Page 100 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

Fiber-Optic Chemical Sensors 813

FIGURE 10 A typical arrangement for remote measurement of sample absorption using optical fibers.

used in such cases, as they provide an additional parameter perature can be measured simultaneously. The degree of
for distinguishing such compounds. For example, phenol, epoxy curing is calculated by taking the ratio of the epox-
−1
toluene, and xylene are single-ring aromatic compounds ide ring stretch at 1240 cm , which is linearly dependent
with similar emission peaks. It is not possible to distin- on the progress of the curing reaction, and the 1186-cm −1
guish between them by simply measuring their fluores- vibration of a fragment not affected by the cure. Raman
cence spectra. It is possible, however, to distinguish be- spectroscopy with optical fibers is also used for on-line
tween them by measuring their fluorescence decay times. monitoring of water in sodium nitrate slurries in the nu-
Also, low concentrations of dissolved polyaromatic hy- clear industry. The Raman method is a nondestructive op-
drocarbons, such as benzene, naphthalene, and pyrene, in tical technique that can also provide detailed information
natural water can be determined selectively by measuring about the molecular composition of tissues and it recently
the fluorescence lifetimes of the individual molecules. The has been used for in vivo determination of the molecular
different techniques for performing fluorescence lifetime composition of an arterial wall.
measurements are described in Section II.C.3.
Remote fiber-optic Raman spectroscopy has found ap-
2. Gas-Phase Sensing
plications in process control. The principal advantage of
this method is that many different molecules can be de- The most common fiber-optic gas sensor is based on spec-
tected directly in solids, liquids, or gases, in complex me- tral transmission analysis. Such analysis is commonly per-
dia, or in harsh environments (i.e., high temperature or formedusingtwowavelengthregions:250–500nmandca.
pressure). This technique is most sensitive to compounds 1–8 µm. Absorption or emission in the lower wavelength
that are IR-active. A large amount of qualitative and quan- region corresponds to electronic transitions within atoms
titative information can be gathered from the Raman spec- or molecules, and is a useful region for measuring the
trum without using any indicator chemistry. The low inten- energy changes associated with the transitions of a large
sity of the Raman-scattered light is the major weakness of number of gaseous species. The longer wavelengths cover
this technique. In addition, organic molecule fluorescence the near- and mid-IR regions of the spectrum, and con-
usually interferes with Raman spectra. This technique has tain information about vibrational absorption bands of the
been widely used for in situ monitoring of chemical reac- gases. Conventional silica fibers are good for measuring
tions under harsh conditions. One application of Raman the near-IR absorption lines but they are not good at the
spectroscopy using optical fibers is to monitor epoxy- lower or mid-IR regions due to high attenuation and an
curing reactions. The extent of curing and the system tem- increase in opacity of the silica, respectively. Therefore, a