Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 120

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.

(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. The probability of light being absorbed and the transition occurring is greatest when the value of hv equals that energy. If light passing through a gas contained in a cell is changed over a range of wavelengths, a detector located on the other side of the gas cell would sense a dip in the light intensity it receives at the wavelengths where these transitions occur. This is shown in Figure 4‐4. This absorption can also be plotted directly as an absorption spectrum as shown in Figure 4‐5. The absorption spectrum offers some advantages for quantitative analysis.

Figure 4‐3 Example of normal vibrations of the SO₂ molecule.

Figure 4‐4 A typical transmission spectrum.

Each electronic state of a molecule will contain many vibrational energy levels and each vibrational energy level will contain many rotational energy levels. This is illustrated in Figure 4‐6, which shows the possible energy states in which a molecule can exist. The molecule's energy state can be modified by supplying a photon of proper energy that can cause a transition from one state to another. Because there are a large number of states, there will also be a large number of wavelengths at which light will be absorbed.

Figure 4‐5 A typical absorption spectrum.

The total energy of a molecule in a specific energy state can be summarized by the approximation

(4‐4)

The fact that transitions can occur between many of these states implies that energy will be absorbed at many different wavelengths. This gives rise to an absorption spectrum that typifies a molecule. As an example, Figure 4‐7 illustrates the vibrational–rotational absorption spectrum of SO₂ in the near‐infrared region. Such spectra are very important in the development of analytical techniques for gas monitoring.

Also, Figure 4‐2 shows that different molecules can absorb light in the same region of the spectrum. This can cause problems in developing an analyzer because it can be difficult to distinguish the relative amounts of absorption from each compound in the overlap region. Water vapor can be particularly troublesome because it absorbs in many regions of the infrared spectrum and is usually present at percent levels in the gas stream, whereas pollutant gases are present at ppm levels. Interfering gases can be removed before entering an analyzer, but this can be difficult and makes the monitoring system more complicated. An alternative to removing interfering gases is to select a region of the spectrum where there is no overlap. The wavelength‐specific light emitted by lasers has enabled a wide variety of instruments to be developed using this technique. High‐resolution instruments, once found only in the laboratory, provide another alternative. Advances in computerization along with advances in the science and technology of gas measurement have since made relatively sophisticated measurement techniques available to field instruments at reasonable cost. Different approaches to such problems are discussed further in the following chapters.

Figure 4‐6 Energy‐level diagram for a molecule.

To this point we have discussed the fact that molecules can absorb light energy. However, the question arises as to how this phenomenon can be expressed quantitatively. The answer lies in a mathematical expression known as the Beer–Lambert law.