Page 310 - gas transport in porous media

309
Chapter 18: Measurement of Vapor Concentrations
amperometric (measurement of current); and (3) conductometric (measurement of
conductivity). A summary of gas and vapor phase analytes that can be detected by
these groups of sensors is summarized in Table 36.3 of Wilson et al. (1995).
In this section, we first review the conductometric class of sensors. These types of
sensors appear to be most relevant for detecting and monitoring VOCs. Then, a brief
overview of potentiometric and amperometric sensors is provided.

18.3.1 Conductometric Sensors
Three different types of conductometric sensors are presented in this section. The first
is a polymer-absorption sensor that indicates a change in resistance in the conductive
polymer electrode when exposed to chemicals. The second is the catalytic bead sen-
sor, which requires elevated temperatures to burn combustible hydrocarbon vapors
and change the resistance of an active element. The third sensor is the metal-oxide
semiconductor sensor, which responds to changes in the partial pressure of oxygen
and requires elevated temperatures to induce combustion of chemical vapors that
change the resistance of the semiconductor.
18.3.1.1 Polymer-absorption chemiresistors
The concept of using polymeric absorption to detect the presence of chemicals in
the vapor phase has existed for several decades. These polymer-absorption sensors
(chemiresistors) consist of a chemically sensitive absorbent that is deposited onto
a solid phase that acts as an electrode. When chemical vapors come into contact
with the absorbent, the chemicals absorb into the polymers, causing them to swell.
The swelling changes the resistance of the electrode, which can be measured and
recorded. The amount of swelling corresponds to the concentration of the chemical
vapor in contact with the absorbent. The process is reversible, but some hysteresis
can occur when exposed to high concentrations. Several companies and organizations
have developed chemiresistors, but the specific attributes and types of absorbents,
which are generally proprietary, vary among the different applications.
Sandia National Laboratories has developed chemiresistors using polymer films
deposited on microelectrodes. Rather than using a single electrode and conductive
polymer, the chips used at Sandia can house an array of chemiresistors. Down-hole
chemiresistors sensors have been developed and field tested in several applications
(see www.sandia.gov/sensor).
Pros: Chemiresistors are small, low power devices that have no moving parts and have
good sensitivity to various chemicals. As a result, they are amenable to being placed
in situ in monitoring wells. Another advantage for chemiresistors in comparison
to the standard electrochemical sensors is that they don’t require liquid water to
work properly. This will be seen more clearly in the section on electrochemical
sensors below, but in brief, standard electrochemistry requires a well controlled liquid
environment for the electrodes to work predictably in detecting analytes. That liquid
(usually water with controlled ionic strength and pH buffers) must be supplied by