Analytical Chemistry Trinidad & 

Tobago Lab Resources

c30f analytical chemistry labscript

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Determination of quinine in commercial soft drink
by molecular fluorescence spectroscopy

1. 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 (1). The medicinal properties of Cinchona bark were first recognized in the seventeenth century and 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 been used in the treatment of heart conditions (2).

The use of quinine in carbonated mineral water began in the mid 1800's for combating malaria. This practice is still continued today, though not for its antimalarial action, but as a beverage 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 (3)..

Molecular fluorescence spectroscopy is based on the emission of light by molecules, which are excited to emit their characteristic spectra by exposure to UV light of specific wavelengths. The wavelength at which a molecule may be excited to emit is referred to as the excitation wavelength, and the spectrum emitted by the sample the emission spectrum(4). .

Unlike UV-Visible spectroscopy, in molecular fluorescence spectroscopy, the emitted spectrum is monitored at right angles to the exciting radiation, which renders the latter technique much more sensitive than the former (4). .

The intensity of emission at a specific emission wavelength is proportional to the concentration of the fluorescent molecule. This relationship forms the basis of quantitative analysis with this technique.

2. Experimental Procedure:

(A) Sample preparation:

Use a 5mL or 10mL microburette and a 50mL burette to prepare your standard and sample solutions..

A standard quinine sulphate solution containing 10mg of quinine sulphate in 1L of distilled water will be provided..

(a) Use the microburette to transfer 0.5, 1.0, 1.5, 2.0, and 2.5 mL respectively of the standard quinine sulphate solution to 25mL volumetric flasks, labeled 0.2, 0.4, 0.6, 0.8, and 1.0 ug/mL respectively..

(b) Using the 50mL burette, add sufficient distilled water to each flask, so that the total volume in each is 12.5mL, then fill each flask to the mark with 0.2N sulphuric acid, to give a final concentration of 0.1N acid for optimal fluorescence. Mix the solutions thoroughly..

(c) Obtain a sample of commercial tonic water and prepare a diluted sample as above..

(see Demonstrator for dilution factor)

(B) Molecular fluorescence spectroscopy:

The Perkin-Elmer LS5OB Luminescence spectrometer is an expensive, highly sensitive, research instrument. 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..

(a) Rinse two matched cuvettes and fill one with the 0.5ug/mL standard solution and the other with 0.1N sulphuric acid reference. .

Avoid fingerprints on the cell faces!.

(b) Scan the emission range of the quinine solution, with excitation wavelength set at 350nm. Determine and note the emission intensity value at the wavelength of maximal emission. Similarly scan the blank solution, to obtain the blank emission value..

(c) Repeat measurements for all standard and sample solutions.

(C) Calculations:

Plot the fluorescent intensity of each standard quinine solution, corrected for blanks, vs concentration. .

Determine the mean quinine (sulfate) concentration and Std. Deviation of the sample.

3. Exercises:

(a) Why is fluorescence spectroscopy far more sensitive than UV-Vis spectroscopy? .

(b) Two fluorescent compounds A and B are present in a mixture. Using a spectrofluorometer 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 spectral region, but A emits fluorescence at different wavelengths to B.

4. References:

1. Chemistry of the Alkaloids, Pelletier, S.W., Nostrand Reinhold (1970) .

2. The Merck Index of Chemicals and Drugs, Stecher, P. Seventh Edition Merck & Co. Inc., Rahway, N.J. (1960)..

3. Qualitative Analytical Chemistry, Schenk, G., Hahn, R., Hartkopf, A., Allyn & Bacon (1978) .

4. Fluorescence and Phosphorescence Spectroscopy: Phsiochemical Principles and Practice, Schulman, S.G. Pergamon Press (1979) .

Anal-Chem Resources
Chem. Dept. UWI. St. Augustine Campus