Introduction: Sample cells (cuvettes) and optical detectors are central to pharmaceutical analysis using UV-Vis spectrophotometry, HPLC detectors and assay instrumentation. B. Pharm students should understand cuvette materials (glass, quartz, plastic), path length, and solvent compatibility plus detector types: phototube, photomultiplier, photovoltaic cell and silicon photodiode. Key concepts include photoelectric effect, quantum efficiency, responsivity, gain, dark current, biasing, spectral response, linearity and noise — all affecting sensitivity and accuracy in drug analysis. Practical knowledge helps in selecting detectors and cells for low-concentration assays, wavelength ranges and fast kinetics. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which cuvette material is best for accurate UV measurements below 200 nm?
- Glass
- Plastic
- Fused quartz
- Polystyrene
Correct Answer: Fused quartz
Q2. What primary physical effect allows phototubes and photomultipliers to detect light?
- Thermoelectric effect
- Photoelectric effect
- Piezoelectric effect
- Magneto-optic effect
Correct Answer: Photoelectric effect
Q3. Which detector provides intrinsic internal gain through a series of dynodes?
- Photovoltaic cell
- Silicon photodiode (unbiased)
- Photomultiplier tube
- PIN photodiode
Correct Answer: Photomultiplier tube
Q4. A photovoltaic cell generates voltage when illuminated. Its output is highest when operated:
- In short-circuit with zero load
- At open-circuit voltage
- At maximum power point (matched load)
- At reverse bias
Correct Answer: At maximum power point (matched load)
Q5. Which detector is most suitable for low-light, fast single-photon counting in a spectrophotometer?
- Photomultiplier tube
- Photovoltaic cell
- Silicon PIN photodiode at zero bias
- Thermopile detector
Correct Answer: Photomultiplier tube
Q6. In a silicon photodiode, reverse bias is applied primarily to:
- Increase dark current
- Reduce depletion region width
- Increase bandwidth and responsivity by widening the depletion region
- Generate a DC voltage for power
Correct Answer: Increase bandwidth and responsivity by widening the depletion region
Q7. Quantum efficiency (QE) of a detector is defined as:
- The number of photons emitted per electron collected
- The ratio of electrons generated to incident photons
- The detector’s electrical power divided by optical power
- The detector’s noise floor relative to signal
Correct Answer: The ratio of electrons generated to incident photons
Q8. Which detector typically has the lowest dark current at room temperature?
- Photomultiplier tube
- Silicon photodiode (reverse biased)
- Photovoltaic cell (open circuit)
- Thermocouple detector
Correct Answer: Photomultiplier tube
Q9. Responsivity of a photodiode is usually expressed in:
- Ohms
- A/W (amperes per watt)
- Watts per square meter
- Volts per lumen
Correct Answer: A/W (amperes per watt)
Q10. Which factor most directly limits the spectral range of a silicon photodiode?
- Bandgap energy of silicon
- Noise from the amplifier
- Size of the depletion region
- Mechanical packaging
Correct Answer: Bandgap energy of silicon
Q11. For routine UV-Vis measurements of drug solutions at 260 nm, which cuvette is appropriate?
- Polystyrene cuvette
- Standard glass cuvette
- Quartz cuvette
- Acrylic cuvette
Correct Answer: Quartz cuvette
Q12. The gain (multiplication factor) of a photomultiplier is mainly controlled by:
- Wavelength of incident light only
- Voltage applied across dynodes
- Ambient temperature alone
- Size of the photocathode only
Correct Answer: Voltage applied across dynodes
Q13. Which detector is most linear over a wide dynamic range without external gain stages?
- Photomultiplier tube
- Photovoltaic cell (solar cell)
- Reverse-biased silicon photodiode with transimpedance amplifier
- Geiger-mode avalanche photodiode
Correct Answer: Reverse-biased silicon photodiode with transimpedance amplifier
Q14. In spectrophotometric measurement, stray light primarily causes:
- Improved sensitivity at low absorbance
- Reduced baseline drift only
- Underestimation of absorbance, especially at high absorbance values
- No measurable effect
Correct Answer: Underestimation of absorbance, especially at high absorbance values
Q15. Which noise source typically dominates at low light levels in a silicon photodiode detector system?
- Shot noise from signal and dark current
- Johnson noise only from the source resistance
- 1/f flicker noise only
- Ambient acoustic noise
Correct Answer: Shot noise from signal and dark current
Q16. A phototube differs from a photomultiplier chiefly because:
- Phototubes use semiconductor junctions while PMTs use dynodes
- Phototubes have only a single electron-emitting cathode and no internal gain stages
- Phototubes operate in reverse bias like photodiodes
- Phototubes are insensitive to visible light
Correct Answer: Phototubes have only a single electron-emitting cathode and no internal gain stages
Q17. For fast kinetic measurements (microsecond response), the preferred detector among these is:
- Thermopile
- Photomultiplier tube with appropriate electronics
- Photovoltaic cell
- Average-response light meter
Correct Answer: Photomultiplier tube with appropriate electronics
Q18. Which of the following increases the signal-to-noise ratio (SNR) of a photodiode detector system?
- Increasing detector temperature
- Using a wider bandwidth amplifier
- Using a transimpedance amplifier and optimizing bandwidth
- Removing optical filters to increase total light
Correct Answer: Using a transimpedance amplifier and optimizing bandwidth
Q19. Avalanche photodiodes (APDs) differ from ordinary photodiodes by:
- Operating at zero bias only
- Providing internal multiplication via impact ionization under high reverse bias
- Having no dark current
- Being based on metal oxide semiconductors
Correct Answer: Providing internal multiplication via impact ionization under high reverse bias
Q20. Which detector would be least suitable for direct UV detection below 190 nm?
- Fused quartz-window photomultiplier with UV photocathode
- Silicon photodiode designed for UV with MgF2 window
- Standard glass-window photodiode
- Photovoltaic cell with quartz window
Correct Answer: Standard glass-window photodiode
Q21. Dark current in photodiodes is mainly caused by:
- Ambient visible light only
- Thermal generation of carriers in the depletion region
- Photons emitted by the detector
- Mechanical vibrations
Correct Answer: Thermal generation of carriers in the depletion region
Q22. When choosing a cuvette for organic solvent measurements, the most important consideration is:
- Cuvette color only
- Material chemical compatibility and optical transparency at the measurement wavelength
- Price of the cuvette exclusively
- Size of the cuvette lid
Correct Answer: Material chemical compatibility and optical transparency at the measurement wavelength
Q23. Which metric describes the smallest optical power change a detector system can reliably measure?
- Bandwidth
- Noise-equivalent power (NEP)
- Quantum efficiency (QE)
- Responsivity
Correct Answer: Noise-equivalent power (NEP)
Q24. In a PMT, the photocathode material selection primarily affects:
- Mechanical durability only
- Spectral sensitivity and quantum efficiency
- Electrical capacitance exclusively
- Operating temperature range only
Correct Answer: Spectral sensitivity and quantum efficiency
Q25. A flow cell in HPLC UV detection is used because it:
- Increases path length but not reproducibility
- Allows continuous measurement of eluate with controlled path length and low dispersion
- Automatically eliminates baseline noise
- Replaces the detector entirely
Correct Answer: Allows continuous measurement of eluate with controlled path length and low dispersion
Q26. Which of the following improves detector linearity at high light intensities?
- Using an unamplified photovoltaic cell only
- Reducing incident light or using neutral density filters
- Increasing supply voltage to maximum
- Cooling the sample only
Correct Answer: Reducing incident light or using neutral density filters
Q27. The primary advantage of silicon photodiodes over PMTs in many modern instruments is:
- Higher gain without power supply
- Lower cost, compactness and robustness for visible-NIR detection
- Superior single-photon sensitivity in UV
- No need for electronic readout
Correct Answer: Lower cost, compactness and robustness for visible-NIR detection
Q28. Which parameter describes how quickly a detector responds to a step change in light?
- Quantum efficiency
- Rise time or response time
- Dark current
- Spectral responsivity
Correct Answer: Rise time or response time
Q29. A transimpedance amplifier in photodiode circuits is used to:
- Convert output resistance to capacitance
- Convert photocurrent to a usable voltage with low noise
- Supply bias current to the photodiode only
- Generate photons for calibration
Correct Answer: Convert photocurrent to a usable voltage with low noise
Q30. In selecting a detector for trace-level drug analysis in UV, the most critical combined considerations are:
- Detector color and size only
- Sensitivity (NEP, QE), spectral match to analyte absorption, and noise characteristics
- Price and brand exclusively
- Whether it is photovoltaic or photoconductive only
Correct Answer: Sensitivity (NEP, QE), spectral match to analyte absorption, and noise characteristics
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