Determination of Water-Soluble Arsenic Compounds in Commercial Edible Seaweed by LC-ICPMS

Overview

Llorente-Mirandes and colleagues determined total arsenic and water-soluble arsenic speciation (arsenosugars, methylated species, and inorganic arsenic) by LC-ICP-MS in six commercial edible seaweeds sold in Spain. Total arsenic ranged 5.8–56.8 mg arsenic (As)/kg dry weight; arsenosugars dominated (>90% of extracted arsenic in four of six species), and inorganic arsenic [As(III)+As(V)] was a small fraction, detected in only three of the six samples at 0.29–0.51 mg/kg and below detection in the two kombu species (Laminaria ochroleuca, L. saccharina) and nori. All six were below the French inorganic-arsenic seaweed limit (3 mg/kg dw). The certified reference material Sargassum fulvellum (a hijiki relative), run throughout, by contrast carried 69.9 mg/kg inorganic arsenic (69% of extracted As). The study refines the row picture: the kombu/Laminaria genus is not uniformly high-iAs — these European-market kombu species were low, in contrast to the elevated L. digitata reported elsewhere.

Key numbers

Total arsenic and inorganic arsenic in mg As/kg dry weight (Table 1; mean ± SD, n=3). Inorganic As = As(III) + As(V). Moisture 7–10%; all results dry mass.

Additional source-reported facts:

• Inorganic arsenic was detected in only three of six edible seaweeds: C. crispus 0.51 mg/kg (3% of extracted As), uranium (U). rigida 0.30 (14%), U. pinnatifida 0.29 (18%); not detected in P. purpurea, L. ochroleuca, or L. saccharina.
• All six samples were below the French inorganic-arsenic limit for edible seaweed (3 mg/kg dw); no sample approached the ANZ limit (1 mg/kg) either.
• Arsenosugars were the dominant arsenic compounds, accounting for 97.0% (C. crispus), 98.3% (P. purpurea), 99.0% (L. ochroleuca), and 98.5% (L. saccharina) of extracted arsenic.
• The CRM Sargassum fulvellum (NIES 9) had 69% of its extracted arsenic as inorganic As (69.9 mg/kg), consistent with the high-iAs pattern of the Sargassum/Hizikia group.
• Method detection limits (mg As/kg): As(III) 0.007, As(V) 0.017, inorganic-As components down to ~0.002; DMA 0.005, MA 0.007.
• Extraction efficiencies were generally high except U. pinnatifida (29%), so part of its arsenic was unextracted; the authors caution that checking inorganic arsenic alone may underestimate risk because much seaweed arsenic is in arsenosugar forms of uncertain toxicity, and that water may not extract tightly (protein-)bound As(III).

Methods (brief)

Six dried edible seaweeds (Galician coast, bought in Barcelona) were oven-dried (40 °C) and milled. Total arsenic was determined in triplicate by ICP-MS (Agilent 7500ce, He collision cell, Rh internal standard) after microwave digestion (Milestone Ethos, concentrated HNO3 + H2O2). Arsenic speciation used a water extraction (end-over-end shaking, 16 h, room temperature) suited to arsenosugars, with LC-ICP-MS on both anion-exchange (Hamilton PRP-X100) and cation-exchange (Zorbax-SCX300) columns; As measured at m/z 75 with m/z 77 monitored for ArCl interference. Arsenosugars lacking standards were quantified against the nearest-eluting standard (PO4-sug vs MA; SO4-/SO3-sug vs As(V); Gly-sug vs AC); an Fucus serratus extract was used to identify arsenosugar peaks. Quality control used CRM NIES 9 Sargassum fulvellum (certified total As 115 ± 9 mg/kg; measured 110.3 ± 0.7). Inorganic arsenic is reported as As(III) + As(V). All values dry mass.

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