Spectral scanning for source identification

All substances other than oil extractable from seawater into a non-polar solvent and emitting fluorescent light at 360 nm when excited at 310 nm are possible sources of positive errors. These inelude components of creosote, which is used as a wood preservative. 

Sources of negative error are substances which quench the fluorescence of PAH. High concentrations of phthalate esters, a group of ubiquitous environmental contaminants, have this effect. However, since a concentration of 10 ng of phthalate ester per mg of oil quenches the fluorescence by approximately 1 %, only very high concentrations of phthalates would impair determinations of seawater-accommodated oil residues. 

With a sample volume of 4 L, a high quality grating spectrofluorimeter will produce interpretable signals from total oil concentrations of approximately 50ng/L. At these very low concentrations extreme precautions to avoid sample contamination are of paramount importance. They determine the successful application of the method to a higher degree than the instrument sensitivity. 

The upper concentration limit as defined by the deviation of the detector response from linearity caused by auto-quenching is approximately 8 mg oil/mL hexane. Thus, at very high concentrations of oil residues in the water that might be encountered under special circumstances in the immediate vicinity of an oil spill, samples may have to be dissolved in larger volumes of n-hexane for measurement or the extract diluted. 

21 Determination of petroleum residues dissolved and/or finely dispersed in surface seawater 

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