Spectrophotometry absorption coefficients

The extraction of chlorophylls and carotenoids from water-containing plant materials requires polar solvents, such as acetone, methanol, or ethanol, that can take up water. These extracts must then be transferred to a solvent such as diethyl ether in order to be stored stably. Samples with very high water content, such as juices and macerated plant material, are usually freeze-dried first, and can then be extracted directly with diethyl ether. After extraction, solutions are clarified and diluted to an appropriate volume to measure chlorophyll content by UV-VIS spectrophotometry. Absorption coefficients and equations needed for quantitative determination are given in unitf4.3. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

This is the fundamental equation of colorimetry and spectrophotometry, and is often spoken of as the Beer-Lambert Law. The value of a will clearly depend upon the method of expression of the concentration. If c is expressed in mole h 1 and / in centimetres then a is given the symbol and is called the molar absorption coefficient or molar absorptivity (formerly the molar extinction coefficient). 

Definitive measurements by fundamental quantities complemented by an empirical factor, e.g. titre (titrimetry), as well as by well-known empirical (transferable) constants like molar absorption coefficient (spectrophotometry), Nernst factor (potentiometry, ISE), and conductivity at definite dilution (conductometry)... 

In spectrophotometry, Y is the absorbance A(X) of the solution at the chosen wavelength L According to the Beer-Lambert law, a and b are the products of the absorption pathlength by the molar absorption coefficients of A and B, respectively. 

The kinetic behavior of solvated electrons has been followed directly using flash radiolysis (44, 45, 58) or flash photolysis technique (62, 94, 107). The former method is more universally applicable owing to the high absorption coefficient of e soiv in a spectral region where most reactants contribute little to the overall optical density. Stopped-flow spectrophotometry has also been applied in the specific case of the eaq + H20 reaction (43), but it is not applicable to reactions where the e soiv half-life is below 0.1 msec. 

As was stressed above, in case of direct excitation of the lanthanides, the differences in the absorption spectra between the solvated ion and the complex formed are usually considered as slight but allow nevertheless the use of spectrophotometry for speciation purposes (for a recent example, see Giroux et al. (2000)). In the case of U(VI), the absorption changes observed upon complexation have often been used to determine equilibrium constants (for the classical example of U(VI) hydrolysed species, see Dai et al. (1998), Meinrath (1998)). For curium, no information of this kind is available the low molar absorption coefficient implies the use of solutions with high total curium concentrations, which are almost impossible to handle nowadays due to safety reasons. The absorption spectra of curium are known only for aqueous solutions of HCIO4 (Carnall et al., 1958), HC1, H2SO4 and HNO3 (Pascal, 1962). 

A stock solution of MDA is prepared as described previously, by dilution of 1 mmol to 100 ml with sulphuric acid (1% v/v). The concentration of MDA is determined by UV-spectrophotometry assuming a molar absorption coefficient of 13700 M-1 cm-1 at 245 nm. This solution is then diluted in 0.1 M Tris buffer (pH 7.0) and brought to... 

In particular, Cabrera et al. (1996), Brandi et al. (1999), and Satuf et al. (2005) have determined optical parameters for Ti02 particles of several commercial brands. The determinations were carried out by means of spectrophotometry experiments involving the measurement of specular reflectance and beam transmittance, as well as hemispherical transmittance and reflectance, of catalyst suspensions (Cabrera et al., 1996). By radiative transfer calculations with the discrete ordinates method (DOM), the values of the extinction and absorption coefficient and of the asymmetry parameter that better fitted the results of measurements were found. Actually, the extinction coefficients of Satuf et al. (2005) are the same as those of Brandi... 

Mass-absorption coefficients at different wavelengths for the various elements are available in various handbooks such as those in the bibliography at the end of this chapter. As in conventional spectrophotometry (Chap. 7), the optimum absorbance range for x-ray absorption analysis is about 0.1 to I. 

UVA IS spectrophotometry is also being applied for in situ analysis of separated spots in TLC. The amount of substance required for an interpretable spectrum (typically 0.01-1.0 /ug) depends on the chromatographic conditions and the absorption coefficient for the compound. 

Horwitz claims that irrespective of the complexity found within various analytical methods the limits of analytical variability can be expressed or summarized by plotting the calculated mean coefficient of variation (CV), expressed as powers of two [ordinate], against the analyte level measured, expressed as powers of 10 [abscissa]. In an analysis of 150 independent Association of Official Analytical Chemists (AOAC) interlaboratory collaborative studies covering numerous methods, such as chromatography, atomic absorption, molecular absorption spectroscopy, spectrophotometry, and bioassay, it appears that the relationship describing the CV of an analytical method and the absolute analyte concentration is independent of the analyte type or the method used for detection. 

A typical example arises from Beer-Lambert s law. In spectrophotometry, it describes the linear relationship between the concentration of a chemical species and the measured absorbance at a particular wavelength. The corresponding coefficients are called molar absorptivities. They are specific for each species and wavelength. We refer to Chapter 3.1, Beer-Lambert s Law, for a more detailed introduction of Beer-Lambert s law. 

Lei et al. reported a method for the indirect determination of trace amounts of procaine in human serum by atomic absorption spectrophotometry [54], The sample was mixed with HCIO4, heated at 85°C for 30 minutes, diluted to a known volume with water, and centrifuged. 1 mL of the supernatant solution was buffered with 0.1 M sodium acetate-acetic acid to pH 3.86, and mixed with 0.2 M Zn(SCN)j reagent to a final concentration of 0.1 M. After dilution to 50 mL with water, the solution was shaken for 1 minute with 10 mL of 1,2-dichloroethane, whereupon the zinc extracted into the organic phase was determined by air-acetylene flame atomic absorption spectrometry for the indirect determination of procaine. The detection limit was found to be 0.1 pg/g, with a recovery of 89-98% and a coefficient of variation (n = 10) equal to 3.2%. 

Equation 18-6, which is the heart of spectrophotometry as applied to analytical chemistry, is called the Beer-Lambert law,6 or simply Beer s law. Absorbance is dimensionless, but some people write absorbance units after absorbance. The concentration of the sample, c. is usually given in units of moles per liter (M). The pathlength, b, is commonly expressed in centimeters. The quantity e (epsilon) is called the molar absorptivity (or extinction coefficient in the older literature) and has the units M l cm-1 to make the product ebc dimensionless. Molar absorptivity is the characteristic of a substance that tells how much light is absorbed at a particular wavelength. 

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