Direct UV Absorption

Direct UV absorbance detection is often applicable for samples where the use of conductivity detection may be problematic. The determination of trace levels of anions in the presence of very high levels of sample matrix ions is a fairly common analytical problem. Seawater, for example, contains approximately 0.50 M 

The eluent chosen for practical separations contained 15 mM sodium chloride and 5 mM phosphate buffer at pH 6.5. Separation of ten anions gave very similar chromatograms for samples in water alone and in 20 000 ppm chloride, although the retention times were slightly shorter in the latter case. 

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As an interesting effect, a 2-50-fold increase in signal intensity for mono- and oligosaccharides using UV detection at 195 nm was observed by applying a borate buffer at pH values between 9 and 10, which enables the detection of these sugars by direct UV absorption. The highest reproducibility could be obtained at elevated temperatures (40-60°C). 

To date, direct UV absorption sensing has been used mainly in environmental applications to monitor pollutants in the atmosphere such as ozone and NO (Wu et al. 2006), hydrocarbons, and volatile organic compounds (VOCs) (Lin et al. 2004). Fiber optic UV systems for gas and vapor analysis have been reviewed by Eckhardt et al. (2007). The strong absorbance of vapors and gases in the UV region is advantageous and has resulted in a compact detection system of good accuracy. 

Ozone in the gas phase can be deterrnined by direct uv spectrometry at 254 nm via its strong absorption. The accuracy of this method depends on the molar absorptivity, which is known to 1% interference by CO, hydrocarbons, NO, or H2O vapor is not significant. The method also can be employed to measure ozone in aqueous solution, but is subject to interference from turbidity as well as dissolved inorganics and organics. To eliminate interferences, ozone sometimes is sparged into the gas phase for measurement. 

A number of diamagnetic defects are also beheved to exist in vitreous siUca. Because there is no direct way to study these species, their identification is either done indirecdy, such as by uv absorption, or by employing esr after the material has been made paramagnetic using ionizing or laser irradiation. 

After treatment with the reagent the detection limits for opium alkaloids are 50-500 ng per chromatogram zone, this is sometimes a somewhat lower sensitivity than that obtained by direct measurement of the UV absorption (cf. Table 1). However, the color reactions provided additional specificity. 

In situ quantitation Direct measurement of the UV absorption at wavelength X = 280 nm was preferred for quantitative in situ evaluation since the reagent treatment did not yield more exact results. 

Radiation from a xenon or deuterium source is focussed on the flow cell. An interchangeable filter allows different excitation wavelengths to be used. The fluorescent radiation is emitted by the sample in all directions, but is usually measured at 90° to the incident beam. In some types, to increase sensitivity, the fluorescent radiation is reflected and focussed by a parabolic mirror. The second filter isolates a suitable wavelength from the fluorescence spectrum and prevents any scattered light from the source from reaching the photomultiplier detector. The 90° optics allow monitoring of the incident beam as well, so that dual uv absorption and fluorescence... 

Once the UV-absorption spectrum of the compound to be irradiated — and whenever possible of the reaction product as well — is known, the main parameter to be selected is the excitation wavelength. In unsensitized — i.e. direct — irradiations the reaction product should not absorb any light as to avoid secondary photoreactions. The wavelength of the light used can be influenced by three factors the light source, filters and the solvent. 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

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