Theory of UV–Visible spectroscopy MCQs With Answer

Theory of UV–Visible spectroscopy MCQs With Answer offers a focused practice resource for M. Pharm students studying Modern Pharmaceutical Analytical Techniques. This set revisits core principles that underpin qualitative and quantitative UV–Vis analyses, including electronic transitions, Beer–Lambert law, chromophore–auxochrome interactions, solvent effects, and instrumental variables such as sources, detectors, slit width, stray light, and spectral bandwidth. It also emphasizes practical aspects like cuvette selection, solvent cut-off, calibration strategies, and advanced tools such as derivative spectroscopy, diode arrays, and double-beam optics. Each question is designed to strengthen conceptual depth and analytical reasoning, aligning with pharmaceutical contexts like assay development, kinetic studies, and impurity profiling. Use these MCQs to diagnose gaps and sharpen exam readiness.

Q1. What is the correct form of the Beer–Lambert law relating absorbance to concentration?

• A = ε b c
• A = T × 100
• A = log c + b
• A = ε / (b c)

Correct Answer: A = ε b c

Q2. Which electronic transition in organic molecules typically exhibits the highest molar absorptivity (ε)?

• n → σ*
• π → π*
• n → π*
• σ → σ*

Correct Answer: π → π*

Q3. A bathochromic shift (red shift) in UV–Vis spectra refers to which change?

• Shift to shorter wavelength with decreased intensity
• Shift to longer wavelength with decreased intensity
• Shift to longer wavelength (increase in λmax)
• Decrease in absorbance without wavelength change

Correct Answer: Shift to longer wavelength (increase in λmax)

Q4. The presence of an isosbestic point in a set of spectra most strongly indicates:

• A three-component equilibrium with constant total concentration
• A simple interconversion between two species with constant total concentration
• Instrumental drift during scanning
• Solvent cut-off interference

Correct Answer: A simple interconversion between two species with constant total concentration

Q5. The most common continuous radiation source for the UV region (≈190–350 nm) is:

• Tungsten–halogen lamp
• Deuterium discharge lamp
• Xenon flash lamp only
• Infrared globar source

Correct Answer: Deuterium discharge lamp

Q6. Increasing the spectral bandwidth (wider slit width) relative to the sample’s natural band width primarily causes:

• Improved resolution and higher apparent peak absorbance
• Loss of resolution and underestimation of sharp peak maxima
• No effect on resolution but increased signal-to-noise
• Blue shift of λmax without intensity change

Correct Answer: Loss of resolution and underestimation of sharp peak maxima

Q7. A common chemical cause of deviation from Beer’s law at high analyte concentrations is:

• Perfect collimation of the beam
• Solute–solute interactions altering activity and refractive index
• Use of quartz cuvettes
• Monochromatic radiation

Correct Answer: Solute–solute interactions altering activity and refractive index

Q8. For recording spectra down to ≈190 nm, which solvent is most suitable due to its low cut-off?

• Ethanol
• Acetonitrile
• Chloroform
• Toluene

Correct Answer: Acetonitrile

Q9. For UV measurements below 300 nm, the recommended cuvette material is:

• Soda–lime glass
• Acrylic (polymethyl methacrylate)
• Quartz (fused silica)
• Polycarbonate

Correct Answer: Quartz (fused silica)

Q10. Hydrogen bonding/protic solvents generally affect the n → π* transition of carbonyls by:

• Bathochromic and hyperchromic shifts
• Hypsochromic and hypochromic shifts
• No change in λmax or intensity
• Bathochromic shift with no intensity change

Correct Answer: Hypsochromic and hypochromic shifts

Q11. The SI-compatible units of molar absorptivity (ε) are:

• cm−1
• L mol−1 cm−1
• mol L−1 cm
• au (arbitrary units)

Correct Answer: L mol−1 cm−1

Q12. Which detector provides very high sensitivity for low-light UV–Vis measurements in scanning instruments?

• Photomultiplier tube (PMT)
• Thermocouple
• Bolometer
• Ionization chamber

Correct Answer: Photomultiplier tube (PMT)

Q13. Stray light in a spectrophotometer most notably leads to:

• Overestimation of absorbance at high A values
• Underestimation of absorbance and compression of dynamic range
• Constant baseline shift without affecting linearity
• Increased spectral resolution

Correct Answer: Underestimation of absorbance and compression of dynamic range

Q14. A key analytical advantage of derivative spectrophotometry is:

• Enhanced light throughput compared to zero-order spectra
• Resolution of overlapping bands and improved baseline correction
• Removal of all noise contributions
• Elimination of solvent effects

Correct Answer: Resolution of overlapping bands and improved baseline correction

Q15. Compared to single-beam instruments, double-beam spectrophotometers primarily help by:

• Eliminating the need for a reference solvent
• Compensating source intensity drift and solvent/baseline variations in real time
• Making cuvettes unnecessary
• Increasing path length automatically

Correct Answer: Compensating source intensity drift and solvent/baseline variations in real time

Q16. Regarding selection rules and intensities, which statement is most accurate for organic chromophores?

• n → π* transitions are strongly allowed and have large ε
• π → π* transitions are typically allowed and show larger ε than n → π*
• σ → σ* transitions always occur in the visible region
• Spin-forbidden transitions always dominate spectra

Correct Answer: π → π* transitions are typically allowed and show larger ε than n → π*

Q17. In a calibration plot of absorbance (A) vs concentration (c) using a 1.00 cm cuvette, the slope corresponds to:

• b
• ε
• 1/ε
• ε b c

Correct Answer: ε

Q18. Which modification most effectively produces a bathochromic shift in an organic molecule’s UV–Vis spectrum?

• Breaking conjugation by hydrogenation
• Extending conjugation (e.g., adding double bonds)
• Switching to a high cut-off solvent
• Decreasing path length

Correct Answer: Extending conjugation (e.g., adding double bonds)

Q19. For opaque or highly scattering solid samples, the appropriate UV–Vis measurement mode is:

• Transmission through a standard cuvette
• Diffuse reflectance using an integrating sphere
• Emissive photoluminescence mode
• Conductivity measurement

Correct Answer: Diffuse reflectance using an integrating sphere

Q20. When significant matrix effects alter the apparent slope of a Beer’s law calibration, the preferred quantitation strategy is:

• External standard calibration with water blanks
• Standard addition method
• Ignoring the matrix and increasing path length
• Using absorbance at the solvent cut-off

Correct Answer: Standard addition method

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