Some Analytical Tools

What is Thermal Analysis (TA)? A group of techniques in which a physical property (Enthalpy, thermal capacity or, Coefficient of heat expansion etc.) is measured as a function of temperature. The sample being analyzed is subjected to a predefined heating or cooling program.

Types of Thermal Analysis: 1. TG (Thermo Gravimetric Analysis): Mass change as function of Temperature 2. DTA (Differential Thermal Analysis): Temperature change as function of Temperature 3. DSC (Differential Scanning Calorimetry): Heat difference as function of Temperature 4. TT (Thermometric Titrations): Temperature difference as function of volume of titrant 5. PTGA (Pressurized Thermo Gravimetric Analysis): Mass change as function of pressure 6. TMA (Thermo Mechanical Analysis): Deformations and dimensions as function of Temp. 7. DIL (Dilatometry): Volume as function of Temperature 8. EGA (Evolved Gas Analysis): gaseous decomposition products as function of Temperature

What is DTA? DTA is a technique in which the temperature difference (ΔT = TS – TR) between sample & thermally inert reference substance is continuously recorded as a function of temperature / time. In this technique, the heat flow to both the test sample & an inert reference material (alumina) remain the same.

If there is zero temperature difference between sample & reference material then it means that sample does not undergo any chemical or physical change. If any reaction takes place then temperature difference (ΔT) will occur between sample & reference material.

Thermal transitions associated with DTA:

What is DSC? Differential scanning calorimetry or 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.

It is used to study thermal transitions of a polymer/sample (the changes that take place on heating). For example: 1. The melting of a crystalline polymer; Pure APIs usually show this. 2. The glass transition (Temp at which an amorphous polymer or an amorphous part of crystalline polymer goes from hard, brittle state to soft, rubbery state.); Amorphous form of crystalline APIs usually show this. 3. The crystallization;

Operating Principle of DSC: The sample and reference pans in the instrument are maintained at the same temperature, even during a thermal event in the sample. The energy required to maintain zero temperature difference between the sample and the reference is measured. During a thermal event in the sample, the system will transfer heat to or from the sample pan to maintain the same temperature in reference and sample pans.

Factors complicating DSC analysis: Endothermic peaks can be created by evaporation and decomposition as well as melting. TGA should be done on all new samples prior to DSC to determine volatile content and decomposition temperature. Dehydyration/Desolvation usually results in loss of crystalline structure. Melting is a thermodynamic transition and therefore, the onset of melting does not change significantly with heating rate. Decomposition is a kinetic (time-dependent) transition and therefore, the onset of decomposition changes significantly with heating rate.

What is TG / TGA? TGA is a technique in which the mass change of a sample is measured with the change in temperature. It gives valuable information on changes in sample composition and thermal stability of the compound. It is often integrated with DSC or DTA to give more definite results about those events where mass of the sample changed.

So, it is obvious that if there is – 1. Melting / Polymorphic Transformation – No change of mass 2. Evaporation of solvent of crystallization – mass will be changed. Example: Decomposition of calcium oxalate monohydrate, a standard material often used to demonstrate TGA performance. It exhibits three (3) weight losses with temperature in an inert atmosphere (e.g. N2)

Applications of Thermal Analysis: 1. To detect impurities in pharmaceuticals: After recording TG, DTA or DSC thermograms of the samples and then comparing with the thermograms of reference standards. Any abnormal mass changes on TG curve & irregular endotherm or exotherm peaks on DTA or DSC curve would indicate the presence of impurity.

2. To identify the correct polymorph during preformulation study: When a substance exists in more than one crystalline form, the different form are designated as polymorphs and the phenomenon as polymorphism. Depending upon their relative stability, one of the several polymorphic form will be physically more stable than others. Stable polymorph represents the lowest energy state, has highest melting point and least aqueous solubility. Metastable form represents the higher energy state, have lower melting point and high aqueous solubility. Metastable form converts to the stable form due to their higher energy state. Metastable form shows better bioavailability and therefore preferred in formulations. Order of dissolution rate: Amorphous > metastable > stable

Example: DTA patterns for two polymorphs and a dioxane solvate form of SQ 10,996 are presented in Fig. Curve (1) shows a melting endotherm at approximately 195°C, followed by a decomposition at 250 to 300°C. Curve (2) shows a melting endotherm at 180°C, followed by a small exotherm characterizing transition to form A, which then melts and decomposes at 190°C and 250 to 300°C, respectively.

Curve (3) is a thermogram for the dioxane solvate. It is similar to that of form B with the exception that it has an extra endotherm at 140°C. This is a de-solvation endotherm; upon desolvation, form B is generated. Other events on the thermogram of the solvate are identical to those seen for form B.

3. To differentiate between true-polymorphism and pseudo-polymorphism: Pseudo means false. The phenomenon in which solvent molecules get incorporated into crystal lattice of solid are known as solvates. If the solvent is water then it is known as a hydrates. This solvates exist in different crystal form called pseuodopolymorph and the phenomenon is called as Pseudo-polymorphism. Desolvation endotherms are not always as distinct as shown in the previous example. In these situations TGA is very useful. The TGA pattern for the dioxane solvate (curve 3) showed a loss in weight that began at 105°C and was complete at about 140°C.

4. It is also important to determine whether polymorphic transitions are possible within the temperature range used for stability studies and during processing (drying, milling, mixing. granulation etc.) & storage. 5. To study drug-excipient incompatibility: Excipients can affect the stability of the active drug. In the absence of any interaction, the thermograms of mixtures show patterns corresponding to those of the individual components. In the event when interaction occurs, it is indicated in the thermogram of a mixture by the appearance of one or more new peaks or the disappearance of one or more peaks corresponding to those of the components.

Figure shows separate thermograms of cephradine and four excipients with the thermograms of the corresponding four mixtures. Only the thermogram for the mixture with anhydrous sodium carbonate retains the significant cephradine exotherm at about 200°C. An investigation of the stability of cephradine in the presence of these excipients also showed that all the excipients with the exception of anhydrous sodium carbonate had deleterious effects on stability.

Conclusion Interpretation of thermal data is not always straightforward like the above example. When two substances are mixed, the purity of each is obliterated. Impure materials generally have lower melting points and exhibit less well-defined peaks in thermograms. The temperature causing thermal events to occur can be too high and the condition may then be too stressful, forcing a reaction that might not occur at lower temperatures. Moreover, if an interaction is indicated, it is not necessarily deleterious. The formation of eutectics, if not occurring at so low a temperature as to physically compromise the final product is acceptable.