Hydrocarbons, mixture analysis

Figure 12.21 SFC-GC heait-cut analysis of chrysene from a complex hydrocarbon mixture (a) SFC ttace (UV detection) (b) GC trace without heait-cut (100% transfer) (c) GC ti ace of heatt-cut fraction (flame-ionization detection used for GC experiments). Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et al., On-line multidimensional supercritical fluid chromatography/capillaiy gas cluomatography , pp. 337-341, 1987, with permission from Wiley-VCFI.

A sample may be characterized by the determination of a number of different analytes. For example, a hydrocarbon mixture can be analysed by use of a series of UV absorption peaks. Alternatively, in a sediment sample a range of trace metals may be determined. Collectively, these data represent patterns characteristic of the samples, and similar samples will have similar patterns. Results may be compared by vectorial presentation of the variables, when the variables for similar samples will form clusters. Hence the term cluster analysis. Where only two variables are studied, clusters are readily recognized in a two-dimensional graphical presentation. For more complex systems with more variables, i.e. //, the clusters will be in -dimensional space. Principal component analysis (PCA) explores the interdependence of pairs of variables in order to reduce the number to certain principal components. A practical example could be drawn from the sediment analysis mentioned above. Trace metals are often attached to sediment particles by sorption on to the hydrous oxides of Al, Fe and Mn that are present. The Al content could be a principal component to which the other metal contents are related. Factor analysis is a more sophisticated form of principal component analysis. 

Analysis for total petroleum hydrocarbons (EPA Method 418.1) provides a one-number value of the petroleum hydrocarbons in a given environmental medium. It does not, however, provide information on the composition (i.e., individual constituents) of the hydrocarbon mixture. The amount of hydrocarbon contaminants measured by this method depends on the ability of the solvent used to extract the hydrocarbon from the environmental media and the absorption of infrared light (infrared spectroscopy) by the hydrocarbons in the solvent extract. The method is not specific to hydrocarbons and does not always indicate petroleum contamination, since humic acid, a nonpetroleum material and a constituents of many soils, can be detected by this method. 

The method of analysis often used for the total petrolenm hydrocarbons (EPA 418.1) method provides a one-number valne of the total petroleum hydrocarbons in an environmental medium. It does not, by any stretch of the imagination, provide information on the composition (i.e., individual constituents of the hydrocarbon mixture). 

Severin, D. Molecular Ion Mass Spectrometry for the Analysis of High- and Nonboiling Hydrocarbon Mixtures. Erdol und Kohle - Erdgas - Petrochemie vere-inigt mit Brennstoff-Chemie 1976, 29, 13-18. 

R. Szostak and S. Mazurek, A quantitative analysis of liquid hydrocarbon mixtures on the basis of FT-Raman spectra registered under unstable conditions, J. Mol. Struct., 704, 235-245 (2004). 

Hase A, Lin PH, Hites RA. 1976. Analysis of complex polycyclic aromatic hydrocarbon mixtures by computerized GC-MS. lu Ereudeuthal R, Joues PW, eds. Carciuogeuesis A compreheusive survey. Volume 1 Polyuuclear aromatic hydrocarbou chemistry, metabolites, aud carciuogeuesis. New York, NY Raveu Press, 435-442. 

Although infrared absorption analysis of hydrocarbon mixtures was described by Lecomt and Lambert (34), the extensive application of this technique to the examination of petroleum products awaited the commercial availability of a practical instrumental unit and adequately described methods such as those of Brattain and Beeck (11) for a two component mixture and Brattain et al. (12) for a multicomponent mixture of hydrocarbon gases. By such methods the possible qualitative constituents of the sample must, of course, be known and their number limited to a maximum probably simultaneously present. 

The development of a commercial mass spectrometer and its application to hydrocarbon gas analysis by the method of Washburn et al. (63) made gas analysis rapid, economical, and, what is even more important, inspired a confidence in the results of routine hydrocarbon gas analysis which was badly lacking. A complex gaseous mixture comprising the atmospheric gases, carbon monoxide, and Ci to C6 hydrocarbons required more than 20 hours of applied time by the previous methods of low temperature fractional distillation coupled with chemical absorption methods. With the mass spectrometer such an analysis is completed in 2 hours or less, about 15 minutes of which is consumed in the... 

R. Rota, F. Bonini, M. Morbidelli, and S. Carra. Experimental Study and Kinetic Analysis of the Oxidation of Light Hydrocarbon Mixtures. Ind. Eng. Chem. Res., 35 2127-2136,1996. 

In the analysis of fatty oils complete expelling of the oxygen by a catalytic high pressure hydrogenation process reduces the problem to the analysis of saturated hydrocarbon mixtures. Such drastic chemical transformations should be executed under strongly controlled conditions only. 

Spectrographic data are gradually becoming more important for the analysis of hydrocarbon mixtures. These methods will not be discussed as they fall outside the scope of this monograph. 

There are mainly two problems that hamper the development of reliable methods for the analysis of branched hydrocarbon mixtures. 

However, in the analysis of saturated hydrocarbon mixtures, D has proved to be not only highly sensitive to the presence of rings (like rmol.), but also to ramifications in the molecules. When comparing D and rmo. of paraffinic or naphthenic hydrocarbons with those of corresponding w-alkanes of equal molecular weight (Z)M-aikane and mol.(w-alkane), respectively), it can be stated that... 

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. 

For the structural analysis of cyclic fatty acid derivatives (polymerized drying oils, copolymerization products of fatty oils with various hydrocarbons), in principle the same graphical methods can be developed as have been described for the investigation of hydrocarbon mixtures. However, the construction of useful graphical representations is hampered by the fact that reliable data on physical constants are restricted to the normal saturated fatty acids and their methyl and ethyl esters the synthesis of pure unsaturated fatty acids is already extremely difficult, to say nothing of more complicated cyclic or branched compounds. 

Another example of the use of deoxidation of oxygen-containing compounds for their structural analysis was given by Wi jnands et al.68, who investigated the structure of novolaks, prepared by polycondensation of formaldehyde with phenol, />-cresol and w-cresol. The novolaks were transformed into saturated hydrocarbon mixtures by direct hydrogenation. Ultimate analysis of the hydrocarbons confirmed the linear structure of the novolaks ... 

The use of physical constants is, however, limited in the case of more complicated chemical processes the more complex the chemical change, the larger the number of physical properties necessary to investigate completely the chemical transformations. This especially holds when catalysts are involved in the reactions. When studying catalytic reactions we are dealing with catalysts as mixtures of a far more complicated nature than is the case, for example, in the structural analysis of hydrocarbon mixtures. The latter can be described, as was shown in the preceding sections, by means of a limited number of physical constants, from which either the chemical composition of the mixture or a series of other physical constants can easily be derived. For the characterization of catalysts completely different principles have to be applied even in simple cases, because in the case of a catalyst it is not chiefly its chemical composition that is important, but its chemical activity, which determines the result obtained by its chemical action. 

---