SPECIAL GUEST EDITOR SECTION Determination of Hydrocarbon Types in Petroleum and Coal-Derived Products by Thin-Layer Chromatography/Densitometry VICENTE L. CEBOLLA and LUIS MEMBRADO Consejo Superior de Investigaciones CientRficas, Instituto de Carboquímica, Departamento de Procesos Químicos, María de Luna, 12, 50015 Zaragoza, Spain JESÚS VELA Universidad de Zaragoza, Facultad de Veterinaria, Departamento de Química Analítica, Grupo de Espectroscopía Analítica y Sensores, Avenida Miguel Servet 177, 50013 Zaragoza, Spain ROSA GARRIGA, 1 PATRICK HENRION,MARÍA P. DOMINGO, and PAULA GONZÁLEZ CSIC, Instituto de Carboquímica, Departamento de Procesos Químicos, María de Luna, 12, 50015 Zaragoza, Spain Different methodologies based on thin-layer chro- matography (TLC)/densitometry were used to sep- arate and quantitate hydrocarbon types in middle distillates (gas oil), heavy distillates (lubricant) from petroleum, and coal-derived products. Thus, petroleum products were separated into saturates and aromatics by development, using n-hexane (9 min) followed by dichloromethane (4.5 min), of silica gel plates impregnated with berberine sul- fate. Detection of saturates and aromatics was per- formed by fluorescence scanning using 365 nm as the excitation wavelength. Alternative detection of aromatics can be performed on either silica gel or berberine-impregnated plates by using ultraviolet (UV) densitometry at 250 nm. On the other hand, polar coal-derived products were separated into aromatics, polar compounds, and uneluted compo- nents by using silica gel plates and development with toluene (12 min), followed by dichloro- methane–methanol (95 + 5, v/v), with detection by UV densitometry at 250 nm. In all cases, external standard calibration was used for quantitation. Re- sults were validated by using standard methods or well-established techniques of the petrochemical in- dustry. The potential usefulness of TLC/densitometry is discussed. E xtensive separations of compounds are sometimes im- practical or simply unnecessary in the petroleum indus- try (1). Because types of hydrocarbons rather than indi- vidual compounds determine the chemical properties of most fossil fuels and oils, determination of hydrocarbon type by chromatography has been very popular for their chemical characterization. These analyses are useful in the petroleum industry for process monitoring, determination of compliance with environmental regulations, and evaluation of product quality, catalyst performance, and feed processability. Thus, processes for upgrading petroleum distillates or coal-derived products, such as hydrogenation, catalytic cracking, lube ex- traction, or coking, can be optimized from quantitative charac- terization of feeds and products by hydrocarbon type. This kind of determination will be increasingly important because of legislation calling for reduction of the aromatic content of fuels by 2005. Liquid chromatography (LC) is usually used for the deter- mination (including separation and quantitation of fuels) of hydrocarbon type for all kinds (1). Types of hydrocarbons in petroleum products can be roughly divided into saturates (n-paraffins, branched paraffins, and cycloalkanes or naph- thenes), olefins (alkenes and cycloalkenes), aromatics (hydro- carbons containing $1 ring of the benzenoid structure), and polars (which include heterocycles and/or compounds of high molecular weight). This simple scheme can become compli- cated, depending on the boiling point range for the many pos- sible combinations of compound types, and on the presence of polar compounds. Sample composition strongly influences the chromatographic technique selected, e.g., open-column LC, high-performance LC, thin-layer chromatography (TLC), etc. Thus, in the petroleum industry, LC techniques are used for separating hydrocarbon types into light or middle petro- leum distillates, whereas TLC, with flame ionization detection (FID), is currently used for heavy and/or polar distillates (or coal products). TLC has been under-used in the analysis of these products in spite of having clear advantages over LC: high sample throughput, simultaneous development of standards and sam- ples, and the possibility of analyzing the whole sample with- out previous fractionation or de-asphalting. TLC does not present the problem of irreversible adsorption of some polar compounds onto the LC stationary phase, which causes in- complete elution and errors in quantitation. The advantages of TLC have been clearly detailed elsewhere (2). 1474 CEBOLLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 6, 2000 Guest edited as a special report on “Thin-Layer Chromatgraphy/ Densitometry” by Joseph Sherma. 1 On leave from Departamento de Química Orgánica y Física, University of Zaragoza, Spain. Downloaded from https://academic.oup.com/jaoac/article/83/6/1474/5656464 by guest on 01 March 2023