Quenching and quenchers MCQs With Answer

Quenching and quenchers MCQs With Answer

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Introduction

For M. Pharm students specializing in Modern Pharmaceutical Analytical Techniques, understanding quenching and quenchers is essential for mastering fluorescence, phosphorescence, chemiluminescence, and liquid scintillation analyses. Quenching processes influence signal intensity and lifetimes, affecting sensitivity, selectivity, and quantitative accuracy across drug assay, biomolecular interaction studies, and radiopharmaceutical analysis. This quiz set deepens your grasp of dynamic and static quenching, Stern–Volmer relationships, inner-filter effects, oxygen and heavy-atom quenching, viscosity and temperature implications, Förster resonance energy transfer (FRET), and liquid scintillation quench assessment. Each MCQ is designed to probe concept, mechanism, and application—helping you diagnose experimental artifacts, choose appropriate quenchers, interpret plots, and apply corrections confidently in pharmaceutical analytics.

Q1. Which statement best defines dynamic (collisional) quenching in fluorescence spectroscopy?

  • Quenching due to ground-state complex formation between fluorophore and quencher
  • Apparent intensity loss caused by reabsorption of emitted light at high absorbance
  • Quenching arising from radiative energy transfer to a distant acceptor without collision
  • Nonradiative deactivation from diffusive encounters with quencher while the fluorophore is in the excited state

Correct Answer: Nonradiative deactivation from diffusive encounters with quencher while the fluorophore is in the excited state

Q2. Which hallmark distinguishes static quenching from dynamic quenching?

  • Fluorescence lifetime remains unchanged, but a ground-state complex alters the absorption spectrum
  • Fluorescence lifetime decreases with quencher concentration; absorption spectrum remains unchanged
  • Both lifetime and absorption spectrum remain unchanged
  • Fluorescence lifetime increases upon quenching due to complex formation

Correct Answer: Fluorescence lifetime remains unchanged, but a ground-state complex alters the absorption spectrum

Q3. In the Stern–Volmer relation for dynamic quenching (F0/F = 1 + KSV[Q]), the slope equals:

  • The intrinsic radiative rate constant (kr)
  • The fluorescence quantum yield (ΦF)
  • KSV = kqτ0, the product of the bimolecular quenching rate and the unquenched lifetime
  • 1/τ0, the inverse of the unquenched lifetime

Correct Answer: KSV = kqτ0, the product of the bimolecular quenching rate and the unquenched lifetime

Q4. How does temperature typically affect quenching constants?

  • Both dynamic and static quenching constants increase with temperature
  • Both dynamic and static quenching constants decrease with temperature
  • Dynamic quenching increases with temperature (faster diffusion); static quenching often decreases (weaker complex)
  • Dynamic quenching decreases, static quenching increases with temperature

Correct Answer: Dynamic quenching increases with temperature (faster diffusion); static quenching often decreases (weaker complex)

Q5. Which quencher is most commonly used to probe solvent exposure of tryptophan residues in proteins via dynamic quenching?

  • Acrylamide
  • Anthracene
  • Rhodamine B
  • Terbium(III)

Correct Answer: Acrylamide

Q6. Molecular oxygen (O₂) quenches fluorescence primarily by:

  • Forming a strong ground-state complex at low temperatures
  • Radiative energy transfer to acceptor states
  • Collisional (dynamic) quenching through spin-allowed interactions with triplet character
  • Increasing solvent refractive index

Correct Answer: Collisional (dynamic) quenching through spin-allowed interactions with triplet character

Q7. Which experimental observation most clearly differentiates dynamic quenching from static quenching?

  • Decrease in absorbance at the excitation wavelength
  • Decrease in fluorescence lifetime with unchanged absorption spectrum (dynamic quenching)
  • Increase in fluorescence lifetime with quencher concentration
  • Appearance of a new emission band

Correct Answer: Decrease in fluorescence lifetime with unchanged absorption spectrum (dynamic quenching)

Q8. In FRET viewed from the donor fluorophore, quenching manifests as:

  • No change in donor lifetime but reduced intensity
  • Reduced donor intensity and reduced donor lifetime due to an additional nonradiative decay pathway
  • Increased donor lifetime with reduced intensity
  • Reduced acceptor intensity and increased donor intensity

Correct Answer: Reduced donor intensity and reduced donor lifetime due to an additional nonradiative decay pathway

Q9. Which is NOT a true quenching mechanism but can mimic quenching in steady-state measurements?

  • Collisional quenching by halide ions
  • Static quenching via ground-state complex formation
  • Inner-filter effect caused by high absorbance at excitation/emission wavelengths
  • Energy transfer to acceptor dyes

Correct Answer: Inner-filter effect caused by high absorbance at excitation/emission wavelengths

Q10. What Stern–Volmer plot behavior suggests combined static and dynamic quenching?

  • Linear plot of F/F0 versus [Q] through origin
  • Linear plot of F0/F versus [Q] with zero intercept
  • Upward curvature in F0/F versus [Q] (concave toward the y-axis)
  • Downward curvature in τ0/τ versus [Q]

Correct Answer: Upward curvature in F0/F versus [Q] (concave toward the y-axis)

Q11. Which pair exemplifies classic heavy-atom quenchers used in fluorescence studies?

  • NaCl and KNO₃
  • KI and KBr
  • MgSO₄ and CaCl₂
  • LiF and NaF

Correct Answer: KI and KBr

Q12. Which ion is a potent paramagnetic quencher of fluorescence via enhanced spin–orbit coupling?

  • Zn²⁺
  • Na⁺
  • Cu²⁺
  • Mg²⁺

Correct Answer: Cu²⁺

Q13. The diffusion-controlled upper limit for the bimolecular quenching rate constant (kq) in aqueous solution at room temperature is approximately:

  • 10⁶ M⁻¹ s⁻¹
  • 10⁸ M⁻¹ s⁻¹
  • 10¹⁰ M⁻¹ s⁻¹
  • 10¹² M⁻¹ s⁻¹

Correct Answer: 10¹⁰ M⁻¹ s⁻¹

Q14. In liquid scintillation counting (LSC), tSIE is a quench-indicating parameter. It stands for:

  • Total Scintillation Index of Efficiency
  • Transformed Spectral Index of the External standard
  • Time-normalized Scintillation Intensity Error
  • Threshold Spectral Integration Energy

Correct Answer: Transformed Spectral Index of the External standard

Q15. Which statement best defines chemical quenching in LSC?

  • Reduction in light yield due to molecular interactions that deactivate excited solvent/fluor states before photon emission
  • Loss of counts solely due to color absorbing emitted photons
  • Electronic noise from the photomultiplier tubes
  • Coincidence loss at high count rates

Correct Answer: Reduction in light yield due to molecular interactions that deactivate excited solvent/fluor states before photon emission

Q16. If τ0 = 5 ns and KSV = 12 M⁻¹ from a Stern–Volmer plot, what is kq?

  • 2.4 × 10⁷ M⁻¹ s⁻¹
  • 2.4 × 10⁹ M⁻¹ s⁻¹
  • 6.0 × 10⁹ M⁻¹ s⁻¹
  • 2.4 × 10¹⁰ M⁻¹ s⁻¹

Correct Answer: 2.4 × 10⁹ M⁻¹ s⁻¹

Q17. The Perrin (sphere-of-action) model of static quenching predicts which relationship?

  • F0/F = 1 + KSV[Q]
  • F/F0 = 1 − KSV[Q]
  • F0/F = exp(V[Q]) where V is the quenching volume
  • τ0/τ = 1 + Ks[Q]

Correct Answer: F0/F = exp(V[Q]) where V is the quenching volume

Q18. How does solvent viscosity influence dynamic quenching?

  • Higher viscosity increases diffusion and hence increases kq
  • Higher viscosity slows diffusion, reducing kq and KSV
  • Viscosity affects static but not dynamic quenching
  • Viscosity primarily changes radiative rate constants

Correct Answer: Higher viscosity slows diffusion, reducing kq and KSV

Q19. Which practice most effectively minimizes oxygen quenching in phosphorescence or fluorescence measurements?

  • Using a shorter excitation wavelength
  • Deoxygenation by nitrogen/argon purging or freeze–pump–thaw cycles
  • Adding sodium chloride to increase ionic strength
  • Raising the sample temperature

Correct Answer: Deoxygenation by nitrogen/argon purging or freeze–pump–thaw cycles

Q20. Which observation most strongly supports static quenching due to complex formation?

  • No change in absorption spectrum upon adding quencher
  • Appearance of a new or shifted absorption band and reduced fluorescence without lifetime change
  • Reduced fluorescence accompanied by reduced lifetime
  • Linear τ0/τ versus [Q] with slope equal to zero

Correct Answer: Appearance of a new or shifted absorption band and reduced fluorescence without lifetime change

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