Continuous Emission Monitoring. James A. Jahnke
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other techniques will have problems in the same application. Also, for proper evaluation of system performance, a knowledge of how an analyzer works is essential to understand the effects of interferences, temperature, pressure, and so on. This understanding will be developed by first reviewing some of the basic properties of light and how these properties can be applied to measure pollutant gas concentrations.
The Wave Nature of Light
Light can be characterized as a wave composed of oscillating electric and magnetic fields. Light waves, or electromagnetic waves as they are better called, are distinguished by their wavelength or frequency. Figure 4‐1 illustrates the typical sinusoidal nature of an electromagnetic wave and its associated wavelength. The length between successive oscillations of a wave is called wavelength (λ). The period of time that it takes a wave to go through an oscillation cycle is called frequency (v). Wavelength and frequency are related by the following expression:
Figure 4‐1 An oscillating electric field and its wavelength.
(4‐1)
where c is the velocity of light, 3.0 × 108 m/s.
Literature describing continuous monitoring instruments often specifies the wavelength to characterize the spectral region used in the analytical method. Different units are often used for wavelength in different regions of the electromagnetic spectrum although the nanometer (nm) = 10−9 m has become the standard unit. Another unit, the Angstrom, Å = 10−10 m, has been used historically in the ultraviolet region. In the infrared region, both the μm = 10−6 m (also called 1 “micron”) and the wavenumber are commonly used by spectroscopists. The wavenumber is expressed as
(4‐2)
Note that the units of
The wavelength of light used in CEM instrumentation ranges from 200 nm in the ultraviolet to 20 000 nm in the infrared. Figure 4‐2 shows the regions of the electromagnetic spectrum and the wavelength bands where molecules interact with light energy. Instruments are designed to measure the effects of these interactions.
The infrared spectral region is particularly important for the measurement of gaseous pollutants, and many instruments have been designed to operate in the infrared region. The infrared region is separated into the near infrared (NIR), mid infrared (MIR), and far infrared. Although an ISO standard does exist (ISO 2015), the boundaries of these regions are not universally agreed upon; there exists some ambiguity as to where the mid infrared ends and the far infrared begins, depending upon the usage by instrument manufacturers, spectroscopists, astronomers, or others.
Absorption of Light by Molecules
Light carries energy. Light has the properties of waves, but in some of its interactions with matter it behaves as if it were composed of discrete packets of energy called photons. In a beam of light having frequency v, each of these photons carries energy defined by the Planck‐Einstein relation of Equation 4‐3.
where h is Planck's constant and has a numerical value of 6.62 × 10−27 erg‐s. Clearly, the energy of a photon is dependent upon the frequency or wavelength of the light. Light (photons) of short wavelengths (such as in the ultraviolet) will carry more energy than light (photons) of longer wavelengths (such as in the infrared). Photons of different energies will have different effects. From a practical sense, sitting on the beach too long in bright sunlight can cause a severe sunburn. On the other hand, sitting under an infrared heat lamp will soothe sore muscles without causing sunburn. The effects of UV radiation are extremely severe under the Antarctic ozone hole, but less severe where the stratospheric ozone layer reduces the number of photons per unit time reaching Earth's surface. Light of different wavelengths (photons of different energies) will have a variety of effects on molecules. In a monitoring instrument, the manufacturer determines the best way to use these effects to make gas concentration measurements.
Molecules are made up of atoms and molecular electrons that are arranged in very specific patterns, which undergo unique and complex motions. If light of a given wavelength should resonate with one of these motions, it will have a high probability of being absorbed by a molecule. The light essentially alters the molecular energy, causing the molecule to act differently than it was acting before the light was absorbed.
Figure 4‐2 The electromagnetic spectrum for continuous emission monitoring analyzers.
If the absorbed light is of low energy (long wavelength, low frequency: [E = hv]), the molecule will rotate differently than it did before. This occurs typically for light wavelengths in the far‐infrared region of the spectrum, at wavelengths greater than 20 μm. Light in the range of 5–20 μm can cause changes in the vibrational characteristics of a molecule. In the range of 0.8–5 μm, the more complex overtones and combinations of fundamental vibrations give rise to light absorption. Figure 4‐2 illustrates the regions over the range of 0.8–20 μm (12 500–500 cm−1) in the infrared spectrum where typical pollutant and combustion gases absorb light due to vibrational–rotational transitions. Figure 4‐3 illustrates some of the specific motions that occur when photons of the right energy (light of the right wavelength) are absorbed by an SO2 molecule.
In the ultraviolet and visible regions of the spectrum, 180–700 nm, impinging light can cause the molecular electrons to change their energy states. Here, higher‐energy photons cause the electrons to become excited, and in the far ultraviolet may even cause the molecules to dissociate. SO2 shows a particularly strong absorption centered at 280 nm, which is taken advantage of in several SO2 analyzers, as we shall see in the next chapter.
Each of these absorption processes requires a precise quantity of radiant energy.