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Chapter 18: Measurement of Vapor Concentrations
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Cons: This unit is not amenable for long-term in-situ operation. The catalytic bead
sensor requires elevated temperatures for operation. Internal pump is required to
sample gas.
18.3.1.3 Metal-oxide semiconductor sensors
The metal-oxide semiconductor (MOS) sensor is comprised of a tin oxide that is
sintered on a small ceramic tube. A coiled wire is placed through the center of
the ceramic tube to act as the sensor heater. Metal wires provide electrical con-
tact between the tin oxide and the rest of the electronics. The MOS sensor requires
between 300 and 600 mW of power to operate the sensor at elevated temperatures
◦
between 200 and 450 C. The combination of the sensor operating temperature and
the composition of the metal oxide yields different responses to various combustible
gases.
When the metal oxide is heated, oxygen is adsorbed on the surface with a nega-
tive charge. Donor electrons are transferred to the adsorbed oxygen, leaving a postive
charge in the layer. Inside the sensor, electrical current flows through the grain bound-
ary of metal oxide micro crystals. Resistance to this electrical current is caused by
negatively charged oxygen at grain boundaries. In the presence of a reducing gas, a
surface catalyzed combustion occurs and the surface density of negatively charged
oxygen decreases, thereby decreasing the resistance of the sensor. The relationship
between the amount of change in resistance to the concentration of a combustible gas
can be expressed by a power-law equation.
Pros: The MOS sensors have high sensitivity to combustible gases (e.g., hydrogen,
carbon monoxide, methane, ethane, propane, alchohols, etc.). They are compact and
durable. The cost is also relatively inexpensive.
Cons: U.S. EPA (1995) performed tests on some Figaro MOS sensors and found that
they had more drift during exposure to xylene than the polymer-absorption sensors.
The MOS sensor has a fair amount of sensitivity to water humidity, which may be
problematic in subsurface environments. Sensitivity to aromatic and halogenated
hydrocarbons is questionable.

18.3.2 Potentiometric and Amperometric Sensors
Potentiometric and amperometric sensors employ an electrochemical cell consisting
of a casing that contains a collection of chemical reactants (electrolytes or gels) in con-
tact with the surroundings through two terminals (an anode and a cathode) of identical
composition. For gas sensors, the top of the casing has a membrane which can be per-
meated by the gas sample. Oxidization takes place at the anode and reduction occurs
at the cathode. A current is created as the positive ions flow to the cathode and the
negative ions flow to the anode. Gases such as oxygen, nitrogen oxides, and chlorine,
which are electrochemically reducible, are sensed at the cathode while electrochem-
ically oxidizable gases such as carbon monoxide, nitrogen dioxide, and hydrogen
sulfide are sensed at the anode. Potentiometric measurements are performed under