Long-path gas cell

The environmental applications of infrared spectrometry are many and varied. Many applications at industrial sites are analogous to those for on-line process analysis waste streams and recycling processes can be monitored in the same way. Commercial infrared stack-gas monitors are based on either an extractive probe attached to a long-path gas cell or an open-path (across stack) configuration (69). Stack plume and flare monitoring can be done externally... 

In the process of developing PRMs, it is necessary to study and establish measurement methods which are used to analyzed the purity of raw gases and verify the stability of the gas mixture kept in the cylinder. Up to now, NRCCRM has been equipped with several series of analytical techniques including atmospheric pressure ionization mass spectrometer, gas chromatograph, infra-red spectrophotometer with long-path gas cell, chemiluminescent, non-dispersive infra-red, minor 02 and H20 analyzer and so on. 

A long-path gas cell containing high-pressure H2 gas (e.g., 0.75-m path length with 6.8 atm of pressure). 

There are two ways to monitor the concentrations of trace gases in the atmosphere by FT-IR spectrometry. The first is to draw the atmosphere in the region of interest into a long-path gas cell, and the second is to measure the spectrum of the atmosphere in situ. The first approach, which is known as extractive monitoring [1], is covered in this section, and the second, known as open-path FT-IR spectrometry (OP/FT-IR) [2], is covered in Section 22.2. 

Hanst, RL. (2002) Long path gas cells. In Handbook of Vibrational Spectroscopy, Vol. 2 (eds J.M. Chalmers and RR. Griffiths), John Wiley Sons, Ltd, Chichester, pp. 960-968. 

The most common sample compartment in high-resolution FT-IR spectrometers is the White cell, where the beam passes a path of 1-1000 m. In a long-path gas cell, however, a small f/number (focal length of the mirror/diameter of the pupil) and a large size of the circular image may lead to a loss of effectivity. In the Oulu interferometer this problem was solved by an alternative multiple-path gas cell, where the images at the focal planes occur on a circle around the principal axis of the cell. The aberrations remain small in spite of a small f/number, because the successive images are relatively near to the principal axis of the optics. 

Long path gas cells nunber of reflections path length... 

Up to now micromachined IR sources with appropriate emitted optical power for use in the NDIR gas analysers have not been fabricated In the gas analyser a commercial available, variable, long path absorption cell with low cost optical components should be used For the wavelength modulated gas sensors it is of interest to fabricate a tuneable Fabry Perot interferometer with Si micromachining technologies The fabrication of a Si etalon array with multispectral filters would provide the head component for a very sensitive and selective multicompound gas analyser based on correlation photometry... 

However, cells of much longer path length are available that use mirrors to deflect the IR beam and to reflect it back and forth many times in the cell chamber before it leaves the cell and reenters the spectrometer. These long-path-length cells are used for detecting very small quantities of gas in pollution studies, for example. 

The soil atmosphere can also be passed into a long-path IR gas sampling cell. Because of the long path length, gases that might otherwise not be detectable are measured and identified. This should not be confused with IR used in remote sensing, which is covered in Section 14.14. 

Very few organic compounds can be examined as gases. The gaseous sample is introduced into a gas cell having the walls made up of sodium chloride. This cell can be directly put in the path of sample beam. The gas cell is usually 10 cm long. 

Well-designed reflectance cells have made this strong absorber of water useful in the process vat. Using long path-length gas cells also allows the analyst to monitor head-space gases in order to follow bioreactions. 

As an interesting adjunct for this last paper, a note by Morrow and Crabb24 actually follows biomass production by using MIR to measure the production of C02 by the reaction. A long path-length gas cell was used to follow the production of the gas and calculate the production of increasing biomass. 

Transfer optics The interface of the IR beam with the sample, especially in a classified hazardous environment, can be a major challenge. Gas samples are not too difficult although it is important to pay attention to the corrosivity of the gases, relative to the windows and any internal optics, as with a folded path length cell. Liquids offer different challenges. For on-line applications users prefer to minimize the use of valves, bypass streams and auxiliary pumps, especially over long distances between the stream and the analyzer. At times there is a benefit to sample the stream either directly or as close as possible... 

Infrared spectra may be obtained for gases, liquids, or solids. The spectra of gases or low-boiling liquids may be obtained by expansion of the sample into an evacuated cell. Gas cells are available in lengths of a few centimeters to 40 m. The sampling area of a standard IR spectrophotometer will not accommodate cells much longer than 10 cm long paths are achieved by multiple reflection optics. 

Infra-red spectra in the vapour phase are rarely determined with dispersive spectrophotometers unless the substance is a gas at room temperature. Special long-path, airtight gas cells are required. The technique has become more widely used with the advent of Fourier transform infra-red spectrophotometers coupled directly to gas chromato- graphs. 

---