Ultraviolet/visible radiation

Photomultiplier tubes are among the most widely used types of transducers for detecting ultraviolet/visible radiation. 

As shown in Section 25A, the components of infrared instruments differ considerably in detail from those in ultraviolet and visible instruments. Thus, infrared sources are heated solids rather than deuterium or tungsten lamps, infrared gratings are much coarser than those required for ultraviolet/visible radiation, and infrared detectors respond to heat rather than photons. In addition, the optical components of infrared instruments are constructed from polished solids, such as sodium chloride or potassium bromide. 

The most common transducers for ICP-MS are electron multipliers. The discrete dynode electron multiplier operates much like the photomultiplier transducer for ultraviolet/visible radiation, discussed in Section 25A-4. Electrons strike a cathode, where secondary electrons are emitted. These are attracted to dynodes that are each held at a successively higher positive voltage. Electron multipliers with up to 20 dynodes are available. These devices can multiply the signal strength by a factor of up to 10. ... 

Electromagnetic radiation is often characterized by its wavelength—the distance between successive peaks in the wave. Radiation with wavelengths between 100 and 700 nanometers (3.94 X 10 and 2.76 X 10 inches) is termed ultraviolet-visible radiation. The transmittance (T) of a sample is the amount of light transmitted (P) by a sample divided by the amount of light transmitted by a blank (Pq)—T=P/Pq. The absorbance (A) of a solution is the negative logarithm of the transmittance— A=-LOG(T). The... 

The measurement of absorption of ultraviolet-visible radiation is of a relative nature. One must continually compare the absorption of the sample with that of an analytical reference or blank to insure the reliability of the measurement. The rate at which the sample and reference are compared depends on the design of the instrument. In single-beam instruments there is only one light beam or optical path from the source through to the detector. This usually means that one must remove the sample from the light beam and replace it with the reference after each reading. Thus, there is usually an interval of several seconds between measurements. 

The extent of absorption of ultraviolet-visible radiation is proportional to the number of molecules capable of undergoing the observed electronic transition therefore. 

The units of wavenumber are almost always chosen as reciprocal centimeters (cm ), so we can picture the wavenumber of radiation as the number of complete wavelengths per centimeter. The frequencies, wavelengths, and wavenumbers of the various regions of the electromagnetic spectrum were summarized in Fig. F.7. In this chapter we concentrate on vibrational and electronic transitions, which can be excited by the absorption of infrared and ultraviolet-visible radiation, respectively. 

Insulators provide a useful trap for optically active ions due to their transparency in the visible region. Nuclear (vibrational) resonances with electromagnetic radiation take place in the infrared part of the spectrum whilst resonances with bound electrons (from the valence band to the conduction band) are usually in the ultraviolet. Visible radiation therefore... 

Chromophore (Section 13 21) The structural unit of a mole cule principally responsible for absorption of radiation of a particular frequency a term usually applied to ultraviolet visible spectroscopy... 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

UV/Vis Spectra for Molecules and Ions When a molecule or ion absorbs ultraviolet or visible radiation it undergoes a change in its valence electron configuration. The valence electrons in organic molecules, and inorganic anions such as oc-... 

The determination of an analyte s concentration based on its absorption of ultraviolet or visible radiation is one of the most frequently encountered quantitative analytical methods. One reason for its popularity is that many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, analytes that do not absorb UV/Vis radiation, or that absorb such radiation only weakly, frequently can be chemically coupled to a species that does. For example, nonabsorbing solutions of Pb + can be reacted with dithizone to form the red Pb-dithizonate complex. An additional advantage to UV/Vis absorption is that in most cases it is relatively easy to adjust experimental and instrumental conditions so that Beer s law is obeyed. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The focus of this section is the emission of ultraviolet and visible radiation following thermal or electrical excitation of atoms. Atomic emission spectroscopy has a long history. Qualitative applications based on the color of flames were used in the smelting of ores as early as 1550 and were more fully developed around 1830 with the observation of atomic spectra generated by flame emission and spark emission.Quantitative applications based on the atomic emission from electrical sparks were developed by Norman Lockyer (1836-1920) in the early 1870s, and quantitative applications based on flame emission were pioneered by IT. G. Lunde-gardh in 1930. Atomic emission based on emission from a plasma was introduced in 1964. 

A UV/Vis absorbance detector can also be used if the solute ions absorb ultraviolet or visible radiation. Alternatively, solutions that do not absorb in the UV/Vis range can be detected indirectly if the mobile phase contains a UV/Vis-absorbing species. In this case, when a solute band passes through the detector, a decrease in absorbance is measured at the detector. 

Optical Properties. Teflon FEP fluorocarbon film transmits more ultraviolet, visible light, and infrared radiation than ordinary window glass. The refractive index of FEP film is 1.341—1.347 (74). 

Radiation from the sun includes significant ultraviolet and infrared radiation in addition to visible radiation. Contributions of each type to the radiation that reaches Earth s surface are reduced significantly... 

Electromagnetic spectrum (Section 12.5) The range of electromagnetic energy7, including infrared, ultraviolet, and visible radiation. 

Luminescence can be defined as the emission of light (intended in the broader sense of ultraviolet, visible, or near infrared radiation) by electronic excited states of atoms or molecules. Luminescence is an important phenomenon from a basic viewpoint (e.g., for monitoring excited state behavior) [1] as well as for applications (lasers, displays, sensors, etc.) [2,3]. 

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