Page 229 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

222 Chapter Ten

Selection
Chlorinated valve UV Source
sample input

Detection system
(photometer)
Chlorine-free
Dechlorination sample
cartridge Absorbance
Waste

Time
Figure 10.9 A useful technique is to derive the absorbance-free reference liquid
from the sample itself, for example via dechlorination.

For example, in the detection of trace absorption changes in a more or less
pure water sample, we could arrange for sequential measurement of an air ref-
erence, a sample of pure water, and the unknown aqueous sample. By measur-
ing at the same point in the tubing with a single source-detector system, tubing
wall contamination errors can be substantially compensated, while the small
differences between sample and solvent can provide a better measure of the
small absorption increases.
In the measurement of very weak absorptions, it is preferable to use as ref-
erence a fluid as similar as possible to the sample itself, so that the detection
system can be designed for high amplification of the small differences. This pro-
motes configuration 3 of Table 10.1, in which the reference is obtained from the
sample stream itself, except purified of the component which is being sought.
A typical example might be an optical chlorine disinfectant measurement
system, where dechlorination is achieved by passing the water through an acti-
vated carbon filter (Fig. 10.9). In practice it is very difficult to remove only
the target species without also removing some of the background matrix
absorption.
Chemical indicators can be used for high sensitivity determinations of a wide
variety of chemical species. The reactions are often selective, and the intense
colors of some of the reagents give much higher sensitivity than when species
absorption is directly measured. Nevertheless, compensation for the absorption
of the sample stream is necessary for the best performance, and FIA is a con-
venient way to automate this. Verma et al. (1992) describe a system for resid-
ual chlorine detection in which the reaction product is a type of azo dye which
absorbs at 532nm. This wavelength is much easier to detect, and water absorp-
tion is much lower there than at the chorine absorption wavelengths of 234nm
and 292nm. The FIA system used performed measurements at a rate of
-1
110 hour .

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