Instrumentation of dispersive IR spectrometer MCQs With Answer

Instrumentation of dispersive IR spectrometer MCQs With Answer

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Instrumentation of Dispersive IR Spectrometer MCQs With Answers

Infrared (IR) spectroscopy remains a cornerstone in pharmaceutical analysis for identifying functional groups, verifying APIs, and checking polymorphism. This blog focuses on the instrumentation of dispersive IR spectrometers, the classic scanning systems that use prisms or gratings to isolate wavelengths. M. Pharm students will find targeted MCQs that probe practical design choices: sources (Globar, Nernst glower), monochromators (prism/grating), slits, choppers, optical materials, detectors (thermocouple, bolometer, Golay cell, MCT), and calibration. We also cover sample handling, nitrogen purging, double-beam optics, and resolution control. Each question is calibrated for depth and real-world use, helping you connect hardware principles to analytical performance in pharmaceutical quality control and research.

Q1. In a classic dispersive IR spectrometer, which source is most commonly used for the mid-IR region (4000–400 cm⁻1)?

  • Globar (silicon carbide)
  • Tungsten-halogen lamp
  • Deuterium lamp
  • Mercury arc lamp

Correct Answer: Globar (silicon carbide)

Q2. The primary function of the monochromator in a dispersive IR instrument is to:

  • Convert polychromatic IR into interferograms
  • Disperse and select a narrow band of wavelengths for detection
  • Modulate the beam for AC detection
  • Amplify thermal noise in the detector

Correct Answer: Disperse and select a narrow band of wavelengths for detection

Q3. Which prism material is most appropriate for mid-IR monochromators but must be protected from moisture?

  • Potassium bromide (KBr)
  • Fused silica
  • BK7 glass
  • Calcium fluoride (CaF₂)

Correct Answer: Potassium bromide (KBr)

Q4. In a double-beam dispersive IR spectrometer, the component that alternately directs radiation through sample and reference paths is typically a:

  • Michelson interferometer
  • Rotating sector chopper
  • Pockels cell
  • Acousto-optic modulator

Correct Answer: Rotating sector chopper

Q5. Which detector is most commonly used for far-IR measurements in dispersive instruments due to its high sensitivity to long wavelengths?

  • Golay cell
  • Photomultiplier tube (PMT)
  • Silicon photodiode
  • Deuterated L-alanine doped triglycine sulfate (DLATGS)

Correct Answer: Golay cell

Q6. The spectral resolution in a dispersive IR spectrometer is primarily improved by:

  • Increasing exit slit width
  • Decreasing slit widths and using higher-dispersion gratings/prisms
  • Switching to tungsten-halogen source
  • Lowering the detector time constant

Correct Answer: Decreasing slit widths and using higher-dispersion gratings/prisms

Q7. Which optical design is commonly used for the monochromator in dispersive IR instruments due to good throughput and resolution?

  • Littrow cavity interferometer
  • Czerny–Turner grating monochromator
  • Fabry–Pérot etalon
  • Refracting telescope

Correct Answer: Czerny–Turner grating monochromator

Q8. The purpose of order-sorting filters in a grating-based dispersive IR spectrometer is to:

  • Increase detector responsivity
  • Eliminate higher-order overlapping wavelengths
  • Stabilize source intensity
  • Convert transmission to absorbance

Correct Answer: Eliminate higher-order overlapping wavelengths

Q9. Why is nitrogen purging (or dry air purging) often used in dispersive IR instruments?

  • To increase thermal noise for better modulation
  • To prevent detector saturation by cooling it
  • To remove water vapor and CO₂ that introduce strong gas-phase absorptions
  • To reduce the grating groove density

Correct Answer: To remove water vapor and CO₂ that introduce strong gas-phase absorptions

Q10. Which window material is most suitable for far-IR sample cells?

  • Polyethylene (PE)
  • Quartz
  • Sapphire
  • Borosilicate glass

Correct Answer: Polyethylene (PE)

Q11. The Nernst glower IR source is composed primarily of:

  • Silicon carbide
  • Rare-earth oxides
  • Tungsten filament
  • Mercury vapor

Correct Answer: Rare-earth oxides

Q12. In dispersive IR, wavenumber calibration is commonly verified using the well-defined bands of:

  • Polystyrene film
  • Holmium oxide glass
  • Caffeine standard
  • Potassium dichromate solution

Correct Answer: Polystyrene film

Q13. Compared to prism monochromators, diffraction gratings in dispersive IR typically offer:

  • Lower throughput and poorer dispersion
  • Higher, more uniform dispersion and better mechanical stability
  • No higher-order diffraction issues
  • Immunity to stray light

Correct Answer: Higher, more uniform dispersion and better mechanical stability

Q14. The effective spectral bandwidth (SBW) in a dispersive IR spectrometer is determined by the slit width and the:

  • Detector bias voltage
  • Source temperature
  • Monochromator dispersion (nm/mm or cm⁻¹/mm)
  • Sample thickness

Correct Answer: Monochromator dispersion (nm/mm or cm⁻¹/mm)

Q15. Which mirror coating is preferred in IR optics due to high reflectance across mid-IR wavelengths?

  • Aluminum (unprotected)
  • Gold
  • Silver with oxide overcoat
  • Dielectric multilayer for visible region

Correct Answer: Gold

Q16. For mid-IR work in dispersive instruments, which detector provides high sensitivity but typically requires cooling?

  • Mercury cadmium telluride (MCT)
  • Photomultiplier tube (PMT)
  • Silicon PIN photodiode
  • Golay cell

Correct Answer: Mercury cadmium telluride (MCT)

Q17. In double-beam dispersive IR, the key advantage over single-beam is:

  • Elimination of the need for a detector
  • Real-time correction for source drift and baseline fluctuations
  • Higher grating groove density
  • Ability to measure in the UV region

Correct Answer: Real-time correction for source drift and baseline fluctuations

Q18. Which sample handling approach is most compatible with mid-IR dispersive instruments for analyzing solid APIs?

  • KBr pellet pressing
  • Quartz cuvette with water solvent
  • Glass microcell
  • Steel microcapillary

Correct Answer: KBr pellet pressing

Q19. The scan mechanism in a dispersive IR spectrometer changes wavelength by:

  • Translating an interferometer mirror
  • Rotating the grating or prism via a synchronous drive
  • Changing detector bias dynamically
  • Switching the source on and off at set intervals

Correct Answer: Rotating the grating or prism via a synchronous drive

Q20. To minimize stray light and improve baseline quality in a dispersive IR instrument, designers commonly use:

  • Wide slits and high lamp current
  • Blackened baffles, optimized slit geometry, and well-aligned enclosures
  • Transparent housings and larger beam diameters
  • Uncoated glass mirrors

Correct Answer: Blackened baffles, optimized slit geometry, and well-aligned enclosures

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