Molar absorptivities of rare

Molar absorptivity of rare-earth ions at analytical wavelengths. ... 

The vapor spectra of the M(thd)3 compounds with M = Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm are shown in Figures 5 and 6. The arrows indicate absorption owing to vibrational overtone and combination bands of the organic chelate moiety. The remaining absorption bands arise from f f transitions of the rare-earth constituents. The energies and molar absorptivities of the f f absorption maxima are shown in Table VII. 

Direct spectrophotometrlc analysis Is rarely used because of the very low molar absorptivity of methyprylon. However, In the absence of any Interfering species, the maximum at 29A nm (In 2-propanol) could be used for direct spectrophotometrlc analysis. 

An additional control for the antibody specificity is the so-called absorption or preabsorption control, in which the primary antibody (prior to its use) is incubated for 1 h with a tenfold molar excess of the purified antigen. Absent or greatly diminished immunostaining should be obtained after application of this preabsorbed antibody. However, it is sometimes difficult to obtain the purified antigen therefore, it is rarely used routinely in immunohistochemical staining. Moreover, absorption of the antibody with the purified antigen does not always indicate that the antibody has bound to the same protein in the tissue (Burry 2000). 

Since UV-visible spectra are broad and lacking in detail, they are rarely printed as actual spectra. The spectral information is given as a list of the value or values of Amax together with the molar absorptivity for each value of Amax. 

Since no accurate vapor pressure data are available for the erbium and thulium hahdes, the molar absorptivities were determined directly from a weighed amount of the respective rare-earth halides. Good results could be obtained from this method if the respective halogen, bromine, or iodine were added to the cell such that its pressure at 1000°C. was 1 atm. This procedure greatly reduced the reaction of the rare-earth... 

In strongly acid solutions (1-10 M HCl) Arsenazo III reacts only with Th, Zr, Hf and U(rV). The molar absorptivities, e, of the complexes with these metals are about 10. At pH 1-4 Arsenazo III reacts with U(VI), Sc, Fe(ni), Bi, and rare earths. The sensitivity of the colour reactions is lower in this case ( -510 ). The use of Arsenazo III in strongly acid medium overcomes difficulties connected with the hydrolysis of some multivalent metals (c.g., Zr, Th, U). In the determination of these metals the high acidity enhances the selectivity of the reagent. 

Even in the absence of 7t-delocalization, azoalkanes absorb in the near-UV region, although the corresponding molar absorption coefficients (n,Jt ) are small. In the absence of competing photoreactions, some rare sterically hindered Z-isomers can be prepared. For example, (Z)-l,l -azodinorbornane (352) was prepared in toluene at 0°C from its / -precursor (Scheme 158).1083... 

In colorimetry, it is preferable that the measurement is made on a chromophore whose absorbance is situated toward longer wavelengths as this reduces the risk of superimposition of the individual absorptions of the different other compounds. Elsewhere, when measurement is preceded by a chemical reaction, the exact structure of the coloured derivative, whose absorbance is measured, is rarely known nevertheless, if it is assumed that the reaction implicated is stoichiometric, its molar absorption coefficient is accessible from the molar concentration of the compound that has been derivatized. 

In very rare cases in which photoionization is the only photoprocess, the absorption observed in a nitrous oxide-saturated solution will be that of the cation radical. However, generally, other processes such as intersystem crossing also occur in parallel. In the presence of oxygen, the hydrated electron and the triplet will be scavenged at diffusion-controlled rates, and the absorption observed in the oxygenated solution will be due to the cation radical. Under these conditions, the cation radical spectrum is easily determined. The molar absorption coefficient of the cation radical can also be calculated using the hydrated electron as an internal standard. The molar absorption coefficient for sulphacetamide cation radical was determined in this manner (Land et al 1982) and later confirmed by pulse radiolysis (see Section 12.2.2.6). 

Prior to the discussion of the spectroscopic characteristics of tetrahydrofuran (THF), it should be noted that THF and many other ether compounds are chemically unstable. Over time, THF breaks down to form peroxides. Peroxides are chemically reactive and unstable. In order to help control peroxide levels in a solvent such as THF, manufacturers typically add a chemical scavenger, called a preservative or stabilizer such as 2,6-di-r-butyl-4-mefhylphenol (2,6-di-r-butyl-p-cresol, BHT). This type of THF is referred to as preserved or nonspectral grade THF. BHT has large molar absorptivity values in the UV below 280nm with the peak maximum at about 270 nm. As a consequence, preserved THF is rarely used in conjunction with UV detectors (see Fig. 1.4). 

Several chromogenic reagents used for the determination of the trivalent rare earths include 8-hydroxyquinoline (e s 5230), Alizarin S (e s lO ), Arsenazo I (e s 10 ), Arsenazo III (e s 5 x lO ). Arsenazo III has been reported by Savvin (1964, 1964a) to form 1 1 complexes with the rare earths and to be more selective than Arsenazo I and II. Cations which have a radius of less than 0.7-0.8 A show no color reaction with Arsenazo III. These include Be, Zn, Al, Ga, In, Ge, Ti, and Sn. Sc, however, with an ionic radius of 0.81 (compared to 0.95 for In) does form a colored complex at pH of 1 to 2 as do the other rare earths, which suggests the ionic radius is not the only criterion to be considered. Arsenazo III also is an extremely sensitive reagent for tetravalent cations and the reaction with these cations can be carried out in strongly acidic media. Molar absorptivities as high as 1.5 x 10 /mole-cm have been reported. 

Figure 6.9.3 Absorption spectra of rare earth elements with simulated concentrations in LiCI-KCI at 773 K. Spectrum "Exp" was experimentally obtained, while "Cal" was superimposed by using the molar absorptivities shown in Figure 6.9.7 for the same concentration to "Exp"...

Figure 6.9.4 Results of concentration analysis of Nd, Pr, and Sm. C, concentration of rare earth elements experimentally added in the system. C p concentration calculated from the absorbance and molar absorptivity. For Sm, Qji for C =0.01 M was not obtained due to the small absorbance...

Variable wavelength UVAds detectors These measure the UV adsorption at a fixed wavelength for samples with chromophores. Since SEC is used to measure the molar masses and the distribution, it is sufficient to measure at one or two fixed wavelengths where the sample shows absorption. Spectra from diode array detectors (typically used for substance identification in HPLC) are only rarely needed, for example, for the analysis and identification of oligomers with special properties. 

The aromatic amino acids - tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe) - have strong deep-UV absorption bands (A, < 230 nm) corresponding to Sq —> S2 transitions, but commonly are excited to the Sj state in fluorescence studies (Agx 260-280 nm) to minimize photoreaction and enhance fluorescence quantum yields (f). At these longer wavelengths, Trp has the largest molar extinction coefficient ( max 5600) and quantum yield (Of 0.2) of the three amino acids for Phe, the values of and Of are so poor that this species is rarely useful in fluorescence studies. When subjected to UV irradiation, proteins with both Trp and Tyr (Figure 1) typically exhibit emission spectra whose shape is characteristic of Trp residues (A ,3x 50 nm) because of nonradiative energy transfer from Tyr to Trp. 

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