定量分析24光谱方法概述Introduction to Spectrochemical Methods

Spectroscopic Measurements
Spectroscopists use the interactions of radiation with mater to obtain information about a sample. The sample is stimulated by applying energy in the form of heat, electrical energy, light, particles, or a chemical reaction. The analyte is predominately in its lowest-energy or ground state. The stimulus then causes some analyte species to undergo a transition to a higher-energy or excited state. We obtain information about the analyte by measuring the electromagnetic radiation emitted as it returns to the ground state or by measuring the amount of electromagnetic radiation absorbed as a result of excitation.
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In emission the analyte is stimulated by the application of heat, electrical energy, or a chemical reaction. Emission spectroscopy usually refers to methods in which the stimulus is heat or electrical energy, whereas chemiluminescence spectroscopy refers to excitation of the analyte by a chemical reaction. Measurement of the radiant power emitted as the analyte returns to the ground state can give information about its identity and concentration. The results of such a measurement are often expressed graphically by a spectrum, which is a plot of the emitted radiation as a function of frequency or wavelength.
Classification
We can classify spectroscopic methods according to the region of the electromagnetic spectrum involved in the measurement. The regions include –ray, X-ray, ultraviolet (UV), visible, infrared (IR), microwave, and radio frequency (RF). Spectrochemical methods have provided the most widely used tools for the elucidation of molecular structure as well as the quantitative and qualitative determination of both inorganic and organic compounds.
E = h = hc/ = hc
where, h is Planck’s constant (6.63 x 10-34 J s). The wavenumber and frequency , in contrast to the wavelength , are directly proportional to the photon energy E. The radiant power of a beam of radiation is directly proportional to the number of photons per second.
The particle Nature of Light: Photons
In many radiation/matter interactions, it is useful to consider light as consisting of photons or quanta. We can relate the energy of photon to its wavelength, frequency, and wavenumber by
Electromagnetic radiation can be treated as discrete packets of energy or particles called photons or quanta. These dual views of radiation as particles and waves are not mutually exclusive but complementary.
The Electromagnetic Spectrum
The electromagnetic spectrum covers an enormous range of energies (frequencies) and thus wavelengths. Useful frequencies vary from > 1019 Hz (-ray) to 103 Hz (radio waves). An X-ray photon ( 3 x 1018 Hz, 10-10 m), for example, is approximately 10,000 times as energetic as a photon emitted by an ordinary light bulb ( 3 x 1014 Hz, 10-6 m) and 1015 times as energetic as a radio-frequency photon ( 3 x 103 Hz, 105 m).
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When the sample is stimulated by application of an external electromagnetic radiation source, several processes are possible. Some of the incident radiation can be absorbed and promote some of the analyte species to an excited state. In absorption spectroscopy, the amount of light absorbed as a function of wavelength is measured, which can give qualitative and quantitative information about the sample. In photoluminescence spectroscopy the emission of photons is measured following absorption. The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy.
The Speed of Light
In a vacuum, the velocity (c) of light is 2.99792 x 108 m s-1 and the velocity of light in air is about 0.03% less. Thus, for a vacuum, or for air, the velocity of light:
c = = 3.00 x 108m s-1 = 3.00 x 1010 cm s-1
In a medium containing matter, light travels with a velocity less than c because of interaction between the electromagnetic field and electrons in the atoms or molecules of the medium. Since the frequency of the radiation is constant, the wavelength must decrease as the light passes from a vacuum to a medium containing matter.
The wavenumber is electromagnetic radiation.
another way It is defined as
thteonu-dmebscerriboef
waves per centimeter and is equal to 1/. The has the
units of reciprocal centimeters (cm-1)
Quantitative Analysis 定量分析
Chapter 24 Introduction to Spectrochemical Methods
ቤተ መጻሕፍቲ ባይዱ
Measurements based on light and other forms of electromagnetic radiation are widely used throughout analytical chemistry. The interactions of radiation and matter are the subject of the science called spectroscopy. Spectroscopic analytical methods are based on measuring the amount of radiation produced or absorbed by molecular or atomic species of interest.
Properties of Electromagnetic Radiation
Electromagnetic radiation is a form of energy that is transmitted through space at enormous velocities. Electromagnetic radiation can be described as a wave with properties of wavelength, frequency, velocity, and amplitude. In contrast to sound waves, light requires no supporting medium for its transmission; thus, it readily passes through a vacuum.