A consortium of international researchers has conducted an inter-laboratory comparative study with a primary focus on accurately quantifying highly toxic arsenic species present in seven distinct biological materials. The outcomes of this endeavor have yielded a high degree of concordance among the submitted results, thereby facilitating the certification of the materials under investigation.
Background:The analysis of arsenic speciation in biological materials ranks among the most frequently examined subjects in the scientific literature related to speciation, spanning several decades. The motivation behind such analyses stems from the varying degrees of toxicity exhibited by different arsenic species. Trivalent arsenic compounds are generally more toxic than their pentavalent counterparts, while inorganic species surpass organic species in terms of toxicity, with some exceptions to this general principle.
Current regulatory frameworks concerning maximum allowable levels of arsenic in food aim to address this complexity by targeting the most toxic arsenic species. To implement such regulations based on species, laboratories must possess the capability to accurately measure the target species. For rice, where regulations are contingent on inorganic arsenic, the competence of laboratories has been scrutinized in various interlaboratory comparisons. The consistent reporting of results has led to the conclusion that the quantification of inorganic arsenic in rice is attainable, making the regulation of iAs feasible.
Regrettably, the arsenic speciation analysis of other food items, such as seafood, proves considerably more challenging. While arsenic in rice typically comprises DMA and iAs, marine animal tissues and algae have exhibited a more diverse array of organic arsenic species. Quantification is further complicated by varying extraction efficiencies, chromatographic recoveries, species interconversion, species-specific detection sensitivities, and matrix effects.
To investigate these effects and assess the accuracy of analytical methods, matrix-certified reference materials (CRMs) serve as invaluable tools. Unfortunately, CRMs certified for arsenic species are limited and predominantly available for plant materials. In such circumstances, speciation data on CRMs published in peer-reviewed literature often serve as informational benchmarks. However, reported results for major As species (iAS, MMA, DMA) displayed minimal agreement when analyzing CRMs like dogfish muscle (DORM-2), lobster hepatopancreas (TORT-2), and fish protein (DOLT-4). Notably, a significant disparity of up to three orders of magnitude in published values of As species in the selected CRMs renders the use of such data as indicators of result accuracy untenable without additional rigorous verification.
The new study:In light of these formidable challenges, an international round-robin study was organized to identify potential sources of discrepancies in the quantification of several arsenic species across diverse biological reference material matrices. Each participating laboratory received identical calibration standard solutions along with seven biological materials: one plant tissue, three marine, and three terrestrial biological tissues. These samples were subjected to analysis using both a standardized group extraction method and individual in-house protocols.
In the group extraction method, 0.25 grams of dried samples were extracted in 10 mL of a 1% v/v H2O2 solution at 95°C, maintained for 60 minutes. Subsequently, the samples underwent centrifugation at 3000 x g, and the supernatant was analyzed for arsenic species. Additionally, six laboratories applied their preferred in-house protocols for sample preparation. The choice of extraction method appeared to have minimal impact on the quantification of As species in plant and terrestrial biological tissues. However, adhering to the prescribed extraction method significantly reduced uncertainties in more complex samples like marine animal tissues.
For total As determination, 0.25 grams of samples were digested using microwave-assisted digestion in 7 mL of concentrated HNO3 and 0.5 mL of 30% H2O2.
One laboratory demonstrated that commercially available salts of arsenic species can exhibit varying levels of purity when using both provided calibration standards and their own. The separation of species relied on HPLC methods chosen by participants, requiring precise separation to mitigate quantification biases. Most samples contained only a few arsenic species, facilitating robust baseline separation. However, more intricate samples, such as marine biological tissues, presented multiple challenges.
Regardless of the diversity of extraction methods employed, a high level of agreement in reported mass fractions was observed for most sample materials. Yet, the behavior of As species was found to be dependent on the extracting agent for the TORT-3 material, necessitating further investigation. The combined consensus values exhibited uncertainties primarily below 15%, enabling their publication as certified reference values.
The authors express optimism that the analyzed materials will serve as appropriate quality control samples for environmentally relevant concentrations of arsenic in food materials and biota.
The original publication
Zuzana Gajdosechova, Patricia Grinberg,
Kevin Kubachka,
Mesay Wolle, Andrea Raab,
Joerg Feldmann, Rebecca Sim, Ásta H. Pétursdóttir, Tomáš Matoušek, Stanislav Musil, Ben Wozniak, Stephen Springer, Nausheen W. Sadiq, Hakan Gurleyuk, Calvin H. Palmer, Indumathi Pihillagawa Gedara, Zoltan Mester,
Determination of inorganic As, DMA and MMA in marine and terrestrial tissue samples: a consensus extraction approach, Environ. Chem., 20/1&2 (2023) 5–17.
DOI: 10.1071/EN23006
Analysed Materials:
National Research Council of Canada (NRC - CNRC) - CAME-1 Canola Meal Certified Reference Material National Research Council of Canada (NRC - CNRC) - KRIK-1 Cricket Flour Certified Reference Material National Research Council of Canada (NRC - CNRC) - VORM-1 Mealworm powder Certified Reference Material Related Studies:
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