Molar difference absorption coefficient

For small molecules, such as aromatic amino acids and model compounds, the molar difference absorption coefficient is frequently used. By analogy with absorbance spec-... 

The different components of the human body have different absorption coefficients. As example the molar extinction curves of oxy-hemoglobin in the arteries and of desoxy-hemoglobin in the veins are shown in Fig. 10.47. 

According to the above-mentioned effect of polarization, in principle each molecule exhibits absorption at different wavelengths and intensity distributions. Bouguer, Lambert, and Beer realized many years ago a correlation between the number of particles, their properties, and the optical pathlength through a cell. It is described by a linear dependency between the attenuation and the concentration, whereby the molar decadic absorption coefficient e is derived as the proportionality constant speciflc to the molecular properties of the molecules analyzed [2,12,13]. This so-called Lambert-Beer law allows quantitative analysis of gaseous, liquid, or solid samples by absorption spectrometry. 

When the concentration of chloride ion was below 3 M, further aquation reactions from Tc(H20)ClJ to Tc(H20)2C14, etc. were observed. Similarly, aquation of hexabromotechnetate(IV) was studied (molar absorption coefficient, s445 nm = 5720 M "1 cm "1). The equilibrium constants K for Eq. (9) at different temperatures are summarized in Table 2. Analysis of the aquation rate gave the following equation ... 

In order to determine the molar absorption coefficient, a pure, dry sample of the compound must be available. Purity is often difficult to check in a routine analytical laboratory but dryness may be achieved by desiccation. Care must be exercised in the choice of desiccant and the temperature used, owing to the potential instability of the compound at even ambient temperatures or the effect of light. It is advisable to determine the value for the coefficient using different samples of the compound and subsequently compare the results. 

Alternatively the absorbance of the mixture can be measured at different wavelengths (usually one for each compound present) and, provided that the molar absorption coefficients for each compound at each wavelength are known, the concentration of each compound can be calculated. Procedure 2.2 illustrates such a calculation. 

It is often difficult to quantitate one particular amino acid in the presence of others because of chemical similarities. Interference from substances other than amino acids is also a problem in many reputedly specific methods. Ultraviolet spectroscopy is of little value in the detection of aromatic amino acids because they have similar absorbance maxima and considerably different molar absorption coefficients. 

To the eye some colors appear similar but may differ in intensity, when c and / are the same. These solutions have a larger molar absorption coefficient, e, meaning they adsorb more. The larger the adsorption coefficient the more the material adsorbs. 

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). 

Table 7 Values of Quantum Yields and Molar Absorption Coefficients for Different Water Pollutants and Oxidants in Water... 

DPD and PYD are the only markers of bone remodeling for which the exactly characterized primary referential material is available. This material is a preparation isolated from bovine bone submitted to intensive purification, and characterized with molar absorption coefficients, elementary organic analysis, mass spectrometry, and NMR spectra (R5). The values of critical differences of DPD/PYD measurements are summarized in Table 2. 

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