Branched alkanes analysis

Contaminants in recycled plastic packaging waste (HDPE, PP) were identified by MAE followed by GC-MS analysis [290]. Fragrance and flavour constituents from first usage were detected. Recycled material also contained aliphatic hydrocarbons, branched alkanes and alkenes, which are also found in virgin resins at similar concentration levels. Moreover, aromatic hydrocarbons, probably derived from additives, were found. Postconsumer PET was also analysed by Soxhlet extraction and GC-MS most of the extracted compounds (30) were thermally degraded products of additives and polymers, whereas only a few derived from the original contents... 

Mineral Oil Hydraulic Fluids. Methods are available for analysis of the hydrocarbon components of mineral oil hydraulic fluids (predominantly straight and branched chain alkanes) in environmental samples. Some of these methods are summarized in Table 6-3. In general, water and sediment samples are extracted with a suitable solvent in a Soxhlet extractor (for solid samples) or in separatory funnel or shake flask (for liquid samples) (Bates et al. 1984 Peterman et al. 1980). The extract is cleaned up on silica gel or Florisil columns using a nonpolar solvent to elute the nonpolar alkanes. Analysis is usually performed by GC/MS (Bates et al. 1984 Kawamura and Kaplan 1983 Peterman et al. 1980). Method performance has not been reported, although 82% recovery of aliphatic hydrocarbons was reported for rainwater (Kawamura and Kaplan 1983). 

The final step in the molecular-mechanics calculation of molecular conformation involves the minimization of the energy Approximations are involved whose importance is not always clear. Usually, all first derivatives with respect to the various internal coordinates are set equal to zero - although these coordinates are often not independent (see Section 10.6). Furthermore, the final conformation obtained depends on the assumed initial structure. Therefore, (he method must be applied with care and a certain amount of chemical intuition. In spite of these uncertainties the molecular mechanics method has been employed with considerable success, particularly in the conformational analysis of branched alkanes. For molecules containing hetero-atoms, it can be applied, but with somewhat less confidence. 

The GP rule (eq. [21]) suffers from the poor statistical analysis for parameters describing methine and quaternary carbon atoms in highly branched alkanes. This was improved by Lindeman and Adams (120), who introduced another equation based on data for 39 alkanes, C5 through C9 ... 

One method (EPA 8020) that is suitable for volatile aromatic compounds is often referred to as benzene-toluene-ethylbenzene-xylene analysis, although the method includes other volatile aromatics. The method is similar to most volatile organic gas chromatographic methods. Sample preparation and introduction is typically by purge-and-trap analysis (EPA 5030). Some oxygenates, such as methyl-f-butyl ether (MTBE), are also detected by a photoionization detector, as well as olefins, branched alkanes, and cycloalkanes. 

It is well established that propionate can be utilized for the methyl branch unit in methyl branched hydrocarbons in insects (4-7), and recent data have shown that the methyl branches are inserted early during chajij elongation rather than toward the end of the process (13,16). C-NMR analysis demonstrated that propionates labeled with C in either the 1, 2 or 3 positions are incorporated into the methyl branched alkanes of insect cuticular lipids. C-3 of propionate becomes the branching methyl carbon, C-2 becomes the tertiary carbon and C-l the carbon adjacent to the tertiary carbon (13,16,17) in these methyl branched hydrocarbons. 

It is quite difficult to find accurate and detailed analyses of single isomers for branched alkanes with more than 10 C-atoms in the literature. Nevertheless, GC analysis of heavy naphtha, kerosene and light gasoils indicates the prevailing presence of isoprenoid structures characterized by an average probability of methyl substitution of about 0.20 (Altgelt and Boduszynski, 1994). 

If a good force field is available, then it is possible to calculate the frequencies of the fundamentals of the different possible conformers with known geometry by normal coordinate analysis (Mizushima, 1954 Woodward, 1972) and to compare them with the experimental spectrum. In this way, Crowder and Lynch (1985, and references therein) have identified the main conformers and their frequencies in a series of branched alkanes up to Cg. 

For each carbon number there is only one normal alkane, while the branched alkanes include all other isomers. The first branched isomer is 2-methylpropane (isobutane) (Table 2). The most important C5 unit, isoprene (alkene) and the saturated isoprane are for the most part photosynthetically produced and are the building blocks of the open-chain and cyclic terpenoids. The importance of the analysis of the mono-, di-, etc. cyclic terpanes will be discussed in the cycloalkanes section. 

A very detailed discussion of branched alkanes MS appears in the literature reviews of biomarkers Most of the compounds were analysed by combined GC/MS. A more detailed examination of the use of fragmentation profiles will be given in our discussion of separation methods and petroleum analysis (Section VII. A). 

The methods discussed for separation of alkanes and cycloalkanes from alkenes, aromatics, resins and other more polar constituents of petroleum can be employed also for synthetic mixtures, asphalts, bitumens, etc. However, for the quantification and identification of such homologous families as n-alkanes, branched alkanes, etc., further analysis is needed. Whereas the GTA relates to functionality and differences in polarity, the separation within the alkane family cannot be based on these characteristics. 

The differences in hydrocarbon patterns in surface waxes and in the components of interior tissue are illustrated by analysis of pupae of Manduca (tobacco hornworm). n-Alkanes only comprised ca 3% of the hydrocarbon fraction of the cuticular wax, the balance being unsaturated compounds. In contrast, internal tissues (fat bodies, muscle, gut) contained the same carbon spectrum (C21 to C41) as in the wax but now branched alkanes made up the bulk of the hydrocarbon fraction ca 80%), followed by n-alkanes (9%) with the residue being unsaturated compounds The proportion of n-alkanes in the hydrocarbon fraction from cuticular wax of a Bombyx silkworm fell from 95 to 35% on passage from the larval to the pupal stage " and similar results have been found for Trichoplusia (cabbage looper) and Drosophilia (fruitfly) species. However, it is likely that the cuticular wax has a more stable composition over the adult life of most insects and is only synthesized at (low) rates sufficient to replace that lost by wear and tear. The site of synthesis has been demonstrated to be in the cuticle in a cockroach species no hydrocarbon synthesis occurred in preparations from fat bodies . 

For paraffins the chemical shift is calculated assuming that the carbon of interest is an alkyl-substituted methane molecule. First, a constant is used which nearly corresponds to that of methane (—1.87 ppm). Then chemical shift contributions are added for each carbon up to five carbons adjacent to the specified carbon. These contributions are described by constants a for the first bonded carbon, for the second carbon, which is two bonds away, / for the third, S for the fourth, and s for the fifth. If there are two a, or other carbons, the constant is multiplied by the appropriate number of adjacent carbons of the type. Values of a through s were obtained by Grant and Paul from a regression analysis of a series of paraffins. The results are shown in Table 6.2. For branched alkanes upheld shifts are observed for branched carbons and carbons next to branches, so corrective terms are required to account for chemical shifts of carbons associated with branching. Quaternary, tertiary, secondary, and primary carbon atoms are designated by 4°, 3°, 2°, and 1°, respectively. In the cor-... 

The composition of lipids from the silk and cuticule has been reviewed by Schulz (1997a, 1999). These lipids consist primarily of alkanes, as found in other arthropods, with 2-methylalkanes with an even number of carbon atoms in the chain being most abundant, with lesser amounts of alcohols, acids, aldehydes, and wax esters. Recently, a thorough analysis of the silk lipids of N. clavipes (Schulz, 2001) revealed a unique class of lipids from spider silk and cuticle, consisting of straight-chain and branched methyl ethers (1-methoxyalkanes, Fig. 4.4) with chain lengths between 25 and 45 carbon atoms. 

Most oils contain low levels of saturated and unsaturated hydrocarbons. In olive oil, the unsaturated hydrocarbon squalene can constitute up to 40% of the unsaponifiable fraction (Boskou, 1996). Other hydrocarbons commonly present in olive oil are straight chain alkanes and alkenes with 13 to 35 carbon atoms, along with very low amounts of branched chain hydrocarbons. Variations are found between different olive varieties but the main hydrocarbons are those with 23, 25, 27 and 29 carbon atoms (Guinda et al., 1996). Olive oil can clearly be differentiated from other vegetable oils on the basis of hydrocarbon components, and levels of 2.6% crude rapeseed oil or crude sunflower oil can be detected by hydrocarbon analysis (Webster et al., 1999). Terpenes have been identified in the volatile fraction of crude sunflower oil (Bocci and Frega, 1996). 

Selectivity of branched(iso)/linear (normal) aldehydes. GC analysis with various high-temperature programs showed no high-molecular-weight products and only trace amoimts of hydrogenated products such as alkanes or alcohols. 

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