Sample handling in IR MCQs With Answer

Sample handling in IR MCQs With Answer is designed to help M. Pharm students master practical decisions that directly affect data quality in infrared spectroscopy. Beyond theory, reliable IR spectra depend on correct sample preparation, choice of accessories, and control of interferences like moisture and CO2. This quiz focuses on real-world pharmaceutical contexts: selecting window materials, optimizing KBr pellets, using ATR, DRIFTS, and gas cells, choosing solvents and pathlengths, and preventing spectral artifacts. You will test nuanced know-how such as when to use CaF2 vs NaCl, how ATR penetration depth changes, how to prepare thin films, and how to avoid common pitfalls. Each question includes a clear answer to reinforce learning.

Q1. What is the primary reason for purging an FT-IR spectrometer with dry air or nitrogen during sample measurement?

• To remove atmospheric water vapor and CO2 bands that can obscure sample spectra
• To cool the IR source and extend its lifetime
• To prevent detector saturation from high source intensity
• To reduce mechanical vibrations of optical components

Correct Answer: To remove atmospheric water vapor and CO2 bands that can obscure sample spectra

Q2. Which is the best practice for preparing a KBr pellet of a hygroscopic drug powder?

• Dry KBr at 110–120 °C, mix 0.5–1% w/w sample, grind finely, and press under vacuum
• Use a few drops of tap water to aid grinding and transparency
• Increase sample load to 10–20% w/w to obtain stronger bands
• Mix with NaCl instead of KBr to avoid moisture problems

Correct Answer: Dry KBr at 110–120 °C, mix 0.5–1% w/w sample, grind finely, and press under vacuum

Q3. For a typical solid sample analyzed by the KBr pellet method, the recommended sample-to-KBr ratio is approximately:

• 0.5–1% w/w (about 1–2 mg sample with 200 mg KBr)
• 5–10% w/w
• 25–30% w/w
• 50:50 w/w

Correct Answer: 0.5–1% w/w (about 1–2 mg sample with 200 mg KBr)

Q4. Which window material is most suitable for transmission IR measurements of aqueous solutions?

• CaF2 windows with a short pathlength liquid cell
• NaCl windows with a 1 mm spacer
• KBr windows without any spacer
• KI windows with a 0.5 mm spacer

Correct Answer: CaF2 windows with a short pathlength liquid cell

Q5. Which change increases the effective penetration depth in ATR-IR for the same sample and angle?

• Using a lower refractive index crystal (e.g., ZnSe instead of Ge)
• Using a higher angle of incidence far above the critical angle
• Using a shorter wavelength (higher wavenumber)
• Increasing clamp pressure on the ATR crystal

Correct Answer: Using a lower refractive index crystal (e.g., ZnSe instead of Ge)

Q6. Which statement about Nujol and Fluorolube mulls is correct?

• Nujol shows strong C–H bands; Fluorolube strongly absorbs below ~1300 cm⁻¹; choose Nujol to access more of the fingerprint region
• Fluorolube is ideal for the C–H stretching region with no interferences
• Both are water-based media that reduce hygroscopicity effects
• Nujol is preferred for aqueous proteins by transmission IR

Correct Answer: Nujol shows strong C–H bands; Fluorolube strongly absorbs below ~1300 cm⁻¹; choose Nujol to access more of the fingerprint region

Q7. For a neat liquid that absorbs strongly in the mid-IR, which transmission setup is generally recommended?

• Demountable liquid cell with a 0.05–0.1 mm spacer and IR-transparent windows
• 1 cm pathlength cuvette used in UV–Vis
• KBr pellet formed directly from the liquid
• Diffuse reflectance (DRIFTS) sample cup

Correct Answer: Demountable liquid cell with a 0.05–0.1 mm spacer and IR-transparent windows

Q8. A KBr pellet looks cloudy and shows poor transmittance. What is the most likely cause and remedy?

• Trapped moisture and air scattering; dry the KBr and evacuate the die before pressing
• Excessive sample dilution; increase the sample percentage
• Over-polished die surfaces; roughen the die to scatter less
• Detector malfunction; increase detector gain

Correct Answer: Trapped moisture and air scattering; dry the KBr and evacuate the die before pressing

Q9. For quantitative analysis by ATR-IR, which approach is essential?

• Calibrate the effective pathlength using standards or apply ATR correction using refractive indices
• Assume Beer–Lambert law directly with the physical pathlength
• Use only a Ge crystal to ensure linearity
• Collect single-beam spectra without a background

Correct Answer: Calibrate the effective pathlength using standards or apply ATR correction using refractive indices

Q10. Diffuse Reflectance (DRIFTS) is most appropriate when:

• Analyzing highly scattering powders with minimal sample preparation
• Measuring a clear liquid quantitatively in transmission
• Obtaining spectra of trace gases at ppm levels
• Recording spectra of thick polymer slabs by transmission

Correct Answer: Analyzing highly scattering powders with minimal sample preparation

Q11. To monitor a carbonyl band near ~1700 cm⁻¹ in solution IR, a good solvent and setup would be:

• CCl4 or CHCl3 (dry), with a short pathlength liquid cell
• Water or methanol with a 1 mm pathlength
• DMSO because it is polar and IR-transparent across the mid-IR
• Benzene in any pathlength because it has no interfering bands

Correct Answer: CCl4 or CHCl3 (dry), with a short pathlength liquid cell

Q12. For transmission IR of a polymer film, which thickness typically yields well-resolved bands without saturation?

• 10–100 μm thin film
• 1 mm thick slab
• 1 cm thick slab
• Thickness is irrelevant in IR

Correct Answer: 10–100 μm thin film

Q13. Which statement about NaCl windows is correct for IR sample handling?

• They dissolve in water; clean only with dry, water-free solvents and avoid aqueous samples
• They are inert to water and dilute acids, so aqueous samples are fine
• They transmit well into the far-IR below 200 cm⁻¹
• They are harder than quartz and can be sonicated in water for cleaning

Correct Answer: They dissolve in water; clean only with dry, water-free solvents and avoid aqueous samples

Q14. Which accessory is preferred to detect trace-level gases (ppm) by IR spectroscopy?

• Multi-pass long-path gas cell (e.g., White or Herriott cell)
• Standard 10 cm single-pass gas cell
• ATR crystal with diamond tip
• DRIFTS cup

Correct Answer: Multi-pass long-path gas cell (e.g., White or Herriott cell)

Q15. Which is the most suitable approach for measuring aqueous protein solutions in the mid-IR?

• ATR with diamond or ZnSe crystal to tolerate strong water absorption
• Transmission using NaCl windows with a 1 mm spacer
• Preparing a KBr pellet of the solution
• DRIFTS of the wet slurry

Correct Answer: ATR with diamond or ZnSe crystal to tolerate strong water absorption

Q16. Particle size effects in IR of solids primarily cause which artifact, and how can it be minimized?

• Baseline sloping and band distortion due to scattering; minimized by fine grinding
• Increased detector noise; minimized by higher source temperature
• Loss of vibrational modes; minimized by cooling the sample
• Enhanced resolution; minimized by widening the slit

Correct Answer: Baseline sloping and band distortion due to scattering; minimized by fine grinding

Q17. Which ATR crystal material is most chemically robust for strongly acidic or aggressive solvents?

• Diamond
• ZnSe
• KRS-5 (TlBr–TlI)
• Ge

Correct Answer: Diamond

Q18. How can CO2 interference near 2350 cm⁻¹ be minimized during IR measurements?

• Purge the system with dry air or nitrogen and collect a fresh background immediately before the sample
• Increase spectral resolution to 0.5 cm⁻¹
• Use NaCl windows for all samples
• Raise the IR source temperature

Correct Answer: Purge the system with dry air or nitrogen and collect a fresh background immediately before the sample

Q19. If a solvent used for transmission IR strongly absorbs in the analyte region, which handling strategy is most effective?

• Switch to ATR to reduce effective pathlength and choose an alternative spectral window
• Increase the transmission pathlength to 1 mm
• Heat the sample to boil off the solvent during acquisition
• Add water to dilute the solvent bands

Correct Answer: Switch to ATR to reduce effective pathlength and choose an alternative spectral window

Q20. If an API reacts upon intimate grinding with halide matrices like KBr, which alternative sampling approach is preferable?

• Record the spectrum by ATR or prepare a Nujol mull between inert windows instead of a KBr pellet
• Increase the proportion of API in KBr to minimize contact time
• Use NaCl as the pellet matrix and add a drop of water
• Add concentrated HCl to stabilize the API during pelletization

Correct Answer: Record the spectrum by ATR or prepare a Nujol mull between inert windows instead of a KBr pellet

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