Petroleum sample, molecular nature

Although much information can be obtained from GC-FID, one of the most diagnostic characteristics of a petroleum sample is the molecular nature of the aromatic hydrocarbon fraction idilch can only be determined by GC-MS (6,18,24,32). The reason for this is that in addition to the many PNA parent compounds, there also exist C. C alkylated homology series for each parent PNA. In addition, each or these alkylated homologs can be present as a number of different isomers for each parent structure. 

This method (EPA 1664) is a liquid-liquid extraction gravimetric procedure that employs n-hexane as the extraction solvent, in place of 1,1,2-trichloroethane (CFC-113) and/or 1,2,2-trifluoroethane (Freon-113), for determination of the conventional pollutant oil and grease. Because the nature and amount of material determined are defined by the solvent and by the details of the method used for extraction, oil and grease method-defined analytes are used. The method may be modified to reduce interferences and take advantage of advances in instrumentation provided that all method equivalency and performance criteria are met. However, n-hexane is a poor solvent for high-molecular-weight petroleum constituents (Speight, 1999, 2001). Thus, the method will produce erroneous data for samples contaminated with heavy oils. 

Petroleum crude and its refinery products have two major component based on distillation. The portion that can be distilled under refinery conditions can be called volatiles and the nondistillables are the nonvolatiles. The volatiles can be analyzed by GC or GC-MS. The crude has both components. The distillate as the names applied, such as naphtha and kerosene contain only volatiles. When GPC is used for analyzing various distillates, the fractions separated by GPC can be characterized by GC or GC-MS. These data can be used to verify the nature of components present in various distillation cuts as a function of GPC elution volume. If the samples such as crude contains both volatiles as well as nonvolatiles, the samples should be separated into volatiles and nonvolatiles. The GPC of both components should be used to calibrate the GPC of the total crude. The parameter that can be obtained from GPC is effective molecular length. It can be used to relate other molecular parameters of interest after calibration. 

Separating a whole sample of a coal liquid or shale oil into classes poses special problems since these materials contain high concentrations of heteroatomic species compared with natural petroleums. Many of these compounds are quite polar and can cause emulsification, precipitation, and may even react to produce artifactual compounds at some stage during a separation procedure. Many liquid chromatographic techniques have been useful in class separations and analyses of petroleums. More often, these have been applied to particular analytical scale operations with fossil-derived liquids. The most common applications are for aromatic-aliphatic and molecular weight types of separations. 

Nucleic-acid approaches, which involve the extraction of microbial DNA and RNA from environmental samples, have been applied to the study of natural attenuation of petroleum products since the 1990s. With these techniques, the specific genes that are responsible for hydrocarbon degrading capabilities in bacteria can be measured. These tools are especially useful for studying microbial diversity at impacted sites (Madsen, 2000). Various recent applications of these techniques in petroleum-contaminated environments are reviewed by Haack and Bekins (2000). For example, Stapleton and Sayler (1999) monitored changes in the molecular microbial... 

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