Light absorption spectrophotometry

S.L. Upstone, UltravioletWisible Light Absorption Spectrophotometry in Clinical Chemistry, Encyclopedia of Analytical Chemistry, John WUey, Chichester, 1699-1714 (2000). 

According to the Pharmacopoeia of the People s Republic of China [7], the method of identification testing of mefenamic acid in capsule and tablet preparations is based on light absorption spectrophotometry. A specific quantity of the powdered contents of capsules (or powdered tablets), equivalent to 0.25 g mefenamic acid, is dissolved in a mixture of 10 mL of 0.1 mL/L hydrochloric acid/methanol (1 99), shaken, and filtered. Then some quantity of the filtrate is diluted with the above-mixed solution to produce a solution having a concentration of about 20 pg/mL. The absorption spectrum of the solution exhibits maxima at 279 nm and 350 nm. 

According to the Pharmacopoeia of the People s Republic of China, the above separated carbon tetrachloride layer is measured by the light absorption spectrophotometry at 435 nm. The absorbance of that solution is not more than 0.35 [7]. 

Ultraviolet-Visible Spectroscopy Ultraviolet-visible (UV-VIS) molecular absorption spectrophotometry (often called light absorption spectrophotometry or just UV-visible spectrophotometry) is a technique based on measuring the absorption of near-UV or visible radiation (180-770 nm) by molecules in solution.35,36 Reference standard characterization by UV-VIS spectophotometry includes determining the absorption spectra and the molar extinction coefficient. These two spectral characterizations are used as identifiers of reference standards. 

Upstone SL. Ultraviolet/visible light absorption spectrophotometry in clinical chemistry. In Meyers RA, ed. Encyclopedia of analytical chemistry Applications, theory, and instrumentation. New York John Wiley Sons, 2000 1699-713. 

Concurrent with requirements for low levels of mercurials in discharge water is the problem of their deterrnination. The older methods of wet chemistry are inadequate, and total rehance is placed on instmmental methods. The most popular is atomic absorption spectrophotometry, which rehes on the absorption of light by mercury vapor (4). Solutions of mercury compounds not stabilized with an excess of acid tend to hydrolyze to form yeUow-to-orange basic hydrates. These frequendy absorb onto the walls of containers and may interfere with analytical results when low levels (ppm) of mercury are determined. 

Maxima of Light Absorptions of Synthetic Food Colorants Determined by UV-Vis Spectrophotometry... 

Chapter 1 is an introduction to the field of molecular fluorescence, starting with a short history of fluorescence. In Chapter 2, the various aspects of light absorption (electronic transitions, UV-visible spectrophotometry) are reviewed. 

The absorption bands due to atoms are very narrow and the use of white light as the incident radiation would swamp even the best monochromating system with unabsorbed radiation on either side of the absorption band. It is fundamental, therefore, to the technique of atomic absorption spectrophotometry... 

Their increased application in light food and drink products has given a new impetus to develop fast and accurate method for their determination. Among computer-controlled instruments multivariate calibration methods and derivative techniques are playing very important role in the multicomponent analysis of mixtures by UV-VIS molecular absorption spectrophotometry [2]. Both approaches ate useful in the resolution of overlapping band in quantitative analysis [3, 4]. 

Atomic absorption spectrophotometry was developed in the 1950s by Dr. Alan Walsh.1 The instrumentation of this method is shown in Figure 6.1. In general, chemical compounds are converted into their atomic constituents, and then the light absorption at a wavelength characteristic of a particular atomic species is determined... 

Chance and associates employed spectrophotometry on intact mitochondria or submitochondrial particles to investigate both the sequence of carriers and the sites of phosphorylation. Using the dual wavelength spectrophotometer, the light absorption at the absorption maximum (Aniax) of a particular component was followed relative to the absorption at some other reference wavelength (Aref). The principal wavelengths used are given in Table 18-6. From these measurements the state of oxidation or reduction of each one of the carriers could be observed in the various states and in the presence of inhibitors. The... 

A205 and A280, spectrophotometry for protein concentrations at, 115-121 Absorbance, see Absorption Absorptivity Absorption, of light, CD spectroscopy of proteins, 219-243 Absorption spectrophotometry amylose, 689-692 lipoxygenase activity, 407-408 meat pigments... 

Absorption spectrophotometry is a satisfactory way of measuring induction time, but is quite unsuitable for the kinetic characterization of subsequent crystallization, except in the early stages minimum transmittance does not correspond to the end of crystallization, transmittance is proportional to the extent of crystallization only when no multiple scattering of the incident light occurs, and an observed decrease in transmittance can be due to light refraction (Marangoni, 1998). 

When a light beam passes through a suspension, the dispersed particles scatter light away from the forward direction, thus reducing the intensity of the transmitted beam. Turbidity, the reduction in light intensity due to such scattering, is directly analogous to the Beer-Lambert relationship used in absorption spectrophotometry,4445... 

The carbon lamp is an intense source of monochromatic UV (193.1 nm) radiation (W4). Other monochromatic light sources can be constructed for a limited range of wavelengths by exciting the resonance emission of an atomic vapor (S24). These have a stable wavelength but the emission is unstable. Lasers provide high intensity monochromatic radiation for a number of wavelengths but they are of limited value in absorption spectrophotometry. 

Because all the rules that apply to absorbance detection apply equally well to CD, it is convenient to think of CD as a modified form of absorption spectrophotometry. Spectra are temperature- and pH-dependent non-linear correlations of signal versus concentration are commonplace and are produced for the same reasons, such as chemical equilibria, polychromatic radiation, stray light, etc. Fluorescence emission CD (FDCD) spectroscopy is observed whenever an analyte meets all... 

Because each element absorbs at very discrete wavelengths, the lamp used for analysis of a particular metal emits light only at the desired wavelengths and is specific for that element. The two kinds of lamps used in atomic absorption spectrophotometry are the hollow cathode lamp (HCL) and the electrodeless discharge... 

The technique of flame atomic absorption spectrophotometry accomplishes this by aspirating the sample solution into a burner chamber, where it is mixed with a fuel gas and an oxidant gas. The mixture is then burned in a specially designed burner head (Fig. 2). The light beam is directed lengthway down the burner, and the absorption of the analyte atoms in the flame is measured. The most commonly used gas mixtures are air with acetylene and nitrous oxide with acetylene. Experimental conditions are well-defined in the literature, and cookbook conditions are available from most instrument manufacturers. In addition, many instruments are computer-controlled, and typical conditions are available directly on the operating screen. 

Spectrophotometry (the measurement of light absorption or transmission), is one of the most valuable analytical techniques available to biochemists. Unknown compounds may be identified by their characteristic absorption spectra in the ultraviolet, visible, or infrared. Concentrations of known compounds in solutions may be determined by measuring the light absorption at one or more wavelengths. Enzyme-catalyzed reactions frequently can be followed by measuring spectrophotometrically the appearance of a product or disappearance of a substrate. 

Flameless atomic absorption spectrophotometry is essentially very simple. A substrate upon which the sample matrix can be deposited is placed in or immediately adjacent to the spectrophotometer light beam, and a means of heating this substrate rapidly to 800°-3500°C is provided. Electrical resistance is usually the heating method used. The substrate... 

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