Individual hydrocarbon analysis

Before reviewing reactions of the individual hydrocarbons, some overall trends will be discussed. In general, post-reaction surface analysis by AES showed the presence of submonolayer coverages of carbon, with a fractional coverage fairly independent of reaction conditions for a given alkane. The amount of carbon left on the surface increased approximately linearly with the size of the parent hydrocarbon molecule. This carbon residue is not believed to be an important intermediate in the hydrogenolysis reactions , however,... 

Radiation Methods of Analysis. The problem of determining individual hydrocarbons in liquids is much more complex than in gases because of the possible greater complexity of the former therefore, radiation methods of analysis are even more important here. Without them, analysis of liquid mixtures for individual hydrocarbons generally would be prohibitive. 

Mass Spectrometry. The use of a quadrupole mass spectrometer as a GC detector for nonmethane hydrocarbon analysis has come of age in recent years. Development of capillary columns with low carrier gas flows has greatly facilitated the interfacing of the GC and mass spectrometer (MS). The entire capillary column effluent can be dumped directly into the MS ion source to maximize system sensitivity. GC-MS detection limits are compound-specific but in most cases are similar to those of the flame ionization detector. Quantitation with a mass spectrometer as detector requires individual species calibration curves. However, the NMOC response pattern as represented by a GC-MS total ion chromatogram is usually very similar to the equivalent FID chromatogram. Consequently, the MS detector can... 

The development of reliable methods for structural analysis of mixtures is very laborious. Physical data of pure compounds may serve as a base for the investigations. It has, however, been proved that not in all cases can these data be simply correlated with those of the mixtures. Thus correlations of physical data of pure, individual hydrocarbons often prove not to be valid in the analysis of mineral oils. In this case physical constants of mineral oil fractions of widely different origin form a more reliable basis for the structural analysis, provided that their structure has been determined by absolute methods. 

It should be pointed out that this method for ring analysis and branching analysis is based exclusively on reliable data of n, d, M and a of pure individual hydrocarbons, and holds, within the limits of accuracy of the determination, for widely differing types of branched as well as non-branched saturated hydrocarbon mixtures. It is particularly recommended for the structural analysis of saturated polymers, where other statistical methods (w- -M-method, v-n-d-method, etc.) fail because they have been developed for mineral oils, and are based on correlations of physical data of mineral oil fractions that always show approximately the same small degree of branching 1-2 branchings per molecular weight = 100. 

F. P. DiSanzo, J. L. Lane and R. E. Yoder, Application of state-of-the-art multidimensional high resolution gas chromatography for individual component analysis of gasoline range hydrocarbons , J. Chromatogr. Sci. 26 206-209 (1988). 

Gasoline. A complete analysis of a Houdry-cracked gasoline in terms of individual hydrocarbon components has been published by Glasgow, Willingham, and Rossini (35). 

Because of the estimated (or real) number of isomers in this carbon number range (Table 7.2), complete speciation of individual hydrocarbons is not possible for middle distillates. Compositional analysis of middle distillates is obtained in terms of hydrocarbon group type totals. These groups are most often defined by a chromatographic separation. 

Oils. Water can be equilibrated with an excess of oil, and the water subsequently analyzed for dissolved hydrocarbons. Individual hydrocarbons at equilibrium in water have concentrations dependent on their mole fractions in the oil and their solubilities in water. Therefore, analysis of the water permits calculation of the amount of each soluble hydrocarbon in the oil phase. 

Although the focus of this book is hydrocarbon analysis, heteroatoms, mainly sulfur and nitrogen compounds, cannot be ignored. Methods for determining the concentration of these elements are well established and Usted in Table 2. The combination of gas chromatography with element selective detection gives information about the distribution of the element. In addition, many individual heteroatomic compounds can be determined. 

Hydrocarbon Test Mixture—A quantitative synthetic mixture of pure hydrocarbons, an example of which is identified in Table 2 is used to tune the instrument analysis conditions and establish that the instrument is performing within specifications. Individual hydrocarbon components, in addition to those listed in Table 2, may be used to aid in the analysis. The concentration level of each component in the hydrocarbon test mixture is not critical as long as the concentration is accurately known. Percentage ranges of 1.0 to 6.0 mass % have been found suitable. Impurities in the individual components may have an adverse effect on the quantitative aspect of the analysis. If an impurity is of the same carbon number and basic molecular structure as the... 

Figure 5 Stacked chromatograms showing the GC analysis of a series of n-hydrocarbons isolated by headspace SPME at various temperatures. The sample was prepared by transferring l-pl of individual hydrocarbon standards into a 4-mL vial and sealing with a Teflon-coated septum. After 10 minutes equilibration at the indicated temperature, the headspace vapor was extracted using a 100-lim polydimethylsiloxane fiber and was analyzed by GC with flame ionization detection. The injector temperature for this sample was set at 300°C, and the fiber was desorbed for 1 minute.

One has seen that the number of individual components in a hydrocarbon cut increases rapidly with its boiling point. It is thereby out of the question to resolve such a cut to its individual components instead of the analysis by family given by mass spectrometry, one may prefer a distribution by type of carbon. This can be done by infrared absorption spectrometry which also has other applications in the petroleum industry. Another distribution is possible which describes a cut in tei ns of a set of structural patterns using nuclear magnetic resonance of hydrogen (or carbon) this can thus describe the average molecule in the fraction under study. 

Figure 13.22 shows the resolution of the surfactants Tween 80 and SPAN. The high resolution obtained will even allow the individual unreacted ethylene oxide oligomers to be monitored. Figure 13.23 details the resolution of many species in both new and aged cooking oil. Perhaps the most unique high resolution low molecular weight SEC separation we have been able to obtain is shown in Fig. 13.24. Using 1,2,4-trichlorobenzene as the mobile phase at 145°C with a six column 500-A set in series, we were able to resolve Cg, C, Cy, Cg, C9, Cio, and so on hydrocarbons, a separation by size of only a methylene group. Individual ethylene groups were at least partially resolved out to Cjg. This type of separation should be ideal for complex wax analysis.

On-line SFE-pSFC-FTD, using formic or acetic acid modified CO2 as an extraction solvent, was used to analyse a dialkyltin mercaptide stabiliser in rigid PVC (Geon 87444) [114]. Hunt et al. [115] reported off-line SFE-pSFC-UV analysis of PVC/(DIOP, chlorinated PE wax, Topanol CA), using methanol as a modifier. Individual additives are unevenly extracted at lower pressures and temperatures, where extraction is incomplete. Topanol CA, the most polar of the three PVC additives studied, could not be fully extracted in the time-scale required (15-20min), even at the highest CO2 temperature and pressure obtainable. However, methanol-modified CO2 enhances extraction of Topanol CA. PVC film additives (DEHP, fatty acids, saturated and aromatic hydrocarbons) were also separated by off-line SFE-preparative SFC, and analysed by PDA and IR [116]. 

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