Oil Spill Identification Methods and Indices

Identification of the source of an oil spill is extremely important to establish responsibility and liability. In the same vein, it is important to establish that an oil is not from an accused, suspected or questioned source.  Conclusions about identity or non-identity of an oil spill are drawn after comparison of the properties of the spilled oil and of oils from sources selected for their possible relationship of the spill.

Most of the identification activity is directed towards components which are intrinsic to the oil.  Identification by this means is called “passive tagging”, “Active tagging” involves the addition of a known and readily identifiable material to the oil.  The added constituent causes contamination problem and raises processing cost which will more than offset the particular advantage claimed for “active tagging” that it can be added at various points during the transportation of individual batches of oil and thus serve to identify the source even of weathered oils.

PAY ATTENTION – Diesel Suppliers In Port Harcourt

Passive tagging establishes the broad characteristics of the oil and is often called characterization. A whole range of methods have been used for crude oil characterization. These will be presented in the following sections.

1. Infrared Spectroscopy: Infrared spectral analysis is probably the most extensive method for oil identification. The method involves detailed comparison of selected absorbance bands or ratio of absorbance bands between an unknown oil and a possible reference suspect oil or oils.  The most probable suspect oil is that whose absorbance bands most closely coincide with those measured on the actual spilled sample. Various statistical methods and pattern recognition techniques have been incorporated into the absorbance band analysis to improve both the capability and reliability of fingerprinting.

Various processes are known to combine and alter the characteristics of oil during exposure at sea. The chemical and physical changes which accompany weathering markedly alter the magnitude of IR absorbance bands, thus the reliability of directly matching a weathered crude oil sample to the correct unweathered crude oil.  Using absorbance data is not as widespread as the use of comparative methods of unweathered crude oil suspects.  Efforts have been made to incorporate the weathering phenomena into oil-identification programmes. These efforts involve both the fundamental investigations of the weathering processes for predictive purposes, and the simulated weathering of suspect samples for analysis.  Since evaporation process constitutes the greatest single effect upon crude oils and related materials and such evaporation can be easily simulated by distillation through 275 degree centigrade, distillation to this temperature represents severe evaporative weathering of crude oil and is consistent with the loss of normal paraffins. Comparative IR analysis by use of reliable and consistent weathering procedure such as distillation to 275 degree centigrade is required to spectrally match similar crude oils in any source identification programme.

2. Chromatographic Method: Geochemical studies and crude oil pricing criteria provide important inputs into oil spill identification studies.  The ability to reliably identify semblance or differentiate crude oils and related materials plays a very dominant role. Improvement of this ability depends strongly upon the improvement of new analytical techniques. Chromatographic analysis has been used prominently in crude oil characterization.

Qualitative interpretation of gas chromatograms involved the determination of key differences between chromatograms for fingerprinting of oil samples.  Quantitative methods of data interpretation have included comparisons of weight percents of normal paraffins, ratios of weight percents of phytane and pristane and quantification of selected compound types such mercaptans and phenols and triterpanes.

The large volume of qualitative data as compared to quantitative data in literature reflects many of the problems associated with chromatographic analysis of complex mixtures.  These include a low level of reproducibility of quantitative data, a low degree of comparability of data obtained with different columns or at different times and a need for rapid data acquisition, data manipulation and interpretation. These problems are further compounded by the natural variations in the analytical data for spilled and suspect crude samples resulting from weathering. Recently, some of these problems were circumvented. Detailed investigations in column selection, column stability, improvement in analytical techniques and instrumentation have led to improved detection, identification AMD quantification of hydrocarbons obtainable in chromatography. These improved methods in chromatography are described below.

Thin-layer Chromatography with Flame Ionization Detector (TLC-ID)

Thin layer chromatography (TLC)  is a well known separation technique for the fractionation of total organic matter extracts or oil into the main compound classes,  namely: aliphatic hydrocarbons, aromatic hydrocarbons and polar fractions.  This low-resolution method can provide quantitative information on the changing relative concentration of the main compound classes of petroleum. The screening capability of TLC is considerably enhanced by combining it with the well known flame ionization detector system. TLC-FID carries out automatically the fractionation of the oil with quantification of the main compound classes.

The heart of the system consists of a pre coated chromatographic rod (chromarod). After this has been spotted with total organic extract sample,  it develops in appropriate solvent system for 15-40 minutes and is automatically transferred into a scanning area where the respective separated bands are sequentially programmed through the flame of the FID for ionization of organic components. The collection of the ions in a collector electrode generates a little current which is subsequently amplified and then output through a recorder as a chromatogram consisting of discrete peaks corresponding to saturates, aromatics/polyaromatics, polar and asphaltenes fractions.

High Performance Liquid Chromatography/Ultra-Violet Fluorescence Detectors.

High performance liquid chromatography (HPLC)  has emerged in recent times as the most suitable technique for the analysis of environmentally important organics. It is a non-destructive method and can be carried out at ambient temperatures which excludes decomposition of thermally labile compounds. Furthermore, because it is not limited by sample volatility, it is superior to gas chromatography for the analysis of polyaromatic hydrocarbons (PAHs). A recent innovation involving HPLC is its combination with fluorescence detector which is connected in series to UV/VIS detector making possible the selective analysis of PAHs at picogram level.  The HPLC/UV-F technique is particularly suitable for forensic studies of PAHs involving characterization and quantification of specific compounds.

Liquid Chromatography/Gas Chromatography – Mass Spectrometry.

The direct coupling of a liquid chromatograph to a mass spectrometer is perhaps the most outstanding achievement in the area of instrumentation for the analysis of organic compounds within the last 15 years. Innovations in the interface and analyzer of the system have further upgraded the sensitivity for many environmental applications involving PAHs. The LC eluant is either introduced directly into the ion source or via a thermospray interface. This is a significant advantage for thermally liable or non volatile compounds such as heavy PAHs.

A variety of soft ionization modes, for example, chemical ionization (CI: positive or negative mode), Fast Atomic Bombardment (FAB), low voltage Electron impact (EI) give rise to strong molecular ion signal compared with the classical high voltage EI mode. This enhances the detectability of specific polyaromatics but there is overall lowering of sensitivity compared with the high voltage EI method.

Gas chromatography/mass spectrometer has remained the most extensively used technique for the identification and quantification of environmental hydrocarbons. The recent introduction of the flexible fused silica column coated with cross-linked (double-bonded) stationary phases has considerably enhanced GC performance and improved overall sensitivity and versatility of the analytical methods.

One important technique of computerized mass spectrometry which has lately become popular in environmental sample analysis is mass fragmentography which is also known as single/multiple ion detection (SIM/MID). This entails the restriction of spectrometric scanning to the atomic mass units corresponding to a pre selected set of ions representing a unique fragmentation pattern of the particular class of compounds. Under this condition, the mass spectrometer functions as a highly specific and sensitive detector enhancing the estimation of the compounds in complex mixtures without pre separation. The computer generated fragmentograms (mass chromatograms) consist of the plots of the intensity of the diagnostic ions against the corresponding chromatogarphic retention times.

Advances in Chromatographic Detection.

The increasing complexity of hydrocarbon mixtures arising from rapid weathering of petroleum spills into the environment necessitates continuous improvements of existing analytical methodologies for rapid detection, quantification and source identification. Recent innovation/modification of chromatographic detector systems have led to the emergence of new generation of analytical tools having vastly increased capabilities in sensitivity, resolution and discriminatory power.  The combination of TLC with flame ionization detector (FID) forms an instrument that automatically fractionates and quantities the main compound classes of total organics.

Other techniques form the basis of the HPLC/UV-F and GC/LC-MS systems which afford detailed profiles and concentration of individual compounds at nanogram-picogram levels. Mass fragmentography has been widely utilized for the characterization and source identification of petroleum spills with little or no masking of quantitative data set generated for discriminatory purposes.

3. Fluorescence Method.

Fluorescence method has achieved widespread use for the determination of background levels of petroleum hydrocarbons in sea water and oil spill sampling as well.

UV/F has been used in conjunction with other analytical techniques to match waterborne oil with suspected cargo in forensic studies.  UV/F spectrometry or spectrofluorometry involves both the ultraviolet (200-400 nm) and visible (350-600 nm) regions of the electromagnetic spectra.  Fluorescence procedures are geared to the determination of aromatic hydrocarbon molecular structures, although other conjugated molecules for example, polyolefinic compounds) and heterocyclic aromatics will fluoresce.  UV/F spectrometry offers advantages of greater sensitivity and selectivity for aromatic molecules than UV absorption techniques.

Quantification of the diverse aromatic components of crude oil and refined products by fluorescence spectrometry has been investigated considerably in recent years, as the diversity of the aromatic compositions of crude and refined oils has resulted in “fingerprintable” spectra characteristics. Excitation, emissions and synchronous modes of fluorometry have been utilized for oil spill studies, but the latter two are of greater utility. Emission spectrofluorometry involves a fixed sample excitation wavelength and a scan of emission wavelengths.  The UV/F of complex mixtures is provided with greater resolution by use of synchronous scanning and both cryogenic low temperature, luminescene and “total luminescene” methods have great potential for further spectral detail.

Indices for Identification.

1. Triterpane Fingerprints.

Petroleum triterpanes fingerprinting of crude oils offers a method of relating crude oil to source rocks and finds application in the identification of individual crude oil pollutants.  A study carried out on one Nigerian crude (Obigbo North) reveals the presence of a high concentration of pentacyclic triterpanes.  The precise molecular structure of a number of these triterpanes has been determined and related to natural triterpanes which are abundantly distributed in the plant world.

Triterpanes reach a maximum concentration in petroleum distillates boiling around 475 degree centigrade. Those with thirty carbon atoms appear to be most abundant.

Fossil fuel will be found well within the boiling range of weathered tarry residue and it is believed that they are unaffected by weathering effects.  Thus, while like the metalloporphyrins, their total concentrations in weathered residue will increase,  the ratio of individual triterpanes, and hence the triterpane fingerprint should remain unaffected by weathering.

A liquid elution chromatographic method has been developed for the segregation of triterpanes concentrates from petroleum.  Gas liquid chromatographic fingerprinting establishes a characteristic pattern for each crude oil suggesting that the fingerprint may provide a very characteristic method of identifying crude oils for oil pollution control or palaeontological correlation.

2. Geological Markers.

The significant presence in crude oil and sedimentary rocks of petroporphyrins, isoprenoid alkanes, phytane and pristine provides a fundamental evidence that these fossils fuel skeletons can survive degradation over millions of years of geological time. Their molecular structure also relate them to biosynthetic sequences. Lipids are also fundamental to living organisms and their structural specificity conferred by mevalonic acid during biosynthesis, make them a natural target for geochemists who seek evidence to link living organisms with the compounds that they may discover in sediments and fossil fuels.

The successful use of such molecules as biological markets presupposes that the probability of their formation in quantity by non biological means is negligible. The revelation of the precise stereochemistry and absolute configuration at any optical centre should provide incontrovertible proof of their biological origin.  Subtle alteration to the skeleton as may be revealed to a perceptive observer, conveying perhaps a finer appreciation of the geological processes involved in the evolution of the earth’s abundant store of fossil fuel.

Fundamental research work has demonstrated that the unusually high optical activity of petroleum distillates which boil around 475 degree centigrade is associated with high concentration of steranes and triterpanes. These hydrocarbons with biologically significant skeletons are also seen in other crude oils.

Spectroscopic evaluation of the hydrocarbons producing this unusual optical activity indicated with fair certainty that they were pentacyclic triterpanes but the data failed to reveal the structure of any of them.  However,  triterpane crystals in monoclinic, triclinic, and orthoclinic forms submitted to x-ray crystallography yield the molecular structure (of the possible sequences in the transformation) of natural triterpenoid. Such transformation of the original natural triterpenoids may limit the chemotaxonomic information of molecular level, but can explain less direct relationships between the natural and fossil triterpenoids.  Furthermore, such transformation may still leave important clues to the processes involved in such molecular rearangements and hence to the very nature of fossil fuel formation and maturation.

3. Trace Metals/Trace Organic Markers.

The chemical composition of crude oils vary tremendously. Crude oils contain thousands of different chemical compounds owing to processes that occurred during petroleum formation. Hydrocarbons are the most abundant compounds in crude oils consisting of alkanes, cycloalkanes (naphthenes), and aromatic compounds. The non hydrocarbon constituents may be grouped into six classes, viz: the sulphur compounds, the nitrogen compounds,  the porphyrins, the oxygen  compounds, asphaltenes and trace metals.

The aromatic compounds contain one or more aromatic rings connected as fused rings or lined rings. Petroleum contains many homologous series of aromatic structures such as phenanthrene and like structures with alkyl side chains that replace hydrogen atoms. The aromatic hydrocarbons are noted for their solubilities and toxicity. Polynuclear aromatic hydrocarbons are important Indices in environmental studies.  These compounds are hazardous to health and occur naturally in petroleum and refined products.  Their fingerprints can be utilized in tracing the source of phenanthrene pollutants in the urban air basin. Consequently fingerprinting of PAHs in whole crude oil is not a versatile method in establishing oil source identification criteria.

The porphyrins are nitrogen containing compounds derived from chlorophyll and consisting of four linked pyrole rings.  Porphyrins occur as organometallic complexes of vanadium and nickel. Vanadium and nickel are the most abundant metallic constituents of crude petroleum, sometimes reaching thousands of parts per million.  The organometallic complexes of vanadium and nickel are called metalloporphyrins.

Petroleum contains a significant fraction (0-20 percent) of materials of higher molecular weight (1000-10,000), consisting of both hydrocarbon and NSO compounds called asphaltenes.  These compounds, consisting of isoprene building blocks. Pristine and phytane are usually the most abundant isoprenoids and their ratio (pristine-phytane ratio) have been widely used in crude oil source correlation.

The naphthenes (cycloalkanes) consist of important major constituents that like isoprenoids, have specific animal and plant precursors such as steranes, diterpanes, triterpanes and serve as important markers in oil spill and geochemical studies.

The stable isotope ratios of nitrogen, carbon and sulphur in whole crude oils, in oil fractions and in the total and lipid fractions of sediments and organisms are being used in geochemical exploration, sourcerock evaluation and oil spill identification.

Comparison of Various Oil Spill Identification Systems.

In earlier times, certain groups of compounds found in crude oil and sedimentary rocks were used as indices(markers) in oil source identification and for palaeontological correlation. These fossil fuel skeletons had not only survived degradation over millions of years of geological time but their molecular structure related them directly to biosynthetic sequences. This group of compounds are the petroporphyrins and the complementary isoprenoid alkanes, phytane and pristane, the steranes and terpanes.

These compounds posses stable carbon to carbon skeleton.  These organic molecules are highly resistant to themal and chemical degradation and biodegradation. Hence, even in highly weathered oils, they still faithfully reflect the composition and distribution of the corresponding compounds in the original oil.  Increasing sophistication of analytical technique has been allowed recognition of the precise molecular structure of these hydrocarbon forms.  The use of x-ray crystallography is required for structural identification and samples should be presented in crystalline forms.

Recent innovations in the techniques of mass fragmentography (fingerprinting) have led to vast improvement in the elucidation of specific individual components in the original crude and sedimentary.  System chemical analyses such as electron capture detection gas chromatography, trace organics/matal analyses,  high performance liquid chromatography coupled with combined ultraviolet/visible and fluorescence detectors, gas chromatography/liquid chromatogarphy-mass spectrometry are significant in terms of the gains in sensitivity, resolution and discriminatory power.  High technology in instrumentation, cost and availability limit the use of these equipments for forensic studies. In some cases, too, considerable sample treatment is required.

Infrared spectrophotometry and discriminant analysis is another useful technique for characterization for crude oil and petroleum products. Infrared spectroscopy at selected absorbance frequencies and thus limits it’s use.

In recent years, ratio of stable isotopes of nitrogen, carbon and sulphur are being exploited in geochemical studies, source rock correlation and oil spill identification. A spurious high technology method, definitely not for forensic studies.