Rotating Crystal Technique MCQs With Answer for M. Pharm Students
The rotating crystal technique is a classical single-crystal X-ray diffraction method used to determine unit cell parameters, lattice type, and symmetry from a well-formed crystal. By rotating the crystal in a monochromatic X-ray beam and recording the diffracted spots on film or an area detector, distinctive layer-line patterns are produced that can be indexed to derive crystallographic information. For M. Pharm students, understanding this technique supports applications such as polymorph identification, solid-state form selection, and quality control of APIs. This quiz consolidates core principles, instrumentation, data interpretation, and pharmaceutical relevance of the rotating crystal method, helping you bridge theory with real-world analytical decisions in modern pharmaceutical analysis.
Q1. The rotating crystal technique primarily relies on which underlying physical principle?
- Bragg’s law of X-ray diffraction
- Lambert–Beer law
- Nernst equation
- Stokes’ law
Correct Answer: Bragg’s law of X-ray diffraction
Q2. In the rotating crystal method, which type of sample is essential?
- A single, well-formed crystal
- A polycrystalline powder
- An amorphous thin film
- A dilute solution
Correct Answer: A single, well-formed crystal
Q3. What pattern is typically observed on the recording medium when a crystal is rotated in a monochromatic X-ray beam?
- Concentric rings
- Layer lines consisting of discrete diffraction spots
- Uniform darkening without features
- Continuous arcs without discrete spots
Correct Answer: Layer lines consisting of discrete diffraction spots
Q4. The spacing between layer lines in a rotation photograph is primarily related to which crystallographic quantity?
- The reciprocal of the lattice parameter parallel to the rotation axis
- Crystal density only
- X-ray tube accelerating voltage
- Sample thickness
Correct Answer: The reciprocal of the lattice parameter parallel to the rotation axis
Q5. Which component is used to ensure a predominantly single-wavelength (monochromatic) X-ray beam reaches the crystal?
- Monochromator
- Collimator
- Filter paper
- Polarizer
Correct Answer: Monochromator
Q6. Compared with the powder (Debye–Scherrer) method, the rotating crystal technique is better suited for:
- Determining complete atomic structures without any further data
- Rapid identification of amorphous phases
- Determining unit cell parameters of a single crystal
- Measuring solution concentrations
Correct Answer: Determining unit cell parameters of a single crystal
Q7. In pharmaceutical analysis, a primary application of the rotating crystal method is:
- Quantifying dissolution rate directly
- Distinguishing polymorphs via differences in unit cell parameters
- Measuring tablet hardness
- Determining pKa of drug molecules
Correct Answer: Distinguishing polymorphs via differences in unit cell parameters
Q8. The Ewald construction used in the rotating crystal technique helps visualize:
- Energy levels of electrons in molecules
- Intersections of reciprocal-lattice points with the Ewald sphere during rotation
- The internal path of X-rays within the tube anode
- Pressure distribution inside the camera
Correct Answer: Intersections of reciprocal-lattice points with the Ewald sphere during rotation
Q9. For modern area-detector rotation scans on organic pharmaceutical crystals, which radiation is commonly chosen to reduce absorption and peak overlap?
- Mo Kα (≈0.71 Å)
- Cu Kα (≈1.54 Å)
- UV light at 254 nm
- Neutron beam only
Correct Answer: Mo Kα (≈0.71 Å)
Q10. Which of the following is a major limitation of the rotating crystal technique?
- It requires high-quality single crystals
- It is not sensitive to X-ray radiation
- It is suitable for complex mixtures
- It provides absolute configuration without any anomalous dispersion
Correct Answer: It requires high-quality single crystals
Q11. The device that holds and spins the crystal around a defined axis is called the:
- Monochromator
- Goniometer spindle
- Collimator
- Anode
Correct Answer: Goniometer spindle
Q12. The zero-layer (equatorial) line in a rotation photograph corresponds to reflections with:
- l = 0 relative to the rotation axis
- h = k = 0 only
- All possible hkl values
- Forbidden reflections only
Correct Answer: l = 0 relative to the rotation axis
Q13. Observation of systematic absences in rotation photographs primarily aids in determining:
- Molecular weight
- Space group symmetry elements
- Melting point
- Hildebrand solubility parameter
Correct Answer: Space group symmetry elements
Q14. When using Cu radiation, Kβ contamination is commonly reduced in rotating crystal experiments by employing a:
- Nickel filter
- Aluminum filter
- Quartz cuvette
- Selenium diode
Correct Answer: Nickel filter
Q15. The angular separation of spots along a given layer line primarily depends on:
- Wavelength and interplanar spacings
- Solvent used for crystal growth
- Chamber pressure during exposure
- Detector pixel size only
Correct Answer: Wavelength and interplanar spacings
Q16. Which historical complementary method was commonly used with rotation photographs to separate overlapping layer lines and simplify indexing?
- Weissenberg camera
- UV–Vis spectrometer
- Karl Fischer titrator
- Differential scanning calorimeter
Correct Answer: Weissenberg camera
Q17. If the rotation axis is aligned with the crystallographic c-axis, the spacing of layer lines is related to:
- c* in reciprocal space
- a* in reciprocal space
- b* in reciprocal space
- Only the unit cell volume
Correct Answer: c* in reciprocal space
Q18. For APIs that show strong preferred orientation in powder form, the rotating crystal technique is advantageous because it:
- Eliminates the need for radiation safety
- Avoids orientation effects by using a single crystal
- Measures dissolution entropy directly
- Allows in situ tablet compression
Correct Answer: Avoids orientation effects by using a single crystal
Q19. A critical practical step before collecting rotation data is to:
- Index the unit cell from an NMR spectrum
- Center the crystal in the X-ray beam using a microscope
- Dry the sample at 300 °C
- Coat the crystal with gold
Correct Answer: Center the crystal in the X-ray beam using a microscope
Q20. Modern area-detector implementations of the rotation method on diffractometers are commonly referred to as:
- φ- and ω-scans
- DSC scans
- Titration scans
- Raman maps
Correct Answer: φ- and ω-scans
