Luis Membrado

Chromatographic Techniques for Petroleum and Related Products

Recent developments in chromatographic techniques for the separation and quantitative characterization of petroleum and related products are highlighted. Specifically, scope, applicability, and versatility of individual techniques such as gas chromatography, liquid chromatography, supercritical fluid chromatography, thin-layer chromatography, and sizeexclusion chromatography are discussed in some detail. In general, analytical approaches vary widely depending on the chromatographic technique, instrumentation, minimum detection limit, and sample type. Specific applications include chromatographic separation followed by identification and determination of individual components, measurement of boiling range distribution, and determination of hydrocarbon group types. With the exception of gaseous samples and light distillates (up to gasoline range materials), the inherent complexity arising from the presence of numerous isomers and compound types, and the finite resolution afforded by chromatographic methods preclude precise identification and quantitative determination of individual components in fossil fuels and oils. For the middle and heavy distillates, chromatographic techniques have been applied mostly for the isolation and determination of compound classes. Several techniques such as open-column liquid chromatography, medium pressure liquid chromatography, high-performance liquid chromatography, and supercritical fluid extraction provide separation of compound classes for subsequent characterization by highresolution chromatography, spectroscopy, or other methods. Hyphenated or multitechnique chromatographic approaches have also been described. These simplify the characterization of complex samples by incorporating multiple separation mechanisms and detection schemes.

Fluorescence detection by intensity changes for high-performance thin-layer chromatography separation of lipids using automated multiple development

Changes in emission of berberine cation, induced by non-covalent interactions with lipids on silica gel plates, can be used for detecting and quantifying lipids using fluorescence scanning densitometry in HPTLC analysis. This procedure, referred to as fluorescence detection by intensity changes (FDIC) has been used here in combination with automated multiple development (HPTLC/AMD), a gradient-based separation HPTLC technique, for separating, detecting and quantifying lipids from different families. Three different HPTLC/AMD gradient schemes have been developed for separating: neutral lipid families and steryl glycosides; different sphingolipids; and sphingosine-sphinganine mixtures. Fluorescent molar responses of studied lipids, and differences in response among different lipid families have been rationalized in the light of a previously proposed model of FDIC response, which is based on ion-induced dipole interactions between the fluorophore and the analyte. Likewise, computational calculations using molecular mechanics have also been a complementary useful tool to explain high FDIC responses of cholesteryl and steryl-derivatives, and moderate responses of sphingolipids. An explanation for the high FDIC response of cholesterol, whose limit of detection (LOD) is 5 ng, has been proposed. Advantages and limitations of FDIC application have also been discussed.

Coal conversion into char: A study of the initial steps

Abstract This paper analyzes the first stages of char formation by different analytical techniques. The chars were obtained by coal pyrolysis carried out at 400°C for 5, 15, 30 and 60 minutes in a nonhydrogenating atmosphere, in absence of catalyst and according to a modified Gray-King method. The aim was to study the changes happening in converting coal into char. FTIR, CP/MAS 13C and FS were applied as spectroscopic techniques together with solvent swelling measurements, proximate and ultimate analysis. The obtained data shows the practical disappearance of the aliphatic hydrogen content with increasing pyrolysis time. The longer the stay in the reactor, the higher the char aromaticity. On the other hand, there is a change from highly condensed structures to pyrene and phenanthrene type molecules.

Application of Thin-layer Chromatography with Fluorescence Scanning Densitometry for Analysing Saturates in Heavy Liquids Derived from Co-pyrolysis of Biomass and Plastics

SummaryTwo alternative methods, based on Thin-Layer Chromatography (TLC) with Fluorescence Scanning Densitometry have been developed for characterization of heavy liquids from copyrolysis of different kinds of biomass and plastics in autoclaves under inert atmosphere.A conventional TLC system, which includes a vertical developing tank, and a High Performance TLC (HPTLC) system, with a horizontal developing chamber and the use of HPTLC plates, have been used. The analytical method involves in both cases the measurement of two chromatograms per sample: the first, on a silica gel berberine-impregnated plate, for detection of saturates using the phenomenon of berberine-induced fluorescence; and the second, on a silica gel plate, for detection of aromatic-polars and polars, by native fluorescence.Although the HPTLC system is more sensitive and faster, both techniques represent an improvement with regard to current methods for analyzing these kinds of products. However their application depends on the particular solubility of each sample and on its slope of the fluorescent response-sample load regression.

Coal-tar pitch characterization by thin-layer chromatography with flame ionization detection

SummaryA rapid and quantitative method for compound class characterization of coal-tar pitches without previous fractionation, using an improved TLC method with an FID system, has been developed. Results show adequate accuracy and precision, including the sampling step.A fast calibration method, based on a variation of the internal normalization procedure, can be used for up to 18 μg of whole sample application, avoiding the usually tedious absolute calibration in such analyses. This range is more than sufficient in view of the small amounts usually spotted in this technique.Speed of sample application by autospotter influences the shape of the peak nearest the point of application. For the coal tar pitch studied (9 wt.% non-eluted), slow application (0.5 μL min−1) gives a non-eluted Gaussianintegratable peak. A faster speed (10 μL min−1) is usable for analysing fossil products with lower non-eluted content. Total analysis time is less than 2 h, a considerably improvement on current methods.

Structural modifications of petroleum asphaltenes by reductive alkylation investigated by TLC-FID

Abstract The reductive alkylation of a French petroleum asphaltene (Lagrave) was studied with potassium in THF, in absence of an electron transfer reagent. From the number of butyl and benzyl groups added to the starting asphaltene, the difference of reactivity of the alkyl reagents, butyl iodide and benzyl bromide, was pointed out. Special attention was focused on the cleavage of C–S and C–O linkages in relation to the structural modifications. From the number of hydrogen atoms added during the reduction step, it was concluded that the sulphide linkages are mainly of aryl-S-aryl, aryl-S-alkyl or alkyl-S-alkyl types. The TLC-FID (Iatroscan) analysis was found to be very suitable to follow the “depolymerization” process of the asphaltene, qualitatively as well as quantitatively, by the application of quantitative hydrocarbon group type analysis.

Changes in fluorescent emission due to non-covalent interactions as a general detection procedure for thin-layer chromatography.

Changes in fluorescence emission due to non-covalent analyte-fluorophore interactions in silica gel plates are studied and used as a general detection procedure for thin-layer chromatography (TLC). The presence of the analyte modifies the microenvironment of the fluorophore and thus changes the balance between radiative (k(r)) and non-radiative (k(nr)) emission constants. A model is proposed for analyte-fluorophore induced electrostatic interactions, which depend on analyte polarizability and are responsible for fluorescence enhancements. As consequence of these induced interactions, the analyte creates an apolar environment that prevents non-fluorescent decay mechanisms, decreasing k(nr). On the other hand, the effect of an increase in refractive index on k(r) is investigated, as it contributes to some extent to fluorescence enhancements in silica gel medium. Changes in fluorescence emission should be regarded as a general property of fluorophores in the presence of analytes, and criteria that fluorophores should meet to be used as sensitive TLC probes are discussed here.

Thin-Layer Chromatography for Hydrocarbon Characterization in Petroleum Middle Distillates

Petroleum is fractionated according to its boiling point by distillation methods. Depending on the boiling point range of the sample, fractionated products fall into three main categories: light, middle and heavy distillates. Middle distillates (from approximately 150°C to 400°C, which approximately corresponds to C9-C24) are mostly used for heating and transportation fuels (e.g., diesel, jet fuel, gas oil, aviation turbine oils, kerosene).

Planar chromatography for the hydrocarbon group type analysis of petroleum middle distillates and coal-derived products

Abstract Different methodologies, based on planar chromatography/detection with densitometry, have been used to analyse compound classes (also known as hydrocarbon group type (HGT)) in samples coming from petroleum and coal conversion. The main problem encountered to analyse these samples is the choice of standard: because of the high variability of the signal that is dependent of molecular structure, one pure hydrocarbon does not reflect the response of a mixture. However, a step based on thin layer chromatography at preparative scale has allowed the fractionation of sample to obtain its derived standards. After this, alkanes have been quantified by fluorescence in presence of berberine sulfate and aromatic compounds have been detected by UV after separation by high performance thin layer chromatography (HPTLC) at analytical scale. The feasibility of the planar chromatography has been tested. The quantitative results obtained for different samples are in agreement with those provided using well-established techniques in the petrochemical industry and the coal-derived product (CDP) analysis.

Evaluation of hydroconverted residues. Rationalization of analytical data through hydrogen transfer balance

Abstract Analytical evaluation of petroleum based materials and processed feeds is a complex task relying on a compromise between tedious in-depth characterizations and fast responding tools for process control. In the present paper, a large number of hydroprocessed vacuum residues, obtained either under catalytic or thermal conditions, have been submitted to the following analytical techniques: Simulated distillation, coupled Simdist/MS, UV spectroscopy, 13 C NMR, quantitative thin-layer chromatography/FID, vapor phase osmometry. A confrontation of analytical data in the light of correlations with hydrogen transfer evaluation is proposed, which accounts for observed variations in aromatic content. Conradson carbon residue largely influences the results obtained with some of the examined techniques. Apparent discrepancies are rationalized and a strategy for a comprehensive analytical evaluation of hydroprocessed feeds is proposed.