Despite the fact that the number of oil spills has been on the decline over the past few decades, it still has a substantial effect on intertidal environments and local marine when it happens. There have been numerous studies on the effect of spilled oil compounds in marine habitats.
Oil spills are subjected to a wide range of weathering and degradation processes resulting in an alteration to the composition of the oil. This compositional modification can conceal the origination of the oil, hence preventing the complete understanding of the processes that oil from a specific spill may be subjected to. Therefore, the ability to track oil from a specific spill will significantly augment the understanding of the ways to deal with and plan bioremediation at the site where the spill has occurred.
The use of EnvirovisION, PetrovisION or isoprime precision helps scientists to analyze such oil spills to establish the ratio of each compound within the spilled oil and also the isotopic composition of each molecule at high levels of accuracy and precision.
A global database of accidental oil spills from tankers, combined carriers and barges is being maintained by the International Tanker Owners Pollution Federation (ITOPF). This database is used to pinpoint the causes of spills and to generate statistics on numbers and sizes of spill.
Analysis of long-term trends is conducted, which is useful for evaluating the risk of oil spills for emergency planning, for assessing the potential consequences of modifications in the design and operations of tankers.
Figure 1. Quantity of oil spilt from 1970 to 2016, published by ITOPF (itopf.com)
Dalian Oil Spill
In 2010, the Dalian oil spill released at least 1500 tonnes of oil into Dalian Bay in the NE of China. This presented a chance to investigate the processes of oil weathering and how these may modify the chemical attributes of the remnants.
A series of weathered oil residues from the coastal zone of the Jinshitan were collected 10 days (DL-1), 30 days (DL-2), 60 days (DL-3), 90 days (DL-4) and 120 days (DL-5) after the occurrence of the oil spill. Furthermore, a fresh crude oil sample (DL-0) was obtained from the oil tank where the spill originated.
To determine the composition and concentration of hydrocarbons, gas chromatography-mass spectrometry (GC-MS) analysis was undertaken. This showed that more than 50% of the oil had been lost within the residue. Of the remaining compounds, the lighter volatile fraction to n-C13 had disappeared within 120 days. This proves that profiling by GC-MS cannot identify the oil's origin due to weathering effects.
Figure 2. Compounds present in crude oil and oil residue 10 days after the spill.
Analysis for compound-specific δ13C ratio was conducted on an isoprime isotope ratio mass spectrometer. The following are the current analytical products capable of this analysis: the isoprime precision, the EnviroVISION, and the PetroVISION systems. These systems benefit from the 100 V head amplifier which allows a considerable dynamic range. This means that there is no issue with compound peaks of greatly differing concentrations within a sample analysis.
Linking the robust analytical performance of the isoprime precision and isoprime visION to the GC5 combustion interface was shown to optimize the performance potential of the systems. With the ability to function at temperatures up to 1450 °C, microbore reactor tubes ensure complete conversion of the sample while preserving the critical chromatographic resolution. Going beyond carbon is simple and effective. If necessary, the systems can be configured for compound-specific isotope analysis of oxygen, nitrogen, and hydrogen.
Figure 4 outlines the δ13C isotope ratios of the individual compounds within the oil sample. These vary drastically with a total range between -30‰ and -24‰. The δ13C value of each compound is preserved over time in spite of the weathering processes.
The standard deviations of δ13C values of individual n-alkanes ranged from 0.07‰ to 0.20‰ for Dl-0 and DL-3 (60 days) and varied from 0.06‰ to 0.36‰ for samples Dl-0 and DL-4 (90 days). The standard deviations of δ13C values of individual n-alkanes ranged from 0.07‰ to 0.20‰, even for the longer weathering samples (DL-5) over 120 days. This showed that the weathering has no major impact on isotopic ratios of individual n-alkanes.
Figure 3. Compounds present in crude oil (a) and 120 days after the spill (b).
Figure 4. Isotopic profile of oil compounds. A combination of crude oil and all residue samples up to 120 days after the spill, demonstrating the conservation of original signature.
Due to the combined processes of evaporation, emulsification and water washing, the more volatile and water soluble compounds are quite rapidly lost in an oil spill. Usually, these processes remove compounds below n-C15. Biodegradation occurs at the same time and is responsible for the reduction in concentration of the heavier fraction.
The isotopic analysis of the samples has revealed that even though there may be alteration to the composition of the oil residue, the compounds’ isotopic ratios are maintained.
The use of the isoprime series of IRMS has proven itself as a key tool in pinpointing the origins of spilled oil over a long period of time from its initial release.
The isoprime precisION and isoprime visION mass spectrometers coupled with the GC5 offer a high level of analytical performance and allow comprehensive characterization of the oil and oil residue samples. In turn, this has shown that the isotopic ratios of the compounds are unchanged even though there may be alteration to the composition of the oil residue.
References and Acknowledgment
We would like to acknowledge the help and support of Dr. Chuanyuang Wang in the production of this application note. The original paper can be found here:
Wang, C., Chen, B., Zhang, B., He, S., & Zhao, M. (2013). Fingerprint and weathering characteristics of crude oils after Dalian oil spill, China. Marine Pollution Bulletin, 71(1-2), 64–8. http://doi.org/10.1016/j.marpolbul.2013.03.034
This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.
For more information on this source, please visit Elementar Analysensysteme GmbH.