Chemistry Reference
In-Depth Information
Bistatic
Source
Detector
Source &
Detector
Monostatic
Reflector
Passive
Sun
Detector
Figure 6.2
Open-path IR spectroscopy geometries available.
the Sun, a preheated plume, hot buildings or process equipment and the OP-FTIR consists of receiving optics
and a detector. The sensitivity of the passive configuration is generally lower than that of the active method,
but the monitored distances are longer (up to several kilometers).
The open path FTIR is a long-path technique with a relatively high sampling volume which means that a
single open-path FTIR system, rather than numerous point samplers, can be used to locate and define a toxic
gas plume. Another advantage is that the open-path FTIR method can do measurements in situ, thus avoiding
sample collection and transport to the laboratory for analysis. Moreover, the open-path FTIR method measures
in near real time, enabling the technology to be used in process control or to detect toxic species which
represent an imminent hazard, thus providing fast information for making decisions.
Obviously, meteorological conditions can affect the monitoring system [6];for example, fog water droplets
scatter infrared radiation precluding data collection. On the other hand, rain degrades the quality of data
because of temperature fluctuations affecting the instrumentation and the large amount of water absorption
observed in the spectra, but the data will be still usable. Variations of temperature also degrade quality of
obtained results. The distance that can be monitored depends on the site geometry and the optical components
used to generate the infrared beam. Although optical systems can be assembled for more than one kilometer
path lengths, the practical limit of monitored distances for most commercially available active systems is
approximately 500 m. Another drawback of the technique is the limit of detection (LOD), which for most of
the molecules under study is between 1 and 100 parts per billion by volume (ppbv), whereas parts-per-trillion
sensitivity is required in some cases [7].
Routine applications include monitoring of toxic chemicals in industrial environments [8] and the
environmental determination of the most abundant and important greenhouse gases, such as H
2
O, CO
2
, CH
4
,
N
2
O and CO [9].
