Fluorescence in Beverages

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Introduction

Quinine is an alkaloid ocuring naturally in the bark of trees or shrubs of the various species of two Rubiaceous genera, Cinchona and Remijia, indigenous to the higher eastern slopes of the tropical Andes in South America. The medicinal properties of Cinchona bark were first recognized in the seventeenth century and became until 1942 the sole source of quinine, a drug which was for years the only existing specific antimalarial remedy. In recent years however, another alkaloid quinidine, the stereoisomer of quinidine has gained reputation in the treatment of heart conditions.

The use of quinine in carbonated mineral water began in the mid 1800's for combatting malaria, and the practice is still continued today, though not for its antimalarial action but for the slightly bitter taste it imparts to the palate.

In this experiment, the technique of fluorescence spectroscopy will be utilized to determine the percentage quinine content in commercial samples of tonic water/bitter lemon.

Fluorescence Spectroscopy of Quinine

Procedure

1. Obtain six 25 ml volumetric flasks, a 5ml or 10ml microburette and a 50ml burette.
   
   
2. Prepare a standard quinine sulphate solution containing 10 mg of quinine sulphate in 1 litre of deionized distilled water.(provided)
   
   
3. Use the microburette to transfer 0.5, 1.0, 1.5, 2.0, and 2.5 ml of the standard quinine sulphate solution to 25 ml volumetric flasks respectively marked 0.2, 0.4, 0.6, 0.8, and 1.0 ppm.
   
   
4. Using the 50 ml burette, add sufficient distilled water to each flask so that the total volume in each is 12.5 ml, then fill each flask to the mark with 0.2N sulphuric acid to give a final concentration of 0.1N acid for optimal fluorescence. Miix the solution thoroughly.
   
   
5. Obtain a sample of commercial tonic water and prepare a diluted sample as above(see demonstrator for dilution factor)
   
   The Perkin-Elmer LS5OB Luminescence spectrometer is an expensive highly sensitive research instrument and under no circumstances should it be operated without proper supervision! The remainder of the procedure must be carried out under guidance from a demonstrator or technician
   
   
6. Carefully rinse two cuvettes and fill one with the standard/test solution (sample) and the other with 0.1N sulphuric acid (reference)
   
   
7. Carry out measurements of all standards/test solutions with the excitation wavelength setting at 350 nm and the emission wavelength setting at 450 nm, with the appropriate scale expansion for optimum signal/noise ratio.

Plot the fluorescent intensity of each quinine solution against concentration and determine the concentration of the unknown.

Questions

1. Two fluorescent compounds A and B are present in a mixture. Using a spectrofluorimeter similar to the one used above, how would you determine the quantity of A in the mixture if

• A absorbs radiation in a region where B does not.
• Both A and B absorb in the same region but A emits fluorescence at differenf wavelengths than B.

References

1. Fluorescence and Phosphorescence Spectroscopy: Phsiochemical Principles and Practice, Schulman, S.G. Pergamon Press (1979)
2. Qualitative Analytical Chemistry, Schenk, G., Hahn,R., Hartkopf, A., Allyn & Bacon (1978)
3. Chemistry of the Alkaloids, Pelletier, S.W., Nostrand Rainhold (1970)
4. The Merck Index of Chemicals and Drugs, Stecher, P. Seventh Edition Merck & Co. Inc., Rahway, N.J. (1960)

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