Researchers from U.S. FDA have analysed 46 seaweed samples purchased from the U.S. market for total arsenic and arsenic speciation. Arsenosugars were dominant in most seaweeds, inorganic arsenic did not exceed 250 ng/g except for knotted wrack, oarweed and hijiki.
Background:Seaweeds are integral parts of the diet in many Asian countries and demand by American markets is increasing, owing in part to actual and perceived health benefits of seaweeds, as well as the growing popularity of international foods. While seaweeds are known to have nutritional values, they also contain higher arsenic than most foods and beverages. Since inorganic arsenic is a known carcinogen, human exposure studies and regulatory guidelines focus on this fraction. However, such approach disregards species of potential or unknown toxicity, especially when applied to foodstuffs of high total arsenic concentrations resulting from a great diversity of arsenic species.
The new study:
Arsenic speciation analysis for the identification of the different forms of arsenic is required to investigate the health risks related to the consumption of seaweeds as part of the diet. In order to do so for the seaweeds offered on the U.S. market, researchers from the U.S. FDA analysed 46 seaweed samples purchased from the U.S. market for total arsenic and arsenic speciation. Selected samples were from 10 different species that belong to brown, green, and red macroalgae.
The seaweed samples were dried in an oven at 70°C, ground to fine powder, and sieved. For the determination of total arsenic, 0.5 g of the powdered samples were digested by HNO3 and H2O2 in a microwave system at 250°C for 15 min with a 30- min linear ramp.
Water-soluble and nonpolar arsenic were extracted from the seaweeds with deionized water after vortexing and heating at 90°C for 30 min with a 45-min linear ramp. Extracts were cooled to room temperature, and centrifuged for 10 min at 3000 rpm. Supernatants were filtered through 0.45 µm pore size syringe filters. Water-soluble arsenicals were speciated by anion and cation exchange high-pressure liquid chromatography−inductively coupled plasma mass spectrometry (HPLC−ICP-MS), and nonpolar arsenic was collectively determined by ICP-MS after the extract was digested in acid.
Analyte quantification was based on external calibration using standards prepared on the day of analysis for a working range between 0.1 and 50 ng/g. The total arsenic concentration in the seaweeds varied greatly, ranging from 3 to 105 µg/g. In general, as observed already in previous studies, brown seaweeds had higher arsenic than red and green algae, with hijiki showing the highest concentrations. While more than 70% of the arsenic in hijiki, kombu, nori, oarweed, and laver was extracted with water, wakame and sea lettuce were found to have significantly lower fractions of water-soluble arsenic. On the other hand, these seaweeds contained the highest fractions of nonpolar arsenic species (38% in sea lettuce and 39% in wakame).
Since seaweeds contain the greatest number of arsenic species among marine samples, a single chromatographic separation mode is not adequate for reliably identifying and quantifying the extracted species. Therefore, in order to achieve efficient separation of anionic, cationic, and neutral species, the authors used anion and cation exchange chromatographic methods. Using the two methods, a total of 15 known and 14 unknown species were identified. The greatest diversity of arsenic species was observed in knotted wrack, dulse and hijiki. Arsenosugars were the major water-soluble arsenic species in most seaweeds. Besides arsenosugars, DMA was found in all the samples. Other species such as arsenocholine (AsC), dimethylarsinoyl ethanol (DMAE), dimethylarsinoyl propionate (DMAP) and trimethylarsine oxide (TMAO) were found in few samples mostly at trace levels. Due to a lack of standards, many peaks could not be identified with certainty. The total amount of such unknown species rarely surpassed 2% of the total arsenic. Inorganic arsenic accounted for 0.1−59% of the tAs in the seaweeds. The highest percentages and concentrations were in hijiki and oarweed at above 1 mg/kg.
The authors concluded, that a low extraction efficiency and poor chromatographic recovery observed for some samples presented challenges to a more complete understanding of arsenic species in seaweeds that should be addressed in future studies.
The original publication
Mesay Mulugeta Wolle,* Todor I. Todorov, and Sean D. Conklin,
Arsenic Species in Seaweeds Commercially Available in the United States, ACS Food Sci. Technol., 1 (2021) 511−523.
DOI: 10.1021/acsfoodscitech.0c00120
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