DTA AND DDTA AS PER PHARMA

DIFFERENTIAL THERMAL ANALYSIS THE TECHINQUE BASED ON TEMPERATURE USED IN INDUSTRIES OR AS A LABORATORY TECHNIQUE TO CHECK THE STATUS OF SAMPLES.

1. Differential Thermal Analysis (DTA) and Derivative Differential Thermal Analysis (DDTA)

2. THERMAL ANALYSIS TECHNIQUES Thermal analysis comprises a group of techniques where the properties of material are studied as they change with temperature . To determine the thermo-physical properties several methods are commonly used: D ifferential thermal analysis (DTA), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), dilatometry (DIL), evolved gas analysis (EGA), dynamic mechanical analysis (DMA), dielectric analyze (DEA) etc.

3.  DTA : DTA may be defined formally as a technique for recording the difference in temperature between a substance and a reference material against either time or temperature . As the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate . This is a comparison method .

4. Analytical method for recording the difference in temperature between a substance and a reference material as a function of temperature or time . Any transformation – change in specific heat can be detected by DTA . In DTA both test sample and an inert reference material (alumina)are controlled by heating or cooling programming. If zero temperature difference between sample and reference material – sample does not undergo any chemical or physical change .

5. Principle : This technique is simple . The basic principle is : It involves the technique of recording the temperature difference (ΔT) between the test sample and an inert reference sample under controlled and identical conditions of heating or cooling is recorded continuously as a function of temperature or time, thus the heat absorbed or emitted by a chemical system is determined.

6. If any reaction takes place in the sample, then the temperature difference will occur between the sample and the reference material  In an endothermic change (such as melting or dehydration of the sample) the temperature of the sample is lower than that of the reference material (i.e) ΔT = ‒ ve (for endothermic process) In an exothermic change or process the sample temperature is higher than that of the reference material. (i.e) ΔT = + ve (exothermic process )

7. The difference in temperature ΔT between the sample temperature and the reference temperature T, (ΔT = Ts - Tr) is then monitored and plotted against sample temperature to give a differential thermo gram. A DTA curve can be used as a finger print for identification purposes. The shape and the size of the peak give information about the nature of the test sample.

8. Sharp endothermic peaks indicate phase changes (such as melting, fusion etc.) transition from one crystalline form to another crystalline form. Broad endothermic peaks are obtained from dehydration reactions. Chemical reactions like oxidative reactions are exothermic reactions. The origin of the temperature difference in the sample lies in the energy difference between the products and reactants .

9. This energy is manifested as enthalpic change either exothermic or endothermic . The area under a DTA peak can be to the enthalpy change and is not affected by the heat capacity of the sample.

10. Phenomena causing changes in temperature Physical: • Adsorption (exothermic) • Desorption (endothermic) • A change in crystal structure (endo – or exothermic) • Crystallization (exothermic) • Melting (endothermic) • Vaporization (endothermic) • Sublimation (endothermic)

11.   Chemical: Oxidation (exothermic) Reduction (endothermic) Break down reactions (endo – or exothermic) Chemisorption (exothermic) Solid state reactions (endo – or exothermic)

12. INSTRUMENTATION: Sample holder: This is used to contain the sample as well as reference material. Material – criteria for selecting material are Cost, ease of fabrication and inertness towards the reactants and products in the temperature range of interest.

13. Metallic - aluminium , nickel, stainless steel, platinum ( generally employed), and its alloys. Gives sharp exotherms and flat endotherms . Non metallic - glass, ceramic, vitreous silica or sintered alumina. Gives flat exotherms and sharp endotherms .

14.   Geometry – cylindrical geometry is used. Types of sample holders: Sample holders with dimples in which thermocouples junctions are inserted. The dimples are called thermocouple wells . It has better contact between sample holder and thermocouple junction. These are common.

15. Furnace: This is device for heating the sample. Its actually a oven enclosed in furnace. In DTA apparatus, tubular furnace is preferred for its good temp regulation and programming. This is constructed material ( wire or ribbon) wound on a refractory tube. These are fairly inexpensive .

16. Temperature controller and recorder : • Temperature controller: • Three basic element required to control temperature are sensor, control element and heater • Two methods: On – off control : If the sensor signal indicates that the temp has become greater than the set point, the heater is turned off .

17. • Not widely used in DTA • Inexpensive Proportional control : Heat in put to the system is progressively reduced as the temperature approaches the desired value . These are widely used in DTA instruments . But in expensive DTA instrument elegant electronic controllers are used .

18. Temperature programming : A time dependent temperature cycling of furnace is required to produce a desired rate of heating or cooling and to maintain the temperature at any desired value. A temperature programmer is employed which transmits certain time based instructions to control unit.  

19. Recorder: The signals obtained from the sensors are recorded by the recorder. And record the DTA curve. Two types Deflection type: The recording pen is moved directly by the input signal. Null type: The input signal is compared with a reference or standard signal and the difference is amplified and used to adjust the reference signal through a servo motor until it matches the input signal .

20. Thermocouples : Thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at different temperatures. Points to be considered while selecting a thermocouple : Thermoelectric coefficient Temperature interval. Chemical compatibility with the sample .

21. Chemical gaseous environment used and reproducibility of the EMF vs. temperature curve as a function of thermal cycling . Availability and cost   Made of chromel P and alumel wires measure and control temp up to 1100 degree C in air . Made from pure platinum and platinum – rhodium alloys wires measure more than 11 . Made from refractory materials like tungsten and rhenium in inert gas or vacuum for up to 2100 degree C .

22. Cooling system : The cooling system is considered separate from the temperature programmer because in most instrument cooling is completely independent from heating .  Its function is to maintain a suitable atmosphere in the furnace and sample holder .

23. Differential temperature sensor To measure the temperature difference between the sample and reference material) the sample and reference holder are kept inside the furnace and the temperature of the furnace and sample holder is controlled by using furnace controller. Heart of the analysis – heating block Identical pair of cavities for the sample, ref.material .

24. Whole unit is set in an oven- control pressure . Thermocouple is place directly in contact with the sample and another in contact with the reference . Temp of the block is raised, the temperature of the sample & reference follow Zero temp. difference – no physical or chemical change . If any reaction – difference in ΔT.

25. FACTORS AFFECTING THE DTA CURVE The various factors affecting the DTA curve are as follows: • Environmental factors. Instrumental factors. Sample factors.

26. ENVIRONMENTAL FACTORS: The DTA technique is more sensitive to the gaseous environment around the sample . Reaction of atmospheric gases with the sample may also produce extra peaks in the curve. In DTA two types of gaseous environment are used Static gaseous atmosphere Dynamic gaseous atmosphere

27. Static gaseous atmosphere • The atmosphere surrounding the sample is changing in concentration chemically due to evolved gases and physically due to convection currents • Studies in it are imprecise. Dynamic gaseous atmosphere • The gases are swept past the sample in a controlled way. • Reliable and reproducible • Sweep gases can be inert or reactive. But should not contain any of the product gases .

28. INSTRUMENTAL FACTORS: Sample holder: The geometry and material with which it is made of affects the DTA curve. • If material has High thermal conductivity – sharp exothermic peaks and flat endothermic peaks are obtained. • Eg . Metals • Poor thermal conductivity - reverse is true. • Eg . Ceramic • the size of holder and the amount of sample should be as small as possible for better resolution.

29. Differential temperature sensing devices: The thickness of thermocouple wires affect the intensity of the peaks, shape of the peaks and the baseline. If wires used are much thick More distortion of peak heights and peak temperatures may take place. If thinner wires are used Less distortion of peak heights and peak temperatures may take place. But the resistance is high and may be unstable in impedance matching .

30. SAMPLE CHARACTERISTICS Physical • Packing density. • Particle size • Peak area decreases with increase in size. • Peak T shifts to higher values with increase in size • Completion T decreases with decrease in size. • Degree of crystallinity.

31. • Amount of sample influence peak area. • As wt of the sample increases peak intensity and temperature. • In order to maintain the heat capacity nearly constant during heating, the sample is generally mixed with diluents. Generally, diluents affects the area, temperature and even resolution of the DTA peaks.

32. Chemical: The chemical reactivity of the sample, the sample holder, thermocouple material, the ambient gaseous environment and added diluents greatly alter the DTA peaks. Therefore, one should make every effort to select these materials as inert chemically as possible with the sample .

33. ADVANTAGES: Instruments can be used at very high temperatures Instruments are highly sensitive Characteristic transition or reaction temperatures can be accurately determined . DISADVANTAGES: Uncertainty of heats of fusion, transition, or reaction estimations is 20- 50%. Sharp thermal changes are unable to predict. Precision not good.

34. APPLICATIONS: A DTA curve can be used only as a finger print for identification purposes but usually the applications of this method are the determination of phase diagrams, heat change measurements and decomposition in various atmospheres. DTA is widely used in the pharmaceutical and food industries.

35. Used to study the characteristic of polymeric material. This technique is used for testing the purity of the drug sample and also to test the quality control of number of substances like cement, soil, glass,etc . Used for the determination of heat of reaction, specific heat and energy change occurring during melting etc.

36. Trend in ligand stability (thermal stability of the ligands) gives the information about the ligands in the coordination sphere. DTA curves may also be used to date bone remains or to study archaeological materials. Using DTA one can obtain liquids & solidus lines of phase diagrams. DTA may be used in cement chemistry, mineralogical research and in environmental studies.

37. Impurities may be detected by depression of the M.P Quantitative identification and purity assessment of materials are accomplished by comparing the DTA curve of sample to that of a reference curve . DTA technique may also be used to study the thermal stability of large no. of inorganic compound and complexes . DTA techniques has been used to study oxalates ,metal amine , carbonates and oxides . DTA investigation have been carried to help identification , purity determination .

38. Quantitative analysis include the evaluation of kinetic parameter of polymers , explosives , pharmaceuticals , oils , fats and other organic chemical . DTA may also be used for industrial controls , such as for determination of the structural and chemical changes occuring during sintering ,fusing and other heat treatments . It is used for identifications of different types of synthetic rubbers.

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