Carbon compounds, identification

Figure 10.16 Total ion current chromatogram obtained for sample 1484 after headspace SPME. Peak labels correspond to compound identification given in Table 10.6. Ax, acid with x carbon atoms. Kx, ketone with x carbon atoms...

Fig. 11.4. Electron ionization mass spectrum of nonanal. Unlike the previous example (toluene, Fig. 11.3), this 9-carbon alkyl aldehyde displays extensive fragmentation and a very low abundance molecular ion at mlz 142. The extensive degree of fragmentation exhibited by many compounds under El conditions makes manual interpretation complex and tedious. Consequently, computerized searches of spectral libraries find extensive use in compound identification.

Phosphorus nuclei have been used for many years in in vivo NMR, especially for intracellular pH measurements. However, because most organic phosphates have similar chemical shifts, compound identification can be difficult without special attention being paid to culture conditions in the NMR tube.15 Carbon NMR also yields significant results because of the large chemical shift dispersion and narrow lines of this nucleus. 13C spectra reflect most of the chemical rearrangements that may take place between substrate and final product. 

Anthony, E. J., Bulewicz, E. M., Dudek, K. Kozak, A. 2002. The long term behaviour of CFBC ash-watersystems. Waste Management, 22,99-111. Bauer, C. F. Natlsch, D. F. S. 1981. Identification and quantification of carbonate compounds in coal fly ash. Environmental Science and Technology, 15, 783-788. 

Besides their use as criteria of purity, melting-point data are extensively used in the identification of carbon compounds. The melting point usually restricts the identification to a few com-poimds which have approximately the same melting point. For this purpose, melting-point curves of mixtures are used for ex-... 

For example the Identification of carbon contamination on an IC surface as a hydrocarbon was based on Its SIMS mass spectrum showing the C H cracking pattern typical of hydrocarbons (52). Both tne"type of Instrument used and the substrate on which the sample Is adsorbed can affect the cracking pattern obtained (12)< thus good standards are necessary for accurate compound Identification. 

Both solid and liquid samples can be analyzed by XRF as described earlier in the chapter. Very flat surfaces are required for quantitative analysis, as discussed subsequently. Liquids flow into flat surfaces, but cannot be run under vacuum. The best solvents are H2O, HNO3, hydrocarbons, and oxygenated carbon compounds, because these compounds contain only low atomic number elements. Solvents such as HCl, H2SO4, CS2, and CCLt are undesirable because they contain elements with higher atomic numbers they may reabsorb the fluorescence from lower-Z elements and will also give characteristic lines for Cl or S. This will preclude identification of these elements in the sample. Organic solvents must not dissolve or react with the film used to cover the sample. 

The carbon compounds individuated and identified in carbon chemistry—the remaining stoichiometric plant and animal substances and the pure carbon compounds isolated from coal tar, as well as the artificial carbon compounds created in the laboratory—were nested in extended networks of experiments and work on paper. In the late 1840s, when the culture of carbon chemistry was firmly established, the individuation and identification of carbon compounds required quantitative elemental analysis, control of stoichiometric purity by studies of the chemical properties and reactions of a substance, experimental examination of their proximate components or constitution (later structure ), and work on paper with chemical formulae to demarcate the substances and to model their constitution and chemical reactions. Analysis of composition (qualitative and quantitative), control of purity, studies of reactions, and modeling on paper allowed chemists to draw ever more sophisticated... 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

Analytical Approaches. Different analytical techniques have been appHed to each fraction to determine its molecular composition. As the molecular weight increases, complexity increasingly shifts the level of analytical detail from quantification of most individual species in the naphtha to average molecular descriptions in the vacuum residuum. For the naphtha, classical techniques allow the isolation and identification of individual compounds by physical properties. Gas chromatographic (gc) resolution allows almost every compound having less than eight carbon atoms to be measured separately. The combination of gc with mass spectrometry (gc/ms) can be used for quantitation purposes when compounds are not well-resolved by gc. 

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