DSC & TGA Thermal Analysis.pptx

M pharm chemistry department

1. R. C. Patel Institute of Pharmaceutical Education and Research, Shirpur. Name - Rina Pandurang Patil Stream - 1 st yr M Pharm (Pharmaceutical Chemistry ) Roll No. 08 DSC & TGA

2. Thermal Analytical Techniques “Thermal Techniques involves the measurement of the dynamic relationship between temperature and some property of a system such as mass, heat of reaction or volume . ” Thermal methods of chemical analysis are as follows - 1) Thermogravimetric analysis (TGA) - Mass of the sample is monitored as a function of temperature or time . 2) Differential Thermal Analysis (DTA) - Temperature difference between sample and reference substance is monitored as a function of either time or temperature as the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate. 3 ) Differential Scanning Colorimetry (DSC) - Difference in the heat that flows into the sample and that which flows into the reference substance is monitored as a function of temperature or time .

3. 5) Thermomechanical Analysis (TMA) – Dimension of sample is measured as the temperature is altered . 6) Derivative Differential Thermal Analysis (DDTA) - A method for precise determination in thermograms of slight temperature changes by taking the first derivative of the differential thermal analysis curve (thermogram) which plots time versus  differential temperature as measured by a differential thermocouple. In thermal analytical techniques the data obtained in the form of continuously recorded curves is known as thermal data . 4) Dynamic Mechanical Analysis (DMA) – Mechanical property of sample is monitored as the sample is subjected to stress .

4. Sr. No. Name of the Technique Abbreviation Instrument Employed Parameter Measured Graph Plotted 1 Thermogravimetric Analysis TGA Thermo balance Mass Mass vs. Temperature 2 Differential Thermal Analysis DTA DTA Apparatus vs . Temperature 3 Derivative Differential Thermal Analysis DDTA DTA Apparatus Differential Temperature vs. Time 4 Differential Scanning Colorimetry DSC Calorimeter dH / dt dH /dt vs. Temperature 5 Dynamic Mechanical Analysis DMA DMA Apparatus Mechanical Property Mechanical Property vs. Stress 6 Thermomechanical Analysis TMA Dilatometer Volume or length Volume or length vs. Temperature Table 1 : Thermal Methods of Analysis

5. 1. Thermogravimetric Analysis (TGA) “In this technique the mass of sample is monitored as a function of temperature . ” The following techniques of analysis are adopted – Isothermal or Static TGA – In this type, sample is maintained at constant temperature for given time during which its weight is recorded. Quasistatic TGA – In this type, sample is heated to constant weight at each of series of increasing temperature . Dynamic TGA – In this type, the sample is subjected to continuous increase in temperature linear with time.

6. Principle TGA is performed by gradually increasing the temperature of sample in a furnance and its weight is measured on an analytical balance that remains outside of furnance. In TGA, mass loss is observed if a thermal event involves loss of a volatile component. A typical TGA curve is shown in Fig. 1 Fig . 1 – Typical TGA curv e

7. Instrumentation A modern thermobalance consist of the components (Fig.2 ) - ( 1) Recording balance (2) Sample holder (3) Furnance (4) Furnance temperature programmer or controller (5) Recorder Fig.2 – Schematic Diagram of Modern Thermobalance

8. (1) Recording balance It is most important component of thermobalance. A good balance must fulfil the following requirements: 1) Its accuracy, sensitivity, reproducibility and capacity should be similar to those of analytical balance. 2) It should have an accurate range of automatic weight adjustment. 3) It should have high degree of mechanical and electronic stability. 4) It should give rapid response to weight changes. 5) It should be unaffected by vibrations. 6) It should be simple to operate and versatile . Recording balances are mainly of two types – (1) Deflection type instrument (2) Null type instrument

9. 1) Deflection type balances – They are of following types (1) Beam Type In these types, there occurs conversion of beam deflection about fulcrum into suitably identifiably weight changes curve by photographic recorded trace, recorded signals generated by suitable displacement measuring transducers or curve drawn electromechanically. (2) Helical type In these types, there occurs elongation or concentration of spring with weight change. The quartz fibre is generally use as spring. (3) The Cantilevered Beam In these types, one end of the beam is fixed and other end on which sample is placed is free to undergo deflection. (4) Torsion wire In these types, beam is attached to taut wire which acts as fulcrum. The wire is firmly fixed at either or both ends so that deflections of beam are proportional to weight changes and torsion characteristics of wire.

10. Fig.3 (a) Deflection Type Balances Fig.3 (b) Null Type Balances 2) Null type instrument – In these types, there should be a sensor to detect deviation of balance beam from its null position. Then, a restoring force, of either electrical or mechanical weight loading, is applied to beam to restore its null position from horizontal or vertical norm. The restoring force is proportional to weight change and the force is recorded directly or by transducer of some type Fig.3(b).

11. (2) Sample holder Depending upon the nature of sample, its weight and quantity to be handled, different sizes and shapes of sample holders known as crucibles are employed. These are constructed from various materials like glass, quartz, aluminium, stainless steel, platinum etc. These generally of two types: 1) Shallow pan for holding sample which eliminates gas vapours or volatile matter by diffusion during heating. 2) Deep crucibles for general purpose are employed in such cases where side reaction and or partial equilibrium is to be desired. 3) Loosely covered crucibles are mainly used in self generated atmosphere studies. 4) Retort cups resembles alchemist’s retort. These are useful in boiling point studies.

12. (3) Furnance The furnance and control system should be designed to produce a linear heating rate over the whole working temperature range of furnance. The choice of furnance heating element and furnance depend upon temperature range being studied. For 1100 C, the material of furnance is “Kanthal” or “Nichrome” wire or ribbon. If a wire is being used, it should be wound “Coiled coil” fashion to accommodate differential thermal expansion of various component. For temperature between 1100 C and 1500 C one should use platinum or any alloy of platinum and rhodium. By using platinum rhodium alloy having rhodium content of 40% one can use furnance up to 1750 C. For temperature above 1750 C, tungsten or molybdenum may be employed in reducing atmosphere.

13. The size of furnance is important factor. For example, a main advantage of low mass furnance is it cools very quickly but its linear temperature rise is very difficult to control. On other hand a high mass furnance may hold isothermal temperature but it requires comparatively more time to achieve the required temperature. It becomes easier to obtain a large uniform hot zone in high mass furnance. With small mass furnance it becomes difficult. Fig4 – Position of Furnance w.r.t. Balance The position of furnance relative to balance is important. With some balances like quartz fibre spring balance, the furnance is below weighing system but with beam balances several choices are possible.

14. (4) Furnance Temperature Programmer Or Controller There are the controllers which can provide gradual rise of temperature at a fixed rate. This device has a course and fine control knobs through which desired temperature with respect to rate/time can be obtained. This controlling is done by increasing voltage through heating element by motor driven variable transformer or by different thermocouple. This can be done by number of ways. The most common method is a thermocouple. For measuring 1100 C, chromel or alumel thermocouples made up of alloys of platinum and rhodium are employed. For higher temperature, tungsten or rhenium thermocouples are used extensively. The position of temperature measuring device relative to sample is very important. This can be adjusted in one of the following ways as in fig.5 (1) In fig. (a), thermocouple is placed near sample container and it has no contact with sample container. This is not good arrangement especially where low pressure is employed.

15. (2) In fig. (b), thermocouple is placed inside the sample holder but not in contact with it. This arrangement is better than (1) because it responds to small temperature changes. (3) In fig.(c), thermocouple is placed either in contact with sample or with the sample container. This is best arrangement of sample temperature detection. Fig.5– Position of thermocouple in thermobalance

16. (5) Recorder The reading systems are mainly of two types a) Time based potentiometric strip chart recorder b) X-Y recorders Electrical supply dully amplified is fed to recorder chart. The speed of recorder is variable and adjusted according to use.

17. Factors Affecting Thermogravimetric Results 1) Instrumental factors 2) Sample Characteristics 1) Instrumental Factors If a system is studied by different thermobalances the sample of TG curve obtained in each class will vary from instrument to instrument. a) Heating Rate - If a substance is being heated at a fast heating rate, the temperature of decomposition will be higher than that obtained at a slower rate of heating. b) Effect of furnance atmosphere - There is marked influence of furnance atmosphere on TG curve. For instance, decomposition of calcium carbonate will take place at much higher temperature if carbon dioxide rather than nitrogen is used as surrounding atmosphere.

18. The common atmospheres involved in thermogravimetry are as follows– 1) Static Air – In this type air from atmosphere is allowed to flow through furnance 2) Dynamic Air – In this type compressed air from a cylinder is allowed to flow through furnance at a measured flow rate. 3) Inert Atmosphere – Nitrogen gas (oxygen free) is used as inert environment . c) Sample Holder - Geometry of the sample holder can change the slope of TG curve. Sample holders range from flat plates to deep crucibles of various capacities. Generally a shallow dish is preferred over a high form cone shape crucible because in former there is rapid gaseous exchange between sample and surrounding atmosphere. When atmosphere is the solely gas involved in reaction the geometry of container has no effect on slope of TG curve.

19. 2) Sample Characteristics a) Weight of sample - If a large sample is used, there occurs a deviation from linearity as temperature rises. This particularly found to be true in the case of fast exothermic reaction. An example of this is the evolution of carbon monoxide during decomposition of calcium oxalate to calcium carbonate. b) Sample particle size – With a particle size of smaller dimension the decomposition takes place earlier, while with greater particle size decomposition proceeds only at higher temperature. The decomposition temperature decreases with decrease in sample particle size. c) Heat of reaction – The effect of heat of reaction of sample on TG curve has been studied by Newkirk. The heat of reaction will alter difference between the sample temperature and the furnance temperature. If the heat effect is exothermic or endothermic, this will cause the sample temperature to lead or lag behind the furnance temperature.

20. d) Compactness of Sample – A compressed sample will decompose at higher temperature than a loose sample. e) Previous History of sample – TG studies shows that magnesium hydroxide prepared by precipitation method has a different temperature of decomposition from that of naturally occurring material. Applications of TGA 1) Automatic Thermogravimetric analysis. 2) Evaluation of gravimetric precipitate. 3) Evaluation of suitable temperatures. 4) Testing of purity of samples. 5) Curie point determination.

21. Sample holder

22. 3. Differential Scanning Colorimetry (DSC) “ Difference in the heat that flows into the sample and that which flows into the reference substance is monitored as a function of temperature or of time .” History The technique was developed by E. S. Watson & M. J. O' Neill in 1962

23. DSC is a thermoanalytical 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. Principle

24. A Typical DSC Curve is shown in fig.10 Fig 10 - A Typical DSC Curve

25. Instrumentation There are three different types of DSC instruments: power-compensated DSC, heat-flux DSC, and modulated DSC. Each produces a plot of power or heat flow versus temperature, called as thermogram. 1) Heat Flux DSC In heat-flux DSC, the difference in 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, as shown in Figure 11. Small aluminium sample and reference pans sit on raised platforms on the constant disk. Heat is transferred through the disks and up into the material via the two pans. The differential heat flow to the sample and reference is monitored by Chromel

26. Fig. 11 - Heat Flux DSC constant area thermocouples formed by the junction between the constant platform and Chromel disks attached to the underside of the platforms. The differential heat flow into the two pans is directly proportional to the difference in the outputs of the two thermocouple junctions. The sample temperature is estimated by the Chromel- alumel junction under the sample disk.

27. 2) Power Compensated DSC In power compensated DSC, the temperatures of the sample and reference are kept equal to each other while both temperatures are increased or decreased linearly. The power needed to maintain the sample temperature equal to the reference temperature is measured. The sample and reference are heated separately in power compensated DSC, as shown in Figure 12. The pan temperatures are monitored using thermocouples attached to the disk platforms. The thermocouples are connected in series and measure the differential heat flow using the thermal equivalent of Ohm’s Law dq/dt=∆T/RD Where dq/dt is the heat flow, ∆T is the temperature difference between the reference and sample, and RD is the thermal resistance of the disk platform. The heat flow to each pan is adjusted to keep their temperature difference close to zero while the furnance temperature is increased linearly.

28. Fig.12 - Schematic of a power compensated DSC system. The sample and reference pans are heated separately. The heat flow to each pan is adjusted to keep their temperature difference close to zero. The difference in heat flow is recorded.

29. 3) Modulated DSC Modulated temperature DSC (MDSC) is an extension of DSC. The same heat flux DSC cell is used for MDSC, but a sinusoidal temperature oscillation (modulation) is overlaid on the conventional linear temperature ramp. This results in the heating rate at times being faster or slower than the underlying linear heating rate. 4) Hyper-DSC - High speed DSC (hyper-DSC) as a tool to measure the solubility of a drug within a solid or semi-solid matrix.   

30. Factors Affecting DSC Sample Shape: The shape of sample has little effect on quantitative aspect of DSC but has more effect on qualitative aspects. However sample in the form of a disc film or powder spread on the pan are preferred. In the case of polymeric sheets a disc cut with a cork borer gives good results. Sample size: About 0.5 to 10 mg is usually sufficient. Smaller samples enable faster scanning, give better shaped peaks with good resolution and provide better contact with gaseous environment. On the other hand, with large samples, smaller heats of transition may be measured with greater precision .

31. A DSC Application in Biology 1 Analysis of proteins 2 DSC of Nucleic Acids. 3 Analysis of Lipids. 4 Analysis of Carbohydrates. 5 Analysis of Ab B DSC Application in Nanoscience 1 Quantification of pharmaceutical Nanosolids. 2 Thermal characteristics of nanostructured lipid carriers (NLC's) 3 Thermoanalysis of colloidal nanoparticles. 4 Glass transition measurement of macromolecule in nanophases. 5 characterization of ion- chelating nano carriers. 6 self- Assembly study of supramolecular nanostructures. Recent Applications in DSC

32. Applications of DSC ..... Glass Transition Temperatures When heating a sample at a certain temp. Plot will be shift downward suddenly. Which means more heat flow & heat capacity increases because of glass transition. ( It is reversible transition in amorphous material from a hard brittle state into molten rubber like state ) Determination of heat capacity It is used to determine heat capacity, when we start heating two pan's, the computer will plot difference in heat output of two heaters against temp. & this is the heat absorbed by substance against temp. Crystallization When polymers fall into these crystalline arrangements, they give of heat. This drop in the heat flow appear as a big peak in the plot of heat flow Vs temp. The temp. At which highest point in the peak is usually considered to be the polymer crystallization temp. Also area of the peak can be measured which tellus us the latent energy of crystallization of polymer. Melting If polymer is heated fast it's Tc eventually reach another thermal transition called melting. Crystallinity and Crystallization Rate Reaction Kinetics It gives a wealth of information regarding both of the physical & energy property of the substances.

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