan thermocouples chromel wafer constantan chromel / alumel wires

20. Differential Scanning Calorimetry  – Principle of Operation A sample is placed inside a crucible which is then placed inside the furnace of the DSC system along with a reference pan which is normally empty (inert gas may be used). By applying a controlled temperature program (isothermal, heating or cooling at constant rates), phase changes can be characterized and/or the specific heat of a material can be determined. Heat flow quantities are calculated based on calibrated heat flow characteristics of the cell.

21. Differential Scanning Calorimetry   – Equipment Two pans Heat transfer disk (almost always made of Constantan – an alloy of 60% Cu and 40% Ni a differential detector a signal amplifier a furnace a temperature controller a gas control device a data acquisition device CHM 342 -

22. Endothermic and exothermic effects According to the thermal transformation, an endothermic or exothermic effect is recorded . In the case of an endothermic effect , it is needed to provide heat to the system for its transformation( sample absorbs energy ). This will result in a decrease of the temperature in the system during the transformation( sample releases energy ). In the case of an exothermic effect , the system provides heat during its transformation. This results in an increase of the temperature in the system.

23. common endothermic effects : melting , sublimation first order and second order phase transitions evaporation , dehydration denaturation ( protein ) gelatinization ( starch with water) common exothermic effects Crystallization , Gelation (gel formation) Oxidation , combustion, Decomposition , ignition, explosion Fermentation, Most of the chemical reactions , Polymerization , reticulation

24. Endothermic and exothermic effects Heat Flux 24 EXO ENDO Time (sec) Temperature (°C) cooling heating

25. Typical results from DSC Analysis of a polymer shows several features due to physical and chemical changes, including: DSC of polymer

26. Summary of Pharmaceutically Relevant Information Derived from DSC Analysis Melting points – crystalline materials Desolvation – adsorbed and bound solvents Glass transitions – amorphous materials Heats of transitions –melting, crystallisation Purity determination – contamination, crystalline/amorphous phase quantification Polymorphic transitions – polymorphs and pseudopolymorphs Processing conditions – environmental factors Compatibility – interactions between components Decomposition kinetics – chemical and thermal stability

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