The defensible identification of spilled crude oils and petroleum products and their correlation to suspected sources is a critical part of many oil spill assessments. Quantitative ‘fingerprinting’ analysis, when evaluated using straightforward statistical and numerical analyses, can provide simple yet defensible means to differentiate among qualitatively similar oils and provide the best assessment of a source(s) for spilled oils. Biomarker concentration data are a particularly useful quantitative measure that can benefit most oil spill investigations. Biomarkers' specificity, diversity, complexity, and relative resistance to weathering make them extremely useful 'markers' in the characterization and differentiation of spilled oils, any candidate source oils, and background contamination. In this paper the methodology and precision of data developed by quantitative gas chromatography-mass spectrometry operated in the selected ion monitoring mode (GCMS-SIM) is demonstrated. These data are evaluated using a statistical and numerical process that results in the defensible identification the source(s) of a spilled oil (heavy fuel oil) from among over 50 candidate sources. Introduction Oil spills of an unknown origin occur routinely in open water and navigable coastal waterways. Often these occur due to accidental or intentional discharges from vessels of dirty (oiled) ballast water, bilge waters, or tank wash slops. The tremendous liability associated with even a small volume oil spill warrants that a through chemical characterization of the spilled oil be conducted by agencies and potentially responsible parties (PRP's) in order todefensibly determine the source of the oil andquantifiably evaluate the extent of impacted ecosystems. In addition, because crude oils and refined products are often transported significant distances from their source along heavily travelled shipping lanes, the potential for a significant number of candidate ‘source’ vessels (PRP's) carrying a wide variety of chemically distinct oils is common. As a result the legally defensible differentiation and characterization of spilled crude oils and refined petroleum products is a critical part of many oil spill assessments. Qualitative chemical ‘fingerprinting’ analysis of spilled oil, candidate sources, and background materials [1–3] can in some instances sufficiently meet the need of an oil spill investigation. However, when the chemical similarity/ difference between spilled oil and candidate source oils is not obvious, the qualitative approach can be deemed subjective (biased), difficult to technically defend, and difficult to convey any complicated details to a lay decision-maker. On the other hand, quantitative chemical ‘fingerprinting’ analysis [4–6], particularly when used in conjunction with multivariate statistical analyses [6–9] can provide an unbiased and defensible means to differentiate among qualitatively similar oils. Biomarker (or biological marker) data are a particularly useful quantitative measure that can benefit oil spill investigations. Biomarkers are naturally occurring, ubiquitous and stable hydrocarbons that occur in crude oils and most petroleum products [10]. They are derived from formerly living organisms whose organic materials were preserved in oil source rocks that upon burial (heat and pressure) generated crude oil over geologic time. Their specificity, diversity, complexity, and relative resistance to weathering make them useful ‘markers’ in the characterization of spilled oils, candidate source oils, and background contamination.
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