Absorption, measurement molar coefficient

ABP = 2-amino-5-bromophenyl(pyridin-2-yl)methanone 226,227 Absorbance, determination of 31 Absorption, measurement of 9,17,31 molar coefficient 36, 40 quantitative relationship 35, 36 recording of spectra 30, 31 -bathochromic/hypsochromic shift 31 -comparison to spectra of solutions 31 scanning curves 17,31,32 ACB = 2-amino-5-chlorobenzophenone 227... 

A standard UV cell was filled with 3.5mL of a 3 X 10"4 M dichloromethane solution of polyether 2. The solution was then treated with 10 uL of trifluoromethanesulfon-ic acid and the changes in UV absorption of the mixture were monitored. Once the reaction was complete the molar extinction coefficient of the product at 276nm was identical to that of naphthalene, therefore conversions during acidolysis were calculated directly from absorption measurements (At/A ). 

FIGURE 8-10 Absorption spectra of the common nucleotides The spectra are shown as the variation in molar extinction coefficient with wavelength. The molar extinction coefficients at 260 nm and pH 7.0 (e26o) are listed in the table. The spectra of corresponding ribonucleotides and deoxyribonucleotides, as well as the nucleosides, are essentially identical. For mixtures of nucleotides, a wavelength of 260 nm (dashed vertical line) is used for absorption measurements. 

To apply the Lambert-Beer law in calculation of the concentrations of the pseudo-enantiomeric substances, the molar coefficients of absorbance need to be determined. For this purpose, solutions of (R)-4 and (S)-13C-4 in cyclohexane at different concentrations have to be prepared. After measuring the corresponding absorbances at the absorption maxima of the carbonyl-stretching vibration, the molar coefficients of absorbance are calculated by applying the Lambert-Beer law E = e c d (Figure 9.8). 

After preparation of a stock solution (0.200 M) of (R)- 1-phenylethyl acetate ((i )-4) and (S)-( 1 -phenylethyl)-1 -13C-acetate ((b1)-l3C-4) in cyclohexane, the solutions are diluted with cyclohexane to concentrations of 0.180, 0.160, 0.140, 0.120, 0.100, 0.080, 0.060, 0.040, and 0.020M (total volume lmL). The absorbance of the resulting samples is measured with a FTIR spectrometer at the corresponding absorption maxima of the carbonyl-stretching vibration ((i )-4 1751 cm-1 (S)-13C-4 1699 cm-1) with a thickness of the layers of 25.0 pm, performing 32 scans at a resolution of 4 cm-1. The molar coefficients of absorbance are determined by linear regression, with correlation coefficients >0.995. Analysis of synthetic mixtures of the pseudo enantiomers of 1-phenylethyl acetate is performed under the same conditions at a concentration of 0.10 M. 

In order to determine the quantum yield A = — dnA/dnp by absorption measurements, the molar absorption coefficients of the starting material A and of the product (mixture) B must be known. The differential amount of light absorbed by the reactant, dnp, during an exposure interval, dt, is then given by Equation 3.18. Replacing the partial absorbance by the reactant A at time t, A Ah by s AnA(t)l/V, where / is the optical pathlength, we obtain... 

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. 

If the four molar absorption coefficients are known and absorption is measured at the two wavelengths, the concentrations Ci and C2 then can be determined by solution of the simultaneous equations (2.18) and (2.19). If three components are present, it is necessary to make absorption measurements at three wavelengths, and three simultaneous equations are solved, The number of wavelengths used must be at least as many as the number of components. 

The major advantage of fluorescence methods is sensitivity. One can see why fluorimetric methods are so sensitive if we compare them with absorption spectroscopy. In absorption methods, the absorbance of a compound in the range of 1.0x10 absorbance units can be measured using a good absorption spectrophotometer. Assuming a very efficient absorber with an extinction molar coefficient of =1 x lO lmol cm, we can calculate a low detection limit Cdl of ... 

The exact amount of UV radiation absorbed at a particular wavelength is expressed as the compound s molar absorptivity or molar absorption coefficient (e). This is a measure of the efficiency of radiation absorption and is calculated from the absorbance of radiation, which is derived from the Beer-Lambert Law. 

Fig. 8.12. Determination of diffusion coefficients of deuterated PE chains in a PE matrix by infrared absorption measurements in a microscope. Concentration profiles 4> x) obtained in the separated state at the beginning and at a later stage of diffusive mixing (the dashed lines were calculated for monodisperse components the deviations are due to polydispersity) (left). Diffusion coefficients at T = 176 °C, derived from measurements on a series of d-PEs of different molar mass (right). The continuous line corresponds to a power law D oc M. Work of Klein (103)...

The basic principle of most colorimetric measurements consists in comparing under well-defined conditions the colour produced by the substance in unknown amount with the same colour produced by a known amount of the material being determined. The quantitative comparison of these two solutions may, in general, be carried out by one or more of six methods. It is not essential to prepare a series of standards with the spectrophotometer the molar absorption coefficient can be calculated from one measurement of the absorbance or... 

If it is known that the compound obeys Beer s Law the molar absorption coefficient e can be determined from one measurement of the absorbance of a... 

The values of the molar absorption coefficients and e2 can be deduced from measurements of the absorbances of pure solutions of substances 1 and 2. By measuring the absorbance of the mixture at wavelengths Xl and X2, the concentrations of the two components can be calculated. 

A) The use of a calibration graph. This overcomes any problems created due to non-linear absorbance/concentration features and means that any unknown concentration run under the same conditions as the series of standards can be determined directly from the graph. The procedure requires that all standards and samples are measured in the same fixed-path-length cell, although the dimensions of the cell and the molar absorption coefficient for the chosen absorption band are not needed as these are constant throughout all the measurements. 

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