Step 1: What is polymorphism?
Polymorphism means a drug can exist in more than one crystalline form. Different polymorphs have different melting points, solubility, and bioavailability, which matters a lot in pharmaceutics.
Step 2: Why study polymorphs?
Identifying the correct polymorph is critical for drug stability and efficacy. For example, ritonavir (an HIV drug) famously failed when an unexpected polymorph appeared.
Step 3: X-ray diffraction (XRD).
Each crystalline polymorph has a unique arrangement of atoms. X-ray diffraction reveals this arrangement by measuring how X-rays scatter off the crystal lattice, giving a unique diffraction pattern (fingerprint) for each polymorph.
Step 4: Dismiss other techniques.
Flame photometry identifies elements in solution (Na, K), not crystal structures. UV spectroscopy detects electronic transitions in molecules, not crystal forms. IR spectroscopy can help detect polymorphs but is less definitive than XRD for crystal structure determination.
Step 5: Confirm.
X-ray diffraction (powder XRD/PXRD) is the gold standard for identifying and characterizing polymorphic forms.
Answer: Option (3) — X-ray diffraction