Page 227 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

220 Chapter Ten

Gas absorption modulation can be combined with another related technique
called nondispersive IR detection. Rather than using a broadband source, only
a small fraction of which is absorbed by the gas’s signature, it would be prefer-
able to have a source spectrum made up of many lines which perfectly match
the absorption spectrum. This is hard to arrange, but it is straightforward
to produce a spectrum which is lacking the absorption lines. All we need to do
is pass the broadband light through a cell of the target gas. As the absorbed
wavelengths are missing, this coded light source should be only faintly absorbed
by the sample gas. Modulation of the unknown sample should not produce a
modulated intensity with the coded light, but a small modulation with the
broadband light. These nondispersive techniques were developed in the 1930s,
and a number of products using them is available, usually for detection of indus-
trially important gases such as CO 2, CO, CH 4, SO 2, NO and H 2O. Originally
detection was via gas expansion and movement of a pneumatic membrane. Dif-
ferent geometries are possible, including parallel comparisons of sample and
reference gases, serial transmission, and high-speed switching of sample and
reference gases.

10.4 Flow-injection Analysis
During the 1960s and 1970s techniques for automated chemical analysis were
developed to handle large numbers of tests performed on blood and urine
samples. These used continuous liquid flows in small (ªmm diameter) tubing,
driven by a multichannel peristaltic pump. Further elements were added to
allow the mixing and reaction of solvents, pH buffers, colorimetric indicators
and liquid standards, the injection of known volumes of the sample liquid under
investigation, residence periods provided by longer transport delay tubes, and
in-pipe detection systems, for example by optical absorbance. A typical system
is shown in Fig. 10.7. Originally samples were kept separate from each other
during their passage through the tubing by interspersing small bubbles of air
or other gas (Fig. 10.8). These segmented-flows serve to reduce hydraulic dis-
persion and intermixing, and to scour the tubing wall to reduce build-up of con-
tamination. Later it was found that in small-bore tubing axial mixing could be
kept to negligible levels even without the use of gas segmentation; many such
flow-injection analysis or FIA systems currently do not segment the sample
between gas bubbles.
Although the primary drivers in development of FIA systems were undoubt-
edly sample throughput and cost-per-test, the idea of mechanically moving
samples past a detector also provides an effective measurand-modulation
scheme. By programming the systems to perform repeated measurements on
chemical standards, interspersed by the samples, slow changes in the fouling,
optoelectronic system baseline drift and even absolute scale-factor can be
removed. Several configurations are possible depending on what reference fluid
is used (Table 10.1).

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