Page 233 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

226 Chapter Ten

solution of benzene in drinking water. The difference spectrum is all negative
here, as the absorption is increased by this treatment. The uppermost curve is
for pure water. Note that there is some absorption change even here, starting
below 240nm wavelength. It is probably caused by modification of nitrate ions
present in large concentrations in this water. However in the 240 to 300nm
region the effects are small. Away from the modulated absorption features, the
difference is also small as the corona discharge has little effect on the back-
ground matrix. See Melbourne (1996) and the author’s papers.
The best feature of electrochemical measurand modulation is the high sta-
bility of the measurement configuration between operations. The corona and
photolytic modification steps are performed within the spectrometer, without
physically replacing samples between measurements, and without mechanical
movement. This helps to reduce other synchronous changes in intensity. All the
other techniques, such as path length variation and FIA liquid movement intro-
duce the risk of significant changes in optical transmission even with no sample
present. As we saw, the LOD is now limited by remaining synchronous para-
sitic variations, which should be minimized by stable geometries, repeatable
optical path losses, and rapid sample/reference measurements.

10.6 Mechanical Scanning
Flow injection analysis uses physical motion of fluid samples in fine-bore
tubing to provide, among other benefits, optical measurand modulation. The
system is a “liquid-chopper.” However, relatively large quantities of buffers,
reagents and sample (ªmL/min) are needed. As the available sample quantities
decrease, it may be more convenient to capture mL samples on solid media
such as filters, paper tapes or synthetic films, and to move the support media
mechanically. Figure 10.14 shows a diffuse reflectance measurement system

Source

Detector

Timebase
Reflected Intensity Ref.
reference mark
w

Time
Figure 10.14 Weak absorptions measured by diffuse reflectance can be
detected at higher resolution by mechanically modulating the samples,
spotted onto a uniform filter paper.

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