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Clearly, a need exists for accurate, inexpensive, real-time, in-situ analyses using
robust sensors that can be remotely operated. Ho et al.
Although a number of chemical sensors are commercially available for field mea-
surements of chemical species (e.g., portable gas chromatographs, surface-wave
acoustic sensors, optical instruments, etc.), few have been adapted for use in geologic
environments for long-term monitoring or remediation applications.
The particular focus of this review is limited to the detection and monitoring of
volatile organic compounds (VOCs). These include compounds such as aromatic
hydrocarbons (e.g., benzene, toluene, xylenes), halogenated hydrocarbons (e.g.,
trichloroethylene (TCE), carbon tetrachloride), and aliphatic hydrocarbons (e.g.,
hexane, octane). As a result, sensors and technologies that detect gas-phase con-
stituents in the vadose zone are emphasized because VOCs are most conveniently
and economically monitored in the gas phase (Looney and Falta, 2000, Ch. 4).

18.1.2 Categories of Sensors
The sensors reviewed in this chapter have been categorized into four general groups:
(1) chromatography and spectrometry; (2) electrochemical sensors; (3) mass sensors;
and (4) optical sensors. The categorization of these sensors is based primarily on the
principal physics and operating mechanisms of the sensor. For example, chromatog-
raphy relies on separation of complex mixtures by percolation through a selectively
adsorbing medium, with subsequent detection of compounds of interest. Electro-
chemical sensors, for the purposes of this chapter, include sensors that detect signal
changes (e.g., resistance) caused by an electrical current being passed through elec-
trodes that interact with chemicals. Mass sensors rely on disturbances and changes
to the mass of the surface of the sensor during interaction with chemicals. Optical
sensors detect changes in visible light or other electromagnetic waves during inter-
actions with chemicals. Within each of these categories, some sensors may exhibit
characteristics that overlap with other categories. For example, some mass sensors
may rely on electrical excitation or optical settings. Nevertheless, these four broad
categories of sensors are sufficiently distinct for the purposes of this review.
In the following sections, the four general categories of sensors are reviewed: (1)
chromatography and spectrometry; (2) electrochemical sensors; (3) mass sensors;
and (4) optical sensors. The following sections are not intended to include exhaustive
reviews of all available sensors and technologies. Instead, the following sections
provide a brief overview of the relevant features and applications of the different
categories of sensors. The intent is to provide guidance for determining the best
options for subsurface vapor-phase monitoring applications.

18.2 CHROMATOGRAPHY AND SPECTROMETRY
(SEPARATION AND DETECTION)
Chromatography is a method for the separation and analysis of complex mixtures of
volatile organic and inorganic compounds. A chromatograph is essentially a highly
efficient apparatus for separating a complex mixture into individual components.