EurOCEAN 2000 - Vlaams Instituut voor de Zee

THE FLUORESCENCE SIGNAL OBTAINED WHEN DIFFERENT SPECIES ARE
PRESENT IN A SAMPLE CONTAINING AN EXCESS OF CHEMOSENSOR CAN BE
EXPRESSED IN THE FOLLOWING WAY:
IT(λ) = x × IZn(λ) + y × ICd(λ) + z × ICh(λ) (1)
In these equations x, y, and z represent the relative molar fractions of the three species present
in the sample. They are respectively: the chemosensor, the chemosensor chelated to zinc and
the chemosensor chelated to cadmium.
IT(λ) represents the signal emitted by the sample; IZn(λ), ICd(λ) and ICh(λ) represent the
emission intensity of the chemosensor saturated by zinc, by cadmium, and the free
chemosensor respectively. The molar fractions can be determined by resolving the equation
system at two different wavelengths. Actually, since the fluorescence spectra are measured at a
large number of wavelengths, this system is overdetermined. The overdetermined equation
system can be resolved by using a so-called QR decomposition. The key step of the QR
decomposition is the orthogonal factorisation of the matrix equation A = Q × R where Q is an
orthogonal matrix and R an upper triangular matrix.
Table I reports some experimental data obtained on samples containing cadmium, zinc and an
excess of {1}.( multiwavelength approach).
Table1
Analytical concentration (nM)
in the mixture
Measured concentration (nM)
Zinc Cadmium Zinc Cadmium
125 125 120(4) 130(4)
95 30 100(5) 30(0)
45 15 50(11) 15(0)
30 95 30(0) 85(11)
15 45 15(0) 40(11)
1.1.1.1 Elimination of interferences
A large number of heavy metals present in variable amounts in seawater interferes with the
fluorescence of the chelated chemosensor with Zn or Cd
Therefore, a separation technique was to be developed in order to prevent fluorescence
quenching by interferents as copper, chromium, mercury, lead and cobalt (Kraus and Moore,
1953).
Using a 1X2 400 Dowex resin in the experimental conditions described by Kraus and Moore,
cadmium and zinc can be successfully separated from a mixture of heavy metals. Furthermore,
a preconcentration factor of 6-10 is observed.
The accuracy of the preconcentration-fluorescence detection technique was examined by the
determination of cadmium concentration in water certified reference material obtained from the
National Research Council of Canada, Marine Analytical Chemistry Standards Programme.
Using our method, we found a cadmium concentration of 169 ± 13 pM in estuarine water
SLEW-2. This result displays good agreement with certified values (170 ± 20 pM).
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