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      and therefore

      (4‐11)

      The expression can be written in terms of particulate concentration c, instead of the particle number density n:

      (4‐12)

      where

      This expression merely states that optical density is directly proportional to the particulate matter concentration and also to the pathlength. It is a very useful relation, because if, for example, the pathlength should increase by a factor of 2, the optical density will increase by the same factor. If the particulate matter concentration is decreased by 1/2, D also decreases by 1/2. Applications of this expression are examined in Chapter 8 on opacity monitors.

COMPONENTS OF A SPECTROMETER: BUILDING AN INSTRUMENT

      The types of analytical techniques used in today's commercially available CEM monitoring analyzers are listed in Table 1‐1 of Chapter 1. Of these techniques, absorption spectroscopy has been the most commonly applied technique in continuous monitoring systems. Spectrometers developed for pollutant and diluent gas monitoring typically incorporate four essential components:

      These components differ depending upon the region of the spectrum in which the instrument operates and the analytical technique itself. The following sections give examples of these components.

      Radiation Sources

      Light sources used in CEM system analyzers emit in the ultraviolet, visible, and infrared regions of the spectrum. Advances in semiconductor electronics have led to the application of light emitting diodes (LEDs) and lasers in CEM analyzers. LEDs have largely supplanted the incandescent lamps once used in opacity monitors, and diode and quantum cascade lasers, alternatives to earlier broadband infrared light sources, have led to a new generation of extractive and in‐situ CEM system gas analyzers.

       UV Light Sources.

      For the ultraviolet region of the spectrum, hollow cathode gas discharge tubes, high‐pressure hydrogen or deuterium discharge lamps, xenon arc, and mercury discharge lamps have been used. UV lamps have shorter lifetimes than those operating in the infrared, and it is sometimes difficult to maintain stable UV intensities over the extended periods of time that they operate.

       Visible Light Sources.

      Visible light is used in opacity monitors, where the peak and mean spectral response is required to be between 500 and 600 nm, with less than 10% of the peak response below 400 nm and above 700 nm. This “photopic” region is established so as to be within the visual range of human observing stack exit opacity. Tungsten lamps, green light emitting diodes, and lasers have been used for this application, however green light emitting diodes are the most commonly used today.

       Broadband Infrared Light Sources.

      Heated materials will emit radiation in the infrared region of the spectrum. Among those used are Nernst Globars (fused hollow rods of zirconium and yttrium oxides, heated to about 1500 °C), Globars (heated rods of silicon carbide), carbon rods, and heated nichrome wire. These sources emit light over a range of wavelengths, from which the analyzer selects to make gas concentration measurements.

       Light Emitting Diodes.

Light emitting diodes are semiconductors that emit light when an electric current is applied. Light is emitted from the recombination of the electron–hole pairs of semiconductor materials. The light wavelength emitted is dependent upon the energy bandgap between the valence and conduction bands of the semiconductor and can range from the infrared to the ultraviolet region as the energy of the bandgap increases. LEDs are not lasers. They do emit over a spectrum narrower than incandescent sources, but the light emitted is incoherent.

       Tunable Diode Lasers.

      Tunable diode lasers (TDLs) are now being used as infrared sources by a number of instrument manufacturers in North America and Europe for both extractive and in‐situ system analyzers in a wide variety of applications (Mettler‐Toledo 2017). Although first introduced for source monitoring applications in the 1970s (Hinkley 1972; Hinkley and Kelley 1971), diode lasers were not incorporated into CEM systems commercially until advances in communications lasers reduced costs for lasers emitting at wavelengths suitable for gas identification (Frisch 1996; Imasaka and Ishibashi 1990; Lerner 1998).

      In a diode laser, an electron from the conduction band combines with a hole in the valence band to emit a photon. In the recombination of electron–hole pairs, some of the excess energy is converted into photons. The photon will stimulate further recombination and will reflect back and forth in the resonant cavity of the laser to emit a coherent beam of light. The laser wavelength is dependent upon the recombination energy, which is a function of semiconductor materials (such as InGaAsP/InP) and the laser design. Operating in the near‐infrared region of the spectrum from 0.5 to 2.5 μm, the wavelengths emitted can be tuned over a narrow range by varying the laser temperature or the injection current.

       Quantum Cascade Lasers.

      Quantum cascade lasers (QCLs), developed in 1994 (Faist et al. 1994), are another option for gas monitoring systems. Operating at room temperature, they emit light in the mid‐infrared region of the spectrum, from 2.5 to over 20 μm, a region where many pollutant molecules strongly absorb. In contrast to diode lasers, where the light emitted depends upon the bandgap of the semiconductor material constituting the laser, quantum cascade lasers incorporate dozens of alternating semiconductor layers. The electric potential varies over the length of the device, where the semiconductor layers form potential “wells.” In QCLs, the output light wavelength is dependent upon the layer structure constructed by design, whereas in tunable diode lasers, it is a function of the material.

      The QCL relies on transitions between excited states in the conduction band (intersubband transitions) for photon generation. In operation, electrons tunnel through the “quantum wells,” generating photons as they cascade through different energy levels. One electron emits a photon in each intersubband transition within the quantum well and then tunnels into the next quantum well to emit another photon, cascading down the quantum wells of the structure to emit multiple photons.

      The flexibility in manufacturing, their ability to emit light in the mid infrared, their high optical power output, and their ability to operate at room temperature have made QCLs increasingly attractive for gas monitoring applications (Kosterev et al. 2008).

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