Introduction to spectral analysis methods and spectral analysis
First, spectral and non-spectral methods
Any analysis method based on the radiation signal or the change caused by the detection of energy acting on the substance to be measured can be called optical spectroscopy, often referred to as optical analysis. According to whether the measured signal is related to the energy level transition, optical analysis methods can be divided into two categories: spectral and non-spectral methods.
Nonspectral methods measure signals that do not contain energy level transitions. They are analytical methods by measuring changes in certain basic properties of electromagnetic radiation, such as refraction, scattering, interference, diffraction, and polarization. Nonspectral methods do not involve transitions in energy within a substance. Without measuring the spectrum, electromagnetic radiation only changes the direction of propagation, velocity, or certain physical properties. Belonging to this type of analysis method are refraction method, polarization method, light scattering method (turbidity method), interference method, diffraction method, optical rotation method, and circular dichroism method.
Spectroscopic analysis is based on the measurement of the wavelength and intensity of emitted, absorbed or scattered radiation produced by the transition between quantized energy levels within a substance when a substance interacts with radiant energy, in order to identify a substance and determine its chemical composition and relative content. The method is based on measuring the wavelength and intensity of radiation. These spectra are generated due to the transition of specific energy levels of atoms or molecules of a substance, and can be qualitatively analyzed according to the wavelength of their characteristic spectra. The intensity of the spectrum is related to the content of the substance and can be quantitatively analyzed. This book mainly introduces spectroscopy.
II. Types of spectra
According to different wavelength regions, spectra can be divided into infrared spectra, visible spectra, and ultraviolet spectra, etc.; According to the nature of the production of the basic particles of the spectrum, the spectrum can be divided into atomic spectra, molecular spectra; According to the apparent form of the spectrum, the spectrum can be divided into line spectra, band spectra, and continuous spectra; According to the way of production, the spectrum can be divided into emission spectra, absorption spectra, and scattering spectra. The following focuses on different spectra from the production mode.
(1) Emission spectrum, the spectrum directly generated by the light of an object is called emission spectrum. Emission spectrum can be divided into three different categories of spectrum: linear spectrum, band spectrum, and continuous spectrum. Linear spectrum is mainly produced in atoms, band spectrum is mainly generated in molecules, and continuous spectrum is mainly generated in hot solid or gas discharge.
(2) Absorption spectrum. When a beam of light with continuous wavelengths passes through a substance, some components of the beam of light will be weakened after the light of certain wavelengths is absorbed by the substance, and the absorption spectrum of the substance will be obtained. Almost all substances have their own unique absorption spectra. The information given by the absorption spectra of atoms about the energy level structure is complementary to that given by the emission spectra. In the absorption spectrum, some absorptions are continuous, called general absorption spectra; some show one or more absorption bands, called selective absorption spectra. All these spectra are generated due to changes in the electronic state of molecules. Selective absorption spectroscopy has a wide range of applications in organic chemistry, including the identification of compounds, the determination of molecular structures, and qualitative and quantitative chemical analysis.
(3) Scattered light spectrum. When light is irradiated on a substance, in addition to the possible absorption of part of the light, scattering also occurs. When the beam of light passes through an uneven medium, part of the beam of light will deviate from the original direction and spread out. There are two kinds of scattering: Tyndall scattering and molecular scattering: when the diameter of the irradiated particle is greater than or equal to the wavelength of the incident ray, Tyndall scattering occurs, and the wavelength of the scattered light is consistent with the wavelength of the incident ray, which is less used for analysis; conversely, when the diameter of the irradiated particle is smaller than the wavelength of the incident ray, molecular scattering occurs. According to whether there is energy exchange when light interacts with molecules, molecular scattering is divided into two types. One has no energy exchange, that is, elastic collision occurs. This scattering is called Rayleigh scattering; the other has energy exchange. This scattering is called Raman scattering. The wavelength of Raman scattering light is inconsistent with the wavelength of incident rays. The latter phenomenon is collectively referred to as the Raman effect, which was discovered by Indian scientist Raman in 1928. Therefore, this scattering of light that produces new wavelengths is called Raman scattering, and the resulting spectrum is called Raman spectroscopy or Raman scattering spectrum.
From the broad concept of spectroscopy, mass spectrometry and various spectroscopic methods related to surface analysis can belong to the category of spectral analysis.
III. Classification of spectral analysis methods
(1) Emission spectroscopy, absorption spectroscopy, and scattering spectroscopy, according to the nature of the interaction between matter and radiation, spectral analysis methods are generally divided into three types: emission spectroscopy, absorption spectroscopy, and scattering spectroscopy.
Emission spectrometry is an analytical method for measuring the characteristic emission spectrum of atoms or molecules, studying the structure of substances and determining their chemical composition. Emission spectroscopy mainly includes: atomic emission spectrometry, molecular phosphorescent spectroscopy, chemiluminescence spectrometry, etc. Since fluorescence spectroscopy measures the characteristic emission spectrum of atoms or molecules, all fluorescence spectra, including atomic fluorescence spectroscopy, molecular fluorescence spectroscopy and X-ray fluorescence spectroscopy, belong to emission spectroscopy.
Absorption spectrometry is a method of analyzing the wavelength and intensity of radiation absorption by measuring substances. Absorption spectroscopy includes atomic absorption spectrometry, ultraviolet-visible spectrophotometry, infrared spectroscopy, electron spin resonance spectroscopy, nuclear magnetic resonance spectroscopy, etc.
Raman spectroscopy is the main method used for material analysis by scattering spectroscopy.
(2) Atomic spectroscopy and molecular spectroscopy, depending on the type of particles that undergo energy level transitions when matter interacts with radiation, spectroscopy can be divided into atomic spectroscopy and molecular spectroscopy. Atomic spectroscopy is generated by the change of the electron energy level in the outer or inner layer of an atom. Since the electron energy level of an atom is quantized, atomic spectroscopy is generally line spectroscopy. Belonging to this type of analysis method are atomic emission spectrometry, atomic absorption spectrometry, atomic fluorescence spectrometry, and X-ray fluorescence spectrometry.
Molecular spectroscopy is generated by the change of electron energy levels, vibration and rotational energy levels in molecules. Since many vibrational energy levels are superimposed on the ground state electron energy levels in molecules, and many rotational energy levels are superimposed on the vibrational energy levels, and the difference between electron energy levels, vibration and rotational energy levels is getting smaller and smaller, therefore, various energy difference transitions in molecules may occur, and the molecular spectrum appears as a basically continuous band spectrum. Belonging to this type of analysis method are ultraviolet-visible spectrophotometry, infrared spectroscopy, molecular fluorescence spectroscopy, and molecular phosphorescent spectroscopy.
IV. Types of Atomic Spectroscopy
According to the different excitation methods of atoms and the detection methods of light, atomic spectroscopy can be divided into atomic emission spectrometry (AES), atomic absorption spectrometry (AAS), and atomic fluorescence spectrometry (AFS).
(1) Atomic emission spectrometry, using flames, arcs, plasma torches, etc. as excitation sources, makes the outer electron excitation process of gaseous atoms or ions obtain energy and become excited state atoms M *. When transitioning from the excited state to the low-energy state or ground state, the characteristic emission spectrum is atomic emission spectrum. The method of qualitative and quantitative analysis using atomic emission spectroscopy is called atomic emission spectrometry. An instrument based on the principle of atomic emission spectrometry for analysis is called atomic emission spectrometer.
M*→M+hυ
(2) Atomic absorption spectrometry, when the electromagnetic radiation energy provided by the light source absorbed by the gaseous atom and the energy required for the transition between the two energy levels of the atom of the substance satisfy the relationship of delta E = h, the atom will produce an absorption spectrum. The method of quantitative analysis using atomic absorption spectroscopy is called atomic absorption spectrometry. An instrument based on the principle of atomic absorption spectroscopy for analysis is called atomic absorption spectrometer, also known as atomic absorption spectrophotometer.
M+hυ→M*
(3) Atomic fluorescence spectrometry. After a gaseous free atom absorbs the characteristic radiation of a light source, the outer electrons of the atom jump to a higher energy level, and then jump back to the ground state or lower energy level. At the same time, the emission of the same or different wavelength as the original excitation radiation is atomic fluorescence (spectroscopy). The method of quantitative analysis using atomic fluorescence spectroscopy is called atomic fluorescence spectroscopy. An instrument based on the principle of atomic fluorescence spectrometry for analysis is called atomic fluorescence spectrometer. Atomic fluorescence is a photoluminescence, which is also a secondary emission. Quantitative analysis is usually carried out by measuring the intensity of fluorescence at an angle (usually 90 °) to the excitation light source.
V. Types of emission spectroscopy and absorption spectroscopy
According to the spectral region where the emission spectrum is located and the different excitation methods, emission spectroscopy is divided into: gamma-ray spectroscopy, X-ray fluorescence analysis, atomic fluorescence analysis, molecular fluorescence analysis, molecular phosphorescent analysis, and chemiluminescence analysis.
According to the spectral region where the absorption spectrum is located, absorption spectroscopy can be divided into Mossbauer spectroscopy, ultraviolet-visible spectrophotometry, atomic absorption spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Absorption spectroscopy can be used to identify the structure of organic compounds, as well as chemical studies such as molecular dynamic effects, hydrogen bond formation, and tautomerism.
Raman scattering spectrum
As mentioned earlier, this kind of scattering with energy exchange and generating new frequencies is called Raman scattering (Raman scattering). This scattering is caused by the energy exchange between photons and matter molecules, that is, not only the direction of motion of the photon changes, but also its energy changes. The frequency of this scattered light is different from the frequency of the incident ray, which is called Raman displacement. The magnitude of the Raman displacement is related to the vibration and rotational energy level of the molecule. The method of using Raman displacement to study the structure of matter is called Raman spectroscopy.
The Raman effect originates from molecular vibration (and lattice vibration) and rotation, and Raman frequency and intensity, polarization, etc. mark the properties of scattered substances. Therefore, molecular vibration levels (lattice vibration levels) and rotational energy level structures can be obtained from Raman spectra, and then the material structure and composition can be derived.
But because Raman scattering is very weak, about one thousandth of Rayleigh scattering, it was not discovered by Indian physicists such as Raman until 1928. This is why Raman spectroscopy was not widely used in the early days. However, since the use of lasers as excitation light sources, especially continuous wave argon-ion lasers and ammonia-ion lasers, Raman spectroscopy technology has undergone great changes. Raman spectroscopy research has become very active again, and its research scope has also been greatly expanded. In addition to expanding the variety of substances studied, Raman spectroscopy has also become a useful tool in studying combustion processes, detecting environmental pollution, and analyzing various materials.
