.
Чтение книги онлайн.
Читать онлайн книгу - страница 61
White Cells.
A White cell is a type of sample cell developed by John White (White 1942) that increases the pathlength of the light beam by reflecting it from three spherical, slightly misaligned concave mirrors (Figure 4‐13). The cell shown in the figure shows the beam making eight traverses across the cell to increase the pathlength eightfold from that of a cell containing only windows. The number of passes can be adjusted by adjusting the mirror positions. White cells can be designed with pathlengths from less than a meter to hundreds of meters (Vidrine 2000; Doussin et al. 1999).
Herriott Cells.
The Herriott cell, named after Donald Herriott (Herriott and Schulte 1965), consists of two opposing concave spherical mirrors, with one mirror having a hole where the light beam enters and exits (Figure 4‐14). The cell can also be designed with an exit hole in the opposite mirror. In the Herriott cell, the number of reflections is changed by changing the separation distance between the two mirrors. More than one light source can be used in the Herriott cell by drilling more holes in the mirrors.
Figure 4‐13 A White multipath gas cell.
Figure 4‐14 A Herriott multipath gas cell.
Figure 4‐15 Integrated cavity output spectrometer (ICOS).
CRDS and ICOS.
The cavity ring‐down spectroscopic (CRDS) technique developed by O’Keefe in 1988 utilizes a sample cell with high reflectivity mirrors to achieve pathlengths on the order of kilometers in conjunction with a laser light source (O’Keefe and Deacon 1988). The off‐axis integrated cavity output spectroscopic (ICOS) method is a similar method; both methods are a breakthrough in trace gas analysis (Figure 4‐15).
Instruments using these methods are able to measure gases at part per trillion levels. The CRDS and ICOS techniques are discussed in Chapter 5.
Optical Components
In most electro‐optical analyzers, other components are used to direct and focus light. The following components can be found in gas monitoring instrumentation:
Neutral density filters
Lenses
Mirrors (concave and triple)
Beam splitters (half‐silvered mirrors)
Optical light fibers
Rotating shutters/window slits/irises
Lenses, slits, and diaphragms are used to focus light on through the system. Plain glass windows are used to separate the primary optical system from flue gases. Optical light fibers can be used to transmit light from the stack to a spectrometer or can route light in a spectrometer to simplify otherwise complex lens or mirror arrangements. Rotating shutters are used to reflect or block light to create oscillating signals.
Figure 4‐16 Constructing a spectrophotometer.
A half‐silvered mirror is a partially silvered mirror which both reflects and transmits light – essentially enabling two functions out of one component in an optical design. Half‐silvered mirrors may be more familiar in their application in mirrored sunglasses, or the “transparent mirror” seen in movies or amusement centers. Half‐silvered mirrors can be found in most double‐pass opacity monitors and in‐situ gas analyzers.
Constructing a Spectrophotometer
The components discussed above are used in various combinations in the construction of pollutant gas monitors. Putting them together, one can construct a spectrophotometer for gas measurements (Figure 4‐16), or a particulate monitor for light scattering or light transmission measurements. Different combinations of these components will be examined in the following chapters that discuss the many unique systems commercially available today.
REFERENCES
1 Doussin, J.F., Ritz, D., and Carlier, P. (1999). Multiple‐pass cell for very‐long‐path infrared spectrometry. Applied Optics 38: 4145–4150.
2 Durham, M.D., Ebner, T.G., Burkhardt, M.R., and Sagan, F.J. (1990). Development of an ammonia slip monitor for process control of NH3 based NOx control technologies. In: Proceedings – Continuous Emission Monitoring – Present and Future Applications (ed. J.A. Jahnke), 227–238. Pittsburgh: Air & Waste Management Association.
3 Ensor, D.S. and Pilat, M.J. (1971). Calculation of smoke plume opacity from particulate air pollutant properties. Journal of the Air Pollution Control Association 21: 496–501.
4 Faist, J., Capasso, F., Sivco, D.L. et al. (1994). Quantum cascade laser. Science 264: 553–556.
5 Frisch, M.B. (1996). The spectra scan family of trace gas monitors based on tunable diode laser spectroscopy. Paper presented at the Air & Waste Management Association meeting, Nashville. June, paper 96‐TP26B.05.
6 Herriott, D. and Schulte, H. (1965). Folded optical delay lines. Applied Optics 4 (8): 883–891.
7 Hinkley, E.D. (1972). Development and Application of Tunable Diode Lasers to the Detection and Quantitative Evaluation of Pollutant Gases. Final Technical Report. Prepared for the Environmental Protection Agency. Research Triangle Park: MIT Lincoln Laboratory.
8 Hinkley, E.D. and Kelley, P.L. (1971). Detection of air pollutants with tunable diode lasers. Science 171: 635–639.
9 Imasaka, T. and Ishibashi, N. (1990). Diode lasers and practical trace analysis. Analytical Chemistry 62 (6): 363A–371A.
10 International Standards Organization (ISO) (2015). Optics and photonics – spectral bands. ISO Standard 20473:(2007). (R2015). Geneva: Central Secretariat.
11 Jahnke, J. A. (1984). Transmissometer Systems – Operation and Maintenance, an Advanced Course. EPA 450/2‐84‐004. СКАЧАТЬ