CHE 230

Lab #8

Quinine in Tonic Water by Fluorescence Spectroscopy

Objectives:

  • Determine the concentration of quinine in tonic water using fluorescence spectroscopy.
  • Using class data, determine if the concentration of quinine in two different brands of tonic water is statistically different.

Tools of the trade: Techniques:

  • Analytical balance
  • Fluorescence spectrometer
  • Volumetric glassware
/
  • Preparing standards
  • Calibration

Procedure-Preparation:

Note: use accurate volume measuring techniques for standard and sample preparation!!

Hint: Do NOT try to measure out the exact volumes calculated. Instead, use an even volume (like 5.0 mL, rather than 4.87 mL) and just calculate the actual concentration.

  1. Prepare 250 mL of 0.05 M H2SO4 from the more concentrated H2SO4 (2.5 M).
  2. Using a calibrated vial (10 or 20 mL), prepare a stock quinine standard that is approximately 10 ppm in 0.05 M H2SO4 from a quinine solution that is 1.50*10-4M quinine. (What is the exact concentration of the stock standard based on the volume used?)
  3. Using calibrated vials (10 or 20 mL), prepare a series of quinine standards (4 or 5) from 0 – 1000 ppb quinine in 0.05 M H2SO4by dilution of the stock solution that you prepared. (You will use these standards to create the calibration curve. What are the exact concentrations based on the dilutions performed?)
  4. Using calibrated vials (10 or 20 mL), prepare one sample of each brand of tonic water. Degas the sample using the sonicator, and dilute to an appropriate volumein 0.05 M H2SO4(ie, make sure they are in your calibration range; you will have to find the typical concentration of quinine in tonic water to start).

**Make sure your notebook contains specific procedures and calculations!

Procedure-Analysis:

  1. Using an excitation wavelength of 350 nm, scan the emission of a standard from 425-475 nm to find the peak maximum.
  2. Using the READ function, record the emission of each standard and each sample at the peak maximum using an excitation wavelength of 350 nm.
  3. Create a calibration curve by plotting fluorescence intensity versus the concentration of quinine of your standards.
  4. Calculate the concentration of quinine in your samples based on the calibration curve and appropriate dilution factors.

Discussion:

  • What is the historical relevance of quinine?
  • Is quinine a “tonic” or a “toxin”? Explain.
  • What is the structure of quinine? What about its structure makes it fluorescent?
  • Compile the class data to determine if the quinine contents of the two brands of tonic water are the same.
  • Are there any outliers in the data?
  • Comment on the accuracy and precision of the class data.
  • Is your calibration curve based on Beer’s Law? Explain.
  • One of the reasons that fluorescence is so sensitive (relative to absorbance spectroscopy) is that the measurements are taken against “a dark background.” Explain what this means and how it enhances sensitivity.