Absorption curves

Comparison of the ultraviolet spectra of analogous sulfur and selenium compounds shows that there is very little difference in the absorption curves, except for a slight bathochromic shift in the case of the selenium derivatives-... 

The shape of the broad absorption curve in Figure 9.17 is typical of that of any dye suitable for a laser. It shows an absorption maximum to low wavelength of the Og band position, which is close to the absorption-fluorescence crossing point. The shape of the absorption curve results from a change of shape of the molecule, from Sq to 5i, in the... 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

A typical absorption curve obtained for a metal-free vitreous sihca after a large dose of y-rays is shown in Eigure 13. The main band is at 215 nm three smaller bands occur at 230, 260, and 280 nm. The 230-nm band may result from an electron trapped at a sihcon atom having an incomplete oxygen bond (205). 

Physical Methods. Vitamins D2 and D exhibit uv absorption curves that have a maximum at 264 nm and an (absorbance) of 450—490 at 1% concentration (Table 8). The various isomers of vitamin D exhibit characteristically different uv absorption curves. Mixtures of the isomers are difficult to distinguish. However, when chromatographicaHy separated by hplc, the peaks can be identified by stop-flow techniques based on uv absorption scanning or by photodiodearray spectroscopy. The combination of elution time and characteristic uv absorption curves can be used to identify the isomers present in a sample of vitamin D. 

Fig. 1 Absorption curve of a chromatogram track with 4 pg of each substance per chromatogram zone. Rhamnose (1), xylose (2), arabinose (3), fructose (4).

Tetrahydroalstonine gives with the Adamkiewicz test, as modified by Harvey et al., a colour similar to that given by yohimbine, which is taken to indicate the presence of a tetrahydro-/3-carboline ring system. Further, the ultra-violet absorption curves for yohimbine and tetrahydroalstonine... 

Rauwolscine gives colour reactions like those of yohimbine and the absorption curves of the hydrochlorides of the two alkaloids are very similar. Heated to 300°/5 mm. rauwolscinic acid forms barman (p. 490) and 3-ethylindole and on fusion with potassium hydroxide decomposes into indole-2-carboxylic acid, isophthalic acid, barman and an unidentified indole derivative. Rauwolscine itself on distillation with zinc dust produces barman, 2-methylindole (scatole) and tsoquinoline. It is suggested that the alkaloid has the skeletal strueture suggested by Seholz (formula XIV, p. 508) for yohimbine, the positions of the hydroxyl and earbomethoxy grouf s being still imdetermined. 

Reference 71 has excellent solubility data and absorption curves for the system. 

The methods dependent upon measurement of an electrical property, and those based upon determination of the extent to which radiation is absorbed or upon assessment of the intensity of emitted radiation, all require the use of a suitable instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such methods are referred to as instrumental methods . Instrumental methods are usually much faster than purely chemical procedures, they are normally applicable at concentrations far too small to be amenable to determination by classical methods, and they find wide application in industry. In most cases a microcomputer can be interfaced to the instrument so that absorption curves, polarograms, titration curves, etc., can be plotted automatically, and in fact, by the incorporation of appropriate servo-mechanisms, the whole analytical process may, in suitable cases, be completely automated. 

As shown in Section 15.17, the location of the end point of a potentiometric titration can often be accomplished more exactly from the first or second derivative of the titration curve, than from the titration curve itself. Similarly, absorption observations will often yield more information from derivative plots than from the original absorption curve. This technique was used as long ago as 1955, but with the development of microcomputers which permit rapid generation of derivative curves, the method has acquired great impetus.9,10... 

Use a recording spectrophotometer to plot the absorption curves of the three separate solutions, in each case using distilled water as the blank. Use silica cells and record the spectra over the range 210-310 nm. 

DETERMINATION OF THE ABSORPTION CURVES OF AROMATIC HYDROCARBONS AND THE ANALYSIS OF BINARY MIXTURES... 

Now, starting with 0.05 mL toluene, repeat the procedure to obtain five working solutions l -5 and use solution 5 to plot the absorption curve of toluene again record the Amax values for the peaks of the curve. There is a well-developed peak at approximately 270 nm, and using the five test solutions, measure the absorbance of each at the observed peak wavelength and test the application of Beer s Law. Measure solution 5 also at the wavelength used for benzene, and solution 5 at the wavelength used for toluene. 

Spectrophotometric determinations 653, 710 analysis of a medicinal preparation, 717 D. of absorption curve, 708... 

Determination of the absorption curve and concentration of a substance (potassium nitrate) 708... 

Determination of the absorption curves of aromatic hydrocarbons and the analysis of binary mixtures 715... 

Fig. 2.4 The spectrum of bacterial luminescence measured with B. harveyi luciferase, FMN, tetradecanal and NADH, in 50 mM phosphate buffer, pH 7.0, at 0°C (dashed line from Matheson et al., 1981) and the absorption and fluorescence emission spectra of LumP (solid lines) and Rf-LumP (dotted lines) obtained from P. leiog-natbi, in 25 mM phosphate buffer, pH 7.0, containing 1 mM EDTA and 10 mM 2-mercaptoethanol, at room temperature (from Petushkov et al, 2000, with permission from Elsevier). LumP is a lumazine protein, and Rf-LumP contains riboflavin instead of lumazine in the lumazine protein. Fluorescence emission curves are at the right side of the absorption curves.

Ultraviolet absorption curves obtained at intervals during irradiation of dilute aqueous solutions until constant (ca. 40 minutes) showing three (left) and two (right) isosbestic points... 

Fig. 7—2. Spectral data to illustrate absorption and enhancement effects for three transition elements. (To avoid crowding, only part of the cobalt absorption curve is shown.) See Table 7-1. Case B. Substitution of A1 for Fe decreases absorption of incident beam and has little effect on analytical line. Net positive absorption effect. Case C. Substitution of Pb for Fe decreases absorption of primary beam but greatly increases absorption of analytical line. Net negative absorption effect. Case D. Note wavelength relationship indicated in figure. Enhancement impossible. Case E. Note wavelength relationship in figure. Enhancement occurs.

In NMR the shape of the absorption curve can be most readily quantified in terms of the even moments of the line shape. For an absorption line described by f(H), the nth moment is defined by... 

Figure 12. Electronic spectra and the results of open-shell PPP-like semiempirical calculations for radical ions. The vertical lines represent the allowed transitions, the wavy lines with arrows the forbidden ones. The right side scales denote the calculated spectral intensities, where f stands for the oscillator strength. Top left the absorption curve (146) redrawn to the log e vs. 0 (cm ) form calculations are taken from (59). Top right taken from (11). Bottom left taken from (143). Bottom right taken from (136), the absorption curve redrawn to the log e vs, 0 (cm" ) form.

Figure 14. Absorption curves of the tetracene radical ions (157) and results of the semiempirical open-shell PPP-like calculations (59). The latter are indicated by vertical lines (allowed transitions) and by wavy lines with arrows (forbidden transitions) f stands for theoretical oscillator strength.

The various porphyrinogens are colorless, whereas the various porphyrins are all colored. In the smdy of porphyrins or porphyrin derivatives, the characteristic absorption spectrum that each exhibits—in both the visible and the ultraviolet regions of the spectrum—is of great value. An example is the absorption curve for a solution of porphyrin in 5% hydrochloric acid (Figure 32-10). Note particularly the sharp absorption band near 400 nm. This is a distinguishing feamre of the porphin ring and is characteristic of all porphyrins regardless of the... 

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