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Chapter 18: Measurement of Vapor Concentrations
18.2.1
Portable Gas Chromatographs
The potential to use an integrated chromatography system as a continuous or in-situ
sensor system depends on the miniaturization of its components. Several portable gas
chromatography systems that can be carried in a backpack have been commercialized
and are suitable for field use. Efforts are also being made to miniaturize the systems
even further into hand-held devices or downhole probes. Micro-analytical devices,
or micro-chem-lab devices, are being developed at Sandia National Laboratories that
are the size of a palm-top computer that incorporate a gas-phase analysis system
for detecting volatile organic compounds and chemical warfare agents (e.g., sarin,
soman, mustard gas) and a liquid-phase analysis system for detecting explosives.
The micro-chem-lab improves the sensitivity and selectivity to individual chem-
icals by using a cascaded approach where each channel includes a sample collec-
tor/concentrator, a separator, and a chemically selective surface acoustic wave (SAW)
array detector. All three components have been developed and demonstrated and can
be fit in an area less than that of a U.S. dime. These small components allow miniature
systems incorporating multiple analysis channels to be produced. Discrimination of
analytes is improved by allowing simultaneous use of more than one channel at a time.
Versatility of the micro-chem-lab can be increased by allowing different channels to
be tailored to detect different chemical analytes.
The collector/concentrator includes a thin-film porous adsorbent to selectively col-
lect chemical analytes to be detected and a heater for thermal desorption of the
collected analyte into a concentrated pulse. The column separator is a long flow
channel coated with a stationary phase material. As analytes pass through the col-
umn, the analytes are separated based on differences in partitioning behavior into the
stationary phase. Detection of the analyte is achieved using an array of SAW sensors
acting as sensitive mass detectors. By coating the SAW devices with chemically dis-
tinct thin film materials, a unique pattern of responses for different chemical analytes
can be used to provide chemically selective detection.
Pros: The bench-top gas chromatograph can provide superior discrimination capabil-
ities (relative to other devices and sensors) with excellent precision, sensitivity, and
reproducibility. Portable and micro-chem-lab units provide faster analysis times and
potentially increased sensitivity.

Cons: Bench-top gas chromatographs are bulky and not portable. They cannot be used
for real-time or in-situ applications. The micro-chem-lab is still under development
and requires additional research and testing. Interferences and long-term stability may
be issues with the smaller systems.

18.2.2 Ion Mobility Spectrometry
The ion mobility spectrometer (IMS) can be considered a sub-class of chromato-
graphic separators. The principle of every IMS is a time-of-flight measurement. After
a gaseous sample has entered the spectrometer it will be ionized by a radioactive