Multielement Analysis and Selenium Speciation in Cattle and Fish Feed using LC-ICP-QQQ

Importance of the Topic

Ensuring the right balance of selenium in animal feeds is crucial for both animal health and food safety. Selenium is an essential trace element involved in antioxidant defense and metabolic regulation, but it poses toxicity risks at elevated levels. Regulatory bodies in the USA and Europe strictly limit total and inorganic selenium in feeds to protect livestock health and prevent environmental contamination.

Objectives and Study Overview

This study aimed to develop and validate analytical methods for measuring total selenium and its chemical forms in commercial cattle and fish feeds. A multielement analysis using triple quadrupole ICP-MS (ICP-QQQ) established total elemental concentrations, while reversed-phase ion-pairing LC coupled to ICP-QQQ (LC-ICP-QQQ) was applied for selenium speciation. The goal was to achieve low detection limits, robust interference removal, and comprehensive profiling of organic and inorganic selenium species.

Methodology and Instrumentation

Samples of two cattle feeds and four aquaculture feeds underwent microwave digestion for total element analysis, followed by enzymatic extraction for selenium speciation. Total concentrations were determined on an Agilent 8800 ICP-QQQ equipped with helium and oxygen cell gases. Speciation was performed using an Agilent 1100 HPLC with a ZORBAX Extend C18 column and a salt-gradient, reverse-phase ion-pairing mobile phase, interfaced online to the ICP-QQQ. Certified reference materials NIST 1547 Peach Leaves and SELM-1 Selenium Enriched Yeast validated accuracy and interference removal.

Results and Discussion

Multielement analysis revealed selenium levels in all feeds (0.55–1.07 mg/kg) far exceeded the 0.3 mg/kg regulatory limit for supplemented feeds, indicating intentional selenium fortification. Certified reference materials showed recoveries of 94–102%, confirming method reliability and low ng/L detection limits for Se and other trace elements.

Selenium speciation demonstrated selenomethionine (SeMet) as the dominant form in both cattle and fish feeds, consistent with the use of selenized yeast or selenium-enriched plant ingredients. Inorganic species (selenite and selenate) were also detected at variable levels (up to 0.13 mg/kg), highlighting the need to monitor toxic species. Fish feeds displayed higher extractable selenium fractions (up to 68%) than cattle feeds.

Benefits and Practical Applications of the Method

• High sensitivity and specificity for total and speciated selenium at trace levels.
• Effective removal of spectral interferences including rare-earth doubly charged ions.
• Validated accuracy using certified reference materials supports compliance testing.
• Detailed speciation informs feed formulation and risk assessment.

Future Trends and Opportunities

Advances may include automated on-line extraction, coupling to high-resolution separation techniques, and integration with other detectors for broader speciation. Expanding this approach to other feed additives, environmental matrices, and real-time monitoring could further improve quality control and regulatory compliance.

Conclusion

The combined use of ICP-QQQ for total multielement analysis and LC-ICP-QQQ for selenium speciation provides a robust, sensitive, and interference-free workflow. This approach ensures accurate quantification of both total selenium and individual species in animal feeds, supporting nutritional optimization and regulatory enforcement.

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