N-Alkane distribution

The Class 4 oils are distinguished by their unusual n-alkane distribution, which is unlike that for any of the other classes these oils contain no n-alkanes heavier than about C15, are not condensates and have API gravities that range from 22.9° to 24.7° (Table III). 

Grimalt, J., Albaiges, J., Al-Saad, H.T., and Douabul, A.A.Z. (1985) n-Alkane distributions in surface sediments from the Arabian Gulf. Naturwissenschaften 72, 35-37. 

Figure 2. Comparison on n-alkane distribution from Beulah lignite, A, and n-alkane distribution from CPU bottoms recycle product, (Beulah lignite feed coal).

Figure 2. GC/MS n-alkane distribution of extracts from (a) reactor recycle cup and (b) reactor outlet line.

Briefly, n-alkane distributions with carbon number maxima in the 17-21 range (Cj y-C j ) originate largely from aquatic algal sources, while maxima characterized by higher carbon numbers are... 

Jovancicevic B, Tasic L, Wehner H, Markovic D, Polic P (1998) n-Alkane Distribution as a tool in the identification of organic type pollution in river sediments. Fresenius Environ Bull 7, 320-326. 

Odd-even ratios are frequently observed to vary from one portion of a given n-alkane distribution to another. For example, see the chromatogram of Core 12-77 from South Shore transect (Figure 6). The CPI ratio for n not greater than 20 is 7.76 and contains a large bias produced by prominent Ci7 and Ci9 biogenic peaks. This is typical of most of the samples listed in Table I. In contrast, the CPI for n not less than 20 is within 20% of 1 in most cases. 

FIGURE 27.5 Quantitative distribution of n-alkanes of the six petroleum products mentioned in Figure 27.4, illustrating distinguishing features of n-alkane distribution patterns between these products. 

FIGURE 27.10 Comparison of n-alkane distribution for the ASMB oil at weathered percentages of 0% and 45%, to illustrate the effects of physical evaporative weathering on the oil chemical composition changes. 

Gas Chromatography. Whereas N.M.R. effectively senses the f mctional groups present in a mixture, differences between retention times enable G.C. to identify individual members (provided they are moderately volatile) of a homologous series such as the n-alkanes. Positive identification requires calibration by co-injection with authentic samples (here n-alkanes) or coupling of M.S. with, preferably, a capillary G.C. column. Although G.C. stick diagrams of n-alkane distributions have been used for comparing extracts from fossil fuels, we advocate acyclic-isoprenoid-hydrocarbon distributions, and particularly the pristane/phytane ratio, as more reliable indicators of fossil fuel maturation 6). 

G.C.-M.S, analysis. The n-alkane distribution is distinctive, with a marked preference of odd over even carbon number (carbon preference index, 1.9, as in low-rank coals) in the range above C25 but with a smooth, nearly Gaussian, distribution for the lower range (peaked at Cie or C17) also, the pristane/phytane ratio is high (U.5), as in sediments of marine origin. 

Fig. 33. n-alkane distribution patterns in samples of profile Valenakker and leaves of Calum vulgaris. 

The marine samples (MAR-P, proximal marine, close to the river mouth MAR-D distal marine) have bimodal n-alkane distributions (Figure 8B), with peaks at C29-C31, fi-om terrestrial carbon discharged by the Amazon River, and broader... 

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