Chemical conversion techniques

The resulting purified material is more stable than the impure samples and could be characterized by a variety of spectroscopic and chemical conversion techniques. In addition, an assay of crude blood-cell extract for the presence of tunichrome was developed (162). Tunichrome blood pigments consist of a number of closely related polyphe-nolic compounds with a central triglycyl unit (Fig. 5). A tunichrome (designated An-1) isolated from the species A. nigra, for example, of... 

The direct accurate measurement of local OH concentrations has been one of the major technical challenges in atmospheric chemistry since the early 1980s. This goal was first achieved in the stratosphere (e.g., Stimpfle and Anderson, 1988), but the troposphere proved more difficult (Crosley, 1995). Nevertheless, early long-baseline absorption methods for OH were adequate to test some basic theory (e.g., Poppe et al., 1994). Current successful direct methods include differential optical absorption near-UV spectroscopy with long baselines (e.g., Mount, 1992 Dorn et al, 1995 Brandenburger et al., 1998), laser-induced fluorescence after expansion of air samples (e.g., Hard et al., 1984, >1995 Holland et al., 1995), and a variety of chemical conversion techniques (Felton et al, 1990 Chen and Mopper, 2000 Tanner et al., 1997). 

The common chemical conversion techniques use a flow reactor in which ambient OH reacts with isotopically labeled SO2 or CO to yield observable products (e.g., Felton et al. (1990) used " CO with radioactive " C02 detection, while Eisele and Tanner (1991) used " S02 with H2 " S04 detection by ion-assisted mass spectrometry). 

There are ill-defined limits on EI/CI usage, based mostly on these issues of volatility and thermal stability. Sometimes these limits can be extended by preparation of a suitable chemical derivative. For example, polar carboxylic acids generally give either no or only a poor yield of molecular ions, but their conversion into methyl esters affords less polar, more volatile materials that can be examined easily by EL In the absence of an alternative method of ionization, EI/CI can still be used with clever manipulation of chemical derivatization techniques. 

Chemical Precipitation. If physical separation techniques do not work, separation may be achieved by chemical conversion to a soluble precipitate. 

Eisele and Tanner (146) have devised a similar scheme for the measurement of [HO ] via the chemical conversion of HO to H2 S04 by the addition of S02 to a flowing reactor followed by chemical ionization of gas-phase sulfuric acid to H S04 . The H 04 ion is uniquely identified and quantified in the flowing gas sample by a mass spectrometer. This technique is capable of sensitive, realtime measurement of [HO ], and although relatively new, appears to be perhaps the best overall technique devised to date. 

While copolymerization/cross-linking of reactive monomers and cross-linking of linear polymers are the main methods for producing hydrogels, there are other techniques which also deserve mention, particularly chemical conversion and the formation of interpenetrating networks. 

Reisinger et al. [21] used the gas chromatographic-atomic absorption spectrometric technique to demonstrate that biomethylation of inorganic lead does not account for the presence of organolead compounds in sediments. Sulphide induced chemical conversion of organic lead(IV) salts into alkyl lead compounds is, however, possible. 

Combination of the results with solvent extraction/liquid chromatography data may elucidate the role of free, unreacted monomer in the post-curing process. The main conclusions have already appeared elsewhere (9). Here we report in much more detail on the experimental techniques as well as on new results on the delay of shrinkage with respect to chemical conversion. 

In chemical terms, the simplest conversion technique is the transformation of coal into a combustible gas by gasification. In its earliest form - introduced in Britain ca. i860 by Sir William Siemens - this involved generation of a producer gas, mainly a mixture of CO, CO2 and nitrogen, by incomplete combustion of coal in air, i.e., by... 

Last but not least, DSC is a powerful technique for many other polymer-relevant aspects such as monitoring of curing reactions, detection of degradation, determination of heat capacity of chemical conversions, monitoring of initiator decomposition, etc. 

MRI to characterize hydrodynamics within reactors is already established. The extent to which the potential of MR to study both hydrodynamics and chemical conversion is fully realized will depend on our ability to integrate the well-established MR spectroscopy techniques in liquid- and solid-state NMR into imaging pulse sequences, and still provide quantitative data in the magnetically heterogeneous environments typical of catalysts and reactors. 

Waste waters containing low concentrations of soluble organic lead in the presence of high concentrations of other diverse ions such as Cl pose a particularly difficult treatment problem. Generally, organic lead exists in solution as the tri- or dialkyl lead chloride species. These salts are not amenable to the conventional methods used to remove inorganic lead, viz., those of pH adjustment followed by settling. The technique of chemical conversion of the... 

The discussion that follows is divided into two sections. The Spectroscopic Methods section includes those measurement techniques that involve the interaction of a photon with a peroxy radical. The Chemical Conversion Methods section describes the measurement of another molecule or radical to which a peroxy radical has been converted. 

Chemical Conversion Methods. Laser-Induced and Resonance Fluorescence of HO. Considerable effort has been applied to the measurement of HO in the stratosphere and troposphere. Ultraviolet fluorescence techniques based on lasers or resonance lamps have received a great deal of attention and study. Because HO concentrations are typically factors of one-tenth to one-hundredth those of H02 in the atmosphere, the difficulties associated with making HO measurements by using fluorescence [low signal-to-noise ratio, laser-generated HO, background fluorescence, etc. see the... 

Chemical Amplification. The measurement of a small electrical signal is often accomplished by amplification to a larger, more easily measured one. This technique of amplification can also be applied to chemical systems. For peroxy radicals, Cantrell and Stedman (117) proposed, as a possible technique, the chemical conversion of peroxy radicals to N02 with amplification (i.e., more than one N02 per peroxy radical). This method has also been used for laboratory studies of H02 reactions on aqueous aerosols (21). The following chemical scheme was proposed as the basis of the instrument ... 

The performance and scalability of the various techniques is most easily compared in a side-by-side format. With respect to experimental procedures, it is now recognized that many chemical conversions (e.g., formation of C-N or C-C bonds) that were reported to require solid supports with catalytic activity and microwave irradiation (and thus introduced environmental concerns) do not require such auxiliaries or irradiation. They occur exothermally at low temperatures with quantitative yields and without solvent-consuming workups even on a large scale. 

This thermal conversion can be compared to the chemical conversion of the investigated reaction as long as the assumptions mentioned above are valid, or the correspondence b etween chemical and thermal conversions has been verified by another analytical technique. 

Analysis of chemical conversion and cure chemistry is another way of studying network structures. Several techniques are used for this purpose, e.g., optical spectroscopy [12], high-resolution NMR spectroscopy and titration of non-reacted functional groups. The spectroscopic methods can be used for quantitative analysis of crosslinks [13-15]. Chemical conversion is usually closely related to the network density. However, no exact quantitative information on the network structure can be obtained because reacted groups can form... 

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