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Worksheet: UV-Vis Sample Preparation

Q1:

When a sample is analyzed by UV-Vis spectroscopy, the results may be unreliable if the signal-to-noise ratio is too low. Which of the following is not a method for avoiding this issue?

  • AUsing a cuvette with a shorter path length
  • BDissolving more of the analyte in the solvent
  • CPre-concentrating the analyte solution
  • DUsing a solvent with a lower UV cutoff
  • EIncreasing the acquisition time for each absorbance measurement

Q2:

𝛽 -carotene is a highly absorbing chromophore. Which solvent would be the best choice for analysis?

  • ABenzene
  • BMethylene bromide
  • CHexyne
  • DEthanol

Q3:

A sample of 0.10 M triethylamine in hexane is prepared for analysis. Which material should be used as the blank reference?

  • ATriethylamine
  • B 0.10 M triethylamine in water
  • CWater
  • DHexane
  • E Air

Q4:

Which of the following would be the best solvent for a compound with a 𝜆 m a x of 281 nm? You may assume that the compound is completely soluble in each.

  • ABenzene (UV cutoff = 278 nm)
  • BToluene (UV cutoff = 285 nm)
  • CAcetone (UV cutoff = 329 nm)
  • DEthanol (UV cutoff = 205 nm)

Q5:

When preparing a calibration curve for sample analysis, which of the following is best practice?

  • AOnly use two data points to ensure a straight line.
  • BPrepare each sample “from scratch” to avoid cross contamination.
  • CUse very concentrated solutions to ensure a good signal.
  • DPrepare a stock solution and use serial dilution for the remaining standards.

Q6:

Why must solutions with high concentrations be diluted prior to analysis via Beer’s Law?

  • AThe photon source is too weak to provide accurate results.
  • BThe detector will reach its detection threshhold.
  • CThe molar absorptivity of a compound is dependent on its concentration.
  • DThe relationship between absorbance and concentration is not linear at high concentrations.
  • EThere is no need to work with dilute concentrations; any concentration will work.