F2
Quinine in Soft Drinks by Fluorimetry
Last Revised: March 2015
1. REAGENTS & EQUIPMENT
1.1 50 mg/L quinine in 0.1 M H2SO4
1.2 0.1 M H2SO4
1.3 Tonic water
1.4 Bitter lemon
2. PROCEDURE
2A. Prework
Typical levels for quinine in bitter lemon and tonic water are 30 and 60 mg/L, respectively.
Determine suitable dilutions in 100 mL. See Step 2.1 for the standard concentrations.
2B. Solution preparation
2.1 From the 50 mg/L standard, prepare 100 mL of 1, 5 and 10 mg/L calibration standards. You
must use 0.1 M H2SO4 as the solvent in all solutions.
2.2 Prepare the sample dilutions in duplicate as determined in Prework 1.
2.3 Prepare duplicate recovery checks by pipetting 5 mL of the 50 mg/L standard into 100 mL
volumetric flasks, together with the same volume of bitter lemon as in 2.2.
2C. Analysis
2.4 Record the UV/VIS spectrum of the 10 mg/L standard from 300-400 nm. Record the
wavelength and absorbance for the peak.
2.5 Determine the appropriate filters for the analysis.
2.6 Zero the instrument with 0.1 M H2SO4 and adjust the gain to at least 1000 on an appropriate
scale using the 10 mg/L standard. Check the zero again. Do not touch the Gain scale controls
again.
2.7 Record the intensities of all solutions.
2.8 For the two samples, dilute roughly 1:1 in the sample tube and re-measure.
3. REPORT
Calculations
•
plot a calibration graph of intensity vs concentration for the standards
•
determine the analyte concentration for each diluted sample
•
apply the dilution factor to determine the concentration in the original samples
•
calculate the % recovery as below, where the concentrations for the recovery check (RC) and
bitter lemon (BL) solutions are those directly from the calibration graph
%RC 100x
AvgeRCconc AvgeBLconc
2.5
Discussion
•
compare your results to the expected values
•
explain why it is necessary to do a ‘rough’ 1:1 dilution on the sample even when its intensity is
within the standards
•
compare the technique to UV/VIS absorption spectrophotometry for this analysis
•
if you have completed V3, compare the two techniques for the analysis of quinine (if you have
already made this comparison in your V3 report, you don’t need to do it again)
Questions
1.
What is the structure of quinine? Explain why it is fluorescent.
2.
What is the natural source of quinine? What was its major use for many years?
3.
What is the meaning of the symbols SC and NB on the filters with this instrument?
4.
List four matrix factors that can cause quenching of the fluorescent intensity.
5.
Chemiluminescence is a recent analytical development, related to fluorescence. Explain how it
occurs, and describe one important analytical application in the area of air pollution
monitoring..
F2
p2
F2. QUININE IN SOFT DRINKS
RESULTS SHEET
Maximum absorbance
Wavelength at max. A
1st filter
2nd filter
Tonic water dilution factor
Bitter lemon dilution factor
Recovery check amount
Solution
1 mg/L
5 mg/L
10 mg/L
BL1
BL2
TW1
TW2
RC1
RC2
Intensity
Gain