Elastic Light Scattering Spectrometry

Cheng Zhi Huang, Jian Ling, Jian Wang

• Publisher: De Gruyter
• Year: 2018
• ISBN:
  9783110573244
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

This book introduces the basics of light scattering and then presents theoretical methods and applications of elastic light scattering spectrometry in the field of analytical chemistry. Different elastic light scattering probes and how to use elastic light scattering probes for the analysis of inorganic ions, organic molecules, nucleic acids, proteins, biological microparticles, water and the atmospheric environment are discussed in detail.

Cheng Zhi Huang, Jian Ling, Yuan Fang Li

1Introduction to light scattering

1.1Synopsis of light scattering

1.1.1Light scattering phenomenon

Why is the sky blue? Why is the deep sea mazarine? And why is the lake green? All these questions are concerned with light scattering and its related optical phenomena.

Light scattering is an optical natural phenomenon that exists extensively. Literally, light scattering refers to a phenomenon that scatters light in all directions. When the light passes through an inhomogeneous medium, photons of different wavelengths interact and collide with the inhomogeneous area of the medium, scattering the light in different directions. Therefore, the light can be observed in other directions besides the direction of the incident light. For example, in a dark night, when you turn on the electric torch, you can observe the light cross, and this is because the electric torch light is scattered by the suspended particles in the air; hence, the light is visible to our eyes. The larger the particles, the clearer would be seen for these floating particles. It is because of the existence of these particles resulted in scattering of the electric torch light that the flashlight could not spread to infinity.

1.1.1.1Light scattering acting as an important form of light decay

The same as the light absorption, light scattering also can make the light passing through a medium decay. For example, when a white light passes through water-diluted milk, in the direction where the light advances, the milk appears pink, while from the side and above, it appears light blue. This is because the white light collides with small particles in milk emulsion, causing variation of different degrees for the light of different wavelengths from the original incidence direction of the white light. A short-wavelength light has more variation, while a long-wavelength light has less variation.

In optical propagation, the flow of photons interacts with the inhomogeneous area of a medium, such as an atom, a molecule, molecular aggregates, or particles of different sizes, and so light intensity changes in terms of spatial distribution, polarization state, or frequency. In other words, when the light propagates through a medium, if there is any inhomogeneity, it can lead to light scattering. Thus, light scattering is a form of interaction between the flow of photons and the inhomogeneous area, which widely exist in nature and our daily life.

1.1.1.2Propagation of sunlight

Propagation of light can be seen in atmosphere, such as blue sky, white cloud, rosy cloud, and fog. Either blue sky, beautiful morning light, pretty sunset glow, or a rainbow after the rain, all these phenomena are the combined result of light scattering, light reflex, and light refraction, produced by the interactions between sunlight and atmospheric suspended solids, and gas molecules. Because of this, on cloudy rainy days or in case of fierce dust storm, sunlight hardly reaches the ground. In addition, because of this, even superstrong laser beam cannot propagate to infinity.

When sunlight passes through the atmosphere, rays of different wavelengths of the sunlight react with the atmosphere, that is, with floating particles and gas molecules in it. For the fact that the light at different wavelengths has different degree of scattering, the shorter the wavelength, the larger the degree of scattering is. A short-wavelength light has a larger degree of scattering; it scatters all over the sky, resulting in pretty blue color of the sky. As a long-wavelength light has a smaller degree of scattering, it directly propagates to the ground, and the color of the light is croci or orange red. For this reason, on a sunny noon, the sun shines directly at the ground with strong short-wavelength light scattering, and we can see the blue sky and nearly white sun. The sun appears white because the sunlight is too strong, so that the scattered intensity distributed all over the wave band (white light) is not enough to change the color of the sun. While in both the morning and at night, sunlight reaches the earth through atmosphere with a large dip angle, there are much more interactions between the sunlight and floating particles and air molecules in the atmosphere, and so the shorter-wavelength including blue light and yellow light are side scattered, and only the longer-wavelength including red light reaches the earth, making the sky blue and the sun yellow to red.

When compared with the wavelength of a visible light, dust, droplet, and other suspended particles have larger sizes. For example, PM2.5 refers to a particle with a size smaller or equal to 2.5 μm; PM10 refers to a particle with a size smaller or equal to 10 μm. Diameters of PM2.5 and PM10 are far larger than the wavelength of the sunlight of visible region (360–760 nm). So, when the sunlight passes through the atmosphere and collides with these larger particles, the sunlight is reflected in different directions. Since this kind of reflected light has no selectivity for light of different wavelength, so these particles reflect white light. On foggy and cloudy days, as there are lots of mist particles, the light of different wavelengths is reflected by these particles, and so the light that reaches the earth appears white, and the light intensity is rather low.

In fact, an analogical observation can also be seen when the white light passes through milk. It has known that milk appears white because it contains a number of 100 nm casein and fat globule particles of large diameters. These particles can equally scatter and reflect the visible light of all wavelengths. If the milk is well-diluted with water, however, large particles are less and so the reflected light is weaker; in such a case, we can see scattering of different colors in different directions of the container.

Consider another example, the motorcycle exhaust is generally light blue in color, especially for a motorcycle with a two-stroke engine. The reason is that gasoline combustion produces some particles. In fact, these particles are not light blue, and the color we see is a result of the scattering of white light.

1.1.1.3Cognitive process of light scattering

In Chinese history, there are many literary quotations on light scattering. Two Kids’ Debate recorded in the book of Tang questions in Liezi Series is an example. It is a story about Confucius who encountered a debate of two kids during a study tour with his students. The debate of the kids focused on: (1) when the sun rises, it appears closer to us, while it gets farther away by the middle of the day. This is because the sun appears as a hood when it first rises; however, it appears as large as a plate by the middle of the day. (2) The sun is far when it first rises and gets closer by the noon. This is because it is cold in the morning and gets hot by the noon. Confucius could not settle the debate because of his lack of knowledge about light scattering and thus, he was mocked by the two kids: “Who says that you know about everything?”

People’s knowledge about light scattering comes from the cognition of life such as dust, smoke, fog, droplet, crystal, suspension liquid, emulsion, colloid, and other common particulate matters in life. In the nineteenth century, people’s cognition of light scattering started from “blue sky phenomenon” or blue sky physics, and finally found that the essence of light scattering is the interaction of light wave electromagnetic field and refractive index inhomogeneous area, led by medium particles, molecules, or density fluctuation. During this period, people mainly focused on scattering by small particles and molecules as well as on scattering intensity variation, but not so much on energy change due to scattering. The scattering involved only included Rayleigh and Mie scattering.

After the twentieth century, people started to focus on the light scattering of particles smaller than the molecules, such as chemical bonds, quasiparticles, atoms, and free electrons, including Compton scattering, Thomson scattering, and so on. People started to pay attention to its energy change. Scientists were especially attracted to different scattering mechanisms involving wavelength and energy changes. They classified the scattering and applied them to molecular structural analysis. These researches mainly include Raman scattering, Brillouin scattering, and so on.

1.1.2Classification of light scattering

Light scattering includes three participants: photons, homogeneous medium, and particles. A photon has energy, it is a kind of particle that flows with momentum, and it collides with suspended particles during transmission in a homogeneous medium and results in light scattering. A medium is a carrier of particles and also collides with particles (except for vacuum), while particles are normally object of the study wherein. The photon serves as a probe. On the basis of the phenomenon of their interactions and their relations between photons and particles, scattering can be divided into different types.

1.1.2.1Classification based on scattering particle size

Light scattering is closely related to the size of the particles and can be classified based on that. Table 1.1 lists the general relationship between the scattered light and incident light, their energy changes, and refractive index.

Table 1.1: Types of light scattering.

Types of elastic scattering

Scatter particles size (d)

Relati...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• List of Contributing Authors
• 1 Introduction to light scattering
• 2 Electromagnetic wave and light scattering theory
• 3 Detection of light scattering signals
• 4 Resonance light scattering spectroscopy
• 5 Light scattering spectral probes of organic small molecule
• 6 Light scattering nanospectral probes
• 7 Nano light scattering spectrometry
• 8 Light scattering microscopic bioimaging
• 9 Light scattering spectrometry of inorganic ions
• 10 Light scattering spectrometry of organic small molecules and drugs
• 11 Light scattering spectrometry of nucleic acids
• 12 Light scattering spectrometry of proteins
• 13 Light scattering spectrometry of bioparticles
• 14 Aquatic environmental light scattering spectrometry
• 15 Atmospheric environmental light scattering spectrometry
• Index