Ultraviolet spectroscopy spectra presentation

In addition to IR and NMR spectroscopy there are many other instrumental techniques that are useful to the organic chemist. Two of these, ultraviolet-visible spectroscopy and mass spectrometry, are discussed in this chapter. Ultraviolet-visible spectroscopy is presented first. The use of this technique to obtain information about the conjugated part of a molecule is described. Then mass spectrometry is discussed. This technique provides the molecular mass and formula for a compound. In addition, the use of the mass spectrum to provide structural information about the compound under investigation is presented. 

The hydroxyl radical, OH, occupies an extremely important position in spectroscopy, in free radical laboratory chemistry, and in atmospheric, cometary and interstellar chemistry. Its ultraviolet electronic spectrum has been described in many papers published over the past seventy years. It was the first short lived gaseous free radical to be studied by microwave spectroscopy, described in a classic paper by Dousmanis, Sanders and Townes [121] in 1955. The details of this work are presented in chapter 10. It was the first free radical to be studied by microwave magnetic resonance, in pioneering work by Radford [141] the microwave and far-infrared laser magnetic resonance studies are... 

DNA is often present in amounts too small to be detected by direct spectroscopy. In this case, the fluorescent dye EtBr can be used to amplify the absorption. EtBr binds to the DNA molecule by intercalating between adjacent base pairs. It absorbs ultraviolet light at 300 nm and emits light at 590 nm in the red/orange region of the visible spectrum. The method can be used to determine the amount of DNA in a test-tube by comparing the EtBr-mediated fluorescence of the sample with that of standards of known amounts of DNA. 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

Analytical absorption spectroscopy in the ultraviolet and visible regions of the elechomagnetic spectrum has been widely used in pharmaceutical and biomedical analysis for quantitative purposes and, with certain limitations, for the characterisation of drugs, impurities, metabolites, and related substances. By contrast, luminescence methods, and fluorescence spectroscopy in particular, have been less widely exploited, despite the undoubted advantages of greater specificity and sensitivity commonly observed for fluorescent species. However, the wider availability of spectrofluorimeters capable of presenting corrected excitation and emission spectra, coupled with the fact that reliable fluorogenic reactions now permit non-fluorescent species to be examined fluorimetrically, has led to a renaissance of interest in fluorimetric methods in biomedical analysis. 

Ultraviolet spectrophotometry is considered a valuable tool as an aid for confirming the identification of pesticide residues. A correlation between the UV spectrum and the structure of several pesticides is discussed. Knowledge of such correlation may provide clues about the general type of chromophore present and may help the analyst to design analytical procedures. The transparency of many groups in the near UV imposes a limitation on interpretations of the absorption bands in this region. However, when taken in conjunction with the information obtained by IR, NMR, and mass spectroscopy, UV spectra may lead to structural proposals of value to the pesticide analyst. A discussion of the methods that have been utilized for the analysis of pesticides on the submicrogram level is also presented. 

The Beer-Lambert rules derived above (equation 7.3) apply equally to absorption of infrared radiation by molecules. Moreover, infrared absorption spectra possess an advantage over the more common ultraviolet absorption in the greater number of bands present. It is often possible to select an absorption band for each component of a mixture such that little or no interference occurs between them. For these reasons, infrared spectroscopy is often used quantitatively in the analytical laboratory to determine drug concentrations in solution. A calibration curve for the assay may be obtained (and Beer s law confirmed) by converting the printed spectrum... 

Pure polyethylene should not absorb ultraviolet radiation of wavelength above 200 nm since pure paraffins are transparent in that region of the spectrum. However, it is well established [ 20] that even carefully purified polyethylene differs from a simple high molecular weight straight chain paraffin in being to some extent unsaturated. The total unsaturation has been estimated to be about 0.25% C=C by weight [21]. Olefinic unsaturation of different types has been detected by infrared spectroscopy [21, 22] it seems to be mainly of the vinyl type in linear polyethylene, while most unsaturation is of the vinylidene type in branched polyethylene [22]. Attention has also been drawn to the fact that a structure seems to be present in low density polyethylene which leads to a triene on ultraviolet irradiation [23]. 

The technique of optical and ultraviolet reflectance spectroscopy, as described earlier, provides still another physical-chemical technique which can be used to study chromia-alumina catalysts. Like ESR and NMR, it has the advantage of being applicable to powdered, commercial catalysts without the necessity of special sample preparation. In the present section, some typical reflection spectra of chromia-alumina catalysts will be presented, and it will be shown that these spectra provide a measure of the amount of chromium present in the 6-f oxidation state. Reflectance spectroscopy is particularly useful here because the Cr + ion is diamagnetic, and hence cannot be detected by the usual magnetochemical or ESR techniques. In addition, it turns out that the intensity of the reflectance spectrum of a chromia-alumina catalyst is directly related to the extent to which the surface of the catalyst is covered with chromia, and thus the surface composition of such a catalyst can be roughly estimated from its optical spectrum. 

The availability of tunable lasers in the visible and infrared wavelength regions has made possible significant advances in atomic and molecular spectroscopy. At the present time, however, there is a lack cjf lasers and especially of tunable lasers in the ultraviolet (UV), vacuum ultraviolet (VUV, from 200 to 100 nm), and extreme ultraviolet (XUV, I from 100 to ZO nm) regions of the spectrum. In fact, only a few lasers have been made to operate at these short wavelengths, in spite of considerable efforts being made in the past decade. The excimer lasers, such as XeF (315 nm), XeCl (308 nm), KrF (248 nm), ArF (193 nm), Xe2 ( 170 nm), and Ara ( 120 nm), and the H2 laser ( llO nm) have been available for some time now, but these emit at discrete wavelengths or are tunable only over their relatively narrow band-widths. 

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