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THE IMPORTANCE OF ARSENIC SPECIES SPECIFIC MASS BALANCE ON THE EVALUATION OF ARSENIC SPECIATION RESULTS IN SEAFOOD MATRICES
Citation:
Creed, J T., P A. Gallagher, B. M. Gamble, A. Heck, D M. Freeman, AND C A. Schwegel. THE IMPORTANCE OF ARSENIC SPECIES SPECIFIC MASS BALANCE ON THE EVALUATION OF ARSENIC SPECIATION RESULTS IN SEAFOOD MATRICES. Presented at International Society of Exposure Analysis 2001, Charleston, SC, November 4-8, 2001.
Impact/Purpose:
The goal is to develop an extraction protocol that mimics the human digestive tract and then to use it to assess the bioavailable fraction of arsenic from complex dietary mixtures such as a daily composite -- to move current methods toward a better human physiologically-based exposure estimate method which approximates the "true" bioavailability of arsenic within an environmental or dietary matrix.
Description:
The two predominant pathways to arsenic exposure are drinking water and dietary ingestion. A large percentage of the dietary exposure component is associated with a few food groups. For example, seafood alone represents over 50% of the total dietary exposure. From a daily dose perspective, these dietary arsenic exposures can easily exceed those from drinking water, especially in populations with high seafood consumption rates. Unlike drinking water, dietary arsenic is a mixture of tox and non-toxic arsenicals and to assess toxicity from dietary exposures, it is necessary to extract and speciate (separate) the arsenic. The quantitative nature of this extraction can dramatically affect risk assessment associated with seafood exposure and thus, the extraction process also needs to be species non-specific. This is essential because some extraction techniques can selectively extract the non-toxic (neutrally charged) species, while allowing the toxic species (anionic) to remain unextracted/undetected. However, the selectivity of an extraction technique is difficult to assess in unknown samples. Therefore, a quantitative extraction is often pursued in which the concentration of each arsenic species are added together and compared to the total digestion (i.e., total arsenic) concentration for that sample. This comparison measures extraction efficiency and is the foundation for species specific mass balance. This mass balance provides some data quality criteria to aid in assessing the reliability of dietary arsenic speciation.
Recently, our laboratory has been investigating various alternatives to quantitatively extracting arsenicals from seafoods. Some of these extractions have been found to be very matrix dependent, with oysters and clams producing extraction efficiencies of only aproximately 50-60%. However, these extraction efficiencies can be improved to greater than 90% by using tetramethylammonium hydroxide as an extraction solvent. In addition, these high extraction efficiencies show little matrix dependence, but the extract has been found to contain unknown arsenic species which do not elute from conventional anion exchange columns until treated with mild acid. The species specific mass balance approach is used to monitor the reappearance of the unknown arsenic species after treatment with the mild acid. Currently, the unchromatographable species are believed to be arsenosugars bound to proteins, lipids or other sugar substrates. Size exclusion chromatography indicates that the unchromatographable species have a higher molecular weight than the acid treated species. The nature of these initially unchromatographable species will be investigated using both atomic and molecular instrumental techniques. These results will be summarized in the context of minimizing uncertainty in assessing risk from seafood exposures.