Rotating crystal technique MCQs With Answer

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

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