Instrumentation of IR spectroscopy – sources, monochromators and detectors MCQs With Answer

Introduction

Instrumentation of IR spectroscopy covers the essential components that determine spectral quality: IR sources, monochromators and detectors. B. Pharm students must understand common IR sources (Globar, Nernst glower, lasers), types of monochromators (dispersive prisms/gratings and the Michelson interferometer used in FT‑IR), beam splitters, and detector classes (thermal detectors like bolometers and pyroelectric, and photon detectors like MCT). Key concepts include spectral range (mid‑IR, far‑IR), spectral resolution, throughput, signal‑to‑noise ratio (SNR), sensitivity, NEP and D* (detectivity), and cooling requirements. This knowledge is crucial for drug analysis, formulation studies and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which IR source is a silicon carbide rod commonly known as?

• Globar
• Nernst glower
• Tungsten filament
• CO2 laser

Correct Answer: Globar

Q2. Which instrument component in dispersive IR selects a single wavelength by angle dispersion?

• Beam splitter
• Monochromator (grating/prism)
• Interferometer
• Detector

Correct Answer: Monochromator (grating/prism)

Q3. What is the primary advantage of FT‑IR over dispersive IR?

• Lower spectral resolution
• Higher throughput and multiplex (Fellgett) advantage
• Requires no detectors
• Eliminates the need for beam splitters

Correct Answer: Higher throughput and multiplex (Fellgett) advantage

Q4. Which detector is a common cooled photon detector for mid‑IR with high sensitivity?

• DTGS (Deuterated triglycine sulfate)
• Thermocouple
• MCT (Mercury cadmium telluride)
• Bolometer

Correct Answer: MCT (Mercury cadmium telluride)

Q5. Which beam splitter material is frequently used in mid‑IR FT‑IR spectrometers?

• Silicon
• CaF2
• KBr
• Potassium bromide (KBr)

Correct Answer: Potassium bromide (KBr)

Q6. What does the term D* (detectivity) describe for an IR detector?

• Detector physical size
• Noise equivalent power per unit bandwidth
• Sensitivity normalized to area and bandwidth
• Maximum operating temperature

Correct Answer: Sensitivity normalized to area and bandwidth

Q7. Which thermal detector generates a voltage due to temperature change of a material with temperature-dependent resistance?

• Photodiode
• Thermistor
• MCT
• Pyroelectric detector

Correct Answer: Thermistor

Q8. In a Michelson interferometer, what determines the spectral resolution?

• Beam splitter material only
• Maximum optical path difference (mirror travel)
• Source intensity
• Detector area

Correct Answer: Maximum optical path difference (mirror travel)

Q9. Why are many photon detectors (like MCT) cryogenically cooled?

• To increase beam splitter efficiency
• To reduce thermal noise and improve SNR
• To change the spectral range
• To avoid chemical contamination

Correct Answer: To reduce thermal noise and improve SNR

Q10. Which IR source is best described as a thermal emitter made from rare‑earth oxides and operates at high temperature?

• Globar
• Nernst glower
• Tungsten lamp
• HeNe laser

Correct Answer: Nernst glower

Q11. What is the main role of a monochromator in dispersive IR instruments?

• Split the beam into reference and sample paths
• Select narrow spectral bands by dispersion for measurement
• Cool the detector
• Increase sample absorption

Correct Answer: Select narrow spectral bands by dispersion for measurement

Q12. Which detector type responds to changes in radiation by producing a current through photon excitation across a bandgap?

• Thermopile
• Photoconductive detector (e.g., MCT)
• Bolometer
• Pyroelectric detector

Correct Answer: Photoconductive detector (e.g., MCT)

Q13. Which factor most directly improves signal‑to‑noise ratio (SNR) in IR spectroscopy?

• Reducing spectral resolution regardless of needs
• Increasing source brightness and detector sensitivity
• Using thicker sample cells only
• Removing the beam splitter

Correct Answer: Increasing source brightness and detector sensitivity

Q14. What is NEP (Noise Equivalent Power) a measure of for detectors?

• Wavelength range of the detector
• Power required to produce a signal equal to noise in 1 Hz bandwidth
• Physical durability under high power
• Detector cooling efficiency

Correct Answer: Power required to produce a signal equal to noise in 1 Hz bandwidth

Q15. Which monochromator element disperses light by wavelength through diffraction rather than refraction?

• Prism
• Diffraction grating
• Beam splitter
• Interferometer mirror

Correct Answer: Diffraction grating

Q16. What advantage does a pyroelectric detector have in IR spectroscopy?

• Operates without chopping for DC signals
• Fast response to modulated IR radiation and works at room temperature
• Requires liquid nitrogen cooling
• Is insensitive to blackbody radiation

Correct Answer: Fast response to modulated IR radiation and works at room temperature

Q17. Which window material is commonly avoided for low‑wavenumber far‑IR measurements due to strong absorption?

• Polyethylene
• KBr
• Germanium
• CaF2

Correct Answer: KBr

Q18. In dispersive instruments, what is the main drawback compared to FT‑IR for pharmaceutical analysis?

• Higher throughput
• Slower data acquisition and lower sensitivity
• Better multiplexing advantage
• No need for reference spectra

Correct Answer: Slower data acquisition and lower sensitivity

Q19. Which detector is ideal when measuring very low-energy far‑IR radiation because of its broadband thermal response?

• MCT
• Bolometer
• Photodiode
• DTGS

Correct Answer: Bolometer

Q20. What is the function of a chopper in IR spectroscopy setups?

• Increase source temperature
• Modulate the beam to allow AC detection and reduce low‑frequency noise
• Change the sample pathlength
• Act as a monochromator

Correct Answer: Modulate the beam to allow AC detection and reduce low‑frequency noise

Q21. Which of the following is true about DTGS detectors?

• They require liquid helium cooling
• They are pyroelectric and operate at room temperature
• They are photon detectors based on semiconductor bandgaps
• They have instantaneous response to DC radiation

Correct Answer: They are pyroelectric and operate at room temperature

Q22. What does the term “throughput” (Jacquinot advantage) refer to in FT‑IR context?

• Amount of optical power delivered to detector without narrow slits
• Number of spectral lines the instrument can separate
• Detector cooling power
• Maximum mirror travel speed

Correct Answer: Amount of optical power delivered to detector without narrow slits

Q23. Which source provides highly monochromatic and coherent IR radiation useful for advanced spectroscopy but is less common in routine FT‑IR?

• Globar
• CO2 laser
• Nernst glower
• Tungsten lamp

Correct Answer: CO2 laser

Q24. Why is the choice of detector important for quantitative pharmaceutical IR measurements?

• Detector type determines sample viscosity
• Detector sensitivity, linearity and noise characteristics affect accuracy and LOD
• Detector material changes chemical structure
• Detector only affects cosmetic appearance of instrument

Correct Answer: Detector sensitivity, linearity and noise characteristics affect accuracy and LOD

Q25. What is one disadvantage of using MCT detectors without cooling?

• They become superconducting
• Thermal noise increases, reducing sensitivity
• They expand and change spectral range
• They emit IR radiation

Correct Answer: Thermal noise increases, reducing sensitivity

Q26. In an FT‑IR, what is the primary role of the beam splitter?

• Absorb unwanted frequencies
• Divide and recombine the source beam to produce an interferogram
• Cool the detector
• Act as a sample holder

Correct Answer: Divide and recombine the source beam to produce an interferogram

Q27. Which parameter directly limits the minimum detectable absorbance in an IR experiment?

• Detector spectral range only
• Signal‑to‑noise ratio (SNR)
• Sample color
• Monochromator physical size

Correct Answer: Signal‑to‑noise ratio (SNR)

Q28. Which detector is insensitive to wavelength but measures total power via temperature rise and is often used in broadband IR?

• MCT
• Bolometer
• Photodiode
• Grating

Correct Answer: Bolometer

Q29. What effect does increasing spectral resolution have on acquisition time in FT‑IR?

• Decreases acquisition time
• Requires longer mirror travel and usually increases acquisition time
• Has no effect on time
• Makes data instantly available

Correct Answer: Requires longer mirror travel and usually increases acquisition time

Q30. For routine pharmaceutical mid‑IR analysis, which combination is most commonly used?

• Globar source, dispersive monochromator, bolometer detector
• Nernst glower, prism monochromator, thermocouple detector
• Globar source, Michelson interferometer (FT‑IR), DTGS or MCT detector
• CO2 laser, grating monochromator, photodiode

Correct Answer: Globar source, Michelson interferometer (FT‑IR), DTGS or MCT detector

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com