Continuous Emission Monitoring. James A. Jahnke

       Interband Cascade Lasers.

      Interband cascade lasers (ICLs) operate in the wavelength range of 3–6 μm and fill the gap in wavelengths emitted between tunable diode lasers and quantum cascade lasers. ICLs are similar to quantum cascade lasers; however, in ICLs, light is emitted from electron–hole recombination in interband transitions rather than in intersubband transitions. In a sense, ICLs are a hybrid between tunable diode lasers and quantum cascade lasers, in that light is emitted by electron–hole recombination from electron injection. Due to the nature of these transitions, light emission can occur at lower electrical input power than in quantum cascade lasers.

      Wavelength Selection

      Lasers, of course, are used in gas analyzers to monitor at specific wavelengths. Before the advent of lasers, it was necessary to restrict or separate the wavelengths at which molecules absorb from the spectra emitted from broadband radiation sources. Optical filters and diffraction gratings were and still are used to do this in spectroscopic CEM system analyzers. To manufacture an analyzer, it is often easier and less expensive to utilize broadband emission sources, coupled with spectral limiters such as gratings and filters.

       Optical Filters.

      An optical filter allows light only in a narrow spectral region to pass through it. Interference filters, constructed by vacuum deposition of metallic films on glass or other materials, are commonly used in the infrared region of the spectrum. Neutral‐density filters are used to attenuate light of all wavelengths equally, being made of quartz or glass with a thin metal coating having a specified optical density.

       Diffraction Gratings.

Diffraction gratings are commonly used in the ultraviolet and visible regions of the spectrum to select specific wavelengths. A grating consists of a reflective surface finely etched with a large number of parallel lines (on the order of 600 lines/mm). Each line will scatter light impinging on it, causing the light to interfere constructively and destructively to spread out a reflected spectrum.

       Prisms.

      Prisms disperse ultraviolet, visible, and infrared radiation due to differences in the wavelength‐dependent index of refraction in the prism material. Prism spectrometers are available in the UV and visible spectral regions; although they have been used in atomic absorption spectrometers for measuring metals, they are not typically used in pollutant gas monitoring applications.

      Detectors

      The type of detector used in an analyzer is very dependent on the energy of the light that it is sensing. Because light in the infrared region is relatively weak in the energy it carries, special stratagems are often devised to detect infrared intensity changes. Pneumatic, microphone‐type detectors (Luft detectors) traditionally have been used in infrared analyzers; however, solid‐state detectors, cooled with Peltier coolers, are common today. Sensitivity is often increased by not overly limiting the spectral region of the analyzer, but using a broader band of radiation to obtain more light for the detector. The special methods of gas filter correlation and Fourier‐transform infrared spectroscopy take advantage of this technique.

      The most familiar detector in the visible region is the human eye, which is, of course, used as the detector in EPA Reference Method 9 for measuring visible emissions. Phototubes, photomultiplier tubes, photovoltaic cells, and photo‐diode arrays are used in instrumented systems. Photo‐diode arrays are being incorporated increasingly into CEM analyzers (Durham et al. 1990; Saltzman 1990). Diode arrays provide a simple way of measuring multiple wavelengths and are useful for monitoring several gases in one analyzer, rather than using a separate analyzer for each gas.

      Multipath Gas Cells

      Infrared and ultraviolet spectrometers, gas filter correlation spectrometer, Fourier‐ transform infrared spectrometers, and systems using tunable lasers make use of the Beer–Lambert law, either in a simple logarithmic relationship, or more complicated expressions. One characteristic in these instruments in which light energy is absorbed by targeted molecules is that when more molecules absorb the light energy, the greater will be the difference between the incident light intensity I_o and the signal intensity, I. For low concentrations or small absorption coefficients, an instrument may not be sensitive enough to provide accurate measurements. In particular, weaker absorption bands in the near infrared make it difficult to apply tunable diode lasers for gas monitoring unless the pathlength is increased.

      By increasing the measurement pathlength, more opportunities are provided for light energy to be absorbed before reaching the detector. There are several ways of doing this; the simplest is to just increase the length of the measurement cell. This was attempted in some early CEM instruments, but led to bulky, problem‐prone installations, most of which have since been replaced. Other approaches include using multipath cells (such as White and Herriott cells), using high sensitivity, ultralong pathlength, absorption techniques such as cavity ring‐down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS), or by measuring across the stack in a double‐pass in‐situ system instead of using a sample cell in an extractive system analyzer.

       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