Applications of IR spectroscopy MCQs With Answer

Applications of IR spectroscopy MCQs With Answer

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Applications of IR Spectroscopy MCQs With Answer

Infrared (IR) spectroscopy is a cornerstone of modern pharmaceutical analytical techniques, enabling rapid, non-destructive characterization of functional groups, solid-state forms, and molecular interactions. For M. Pharm students, mastering IR applications is essential for tasks such as API identification, polymorph screening, excipient compatibility studies, moisture assessment, and in-line process monitoring. This quiz compiles thoughtfully designed questions spanning ATR, DRIFTS, NIR/MIR, chemometrics, micro-FTIR imaging, and hyphenated techniques like TGA-FTIR. Emphasis is placed on practical decision-making—choosing the right accessory, interpreting key bands (e.g., carbonyl, OH/NH), understanding hydrogen bonding effects, and leveraging IR for quantitative work. Use these MCQs to solidify both conceptual understanding and real-world problem-solving in pharmaceutical analysis.

Q1. In routine pharmaceutical identification, the primary application of mid-IR spectroscopy is to:

  • Quantify trace impurities below ppm levels without calibration
  • Confirm API identity via functional-group analysis and fingerprint region matching
  • Determine particle-size distribution of powders directly
  • Measure optical rotation in chiral drugs

Correct Answer: Confirm API identity via functional-group analysis and fingerprint region matching

Q2. For rapid, non-destructive spectral acquisition directly from an intact tablet surface, the most suitable IR approach is:

  • Transmission FTIR using KBr pellet
  • ATR-FTIR with a diamond crystal
  • GC-IR hyphenation
  • IR microscopy with thin-section microtoming

Correct Answer: ATR-FTIR with a diamond crystal

Q3. In the KBr pellet method for solid samples, a critical requirement to avoid spurious bands is:

  • Use of anhydrous, finely ground KBr to minimize water absorption bands
  • Using sample at 50–80% w/w in KBr for stronger signals
  • Pelletizing under ambient humid conditions to prevent static
  • Adding glycerol to improve pellet transparency

Correct Answer: Use of anhydrous, finely ground KBr to minimize water absorption bands

Q4. Which IR spectral change best indicates strengthened hydrogen bonding in a drug–polymer solid dispersion?

  • Blue shift and narrowing of OH/NH stretching bands
  • Red shift and broadening of OH/NH stretching bands
  • Appearance of sharp peaks at 2200–2300 cm⁻¹
  • Complete disappearance of CH stretching bands near 2900 cm⁻¹

Correct Answer: Red shift and broadening of OH/NH stretching bands

Q5. The fingerprint region, most useful for confirming molecular identity due to its uniqueness, typically spans:

  • 4000–2500 cm⁻¹
  • 2500–2000 cm⁻¹
  • 2000–1500 cm⁻¹
  • 1500–500 cm⁻¹

Correct Answer: 1500–500 cm⁻¹

Q6. To follow API polymorphic transitions during wet granulation as part of PAT, the most appropriate strategy is:

  • Off-line KBr pellet FTIR after drying granules
  • In-line ATR-FTIR with a chemometric model tracking polymorph-specific bands
  • UV-Vis fiber optic probe in transmission mode
  • Raman microscopy of isolated particles post-process

Correct Answer: In-line ATR-FTIR with a chemometric model tracking polymorph-specific bands

Q7. When distinguishing an amide from an ester in an unknown, which statement regarding carbonyl stretching is most accurate?

  • Amide C=O typically absorbs higher than ester C=O near 1780–1820 cm⁻¹
  • Ester C=O typically appears lower than amide C=O near 1620–1650 cm⁻¹
  • Amide C=O usually appears lower (≈1640–1690 cm⁻¹) than ester C=O (≈1735–1750 cm⁻¹)
  • Both amide and ester C=O always appear at exactly 1700 cm⁻¹

Correct Answer: Amide C=O usually appears lower (≈1640–1690 cm⁻¹) than ester C=O (≈1735–1750 cm⁻¹)

Q8. For non-destructive moisture determination in pharmaceutical powders using IR, the best-suited approach is:

  • Mid-IR transmission FTIR focusing on 3500 cm⁻¹ OH stretches
  • NIR diffuse reflectance targeting OH overtone/combination bands near 1450 and 1940 nm
  • Far-IR spectroscopy below 400 cm⁻¹
  • GC-IR analysis of headspace water

Correct Answer: NIR diffuse reflectance targeting OH overtone/combination bands near 1450 and 1940 nm

Q9. In ATR-FTIR, the effective penetration depth of the evanescent wave generally increases with:

  • Increasing wavenumber (shorter wavelength)
  • Increasing wavelength (lower wavenumber)
  • Higher refractive index of the ATR crystal
  • Greater contact pressure only

Correct Answer: Increasing wavelength (lower wavenumber)

Q10. Which IR observation most strongly supports ester formation during drug–excipient incompatibility stress?

  • Disappearance of C–H stretching near 2950 cm⁻¹
  • Appearance of a new sharp band near 1735 cm⁻¹ with reduction of acid OH broad band
  • New band at 2250 cm⁻¹ characteristic of nitriles
  • Splitting of amide II band near 1550 cm⁻¹

Correct Answer: Appearance of a new sharp band near 1735 cm⁻¹ with reduction of acid OH broad band

Q11. To visualize spatial distribution of an API within a tablet cross-section, the most appropriate IR tool is:

  • Single-bounce ATR without imaging
  • FTIR imaging microscope with a focal plane array detector
  • Raman handheld spectrometer
  • Transmission FTIR using thick sections

Correct Answer: FTIR imaging microscope with a focal plane array detector

Q12. For quantifying an API in a semi-solid (e.g., ointment) without extraction, a robust IR-based strategy is:

  • Transmission FTIR through a liquid cell and direct Beer–Lambert calculation
  • ATR-FTIR with multivariate calibration (e.g., PLS) on a characteristic band
  • Polarimetry of the bulk sample
  • Thin-layer chromatography followed by IR scraping

Correct Answer: ATR-FTIR with multivariate calibration (e.g., PLS) on a characteristic band

Q13. In counterfeit screening of tablets using IR, the most discriminating approach is to:

  • Compare only the CH stretching region (3000–2800 cm⁻¹)
  • Rely solely on intensity of a single carbonyl band
  • Perform full fingerprint region matching with library search plus PCA classification
  • Use far-IR lattice modes alone

Correct Answer: Perform full fingerprint region matching with library search plus PCA classification

Q14. Which factor typically does not cause a significant shift in IR band position for a given functional group?

  • Hydrogen bonding
  • Conjugation with adjacent double bonds
  • Isotopic substitution (e.g., H to D)
  • Sample path length/thickness

Correct Answer: Sample path length/thickness

Q15. Regarding complementarity of IR and Raman in pharmaceutical analysis, which statement is correct?

  • Vibrations that change dipole moment are strong in Raman and weak in IR
  • Symmetric, non-polar vibrations often appear strong in Raman but weak in IR
  • IR and Raman always produce identical selection rules
  • Raman cannot be used for solid-state characterization

Correct Answer: Symmetric, non-polar vibrations often appear strong in Raman but weak in IR

Q16. Diffuse reflectance IR spectroscopy (DRIFTS) is especially useful for:

  • Measuring aqueous solutions in transmission cells
  • Analyzing finely powdered solids without pelletizing, including moisture-sensitive samples
  • High-spatial-resolution chemical imaging
  • Gaseous sample analysis at low pressure

Correct Answer: Analyzing finely powdered solids without pelletizing, including moisture-sensitive samples

Q17. Evidence for carboxylate salt formation (vs. free carboxylic acid) in a drug–excipient mixture is best indicated by:

  • A single strong C=O band near 1710 cm⁻¹
  • Disappearance of all bands below 800 cm⁻¹
  • Two COO⁻ stretching bands near ~1600 cm⁻¹ (asymmetric) and ~1400 cm⁻¹ (symmetric)
  • New band near 2250 cm⁻¹

Correct Answer: Two COO⁻ stretching bands near ~1600 cm⁻¹ (asymmetric) and ~1400 cm⁻¹ (symmetric)

Q18. Distinguishing an anhydrous form from a hydrate of an API by IR commonly relies on:

  • Absence of any CH bands in the hydrate
  • Appearance of water bands near ~3400 cm⁻¹ (OH stretch) and ~1640 cm⁻¹ (HOH bend)
  • Shift of nitrile band to 2100 cm⁻¹
  • Complete overlap of fingerprint region

Correct Answer: Appearance of water bands near ~3400 cm⁻¹ (OH stretch) and ~1640 cm⁻¹ (HOH bend)

Q19. The primary application of TGA–FTIR in pharmaceutical analysis is to:

  • Map API distribution in a tablet cross-section
  • Identify and monitor evolved gases/solvents during thermal decomposition or drying
  • Determine crystalline domain size
  • Measure optical activity of decomposed products

Correct Answer: Identify and monitor evolved gases/solvents during thermal decomposition or drying

Q20. When a sample shows saturated absorbance in ATR-FTIR due to strong bands, a practical way to reduce band intensity without altering chemistry is to:

  • Increase contact pressure to maximize coupling
  • Switch to a germanium ATR crystal to decrease penetration depth
  • Add water to the sample
  • Run more co-added scans

Correct Answer: Switch to a germanium ATR crystal to decrease penetration depth

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