Absorption co-efficient

It I0 is the intensity of light which passes through the solution and If the intensity of the same light which passes through the solvent, then IQ/It is called transmittance. From the thickness of the cell and I0/It, absorption co-efficient or extinction co-efficient is calculated. Then log I0/It is plotted against X. The maximum absorbance will be given by the maxima in the curve. 

Fig. 17.16. Plots of mass spectral intensities and relative absorption co-efficients as a function of temperature for [Y(TMHD)j] vapour without oxygen [66].

The a,ft unsaturated carbonyls exhibit an ultraviolet absorbance with an intense maximum in the range 220 nm and a weak one at 280 nm, depending on the polarity of their solvent (hexane 215 nm, methanol 221 nm, water 224 nm) (Fig. 5.5). This characteristic has been extensively exploited for the characterization of 4-hydroxynonenal produced during the peroxidation of hepatic tissues. Table 5.1 lists the molar absorption co-efficients for free hydroxyalkenals (average e= 10000-14000 M- cm-1 (Esterbauer and Weger, 1967). 

Compound Absorption peak ( mJ nm Molar extinction co-efficient T max... 

An earlier procedure described by Katz and Comb (1963) separates Up, Gp and (Ap + Cp) in the first stage and then Ap and Cp in the second stage. Because of the different absorption spectra of Ap and Cp only the first stage may be necessary for nucleotide composition determination. Moderate volumes of eluant are used so sensitivity is reasonable (50 to 100 nmoles of each nucleotide, or about 0.1 mg RNA) (Katz and Comb 1963 Nishimura et al. 1967). The first column is 0.9 x 5.0 cm of Dowex 50 H " ( x 4, 200-400) washed with 3 N HCl, water until neutral, and finally with 20 ml 0.05 N HCl. The sample of nucleotides, in 0.05 N HCl, is loaded onto the column and washed in with 1 ml 0.05 N HCl. Up is eluted with 5 ml 0.05 N HCl. The column is then eluted with water at 1 ml/min or less. Gp is eluted in the first 7.5 ml of water effluent and Ap and Cp in the next 25 ml. The nucleotide composition of the sample may be determined from the extinction co-efficients of the fractions (at 2 wavelengths for the Ap and Cp fraction). 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

Stability may not be as much of a problem as with a diode source. However, there are problems with this method as well. The range of tunability is limited by the absorption properties of the nonlinear crystal which generates the difference frequency. At present, tunability is limited to wavenumbers >2500 cm-1 and conversion efficiencies are low. Typical laser powers in the CH2 experiments (82) were 20 n W (compared to the power of the CO lasers, 10 mW-1 W). This produces a situation where IR detectors, particularly fast ones, may be close to or background noise limited. However, it is clear that more applications of this technique will appear in the future. 

In search of new sensitizers that absorb strongly in the visible region of the spectrum, Arakawa and co-workers have developed several sensitizers based on 1,10-phenanthroline ligands. Among these new compounds, (29) and (30) are noteworthy they show an intense and broad MLCT absorption band at 525 nm in ethanol. The energy levels of the LUMO and HOMO for (29) were estimated to be —1.02 and 0.89 eV vs. SCE, respectively, which are slightly more positive than those of the sensitizer (22). These sensitizers, when anchored onto a Ti02 surface, yield more than 85% photon to electron injection efficiencies.8,52,53... 

---