Sample handling techniques for IR spectroscopy MCQs With Answer

Sample handling techniques for IR spectroscopy MCQs With Answer

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Sample handling techniques for IR spectroscopy are essential for accurate spectral interpretation in pharmaceutical analysis. This introduction reviews practical methods such as KBr pellet preparation, Nujol mulls, attenuated total reflectance (ATR), thin films, liquid cells, and diffuse reflectance, emphasizing steps to avoid contaminants, moisture, and scattering artifacts. Key considerations include choice of window materials (NaCl, KBr, CaF2, ZnSe), sample grinding, dilution, pellet pressure, solvent IR cutoffs, and instrument purging with dry gas. Mastering these techniques improves reproducibility, sensitivity, and quantitative reliability in drug analysis using FTIR. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which sample preparation technique is most suitable for recording high-quality spectra of powdered solids while minimizing sample scattering and achieving uniform dilution?

  • Preparing a KBr pellet by thoroughly mixing finely ground sample with dry KBr
  • Placing the powdered sample directly on a NaCl plate without mixing
  • Mixing the sample with water and air-drying on a glass slide
  • Applying the powder as a thick layer on a metal mirror for reflection

Correct Answer: Preparing a KBr pellet by thoroughly mixing finely ground sample with dry KBr

Q2. What is the primary disadvantage of using Nujol mulls for IR analysis of pharmaceutical solids?

  • They require expensive window materials
  • Nujol shows strong IR absorption bands that can overlap sample signals
  • Mulls eliminate sample heterogeneity completely
  • They are incompatible with ATR accessories

Correct Answer: Nujol shows strong IR absorption bands that can overlap sample signals

Q3. Which IR technique is best for rapid analysis of surface films and requires minimal sample preparation and contact pressure control?

  • KBr pellet transmission
  • Attenuated Total Reflectance (ATR)
  • Nujol mull between salt plates
  • Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) without an accessory

Correct Answer: Attenuated Total Reflectance (ATR)

Q4. Why are KBr pellets typically prepared in a dry atmosphere or using dry KBr?

  • Moisture increases pellet transparency
  • KBr is hygroscopic and absorbs water, causing broad OH bands and scattering
  • Wet KBr enhances the mechanical strength of the pellet
  • Dry KBr shifts absorption bands to higher wavenumbers

Correct Answer: KBr is hygroscopic and absorbs water, causing broad OH bands and scattering

Q5. Which window material should be avoided when analyzing acidic pharmaceutical compounds that may react with the window surface?

  • CaF2
  • ZnSe
  • NaCl
  • KBr

Correct Answer: NaCl

Q6. For liquid samples, what is an advantage of using an IR liquid cell with fixed path length compared to creating a thin film on a salt plate?

  • Liquid cells are cheaper than salt plates
  • Fixed path length allows reliable quantitative measurements using Beer-Lambert law
  • Thin films provide more reproducible path lengths
  • Liquid cells prevent all solvent absorption bands

Correct Answer: Fixed path length allows reliable quantitative measurements using Beer-Lambert law

Q7. When using ATR-FTIR, why is contact pressure between sample and crystal important?

  • Pressure determines the spectral resolution of the instrument
  • Insufficient contact reduces effective penetration depth and signal intensity
  • Too much pressure eliminates all baseline noise
  • Contact pressure changes the refractive index of the crystal permanently

Correct Answer: Insufficient contact reduces effective penetration depth and signal intensity

Q8. Which practice helps reduce spectral artifacts caused by atmospheric CO2 and H2O during IR measurements?

  • Recording spectra at lower spectral resolution
  • Purge the spectrometer with dry nitrogen or dry air and run background scans frequently
  • Always use Nujol mulls to mask atmospheric bands
  • Use thicker KBr pellets to dilute the effect of atmosphere

Correct Answer: Purge the spectrometer with dry nitrogen or dry air and run background scans frequently

Q9. What is the main reason to grind solid samples to a fine powder before mixing with KBr?

  • To increase sample hygroscopicity
  • To reduce scattering and improve homogeneity for better transmission
  • To produce larger particle sizes for DRIFTS
  • To chemically modify the sample for better absorption

Correct Answer: To reduce scattering and improve homogeneity for better transmission

Q10. Which solvent should be avoided when preparing a thin film for IR because it has strong IR absorptions across the mid-IR region?

  • Carbon tetrachloride (CCl4)
  • Chloroform (CHCl3)
  • Water (H2O)
  • Hexane

Correct Answer: Water (H2O)

Q11. When using reflection techniques, what artifact arises from particle size and surface roughness and how can it be minimized?

  • Baseline drift; minimize by increasing spectrometer temperature
  • Mie scattering leading to distorted baseline and band shapes; minimize by reducing particle size and using dilution
  • Excessive absorbance; minimize by using thicker samples
  • Peak sharpening; minimize by increasing particle size

Correct Answer: Mie scattering leading to distorted baseline and band shapes; minimize by reducing particle size and using dilution

Q12. For quantitative FTIR analysis using KBr pellets, why is sample dilution with KBr important?

  • Dilution increases the sample’s refractive index uniformly
  • To keep absorbance within the linear range and avoid saturation of strong bands
  • To make pellets more hygroscopic for better contact
  • Dilution increases chemical reactivity with KBr for clearer peaks

Correct Answer: To keep absorbance within the linear range and avoid saturation of strong bands

Q13. Which precaution is most important when using NaCl windows for liquid sample cells?

  • Avoid acidic or basic solutions that can etch NaCl
  • Always heat NaCl to 200°C before use
  • Use NaCl only for ATR measurements
  • Store NaCl in water to prevent cracking

Correct Answer: Avoid acidic or basic solutions that can etch NaCl

Q14. Why are ZnSe windows preferred for ATR crystals in many pharmaceutical applications?

  • ZnSe is highly hygroscopic and absorbs moisture for better spectra
  • It has a broad spectral range, good mechanical strength, and chemical resistance to many samples
  • ZnSe is inexpensive and disposable after single use
  • It reacts with organic compounds to produce sharper peaks

Correct Answer: It has a broad spectral range, good mechanical strength, and chemical resistance to many samples

Q15. When performing DRIFTS (diffuse reflectance), what sample preparation step improves signal reproducibility for powders?

  • Using large particle sizes and no dilution
  • Diluting with non-absorbing matrix like KBr and thoroughly mixing to achieve uniform particle distribution
  • Applying oil to the powder to enhance reflectance
  • Heating the powder to remove volatile components during measurement

Correct Answer: Diluting with non-absorbing matrix like KBr and thoroughly mixing to achieve uniform particle distribution

Q16. What is the effect of pressing a KBr pellet at excessively high pressure?

  • It always improves spectral resolution without drawbacks
  • It may cause sample decomposition, induce chemical changes, or produce cracks leading to scattering
  • It reduces pellet transparency and increases hygroscopicity beneficially
  • It eliminates the need to dry the pellet before measurement

Correct Answer: It may cause sample decomposition, induce chemical changes, or produce cracks leading to scattering

Q17. For thin polymer films, which preparation technique yields uniform, reproducible IR spectra and avoids solvent residues?

  • Drop-casting a concentrated polymer solution and letting solvent evaporate rapidly at room temperature
  • Spin-coating a polymer solution under controlled conditions and drying under vacuum to remove solvent
  • Smearing polymer powder between salt plates without solvent
  • Heating the polymer above its decomposition temperature on a metal plate

Correct Answer: Spin-coating a polymer solution under controlled conditions and drying under vacuum to remove solvent

Q18. Which internal standard practice helps correct for pellet-to-pellet variation in quantitative KBr pellet IR measurements?

  • Add a volatile compound that will evaporate during pellet pressing
  • Add a chemically inert compound with a non-overlapping IR band as an internal reference for normalization
  • Do not use any internal standard to avoid contamination
  • Use water as an internal standard because it’s always present

Correct Answer: Add a chemically inert compound with a non-overlapping IR band as an internal reference for normalization

Q19. Why is baseline correction often necessary after collecting IR spectra from pellets or mulls?

  • Baseline correction makes peaks disappear for easier interpretation
  • Scattering, inhomogeneity, and instrument drift can cause sloping or curved baselines that must be corrected for accurate peak integration and comparison
  • It increases the wavenumber range of the spectrum
  • Baseline correction is only required for ATR spectra

Correct Answer: Scattering, inhomogeneity, and instrument drift can cause sloping or curved baselines that must be corrected for accurate peak integration and comparison

Q20. Which procedure minimizes contamination of IR windows with grease while handling samples?

  • Handling windows with bare fingers to transfer oils beneficially
  • Wiping windows with organic solvents like ethanol or acetone and using lint-free gloves and tools
  • Rinsing windows in tap water only
  • Using petroleum jelly on windows to improve contact

Correct Answer: Wiping windows with organic solvents like ethanol or acetone and using lint-free gloves and tools

Q21. For measuring aqueous pharmaceutical formulations by IR, which approach reduces strong water absorption interference?

  • Use a very long path length liquid cell
  • Use ATR with a shallow penetration depth and minimize path length to reduce water band dominance
  • Freeze the sample and measure as a solid without changing conditions
  • Mix the sample with KBr and prepare a pellet

Correct Answer: Use ATR with a shallow penetration depth and minimize path length to reduce water band dominance

Q22. When preparing KBr pellets, why is it advisable to store KBr and pellets in a desiccator?

  • To increase pellet brittleness
  • To prevent absorption of atmospheric moisture which causes OH bands and degrades pellet quality
  • To chemically activate KBr for improved spectra
  • To allow KBr to react with CO2 for clearer baselines

Correct Answer: To prevent absorption of atmospheric moisture which causes OH bands and degrades pellet quality

Q23. What is a common method to prepare a microtomed thin section for IR microspectroscopy of tablets or polymers?

  • Embedding the sample in resin and cutting thin sections with a microtome to produce smooth, thin slices for transmission or mapping
  • Grinding into coarse powder and pressing into a thick pellet
  • Smearing the sample on a rough glass slide
  • Heating the sample until it melts and pouring onto a metal plate

Correct Answer: Embedding the sample in resin and cutting thin sections with a microtome to produce smooth, thin slices for transmission or mapping

Q24. In ATR-FTIR, how does the refractive index mismatch between crystal and sample affect the spectrum?

  • It has no measurable effect on ATR spectra
  • Mismatch affects penetration depth and therefore relative band intensities; matching indices improves signal strength and reproducibility
  • Higher mismatch always sharpens all bands equally
  • Refractive index only affects UV spectroscopy, not IR

Correct Answer: Mismatch affects penetration depth and therefore relative band intensities; matching indices improves signal strength and reproducibility

Q25. Why should organic solvents with strong IR absorption regions be avoided when preparing solutions for IR analysis?

  • They increase the sample’s refractive index beneficially
  • Solvent absorption can mask sample bands; choose solvents with minimal mid-IR absorptions or use dilution/evaporation techniques
  • Strongly absorbing solvents enhance sensitivity for all analytes
  • They prevent formation of KBr pellets

Correct Answer: Solvent absorption can mask sample bands; choose solvents with minimal mid-IR absorptions or use dilution/evaporation techniques

Q26. When analyzing hygroscopic pharmaceuticals, which sample handling technique reduces water-related artifacts?

  • Expose the sample to ambient humid air briefly before measurement
  • Dry the sample under vacuum or in a desiccator and handle quickly in low-humidity conditions
  • Mix the hygroscopic sample with water to stabilize it
  • Prepare mulls using water as the mull medium

Correct Answer: Dry the sample under vacuum or in a desiccator and handle quickly in low-humidity conditions

Q27. Which of the following is a valid reason to choose transmission FTIR over ATR for a particular sample?

  • The sample is thin and transparent enough to allow transmission, yielding better quantitative absorbance data with known path length
  • Transmission requires no sample preparation for all samples
  • ATR always gives higher sensitivity for trace analytes
  • Transmission completely eliminates scattering artifacts

Correct Answer: The sample is thin and transparent enough to allow transmission, yielding better quantitative absorbance data with known path length

Q28. What is the main cause of chemical reaction artifacts when preparing pellets or pressing samples, and how can it be avoided?

  • Insufficient pressure; avoid by pressing at maximum pressure
  • Heat and pressure inducing degradation or reaction with KBr; avoid by using gentle pressing, low temperature, or alternative sample holders like ATR
  • Using too fine a powder; avoid by using coarse particles
  • Storing pellets in desiccators; avoid by leaving them in air

Correct Answer: Heat and pressure inducing degradation or reaction with KBr; avoid by using gentle pressing, low temperature, or alternative sample holders like ATR

Q29. For IR imaging or mapping of pharmaceutical tablets, what sample handling consideration improves spatial resolution and spectral quality?

  • Using thick pellets to enhance signal from entire tablet depth
  • Preparing smooth, flat sections (microtomy or polishing) to avoid scattering and ensure uniform contact with the microscope accessory
  • Coating the tablet surface with oil to improve contact
  • Measuring uncut tablets with uneven surfaces for better heterogeneity sampling

Correct Answer: Preparing smooth, flat sections (microtomy or polishing) to avoid scattering and ensure uniform contact with the microscope accessory

Q30. Which routine maintenance and handling practice most reduces the likelihood of contamination and poor spectra in a teaching lab performing many IR measurements?

  • Allowing students to handle windows and crystals without gloves to learn proper technique
  • Regularly cleaning windows and ATR crystals with appropriate solvents, using gloves and clean tools, and performing frequent background checks
  • Never performing background scans to save time
  • Storing all samples on the instrument stage to reduce handling

Correct Answer: Regularly cleaning windows and ATR crystals with appropriate solvents, using gloves and clean tools, and performing frequent background checks

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