All analysis was performed using the AMI DSC 600 and AMI TGA 1000, part of AMI’s range of thermal analysis instruments. For background on AMI’s broader thermal characterization capabilities, see our thermal properties analysis overview.
Polymorph identification in pharmaceutical development rarely comes down to a single clean measurement. Heating rate, crucible configuration, and the combined use of two complementary techniques can all change whether a second crystal form is detected — or missed entirely. This case study demonstrates DSC TGA pharmaceutical polymorphism analysis applied to revefenacin, a long-acting muscarinic antagonist (LAMA) used as maintenance therapy for chronic obstructive pulmonary disease (COPD), showing exactly how methodology choices affect what you see — and what you might miss.
Thermal analysis techniques are applied throughout pharmaceutical research to evaluate polymorphism, phase transitions, hydration states, decomposition behavior, purity, compatibility, and stability. Among these techniques, DSC and TGA are the most frequently used and are officially recognized in many pharmacopeias for drug quality control.
Differential scanning calorimetry measures heat flow associated with thermal events during a controlled temperature program — including endothermic and exothermic transitions such as melting, glass transitions, recrystallization, and polymorphic transformations. In pharmaceutical work, DSC is most often used to determine melting points, glass transitions, drug purity, polymorphic form, and compatibility between active pharmaceutical ingredients (APIs) and excipients.
Thermogravimetric analysis records changes in sample mass as a function of temperature under a defined atmosphere. TGA is valuable for characterizing adsorbed water, crystallization solvents, thermal stability, and decomposition behavior. For hydrates and solvates specifically, TGA can differentiate between loosely bound and tightly bound solvent molecules by observing distinct weight-loss steps across different temperature ranges.
Used together, DSC and TGA provide complementary evidence: DSC reveals thermal events, while TGA confirms whether those events involve mass change — directly resolving ambiguity that neither technique can resolve alone. For a deeper look at how DSC-measured calorimetric data supports quantitative solubility modeling for polymorphic drug forms, see our article on differential scanning calorimetry application for pharmaceutical polymorphism.
Revefenacin is used as a once-daily, long-acting muscarinic antagonist for nebulized maintenance therapy in COPD patients, improving lung function and slowing disease progression. Revefenacin exhibits polymorphism, with multiple crystalline forms (I–IV) reported in the literature. Only Form III is currently used in commercial formulations due to its superior processability, but challenges including solvate formation and residual solvent during crystallization continue to drive development of more stable forms.
These considerations make revefenacin a representative case for demonstrating how complementary DSC and TGA techniques systematically investigate the thermal behavior, hydration state, and stability of pharmaceutical solid forms during development.
| Parameter | DSC Method | TGA Method |
|---|---|---|
| Instrument | AMI DSC 600 | AMI TGA 1000 |
| Crucible | Aluminum (sealed and open compared) | Platinum |
| Atmosphere | Nitrogen purge, 50 mL/min | Nitrogen, 50 mL/min |
| Heating rates tested | 5°C/min and 10°C/min | 5°C/min |
| Temperature range | 5°C to 200°C | Room temperature to 300°C |
DSC curves collected at different heating rates (Figure 1a; alt text: DSC curves of revefenacin at 5°C/min and 10°C/min showing a secondary melting peak resolved only at the slower heating rate) revealed a clear methodology-dependent effect. At the faster heating rate (10°C/min), the melting peak appeared larger and sharper due to faster energy input — but only a single transition was visible. At the slower heating rate (5°C/min), a secondary melting peak became visible, indicating the presence of a second polymorphic form that the faster scan had missed entirely.
| Key finding: Slower heating rates improve the resolution of closely spaced thermal transitions and help distinguish between multiple crystalline forms. Appropriate heating rate selection is therefore essential for accurate polymorph identification — a fast routine scan can produce a false negative for polymorphic impurity. |
Melting behavior was also strongly influenced by crucible configuration (Figure 1b; alt text: DSC curves of revefenacin comparing sealed crucible clean melting peak versus open crucible broader profile from moisture loss beginning near 30°C). In sealed crucibles, a single clean melting peak was observed. In open (unsealed) crucibles, adsorbed and crystallization water evaporated during the initial heating ramp, producing a broader melting profile that began as early as 30°C.
This early thermal event in the open-crucible run is attributable to moisture loss — not actual melting — and could be misinterpreted as a low-melting polymorphic impurity or degradation event if crucible configuration is not accounted for during data interpretation. This finding underscores why crucible type is a methodology choice with direct consequences for polymorph data accuracy, not merely a procedural detail.
The TGA curve (Figure 2; alt text: TGA curve of revefenacin showing stepwise mass loss totaling 8.243% from removal of adsorbed and crystallization water) supported the DSC findings directly, showing a stepwise mass loss corresponding to the removal of adsorbed and crystallization water. A total weight loss of 8.243% was recorded across the heating program.
From the magnitude of weight loss between thermogram plateaus, the molar ratio of crystallization water can be calculated directly — confirming both the hydrate content of the sample and its thermal release characteristics. This quantitative TGA confirmation directly explains the moisture-related broadening observed in the open-crucible DSC run, tying the two techniques’ results together into a single coherent picture of the material’s solid-state behavior.
| Related reading: For a detailed kinetic study of how crystalline hydrates release water of hydration — including activation energy and diffusion mechanism determination from isothermal TGA — see our article on TGA crystalline hydrates dehydration kinetics. |
This case study demonstrates several practical lessons that apply broadly to DSC TGA pharmaceutical polymorphism analysis, beyond the specific revefenacin result:
The DSC 600 provided the heat-flow precision needed to resolve closely spaced polymorphic melting transitions across multiple heating rates, while the TGA 1000 provided the mass-loss sensitivity required to quantify hydrate water content with confidence. Used together — as in this revefenacin study — the two instruments provide a complete, mutually validating picture of polymorphic form and hydration state that neither technique alone can deliver.
Accurate DSC TGA pharmaceutical polymorphism analysis depends on more than instrument capability — methodology choices including heating rate and crucible configuration directly determine whether polymorphic forms and hydration-related artifacts are correctly identified or missed. In this revefenacin case study, the AMI DSC 600 and AMI TGA 1000 provided complementary thermal and gravimetric data: DSC enabled clear differentiation between polymorphs and revealed the influence of heating rate and crucible type, while TGA verified the presence and quantity of crystallization water, supporting accurate formulation development and process control.
These methods play a vital role in the development and quality assurance of pharmaceutical compounds where polymorphic stability, solvate formation, and residual solvent content are major concerns. Explore AMI’s full range of thermal analysis instruments, or visit the AMI Technical Library for further application notes on DSC, TGA, and pharmaceutical thermal characterization methodology.
DSC and TGA provide complementary, mutually validating evidence. DSC measures heat flow and reveals thermal events such as melting, glass transitions, and polymorphic transformations, but cannot by itself distinguish whether an observed event involves mass change. TGA measures mass directly as a function of temperature, confirming whether a thermal event corresponds to water or solvent loss versus a true solid-state transition. Used together, as demonstrated in this revefenacin case study, the two techniques resolve ambiguities that neither can resolve alone — for example, distinguishing a genuine polymorphic melting transition from an artifact caused by moisture evaporation.
Slower heating rates allow more time for thermal events to develop and separate in temperature, improving resolution between closely spaced transitions. In this study, a 5°C/min scan resolved a secondary melting peak — indicating a second polymorphic form — that was not visible at 10°C/min, where faster energy input produced a single, larger, sharper peak that obscured the underlying complexity. This means polymorph screening protocols that rely on a single fast heating rate risk false-negative results for polymorphic impurities.
In an open (unsealed) crucible, adsorbed and crystallization water can evaporate during the initial heating ramp, producing a broad thermal event beginning at low temperature that can be mistaken for a polymorphic transition or degradation. In a sealed crucible, this moisture cannot escape during heating, producing a cleaner melting profile that more accurately isolates true melting behavior from moisture-related artifacts. Comparing both crucible configurations — as in this revefenacin study — helps confirm whether an observed low-temperature DSC event reflects genuine solid-state behavior or simply moisture loss.
The molar ratio of crystallization water is calculated from the magnitude of mass loss observed between distinct plateaus on the TGA thermogram, using the known molecular weight of the compound and water. In this revefenacin study, a total weight loss of 8.243% was recorded across the heating program, from which the stoichiometric water content of the hydrate form was determined directly — providing quantitative confirmation of hydration state that complements the qualitative thermal evidence from DSC.
Revefenacin is a long-acting muscarinic antagonist (LAMA) used as a once-daily, nebulized maintenance therapy for chronic obstructive pulmonary disease (COPD). It exhibits polymorphism, with multiple crystalline forms (I–IV) reported, of which only Form III is currently used commercially due to superior processability. Ongoing challenges with solvate formation and residual solvent during crystallization make it a representative, practically relevant case study for demonstrating combined DSC and TGA methodology in pharmaceutical solid-form characterization.
