Introduction to IR spectroscopy MCQs With Answer

Introduction to IR Spectroscopy MCQs With Answer

Infrared (IR) spectroscopy is a cornerstone technique in Modern Pharmaceutical Analytical Techniques, enabling identification, structural elucidation, and even quantitative analysis of drug substances and excipients. By probing molecular vibrations that involve a change in dipole moment, IR provides characteristic group frequencies and a rich fingerprint region invaluable for confirming identity and purity. This MCQ set is tailored for M. Pharm students to deepen understanding of IR principles, instrumentation (FTIR vs. dispersive), sample preparation (KBr pellet, ATR, thin films), data interpretation (band positions, intensities, hydrogen bonding, isotopic shifts), and practical method considerations (resolution, apodization, calibration, solvent and window selection). Each question is designed to challenge and reinforce concepts required for advanced pharmaceutical analysis.

Q1. Which fundamental requirement makes a molecular vibration IR-active?

• A change in molecular polarizability during vibration
• A change in dipole moment during vibration
• A high molar absorptivity in the UV region
• A large molecular mass

Correct Answer: A change in dipole moment during vibration

Q2. The mid-IR spectral region commonly used for organic functional group analysis spans which wavenumber range?

• 8000–4000 cm⁻¹
• 4000–400 cm⁻¹
• 400–10 cm⁻¹
• 25000–10000 cm⁻¹

Correct Answer: 4000–400 cm⁻¹

Q3. In FTIR instruments, which component provides the wavelength scale accuracy (Connes advantage)?

• He–Ne reference laser
• Globar source
• DTGS detector
• KBr beam splitter

Correct Answer: He–Ne reference laser

Q4. Which source is most commonly used for the mid-IR region in modern FTIR instruments?

• Tungsten–halogen lamp
• Deuterium arc lamp
• Globar (SiC) source
• Nernst glower exclusively

Correct Answer: Globar (SiC) source

Q5. For a non-linear molecule with N atoms, the number of fundamental vibrational modes is:

• 3N
• 3N − 6
• 3N − 5
• 2N − 1

Correct Answer: 3N − 6

Q6. Which of the following statements about ATR-FTIR is TRUE?

• Penetration depth decreases with increasing wavelength
• Effective path length increases with increasing angle of incidence away from the critical angle
• Penetration depth increases as the refractive index of the ATR crystal increases
• Penetration depth increases with increasing wavelength and decreases with higher crystal refractive index

Correct Answer: Penetration depth increases with increasing wavelength and decreases with higher crystal refractive index

Q7. Which detector is highly sensitive in the mid-IR but typically requires cooling (often liquid nitrogen)?

• DTGS (deuterated triglycine sulfate)
• MCT (HgCdTe)
• PMT (photomultiplier tube)
• Si photodiode

Correct Answer: MCT (HgCdTe)

Q8. The strong, broad absorption band of a carboxylic acid O–H stretch typically appears around:

• 2500–3300 cm⁻¹
• 3600–3800 cm⁻¹
• 2100–2260 cm⁻¹
• 1650–1750 cm⁻¹

Correct Answer: 2500–3300 cm⁻¹

Q9. The absorption near 2250 cm⁻¹ in an IR spectrum is most characteristic of:

• Aldehyde C–H stretch
• Nitrile (C≡N) stretch
• Aromatic C=C stretch
• Ether C–O–C stretch

Correct Answer: Nitrile (C≡N) stretch

Q10. Which sample preparation is BEST for a crystalline, non-volatile solid that is strongly absorbing and difficult to dissolve?

• Solution cell with water as solvent
• Thin film on NaCl plate
• KBr pellet (press) method
• Gas cell with long pathlength

Correct Answer: KBr pellet (press) method

Q11. Which statement about KBr pellets is CORRECT?

• KBr is non-hygroscopic and compatible with aqueous samples
• KBr is hygroscopic; water absorption can distort spectra
• KBr transmits only above 2000 cm⁻¹
• KBr pellets are unsuitable for mid-IR measurements

Correct Answer: KBr is hygroscopic; water absorption can distort spectra

Q12. What is the main advantage of FTIR over dispersive IR described as the Fellgett advantage?

• Improved wavelength accuracy using a laser
• Higher throughput due to larger apertures
• Simultaneous measurement of all frequencies (multiplexing)
• Reduced sensitivity to CO₂ and H₂O interferences

Correct Answer: Simultaneous measurement of all frequencies (multiplexing)

Q13. The resolution (Δν, cm⁻¹) of an FTIR is primarily determined by:

• Detector response time
• Maximum optical path difference in the interferometer
• Source temperature
• Beam splitter material only

Correct Answer: Maximum optical path difference in the interferometer

Q14. Which window material is commonly used for solution cells in the mid-IR but should not be used with aqueous samples?

• Diamond
• NaCl
• ZnSe
• CaF₂

Correct Answer: NaCl

Q15. Hydrogen bonding in a carbonyl-containing compound typically causes the C=O stretch to:

• Shift to higher wavenumber and narrow
• Shift to lower wavenumber and broaden
• Remain unchanged
• Disappear due to IR inactivity

Correct Answer: Shift to lower wavenumber and broaden

Q16. Isotopic substitution of 12C by 13C in a carbonyl group will most likely:

• Shift the C=O stretch to higher wavenumber due to lower reduced mass
• Shift the C=O stretch to lower wavenumber due to higher reduced mass
• Have no effect on the C=O frequency
• Eliminate the IR activity of the C=O stretch

Correct Answer: Shift the C=O stretch to lower wavenumber due to higher reduced mass

Q17. The fingerprint region in IR spectroscopy is typically defined as:

• 4000–2500 cm⁻¹
• 2500–2000 cm⁻¹
• 2000–1500 cm⁻¹
• 1500–400 cm⁻¹

Correct Answer: 1500–400 cm⁻¹

Q18. Which of the following molecules is IR-inactive due to its symmetry?

• CO
• CO₂ asymmetric stretch
• N₂
• HCl

Correct Answer: N₂

Q19. In practice, IR spectra are often ratioed against a background. A common atmospheric interference that must be corrected is:

• Argon absorption at 3000 cm⁻¹
• CO₂ bands around 2350 cm⁻¹ and H₂O bands near 3700–3500 and 1600 cm⁻¹
• Ozone absorption at 1720 cm⁻¹
• Nitrogen absorption at 2100 cm⁻¹

Correct Answer: CO₂ bands around 2350 cm⁻¹ and H₂O bands near 3700–3500 and 1600 cm⁻¹

Q20. For wavenumber calibration and instrument qualification in mid-IR, a commonly used standard is:

• Holmium oxide filter
• Neon emission lamp
• Polystyrene film with characteristic bands
• Caffeine reference solution

Correct Answer: Polystyrene film with characteristic bands

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