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
