Page 231 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

224 Chapter Ten

0.04
100ppb
0.03

0.02 50ppb
Absorbance Difference (au) 0.01 10ppb

0

-0.01

-0.02

-0.03
200 210 220 230 240 250 260 270 280 290 300
Wavelength (nm)
Figure 10.11 Difference spectra of 10, 50, 100ppb atrazine in water formed by in-situ flash
photolysis.

teristic absorption is simply bleached out by the photolysis reactions. This is
just what happens to swimming pool water in sunlight. In other cases,
including many of the herbicides, the absorption features of the herbicides are
not just removed, which would lead to a positive absorbance value for the
before-after difference, but modified by the photolytic action, giving both posi-
tive and negative spectral features. This can aid determination of the types of
contaminants present. Figure 10.11 shows difference spectra of three concen-
trations (10, 50, 100ppb) of Atrazine in water, modified by a UV/visible
flash pulse.
This photolytic modulation is just one example of electrochemical modulation
schemes which are useful for modulating the intrinsic absorption of active
species such as benzene, toluene, ethyl-benzene, xylene (“BTEX”), phenol,
-
aqueous chlorine (HOCl/OCl ), and many common indicator dyes. Other mod-
ulation techniques include thermal degradation, pH changes generated at elec-
trodes, and the use of high voltage corona discharges. Figure 10.12 shows an
electrochemical reactor in which a corona-wind of electrons and charged species
is injected into the surface of a contaminated water sample. The liquid is
grounded via an electrical connection in the silica cuvette base. The corona-
wind is generated by a high tension power supply providing current of a few
microamperes. The injected species can modify contaminant molecules as does
the photolytic illumination. Figure 10.13 gives results for a 0.0, 0.5, 1, 2ppm

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