Optical Spectroscopic Techniques

As described in Chapter 3, the products of some chemical reactions are initially produced in electronically excited states. If the excited state has a sufficiently short radiative lifetime, it will emit light faster than collisional quenching by air molecules can occur (see Problem 1). The effective concentration of the emitting species (and hence emitted light intensity) is proportional to the concentrations of the reactants. As a result, the chemiluminescence intensity can be used to monitor one of the reactants if the second reactant is kept at a constant (excess) concentration. 

For example, ozone reacts with nitric oxide to form electronically excited NOz  

The light emission from electronically excited NOz extends from 590 nm out to 2800 nm, a region that is relatively easily and sensitively monitored using con- 

To measure NO using chemiluminescence, the air is mixed with a stream containing excess 03 and the chemiluminescence intensity monitored. In principle, the reverse procedure can be used i.e., excess NO can be added and the chemiluminescence intensity used to measure 03. However, this requires a source of NO such as a gas cylinder, which is often not convenient for field studies. 

Another chemiluminescence method for monitoring ozone involves the production of electronically excited formaldehyde in the 03 reaction with ethene  

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The identification and quantitative determination of specific organic compounds in very complex samples is an area of intense current research activity in analytical chemistry Optical spectroscopy (particularly UV-visible and infrared absorption and molecular fluorescence and phosphorescence techniques) has been used widely in organic analysis. Any optical spectroscopic technique to be used for characterization of a very complex sample, such as a coal-derived material, should exhibit very high sensitivity (so that trace constituents can be determined) and extremely great selectivity (so that fractionation and separation steps prior to the actual analysis can be held to the minimum number and complexity). To achieve high analytical selectivity, an analytical spectroscopic technique should produce highly structured and specific spectra useful for "fingerprinting purposes," as well as to minimize the extent of overlap of spectral bands due to different constituents of complex samples. 

It should be emphasized that the objective of combining high-resolution spectroscopic detection with chromatographic separations is to minimize the chromatographic resolution required to achieve identification and quantitation of individual components in complex samples. The advantage of optical over mass spectrometry for this purpose lies in the ability of optical spectroscopic techniques to distinguish readily between isomeric compounds. The combination of separation with spectroscopic analytical techniques, termed "hyphenated methods" by Hirschfeld (31)f is a field of intense activity at the present time it.is our belief that matrix isolation sampling adds an extra analytical dimension to the already powerful techniques achieved in this manner. 

Figure 5. Frequency scales of the optical spectroscopic techniques discussed in the text. The most widely used Raman and interferometric methods are shown in bold.

This laboratory has examined the many techniques which are available. These range from the classic organic analytical methods of the 19th century through chemical spot tests, solution spectrophotometry, infrared, and other optical spectroscopic techniques through mass spectrometry. Thin-layer chromatography is, by itself, a separation technique which allows identification of the separated components by some appropriate technique. In many cases the patterns obtained may be sufiicient for identification, and in the hands of such workers as... 

Studies on complex systems, such as metalloen-zymes, increasingly require the measurement of a minor spectral component in a far larger assembly. Basic aspects of optical spectroscopic techniques that make them particularly effective are the speed, sensitivity, and linearity of light detectors as well as the intensity, stability, and precision of conventional and laser light sources. 

Chemical Reactions. Products from gas-phase chemical reactions can also be trapped in rare gas matrices, and those products which absorb light can be studied by optical spectroscopic techniques. For example (31), the products from a low pressure 1 mm. of Hg) atomic flame of oxygen atoms plus acetylene were allowed to leak through a small oriflce in a borosilicate glass reaction chamber, where they were mixed with an excess of gaseous krypton at 1(H mm. of Hg pressure. The mixture was condensed on a quartz window cooled to liquid helium temperature. The only detectable small free radical found was HCO, but it was present in considerable quantities. Similar experiments by Harvey and Brown (23) showed that HNO could be easily produced and trapped from the gas-phase reaction of hydrogen atoms plus nitric oxide. 

The conductivity of such layers can be enhanced by replacing alkane thiol with an aromatic thiol in situ [200, 201], That the interaction energy of nanocrystals in such organizations can be continually varied by changing the interparticle distance was exploited by Heath and co-workers [202, 203], who prepared a monolayer of Ag ( 3 nm) nanocrystals at the air-water interface in a LB trough and varied the interparticle distance by applying pressure. A host of measurements including reflectivity and non-linear optical spectroscopic techniques were carried out in situ. 

The optical spectroscopic techniques are feasible only for suitably transparent solids except where diffuse reflectance spectroscopy can be used. ESR requires nonconducting samples, so that metals can generally not be studied by either type of spectroscopy. 

Complexes of nitronyl nitroxide radicals with transition metal ions of the 3d series have been more thoroughly investigated by optical spectroscopic techniques than the lanthanide complexes or the uncoordinated radicals discussed in the preceding sections. We limit this section to a short summary... 

This technique represents an improvement in the optical spectroscopic techniques which is particularly relevant for the study of the polymer electrochemical equilibra-... 

Metabolomics is the newest of the functional genomic technologies and also the least well defined in terms standard experimental approaches. A number of different spectroscopic methods can be used to obtain metabolite profiles from tissue, cellular, or extracellular fluid samples. These include NMR, mass spectroscopy, liquid chromatography, and optical spectroscopic techniques [26]. Of these, NMR approaches, such as magic angle spinning NMR, which can be used with intact cells and tissues, and mass spectrometric approaches appear to be the most promising. 

Because of the different capabihties of the various optical spectroscopic techniques and their distinct demands, special types of microscopes have been developed. 

The past decade has seen an explosion in investigations of molecular ions using a variety of optical spectroscopic techniques in conjunction with trapping mass spectrometers. The mass selection and ion storage capabilities of instruments such as 3-D QITs and FT-ICR mass spectrometers provide valuable control over the ion population under investigation. Moreover, thanks to modem ion sources, the size of molecules is no longer a limitation for gas-phase ion spectroscopic studies. A number of spectroscopic techniques have been developed to probe gas-phase molecules that will be fruitful when applied to the spectroscopy of trapped ions. 

Optical spectroscopic techniques are attractive for online analysis as remote sampling by fiber optics can... 

Optical spectroscopic techniques have been applied to a wide variety of coordination compounds in the solid state, in solution and in the gas phase, underlining the general applicability and the detailed information that can be gained from these techniques. They provide very important photochemical information, as described in Section 7.3 (Volume 1) of the first edition of this work. An illustrative recent example is the photodissociation reactions of gas-phase coordination compounds that reveal the highly resolved optical spectroscopic signatures of unexpected photofragments. " ... 

Most charge-transfer transitions show less vibronic resolution than the examples in Figure 2. Resonance Raman spectroscopy has often been used in these cases to analyze the structural changes between the initial and final states of the transition, an approach especially relevant to metal centers in enzymes and to bioinorganic model compounds. The full ensemble of optical spectroscopic techniques has been applied to the study of the lowest-energy metal-to-ligand charge-transfer (MLCT) bands in Ru(bipyridine)3 and related complexes. Other well-studied cases of MLCT transitions with resolved vibronic structure include a number of W(CO)sL complexes. "... 

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