The analysis of various sample matrices (seawater, sediment, tissue) to determine concentrations of Corexit dispersants used in the Gulf spill requires the use of one or more of the components in the dispersant as a tracer. The primary active ingredient (i.e. the primary surfactant) in the Corexit products is Dioctyl Sulfosuccinate Sodium Salt (DOSS). The other primary ingredients function as carriers until the dispersant is applied. At that point, the lighter fractions evaporate and the water soluble component(s) dissolve in the seawater and rapidly dissipate.
As noted from the structure, DOSS has two R groups [each is an identical branched octyl group or bis(2-ethylhexyl)]. The R groups are what give the compound its ability to disperse non-polar compounds (e.g. components of the Gulf oil from the Deepwater Horizon well). The other portion of the molecule are two ester linkages with a common carbon between them (linking one R-ester group with the other R’-ester group). However, the other key to behavior of the surfactant is the sulfonic acid group that is substituted on the common carbon associated with the esters.
The sulfonic acid is a strong acid (readily dissociable in water). The compound is added to the Corexit dispersants as the sodium salt, so once they are released to aqueous medium, the compound ionizes to leave a fully charged sulfonate. This (with a little help from the carbonyls in the ester linkages providing hydrogen bonding with water) is what enables the molecule to be miscible with water when carrying petroleum hydrocarbon with it (i.e. bonded via partial charges with the R groups).
The key to the analysis is twofold:
- Separation (i.e. extraction) from seawater, oil, sediment, and possibly tissue is the challenge. (Note: The analysis for DOSS in tissue might not be applicable. Instead, metabolites of DOSS may end up being the targeted species if data shows DOSS is readily metabolized by fish, mollusks, crustaceans, benthic organisms, mammals, etc.).
- Liquid Chromatography w/Tandem Mass Spectrometry (LC/MS/MS) determination.
Chemical separations to isolate DOSS from the sample matrix must be performed prior to determination by LC/MS/MS. The determination in seawater is relatively easy when taking advantage of the anionic character of DOSS. Anion exchange resin is used to separate DOSS from the salt matrix prior to instrumental analysis, followed by elution using an appropriate solvent.
The analysis of sediment and tissue requires a preliminary solvent extraction followed by separation of DOSS via ion exchange treatment. For sediment, DOSS must be manipulated into a relatively polar solvent for the ion exchange treatment to function effectively. If relatively large amounts of petroleum material are present, the chemical separation is more complex, as the petroleum: DOSS specie requires complete dissolution to assure quantitative results. Thus, DOSS must be taken from a non-polar solvent to a polar solvent prior to ion exchange treatment.
Determinations of DOSS in tissue introduce at least two more significant considerations. The first is whether a direct solvent extraction of homogenized tissue will yield quantitative removal of DOSS. Perhaps a mild alkaline digestion will be necessary to allow efficient solvent extraction.
The other bigger question is related to the fate of DOSS once ingested. The compound is likely metabolized after ingestion. Thus, the metabolites then become the target compounds for the analysis (i.e. new tracers for Corexit dispersants). No work has been done to date for DOSS in tissue at Columbia Analytical Services, Inc., nor has a literature search revealed anything significant to date. Further research is required to identify the fate of DOSS after ingestion into various organisms.
Once DOSS has been isolated in an appropriate solvent, the analysis is performed by LC/MS/MS. Due to superior selectivity and sensitivity coupled with the fact that DOSS lends itself best to liquid chromatography, LC/MS/MS is the most appropriate choice of instrumentation. The tandem mass spectrometers allow a determination relatively free of matrix interferences. The initial electrospray ionization (“soft” ionization) simply removes a proton from DOSS to produce a parent ion and is followed by a second ionization that produces a daughter ion. The combination of the two ionizations and subsequent mass filtering serves to remove interference by selectively allowing only (ions specific to) the target compound to reach the detector. The result is high selectivity, as well as signal:background.
A typical reporting limit for seawater is 0.1 ng/ml.
Structure: (see http://www.caslab.com/News/corexit-dispersants.html)
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