UV cutoff

In and of itself, the UV cutoff is typically not a critical parameter on which to base solvent selection. Rather, the UV cutoff is a rapid way of assessing whether or not the gross characteristics of the solvents (1) make it an appropriate choice based on the system s operating wavelength, and (2) have changed from lot to lot. Most manufacturers provide this datum for each lot of solvent as part of the printed label afBxed to each solvent bottle, since it is an easy test to perform. 

To be sure that the UV cutoff value is properly interpreted, the working definition of the UV cutoff is The wavelength at which the absorbance of the solvent in a 1-cm cell (versus air as reference) is equal to unity. The mathematical relationship between the absorbance, the incident beam intensity, and the transmitted beam intensity at a specified wavelength is represented by Beer s law as 

Close examination of Eq. 1.1 leads to the discoveiy of a misnomen the UV incident radiation is not truly cut off when A = I (i.e., T 0 at = 1), but rather is markedly attenuated. As a result, the cutoff, as defined above, occurs when the transmitted beam intensity reaching the detector is attenuated to 10% of the incident radiation on the sample. 

When impurities are present in any solvent, the overall absorbance fi r the solvent at a given wavelength, 2, is the sum of the absorption contributions of each component  

Consequently, for ai given 2, low concentrations of in urities with large e values or high concentrations of impurities with low. values will create absorbance problems. Consistent removal of such impurities from solvents is therefore critical to reproducible chromatographic and analytical performance. 

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Solvent Boiling point, °C Solvent strength parameter Viscosity, mN s m (20°C) Refractive index (20°C) UV cutoff, nm... 

Type Method OH Impurities, ppm Cl Cations Uv cutoff, nm Representative glasses... 

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). 

When equimolar quantities of a given modifier oxide are added to Si02 and the uv cutoff of the siUcate glass is at a lower energy than that of... 

Different dianhydrides have been syndiesized or are commercially available, and some structures arc shown in Fig. 5.9.64-66 An improved method for preparation of cyclobutanetetracarboxylic dianhydride (CBDA) by photochemical dimerization of die maleic anhydride has been developed by Nissan.67 The polyimide obtained by condensation of CBDA widi oxydianiline gives a transparent and colorless material. The transmittance of 50-pm-thick film is 82% and the UV cutoff is 310 nm. 

Note that the chemical potential // serves here as a UV cutoff. Thus the integral is finite. Even if p, oc. the integral is at least conditionally convergent, because of the oscillating nature of 5pc(E) as a function of E. Note that it does not matter whether a canonical-ensemble formulation or a... 

Some common solvents and their UV cutoff wavelengths... 

For HPLC, some fairly broad generalizations can be made about the selection of certain preferred solvents from the large number available. A suitable solvent will preferably have low viscosity, be compatible with the detection system, be readily available in pure form, and if possible have low flammability and toxicity. In selecting organic solvents for use in mobile phases, several physical and chemical properties of the solvent should be considered. From the standpoint of detection, the refractive index or UV cutoff values are also important. 

Since the UV cutoff of the HDET must be larger than ms, we have... 

TABLE 2 Common HPLC Buffers and their Respective pK, and UV Cutoffs... 

Typical UV Cutoff Wavelengths for Some Commonly Used HPLC Solvents and Buffer Components, Taken Primarily from Manufacturer Product Specifications... 

Generally, 1 to 5 mg of sample dissolved in 100 pi of mobile phase were injected for each SEC run. For the Si-H determinations tetrachloroethylene (TCE) mobile phase was used at 55°C. It was Dowper grade (Dow Chemical Co.) and was dried by passage through activated Molecular Sieves (Linde Co.). The Si-OH analyses employed 1,4-dioxane/ TCE (90/10 by volume) at the same temperature. The dioxane was purchased from Burdick Jackson, with UV cutoff of 211 nm and water content of 0.036%. The silicone-phenyl analyses used methylene chloride (Burdick Jackson, UV cutoff of 230 nm, water content 0.003%) at 35°C and tetrahydrofuran (THF, Burdick Jackson, UV cutoff 212 nm, water content 0.02% or less) at 50 C. Each of the mobile phases was continually purged with pure helium during use. 

This method is used to detect impurities that show UV absorption. The UV cutoff points for various solvents are listed in Table 10.1. Usually it is not easy to identify the impurities that cause UV absorption. However, if the impurities are removed by purification, better results can be obtained in electrochemical mea-... 

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