Ultraviolet cutoffs

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

The solvent chosen must dissolve the sample, yet be relatively transparent in the spectral region of interest. In order to avoid poor resolution and difficulties in spectrum interpretation, a solvent should not be employed for measurements that are near the wavelength of or are shorter than the wavelength of its ultraviolet cutoff, that is, the wavelength at which absorbance for the solvent alone approaches one absorbance unit. Ultraviolet cutoffs for solvents commonly used are given in Table 7.10. 

TABLE 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Absorbance of 1.00 in a 10.0 mm cell V5. distilled water. 

The ultraviolet cutoff or the absorption edge for pure vitreous siUca is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkaU metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe " ) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

Table 25-2 Eluotropic series and ultraviolet cutoff wavelengths of solvents for adsorption chromatography on silica...

Fig. 11.2. Transmission of Pyrex, Vycor, and fused quartz. Transmission curves for 2-mm-lhick samples of (a) fused quariz (GE type 102), (b) Vycor (Corning 7910—note that this is a specially controlled grade the more common 7900 has an ultraviolet cutoff similar to 7740), and (c) Pyrex (Corning 7740). (Adapted from data supplied by Corning Glass Works and General I-ledlie Co.) Certain glasses and types of fused quartz are available which lack the infrared absorption at about 2.7 microns.

Table 6.2 lists the ultraviolet cutoff for a variety of solvents commonly used in UV-VIS spectroscopy. The solvent chosen must dissolve the sample, yet be relatively transparent in the spectral region of interest. Typically, very low concentrations of sample will be present in the solvent. It is therefore important to avoid solvents that have even weak absorptions near the solute s bands of interest. Methanol and ethanol are two of the most commonly used solvents. Care must be exercised when using the latter that no benzene (an azeotropic drying agent) is present as this will alter the solvent s transparency. Normally, this will not be a problem in spectral grade solvents. 

Solvents for spectroscopic use need to be transparent in the wavelength or wavenumber ranges where the desired spectral information is to be obtained. All liquids have an ultraviolet cutoff, meaning that at and below some wavelength in the ultraviolet they absorb so much of the UV light that they cannot be used as solvents for spectroscopic purposes in this range. 

This calculation is in principle valid till the breakdown of quantum held theory itself, i.e. till Planck scale, so that one can put an ultraviolet cutoff at kp 1/Lpi in the above equation and the result is... 

TABLE 7.4 Ultraviolet Cutoff of Spectroquality Organic Solvents"... 

Solvents used for visible-ultraviolet spectroscopy may be used only for wavelengths greater than some ultraviolet cutoff wavelength Xc, below which the solvent absorbs strongly. These cutoff wavelengths /,c are listed with some other useful data in Tables 11.3 and 11.4. 

Table 11.3 Ultraviolet Cutoff Wavelength Ac (at which the Solvent in a Cell of Path Length L = 1 cm has an Absorbance of 1 unit). Dielectric Constant e, Scalar Refractive Index nD (Measured at 589 nm, the Na D-line), Dipole Moment ft (Debyes30), and Reichardt s31 Solvent Polarity Index ET...

ULTRAVIOLET SPECTROSCOPY 12.2.1 Ultraviolet Cutoff Limits for Solvents... 

Chemicals that have been prepared for a specific application are also available. Included among these are solvents for spectrophotometry and high-performance liquid chromatography. Information pertinent to the intended use is supplied with these reagents. Data provided with a spectrophotometric solvent, for example, might include its absorbance at selected wavelengths and its ultraviolet cutoff wavelength. 

Dioxane from a variety of manufacturers and of various degrees of purity were all unsatisfactory without extensive purification the measure of quality was a sharp ultraviolet cutoff at 203 n.m. and a low and reproducible rate of DMU photolysis in the neat solvent. This could only be achieved in dioxane fractionated in a nitrogen atmosphere five times from sodium through a ten plate packed column and distilled immediately into flask containing the pyrimidine. Dioxane was not considered a satisfactory solvent. 

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