Alkanes sampling

Table 2 The ideal crystal thickness for differently folded forms of the alkane samples studied, assuming six carbons per fold ...

The radiation cells were made from 9-mm. o.d. borosilicate glass tubing. The concentration of nitrous oxide was calculated by assuming that the amount in the gas phase was negligible since the solutions filled approximately 90% of the cells. The samples were thermostatted at 35°C. and irradiated for one hour in a G0Co y-ray facility at a dose rate of approximately 1.3 X 1019 e.v. liter1 sec."1. Ferrous sulfate dosimetry was used. Since we wished to compare different alkanes, samples were prepared in pairs one sample of the pair always contained cyclohexane and the other some other alkane. Each sample of each pair contained the same amount of nitrous oxide. 

Theoretically, optimum separation is achieved when the sample dimensionality and system dimensionality are equivalent, resulting in an ordered separation (Figure 3). In the above example, only one separation dimension would be required to analyze the alkane sample. If, however, the dimensionality of the sample exceeds that of the system, sample components will not be resolved in an orderly fashion, but rather a disordered or chaotic separation will result. In Figure 3, three descriptors are required to define the sample - shape, pattern, and size. In a chemical sense, these might be molar mass, polarity, and molecular shape. As more dimensions of separation are applied, greater definition of the mixture components is achieved. Unfortunately, for very complex samples, the sample dimensionality will be... 

Within the VGO saturates, distribution of paraffins, isoparaffins, and naphthenes is highly dependent on the petroleum source. The naphthenes account for roughly 60% of the saturates in a normal cmde oil. However, samples can be found having paraffins from <20 to >80%. In most samples, the / -paraffins from C2Q—are still present in sufficient quantity to be detected as distinct peaks in gc analyses. Some cmde oils show a nearly symmetric pattern of peaks such that each carbon number is present in regular progression up to a maximum around C -j. Other cmde oils show a similar distribution, but have preference for odd-numbered alkanes. Both the distribution and the selectivity toward odd-numbered hydrocarbons are considered to reflect differences in petrogenesis of the cmde oils. Although / -paraffins are distinct in the gc, these usually account for only a few percent of the saturates measured by gc. 

Figure 2.6 Gas cluotnatograni of a 10 ml test sample containing C I4 C26 alkanes in -hexane (about 1 ppb each) the earner gas (H2) inlet pressure was 2.5 bar for a 22 m X 0.32 mm id separation column coupled with a 2 m X 0.32 mm id uncoated precolumn (no vapour exit). Reprinted from Journal of High Resolution Chromatography, 9, K. Grob et al., Concunent solvent evaporation for on-line coupled HPLC-HRGC , pp. 95-101, 1986, with peimission from Wiley-VCH.

With comprehensive GC, we can now choose a rational set of columns that should be able to tune the separation. If we accept that each column has an approximate isovolatility property at the time when solutes are transferred from one column to the other, then separation on the second column will largely arise due to the selective phase interactions. We need only then select a second column that is able to resolve the compound classes of interest, such as a phase that separates aromatic from aliphatic compounds. If it can also separate normal and isoalkanes from cyclic alkanes, then we should be able to achieve second-dimension resolution of all major classes of compounds in petroleum samples. A useful column set is a low polarity 5 % phenyl polysiloxane first column, coupled to a higher phenyl-substituted polysiloxane, such as a 50 % phenyl-type phase. The latter column has the ability to selectively retain aromatic components. 

Figure 12.2 Cliromatograms of an ABS copolymer sample (a) microcolumn SEC ti ace (b) capillaiy GC trace of peak x . Peak identification is as follows 1, C,4 alkanes 2, C,5 alkanes 3, C 8 alkanes 4, nonylphenol 5, palmitic acid 6, styrene-acetonitrile firmer 7, stearic acid 8, styrene-acetonitrile trimer 9, Irganox 1076 10, fiinonylphenyl phosphate 11, Ethanox 330. Reprinted with permission from Ref. (12).

The main drawback of GC is sample introduction and this is especially important when analytes are to be determined at trace levels. Today, however, there is no problem with introducing 10 -100 p.1 of Organic solvents such as ethyl acetate or alkanes... 

Figure 14.18 Typical GC cliromatogram of the separated mono-aromatics fraction of a middle distillate sample the numbers indicate the retention time of the various -alkanes.

Impurities consist of unreacted material, including alkanes and internal or branched alkenes, and other material which can be detected in the neutral oil fraction of AOS. Examination of this fraction also indicates the amount of unhydrolyzed material (sulfonate esters and sultones) and byproducts (secondary alcohols, unsaturated and 2-chloro-y-sultones) in the sample. Salt calculations are made to determine inorganic sulfates and sodium chloride. Determinations for alkalinity, color, and water are required to meet product... 

A solution of sample in carbon tetrachloromethane is injected directly into the column and alkane- and monochloroalkanesulfonyl chloride determined as chloroalkane and dichloroalkane, respectively. 

The procedure for determination of neutral oils in AOS measures petroleum spirit-extractable material from an aqueous alcoholic solution. Normally a solution of 1 1 ethanol/water is used as to dissolve the sample prior to extraction. For higher molecular weight materials 2 1 propan-2-ol/water is preferred. The petroleum ether is removed on a steambath and finally under vacuum. The amount of neutral oil is then determined gravimetrically. Due to the slight volatility of alkenes, alkanes, and alcohols, especially in the C,2 materials, the neutral oil tends to be underestimated by these procedures. 

EXAMPLE 18.1 Sample exercise Naming alkanes and cycloalkanes... 

In this contribution, in order to illustrate tlie importance of shake-up bands for extended systems, we simulate and compare on correlated grounds the ionization spectra of polyethylene and poly acetylene, the most simplest systems one can consider to represent insulating or semi-conducting polymers. Conclusions for the infinite stereoregular chains are drawn by exU apolation of the trends observed with the first terms of the related n-alkane or acene series, CnH2n+2 and CnHn+2. respectively, with n=2, 4, 6 and 8. Our simulations are also compared to X-ray photoionization spectra (7) recorded on gas phase samples of ethylene, butadiene and hexatriene, which provide a clear experimental manisfestation of the construction of correlation bands (8-12). 

Figure 1.10 Aldehydic regions of 400 MHz H-NMR spectra of unheated (a) and repeatedly used samples (b) of culinary frying oil obtained from a fast-food/take-away establishment. A total of 0.30 ml of each sample was diluted to a final volume of 0.90 ml with C HCls. High levels of n-alkanals, trans-2-alkenals and alka-2,4-dienals are detectable in spectrum (b).

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