Page 309 - gas transport in porous media

Ho et al.
308
source. The resulting positive and/or negative charged species will be accelerated
over a short distance in an electrically charged field, and the time-of-flight will
be determined. The IMS is different than the mass spectrometer in that it operates
under atmospheric conditions and does not need large and expensive vacuum pumps.
Because of this, IMSs can be easily miniaturized.
IMS devices are currently used in airports to screen passengers for trace explosives
(negatively charged ions). They can also be used by law enforcement personnel to
detect narcotics and other drugs (positively charged ions). These devices are suited
for the screening of traces in gas and for the detection of toxic industrial compounds
and chemical warfare agents down to the ppb-range. Some devices are equipped with
automatic polarity switching, and some instruments enable continuous monitoring.
The built-in microprocessor evaluates the recorded ion mobility spectra. The results,
identified substances and their concentrations, are shown on the display.An integrated
alarm function responds according to programmed threshold values.
Pros: IMS systems can be miniaturized because they operate at atmospheric pressure.
Can be used to detect toxic industrial compounds and chemical warfare agents in
ppb-range. Integrated alarm for threshold detection.

Cons: Cannot currently be used in situ. Contains radioactive ionizing source.

18.2.3 Mass Spectrometry

The principle of the mass spectrometer is similar to the ion mobility spectrometer,
exceptavacuumisrequired. Sampledgasmixturesareionized, andchargedmolecular
fragments are produced. These fragments are sorted in a mass filter according to their
mass to charge ratio. The ions are detected as electrical signals with an electron
multiplier or a Faraday plate.
Low mass ions are displayed as a vertical line at the left end of a scale while heavy
ions are displayed towards the right. The length of a line represents the quantity of
that ion in the gas mixture.

Pros: The mass spectrometers have good discrimination capabilities and can detect
a wide range of chemicals. Some of the mass spectrometers are portable enough to
carry into the field.
Cons: The units are quite expensive and cannot be used in situ. Spectral overlaps can
be a problem in detecting mixtures of unknown composition. Contains radioactive
ionizing source.

18.3 ELECTROCHEMICAL SENSORS
Electrochemical sensors have been categorized by Wilson et al. (1995, Ch. 36) and
Janata (1992) into three groups: (1) potentiometric (measurement of voltage); (2)