Trace, constituents identification

Currently, the preferred method for the analysis of liquefied petroleum gas, and indeed for most petroleum-related gases, is gas chromatography (ASTM D2163 IP 264). This technique can be used for the identification and measurement of both primary and trace constituents. However, there may be some accuracy issues that arise in the measurement of the higher-boiling constituents due to relative volatility under the conditions in which the sample is held. 

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. 

The precursor analysis approach should now be seen as a useful complement to traditional methods of flavor analysis of fruits. The latter methods are often limited to the painstaking processes of isolation and identification of those trace constituents which are directly responsible for flavor. The precursor analysis approach takes advantage of the evidence provided by Nature vhen secondary metabolites, including flavor corpounds, are biochanically transformed and accumulated by the fruit. 

Much more emphasis must be placed upon this subject of the identification of trace constituents. The problem that the polychlorinated biphenyls present in the analysis of organochlorine pesticides has been illustrated. This is just one example of the need to identify accurately a very wide range of chemical compounds in extremely small concentrations. 

Hydrocarbon gases are amenable to analytical techniques, and there has been the tendency (and it remains) for the determination of both major constituents and trace constituents more than is the case with the heavier hydrocarbons. The complexity of the mixtures that are evident as the boiling point of petroleum fractions and petroleum products increases makes identification of many of the individual constituents difficult, if not impossible. In addition, methods have been developed for the determination of physical characteristics such as calorific value, specific gravity, and enthalpy from the analyses of mixed hydrocarbon gases, but the accuracy does suffer when compared with the data produced by methods for the direct determination of these properties. 

Qualitative analysis is, in principle, very simple with XRF and is based on the accurate measurement of the energy, or wavelength, of the fluorescent lines observed. Since many WD-XRF spectrometers operate sequentially, a 20 scan needs to be performed. The identification of trace constituents in a sample can sometimes be complicated by the presence of higher order reflections or satellite lines from major elements. With energy-dispersive XRF, the entire X-ray spectrum is acquired simultaneously. The identification of the peaks, however, is rendered difficult by the comparatively low resolution of the ED detector. In qualitative analysis programs, the process is simplified by overplotting so called KLM markers onto... 

Boldingh, j., and R. J. Taylor Trace Constituents of Butterfat. IV. Isolation and Identification of Unsaturated Aliphatic Lactones. Nature 194, 909 (1962). 

Figure 21 shows a comparison of the neutral cheese fractions isolated not only by SAFE but also by the SDE technique. There were differences in both the qualitative and quantitative composition. Many heat-induced alterations of the aroma profile in the SDE sample were observed (data not presented). Identification of cheese volatiles was based on GC/MS. We found, based on our spectroscopic investigation of the acidic fraction, that the SAFE technique was more effective in extracting less volatile and polar constituents such as 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 4-hydroxy-5-methyl-3(2H)-furanone, and 5-ethyl-4-hydroxy-2-methyl-3(2H)-furanone. As a result, it should be possible to quantify these organoleptically important trace constituents by special techniques—e.g.,... 

Carbon in the forms of charcoal and soot must certainly have been known even to prehistoric races, and in Pliny s time the former was made, much as it is today, by heating wood in a pyramid covered with clay to exclude the air (21). The recognition of carbon, the chief constituent of charcoal, as a chemical element, however, is much more recent. In an interesting article in Osiris, entitled The discovery of the element carbon, Theodore A. Wertime traced the development of this concept (276). In his opinion the identification of carbon as an element was worked out step by step by R.-A.-F. de Reaumur, H.-L. Duhamel du Monceau, Torbern Bergman, C. W. Scheele, C.-L. Berthollet, A.-L. Lavoisier, and others. 

In summary, model studies are very efficient for the identification and structure elucidation of important flavor components. Most of the compounds reported here have not been identified in meat and have not yet been reported as constituents of food volatiles. Nevertheless, there are good reasons to believe that minute traces of these sulfur-containing components are present in roasted and/or cooked meat volatiles because our model system was based solely on naturally occurring precursors. We believe that only minute trace amounts of these types of components need to be present in natural products to be of prime significance due to their extremely low odor threshold values. 

In drug preparations, sensitivity is not a particular problem as there is always sufficient of the active constituent present. Suitable extraction procedures should be carried out and each fraction should then be subjected to chromatographic analysis. The mass spectra, together with the appropriate retention parameters, may then be used for identification purposes. In many cases mass spectrometric analysis is undertaken as confirmation of an identification based on the physical characteristics such as size, colour, markings, etc., of the tablet or capsule. Mass spectrometry may be used not only to identify the major components but also the trace impurities. This information may be used for quality control purposes, and may enable the route of manufacture of certain drugs to be determined. 

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