Spectrophotometry Knowledge-Based Practice

Questions

1. 1.

   In absorption spectrophotometry, how does stray light reaching the detector affect the measured absorbance of a highly absorbing sample?

   • A) It causes an apparent negative deviation, producing a measured absorbance that is lower than the true absorbance.
   • B) It causes an apparent positive deviation, producing a measured absorbance that is higher than the true absorbance.
   • C) It has no effect on the measured absorbance because the detector automatically filters out unabsorbed stray photons.
   • D) It increases the apparent molar absorptivity of the analyte proportionally as the solution concentration rises.
2. 2.

   Why does the Beer–Lambert law typically fail to maintain linearity at analyte concentrations exceeding approximately 0.01 M?

   • A) High concentrations cause the effective path length of the cuvette to contract due to increased solution density.
   • B) Solvent molecules undergo accelerated photodecomposition that increases background transmittance.
   • C) High concentrations cause electrostatic interactions among neighboring absorbing particles, altering their electron distribution and molar absorptivity.
   • D) Monochromator diffraction gratings experience thermal drift caused by radiation re-emitted from concentrated samples.
3. 3.

   What is the primary function of a reagent blank in quantitative absorption spectrophotometry?

   • A) To calibrate the wavelength accuracy of the monochromator across the operating spectral range.
   • B) To amplify the intensity of the incident radiation before it impinges on the detector.
   • C) To compensate for background absorbance and optical attenuation caused by the solvent, reagents, and sample cell.
   • D) To chemically stabilize the analyte against photochemical decomposition during light exposure.
4. 4.

   In quantitative absorption spectrophotometry, why is measurement typically conducted at the analyte's wavelength of maximum absorbance ($\lambda_{\max}$)?

   • A) It completely eliminates light reflection and scattering at the cuvette interfaces.
   • B) It ensures that the molar absorptivity coefficient remains zero across all concentration levels.
   • C) It eliminates the necessity of using an optical blank to set the baseline absorbance.
   • D) It yields the highest measurement sensitivity and minimizes deviations from Beer's law caused by slight wavelength inaccuracy.
5. 5.

   Why must fused quartz or silica cuvettes be used instead of standard optical glass cuvettes when performing spectrophotometric measurements in the ultraviolet region below 350 nm?

   • A) Optical glass scatters visible wavelengths and causes severe diffraction within the sample compartment.
   • B) Fused quartz alters the molar absorptivity of the analyte by stabilizing excited electronic states.
   • C) Optical glass strongly absorbs radiation in the ultraviolet region, preventing sufficient light from reaching the detector.
   • D) Fused quartz selectively filters stray light emitted by the deuterium discharge lamp.

Answers and step-by-step solutions appear after you finish the test.