Solvents 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. 

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

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. 

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. 

Normal-phase HPLC (NP-HPLC) and reverse-phase HPLC (RP-HPLC) are used to purify the final products of these chemical derivatives. For NP-HPLC a gradient of hex-ane/MeOH/THF works well for separations. Usually, acetogenins are placed on RP-HPLC because they are more easily detected in the low-ultraviolet range. The weak UV absorbance at 210 nm for the a,p-unsaturated y-lactone is one reason to use an RP-HPLC system. This weak absorbance is often overshadowed by impurities that have larger absorbances. Another reason for choosing RP-HPLC is because the solvent cutoff for the RP solvents is lower than NP solvents. In their pure form, acetogenins are white, waxy substances. 

The following table lists some useful solvents for ultraviolet spectrophotometry, along with their wavelength cutoffs and dielectric constants.16... 

The choice of the solvent to be used in ultraviolet spectroscopy is quite important. The first criterion for a good solvent is that it should not absorb ultraviolet radiation in the same region as the substance whose spectrum is being determined. Usually solvents that do not contain conjugated systems are most suitable for this purpose, although they vary as to the shortest wavelength at which they remain transparent to ultraviolet radiation. Table 7.1 lists some common ultraviolet spectroscopy solvents and their cutoff points, or minimum regions of transparency. 

Ultraviolet (UV) and visible (Vis) absorbance are commonly used for detection in CE. The wavelengths available are limited by the UV-cutoff of the solvent and the fused silica capillary, allowing wavelengths as low as 190 nm to be used in aqueous solvents. 

Table 14-3 lists some common solvents and the approximate wavelength below which they cannot be used because of absorption. These wavelengths, called the cutoff wavelengths, depend strongly on the purity of the solvent. Common solvents for ultraviolet spec-... 

Briefly, TEA can react with either the residual exposed silica or with the attached phases of reverse phase columns. This causes the TEA concentration in the mobile phase to fluctuate. When the mobile phase content is fluctuating or incompletely mixed, the TEA equilibrium on the column is altered, which can cause amplification of the mixing ripples that are already normally caused by the column. This is further increased in an ultraviolet light (UV) detector, because TEA absorbs more UV light than most other mobile phase components (see Table 2.4 for the UV cutoff of common chromatographic mobile phase solvents and Figure 2.6 for the UV spectrum of TEA). 

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