Evaluation of stable and metastable forms of acetaminophen using a combination of X-ray diffraction, thermal analysis, and electron diffraction

Crystal polymorphism occurs when a compound crystallizes into two or more different crystal structures while maintaining the same chemical composition. Each polymorph can exhibit distinct physical properties such as solubility, dissolution rate, mechanical strength, thermal stability, and bioavailability. Because these differences directly affect manufacturing processes and drug performance, identifying and controlling polymorphs is an essential part of pharmaceutical development and quality assurance.

Each technique provides different information. X-ray diffraction identifies crystal structures and phase composition, while DSC measures thermal events such as melting, crystallization, glass transitions, and polymorphic transformations. Simultaneous XRD-DSC allows these structural and thermal changes to be observed under identical experimental conditions, making it possible to directly correlate a thermal event with the formation, disappearance, or transformation of a specific crystal phase.

A two-dimensional detector captures a broad angular range in a single exposure without continuously scanning the goniometer. This enables rapid observation of structural changes during heating or cooling and minimizes the chance of missing transient phase transformations. It also records diffraction information from coarse-grained or preferentially oriented samples more effectively than conventional one-dimensional detectors, improving interpretation of challenging pharmaceutical materials.

DSC records heat flow associated with thermal events. Exothermic peaks typically indicate crystallization or certain phase transitions, while endothermic peaks often correspond to melting. When DSC results are interpreted alongside simultaneous diffraction data, each thermal peak can be assigned to a specific structural change, allowing researchers to distinguish between new crystal formation, crystal-to-crystal transitions, and melting events with much greater confidence.

Metastable polymorphs frequently exist only within narrow temperature or processing windows and often transform rapidly into more stable crystal forms. Their instability makes it difficult to isolate suitable crystals for conventional structural analysis. Capturing these transient phases requires analytical techniques that combine rapid in situ measurements with high sensitivity to small or short-lived crystals.

Electron diffraction can determine crystal structures from crystals that are only a few hundred nanometers in size, far smaller than those typically required for single-crystal X-ray diffraction. Because electrons interact much more strongly with matter than X-rays, electron diffraction enables structural analysis of crystals that are too small or too unstable for conventional crystallographic methods, making it particularly valuable for pharmaceutical polymorph characterization.

Mapping the sequence of melting, glass formation, recrystallization, and polymorphic transformation helps researchers identify the processing conditions that favor specific crystal forms. This knowledge supports improved control of manufacturing, enhances product consistency, reduces the risk of unintended polymorphic changes during production or storage, and contributes to more reliable drug performance.