Factors affecting fluorescence MCQs With Answer

Factors affecting fluorescence MCQs With Answer

by

Factors Affecting Fluorescence MCQs With Answer

Fluorescence-based methods are central to Modern Pharmaceutical Analytical Techniques due to their exceptional sensitivity and selectivity. However, fluorescence intensity and spectra are highly dependent on the physicochemical environment and instrumental conditions. This MCQ set helps M. Pharm students deepen their understanding of the factors that modulate fluorescence, including molecular structure, solvent polarity, pH, temperature, concentration, quenchers (oxygen, halides, heavy atoms), micellar effects, viscosity, and common instrumental parameters. You will also test your grasp of mechanistic concepts such as inner filter effects, static vs dynamic quenching, Stern–Volmer analysis, FRET, excimer formation, and spectral interferences like Raman scattering. Each question is designed to reinforce both conceptual insights and practical considerations in quantitative fluorimetric analysis.

Q1. Which molecular feature most strongly favors high fluorescence quantum yield in organic molecules?

  • Rigid, conjugated aromatic systems with restricted intramolecular rotation
  • Saturated aliphatic chains with multiple single bonds
  • Highly flexible molecules with many rotatable single bonds
  • Molecules dominated by n→π* transitions (e.g., isolated carbonyls)

Correct Answer: Rigid, conjugated aromatic systems with restricted intramolecular rotation

Q2. How do strong electron-withdrawing groups (e.g., –NO2) typically affect fluorescence of aromatic compounds?

  • They generally enhance fluorescence by increasing radiative decay
  • They often quench fluorescence by promoting nonradiative pathways (e.g., charge-transfer, ISC)
  • They have no effect on fluorescence
  • They invariably shift emission to shorter wavelengths but keep intensity constant

Correct Answer: They often quench fluorescence by promoting nonradiative pathways (e.g., charge-transfer, ISC)

Q3. Increasing solvent polarity for a polar fluorophore commonly causes which spectral change?

  • A bathochromic (red) shift of the emission maximum due to stabilization of the excited state
  • A hypsochromic (blue) shift of the emission maximum
  • No change in emission wavelength, only intensity changes
  • Disappearance of fluorescence in all cases

Correct Answer: A bathochromic (red) shift of the emission maximum due to stabilization of the excited state

Q4. How does pH influence fluorescence of ionizable fluorophores?

  • pH only affects absorbance, not fluorescence
  • pH can change ionization state, altering absorption/emission spectra and quantum yield
  • Fluorescence increases linearly with pH regardless of structure
  • Fluorescence is independent of pH in aqueous media

Correct Answer: pH can change ionization state, altering absorption/emission spectra and quantum yield

Q5. What is the usual effect of increasing temperature on fluorescence intensity of most organic fluorophores?

  • Intensity increases due to enhanced radiative decay
  • Intensity decreases due to enhanced collisional (nonradiative) deactivation
  • No change in intensity
  • Intensity oscillates with temperature

Correct Answer: Intensity decreases due to enhanced collisional (nonradiative) deactivation

Q6. Dissolved oxygen reduces fluorescence primarily through which mechanism?

  • Static quenching via ground-state complex formation
  • Dynamic collisional quenching facilitated by oxygen’s paramagnetism and triplet interactions
  • Enhancement of radiative transitions
  • Formation of fluorescent excimers

Correct Answer: Dynamic collisional quenching facilitated by oxygen’s paramagnetism and triplet interactions

Q7. The external heavy-atom effect (e.g., due to iodide or bromide) generally leads to:

  • Increased fluorescence quantum yield by suppressing intersystem crossing
  • Decreased fluorescence by enhancing spin–orbit coupling and intersystem crossing
  • No effect on fluorescence processes
  • Only a shift in emission wavelength, intensity remains unchanged

Correct Answer: Decreased fluorescence by enhancing spin–orbit coupling and intersystem crossing

Q8. The general order of halide quenching efficiency for many fluorophores is:

  • F− > Cl− > Br− > I−
  • I− > Br− > Cl− > F−
  • Cl− > I− > F− > Br−
  • Br− > I− > Cl− > F−

Correct Answer: I− > Br− > Cl− > F−

Q9. Which is the primary reason fluorimetric calibration curves deviate from linearity at high analyte concentrations?

  • Detector saturation independent of sample absorbance
  • Inner filter effect and self-absorption due to high absorbance at excitation/emission wavelengths
  • Thermal lensing of the sample
  • Increased photobleaching resistance

Correct Answer: Inner filter effect and self-absorption due to high absorbance at excitation/emission wavelengths

Q10. For right-angle fluorescence measurements, an approximate inner-filter correction uses which relationship?

  • Fcorr = Fobs × 10^((Aex + Aem)/2)
  • Fcorr = Fobs × 10^(Aex − Aem)
  • Fcorr = Fobs × 10^(Aem)
  • Fcorr = Fobs × 10^(−(Aex + Aem))

Correct Answer: Fcorr = Fobs × 10^((Aex + Aem)/2)

Q11. In purely dynamic (collisional) quenching, the Stern–Volmer plot shows:

  • F0/F vs [Q] is linear with an intercept of 1
  • F0/F vs [Q] is curved upward with zero intercept
  • F vs [Q] is linear with an intercept of zero
  • Lifetime increases with [Q]

Correct Answer: F0/F vs [Q] is linear with an intercept of 1

Q12. Which observation is most diagnostic of static quenching rather than dynamic quenching?

  • Fluorescence lifetime remains unchanged while steady-state intensity decreases with quencher
  • Both intensity and lifetime decrease identically with quencher
  • Emission maximum undergoes a red shift
  • Quantum yield increases with quencher concentration

Correct Answer: Fluorescence lifetime remains unchanged while steady-state intensity decreases with quencher

Q13. How does increasing solution viscosity generally affect fluorescence for rotor-like or flexible fluorophores?

  • Decreases fluorescence by promoting internal conversion
  • Increases fluorescence by restricting intramolecular rotations (reducing nonradiative decay)
  • No effect on fluorescence
  • Eliminates excimer formation

Correct Answer: Increases fluorescence by restricting intramolecular rotations (reducing nonradiative decay)

Q14. What is the typical effect of anionic or nonionic micelles on the fluorescence of hydrophobic probes?

  • Strong quenching due to increased polarity
  • Enhanced fluorescence by providing a less polar, protected microenvironment
  • No change because micelles do not interact with fluorophores
  • Complete suppression due to static quenching in the micellar core

Correct Answer: Enhanced fluorescence by providing a less polar, protected microenvironment

Q15. Increasing the monochromator slit width in a spectrofluorimeter primarily:

  • Decreases signal and improves spectral resolution
  • Increases signal but reduces spectral resolution and may raise stray light
  • Has no effect on measured intensity
  • Shifts the emission maximum to longer wavelengths

Correct Answer: Increases signal but reduces spectral resolution and may raise stray light

Q16. Which action commonly reduces photobleaching during fluorescence measurements?

  • Increasing excitation intensity and acquisition time
  • Deoxygenating the solution and minimizing excitation exposure
  • Raising the temperature
  • Adding heavy-atom salts

Correct Answer: Deoxygenating the solution and minimizing excitation exposure

Q17. In emission spectra of aqueous samples, which statement best describes Raman scattering from the solvent?

  • It appears at a fixed emission wavelength regardless of excitation wavelength
  • It shifts with excitation wavelength but maintains a nearly constant frequency shift (in cm−1)
  • It always overlaps the analyte’s emission maximum
  • It only occurs in nonpolar solvents

Correct Answer: It shifts with excitation wavelength but maintains a nearly constant frequency shift (in cm−1)

Q18. Which change increases FRET (Förster resonance energy transfer) efficiency from a donor to an acceptor?

  • Increasing donor–acceptor distance
  • Increasing spectral overlap between donor emission and acceptor absorption
  • Decreasing donor quantum yield
  • Randomizing dipole orientation to κ² ≈ 0

Correct Answer: Increasing spectral overlap between donor emission and acceptor absorption

Q19. For highly absorbing or turbid samples, the preferred measurement geometry to minimize reabsorption and scattering artifacts is:

  • Right-angle (90°) geometry with standard cuvettes
  • Front-face illumination/detection geometry
  • Colinear transmission geometry
  • Integrating sphere with reflective interior

Correct Answer: Front-face illumination/detection geometry

Q20. Which measurement most directly distinguishes dynamic from static quenching mechanisms?

  • Steady-state emission intensity only
  • Absorption spectrum of the donor only
  • Time-resolved fluorescence lifetime measurement
  • Measurement of Raman scattering intensity

Correct Answer: Time-resolved fluorescence lifetime measurement

Leave a Comment