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being detected. When gas in the path absorbs energy from the source, the detector
receives less radiation than without the gas present, and the detector can quantify the
difference.
Pros: These devices can be made to identify specific gases; they require less
calibration than other sensors; good durability with minimal maintenance.
Cons: They can only monitor specific gases that have non-linear molecules; they can
be affected by humidity and water; they can be expensive; dust and dirt can coat the
optics and impair response, which is a concern in in-situ environments.

18.6 SUMMARY AND DISCUSSION
Four general categories of technologies were reviewed for their potential application
in real-time, in-situ chemical sensing applications. The first category reviewed was
chromatography/spectrometry, which included ion-mobility spectrometry and mass
spectrometry. The gas chromatographs reviewed provide excellent discrimination
among various chemicals of interest. The size of gas chromatographs range from
large bench-top systems to portable hand-held systems and microchips the size of a
coin.Although the portable GCs can be taken to the field and used manually to sample
monitoring wells, the majority of these devices are not yet amenable to real-time, in-
situ downhole applications. The micro-chem-lab might be a potential candidate, but
it requires micro-pumps to circulate gas through the system, and these moving parts
may not be able to withstand long periods in geologic environments. The ion-mobility
and mass spectrometers also have excellent discrimination capabilities, but like the
gas chromatographs, they are not currently amenable for in-situ applications.
The second category reviewed was electrochemical sensors, which included con-
ductometric, amperometric, and potentiometric sensors. The amperometric and
potentiometric devices traditionally are used to monitor oxygen, carbon monoxide,
chlorine, and other constituents for air quality purposes. Water quality parameters
such as pH can also be measured with these devices. However, the amperometric
and potentiometric devices are not widely used for detection of VOCs. The con-
ductometric sensors reviewed include polymer-absorption chemiresistors, catalytic
bead sensors, and metal oxide semiconductors. These devices are sensitive to VOC
exposure, resulting in large changes to resistance in the device. However, current
commercial devices intended for use in situ (primarily polymer-absorption sensors)
cannot discriminate different constituents in a mixture. Some hand-held polymer
absorption devices can discriminate different species because of the use of arrays
of chemiresistors, but they are not amenable to in-situ applications. Catalytic bead
sensors and metal-oxide semiconductors require elevated temperatures for operation,
and they may not be amenable for prolonged periods in situ.
The third type of technology reviewed was the mass sensor. These devices typically
absorb the chemical of interest onto a surface, and the device detects the change in
mass. The detection can be accomplished through changes in acoustic waves prop-
agated along the surface (SAW devices) or by actual bending or a change in shape