Extinction coefficient, definition

Mohr procedure exptl. details of, 349, 351 Molar absorption coefficient 649 Molar conductivity 520 Molar extinction coefficient see Molar absorption coefficient Molar solution definition, 260 Molarity 259 Mole 259... 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m -1 cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M -1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts max to 270 nm with extinction coefficients of 20,000-40,000 M 1 cm 1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

The compound [Pt(terpy)(SCH2CH20H)]N03 is a dark red-purple microcrystalline material that is extremely water soluble. The complex is stable in cold aqueous solution for periods up to several weeks. High pH or heating (T >60°) leads to decomposition, however. The electronic absorption spectra of the ter-pyridine thiolato compounds are characteristically definitive in the 300-350 nm region and may be used to determine product purity. Solutions of [Pt(terpy)-(SCH2 CH2 OH)] N03 of less than 15 pM obey Beer s law and exhibit the following absorption maxima and molar extinction coefficients 475 (890), 342 (12,900), 327 (10,700), 311 (10,300), 277 (20,300), and 242 (28,700) nm. 

MOLAR EXTINCTION COEFFICIENT (e) (see its definition under Absorbance). 

Another factor that influences the electronic structure of the complexes is distortion from Oh symmetry of the donor atom positions. There seems to be a general trend toward higher extinction coefficients as the complexes are distorted away from octahedral symmetry. One measure of the distortion is the twist angle . The angle is defined for the tris-bidentate group of complexes in Fig. 5. This definition can be extended to include any complexes with a real or pseudo threefold axis, such as the bis-tridentates of fac symmetry and many sexadentates. 

It is useful to collect here a few more definitions normally used in the discussion of optical properties. One is the molar extinction coefficient Gm = g/c, where c is the concentration in moles/liter of the absorbing material which implies the assumption that the absorption of light is due to specific light absorbing species. Absorbance (A) and optical density (D) are the other two quantities. They are related as follows ... 

In this case, the concentration of DNA was kept at a definite value of 4X10 mol/1 of phosphate units of DNA while for the dye solution various amounts of dye were used. The DNA concentration was determined from molar extinction coefficient at 260 nm22 Ep= 7400, 6500 and 7000 for Cl. perfrlngens DNA (DNA 1), E. coli DNA (DNA II) and M. lysodelktlcus DNA (DNA III), respectively. 

Since the redox changes in P700 are often monitored rather easily spectrophotometrically, its extinction coefficient would be a useful parameter for various quantitative measurements that involve P700, such as reaction rate and quantum yield. An initial attempt to determine the extinction coefficient was made by Ke, Ogawa, Hiyama and Vernon in 1971, and a more definitive follow-up measurement was reported by Hiyama and Ke in 1972. 

In principle, a definitive identification of the optically detected species P430 and the species responsible for the EPR, iron-sulfur signal would be best made with spectro-kinetic measurements. However, this approach is limited by the fact that although spectro-kinetic measurements may be performed on the optical species P430 with relatively high time resolution, in spite of its rather small extinction coefficient. 

Thus there now exists overwhelming experimental evidence that the quarter millimolar extinction coefficient of HiCN at X = 540 nm is very near 11.0. This value was therefore proposed by the standardization committee of the European Society of Hematology at its 1963 meeting in Lisbon (B2) and was definitely adopted at the next meeting of this committee (Stocldiolm, 1964, B3). 

The relationships between measurable quantities related to absolute transition probability (e.g. absorption cross section, molar absorption or extinction coefficient, radiative lifetime) and the fundamental quantities used to describe and inter-relate the observable quantities axe fraught with difficulties of unit conversions and internally consistent treatments of initial- and final-state degeneracies. Several excellent papers on this subject exist (Hilborn, 1982 and 2002, Larsson, 1983, Tatum, 1967, Schadee, 1978, and Whiting, et al., 1980). Much of Section 6.1.1 is based on or checked against Hilborn (1982 and 2002), although slightly different notation and definitions are used. 

When concentration C, is expressed as moles per liter, e is the molar extinction coefficient. Although the extinction coefficient should be a constant by the above definition, we will see that it is often dependent on the environment. 

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