Hexavalent selenium, present in dairy products primarily as the selenate anion, requires species-specific analysis because its toxicity and bioavailability differ significantly from organic selenium compounds. This article explains how the determination of Se(VI) in dairy matrices is performed within the framework of the Chinese national standard GB 5009.93-2017, using anion-exchange liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). It covers the technical principle, sample types, sample preparation, chromatographic and detection conditions, quality control, regulatory applications, and frequently asked questions.
Technical Overview
Selenium is an essential trace element with a notably narrow margin between nutritional adequacy and excessive intake. In dairy products, selenium originates from feed-chain transfer, environmental background levels, and deliberate fortification with permitted selenium sources such as sodium selenite, sodium selenate, or selenium-enriched yeast as regulated by GB 14880 (Standard for Use of Food Nutrition Fortification Substances).
From a chemical perspective, selenium in dairy matrices exists in several forms, and the oxidation state and chemical species strongly influence both bioavailability and toxicological profile:
- Inorganic tetravalent selenium — selenite (Se(IV), SeO3 2-)
- Inorganic hexavalent selenium — selenate (Se(VI), SeO4 2-), which is highly water-soluble and chemically persistent
- Organic species — primarily selenomethionine and selenocysteine incorporated into milk proteins
- Colloidal or elemental selenium, generally of minor relevance in fluid dairy systems
GB 5009.93-2017 (National Food Safety Standard — Determination of Selenium in Food) establishes hydride generation atomic fluorescence spectrometry (HG-AFS) and inductively coupled plasma mass spectrometry (ICP-MS) as the official techniques for total selenium. However, because total selenium cannot distinguish between the more toxic inorganic species and the nutritionally preferred organic species, speciation analysis is required for hexavalent selenium. The accepted technical route, applied under the quality framework and method validation principles of the national standard system, is anion-exchange high performance liquid chromatography coupled online with ICP-MS, and this work is typically performed by accredited third-party laboratories holding CMA and CNAS accreditation.
Testing Samples
Liquid Dairy Products
Raw milk, pasteurized milk, UHT sterilized milk, modified milk, and milk-based beverages. Liquid samples are homogenized and either analyzed directly after protein precipitation or diluted to a defined concentration before extraction.
Milk Powders and Infant Formula
Whole milk powder, skimmed milk powder, whey powder, infant formula, follow-up formula, and selenium-fortified formulated milk powders. Powders are reconstituted with ultrapure water (typically 1:10, w/v) before extraction, and fortification control makes this category the most frequent object of Se(VI) testing.
Fermented Dairy Products
Set and stirred yogurts, fermented milks, and lactic acid bacteria beverages. The acidic matrix requires careful pH control during extraction, because selenite and selenate interconversion is promoted under inappropriate pH conditions.
Cheese and Dried Dairy Ingredients
Natural and processed cheese, butter, cream, whey protein concentrates and isolates, caseinates, lactose, and permeate powders. High-protein or high-fat matrices benefit from enzymatic hydrolysis to release selenium species bound or associated with the protein fraction.
Testing Items
The core testing item is the mass fraction of hexavalent selenium, calculated and reported as selenium (Se) in mg/kg or μg/kg. In practice, accredited laboratories frequently deliver a speciation package that includes:
- Selenate — Se(VI) content, the primary deliverable
- Selenite — Se(IV) content
- Organic selenium estimated by difference or determined by complementary separation modes
- Total selenium by GB 5009.93-2017 (HG-AFS or ICP-MS) for mass-balance verification
- Extraction recovery and species-stability indicators as data-quality supplements
Testing Methods
Method Principle
Water-soluble selenium species are extracted from the dairy matrix under mild, non-oxidizing aqueous conditions. After removal of proteins and fat by centrifugation and membrane filtration, the extract is injected onto a strong anion-exchange LC column. Se(IV) and Se(VI) are separated chromatographically based on their different anion-exchange affinities, with Se(VI) eluting after Se(IV). The column effluent is directed directly into the ICP-MS, where selenium isotopes are monitored in time-resolved mode. Each species is quantified against an external calibration curve prepared from sodium selenate reference standards, and the result is expressed as Se(VI) calculated as selenium.
Extraction and Sample Preparation
Typical sample preparation includes the following steps, all designed to preserve the native speciation:
- Weighing 0.5 g to 2 g of sample (or the equivalent reconstituted liquid) into polypropylene centrifuge tubes
- Aqueous extraction with ultrapure water or dilute ammonium hydroxide solution, assisted by protease hydrolysis where protein-bound species must be released
- Gentle shaking in a 40–60 °C water bath and/or ultrasonication for 20–40 minutes
- Centrifugation and filtration through 0.45 μm or 0.22 μm membranes
- Analysis as soon as practicable, with extracts held refrigerated to suppress species interconversion
Strong acids, oxidants, and high temperatures are deliberately avoided before chromatographic separation, since they can alter the original Se(IV)/Se(VI) distribution and invalidate the speciation result.
Chromatographic Conditions
A strong anion-exchange column with quaternary ammonium functionality (250 mm × 4.1 mm, 10 μm, or equivalent) is used. Common operating conditions include an isocratic mobile phase of ammonium carbonate or ammonium citrate buffer at pH 8.5–10.0, a flow rate of approximately 1.0 mL/min, and an injection volume of 20–100 μL. Baseline separation of Se(IV) and Se(VI) is typically achieved within 8–12 minutes, with species identification confirmed by retention time matching against standards.
ICCP-MS Detection Conditions
Selenium is monitored at isotopes m/z 78, 80, and/or 82. Because argon dimer polyatomic interferences overlap the selenium mass range, a collision/reaction cell (hydrogen or helium mode, or oxygen mass-shift to SeO+) is employed. Germanium or tellurium serves as the internal standard delivered post-column. Under optimized conditions, instrument detection limits in the low sub-μg/L range are attainable, supporting sample-based limits of quantification on the order of 0.5–5 μg/kg.
Total Selenium Confirmation (GB 5009.93-2017)
A representative portion of the sample is subjected to closed-vessel microwave digestion with nitric acid and hydrogen peroxide, followed by determination of total selenium by ICP-MS (Method II of GB 5009.93-2017) or hydride generation atomic fluorescence spectrometry (Method I). The sum of the separated species is compared against total selenium as a mass-balance check, with agreement typically expected within approximately 80–120%.
Quality Assurance and Method Validation
- Method blanks and reagent blanks carried through the full procedure
- Matrix spikes at multiple fortification levels, with recoveries targeted at 80–120%
- Retention-time confirmation of Se(VI) within ±2.5% of the calibration standard
- Linearity verification with correlation coefficients typically r ≥ 0.999
- Repeatability with relative standard deviation generally below 10%
- Certified reference materials, where available, and participation in interlaboratory comparisons
- Species-stability assessments on fortified extracts held over the analytical timeframe
Applications
Speciation-based determination of hexavalent selenium in dairy products supports a wide range of regulatory, commercial, and scientific objectives:
- Compliance verification of selenium-fortified dairy products against the fortification ranges and permitted selenium sources specified in GB 14880
- Support for nutrition labeling under GB 28050 and for product registration dossiers, particularly for infant formula and foods for special medical purposes
- Import and export conformity assessment and market surveillance testing performed by accredited third-party laboratories
- Verification that fortification was achieved with organic selenium sources rather than inorganic selenate or selenite salts, where product claims require it
- Investigation of the influence of dairy processing — heat treatment, fermentation, spray drying — on selenium species transformation
- Research into geographical origin signatures, feed-chain transfer, and bioavailability studies in nutritional science
FAQ
Does GB 5009.93-2017 directly determine hexavalent selenium?
No. GB 5009.93-2017 determines total selenium regardless of chemical form. Species-specific determination of Se(VI) is achieved by anion-exchange HPLC coupled with ICP-MS, executed under the validation and quality requirements consistent with the national standard system, with the total selenium result serving as an essential mass-balance cross-check.
Why is hexavalent selenium monitored separately in dairy products?
Inorganic selenium species such as selenate carry a higher toxicological concern and different bioavailability compared with organic species such as selenomethionine. Regulators, brand owners, and registration authorities increasingly require species-level data to substantiate fortification source compliance and safety assessments.
What is the typical limit of quantification?
With validated HPLC-ICP-MS procedures, limits of quantification for Se(VI) in dairy matrices typically fall in the range of 0.5–5 μg/kg, depending on the sample mass, extraction volume, and instrument sensitivity.
Could the selenium species convert during testing?
Species interconversion is the principal technical risk. It is controlled through mild aqueous extraction, avoidance of oxidants and strong acids before separation, low-temperature handling, prompt analysis, and mandatory spike-recovery and stability verification within each analytical batch.
What sample quantity and turnaround time are typical?
Most laboratories request approximately 100–500 g of representative sample, and routine speciation testing is completed within about 7–10 working days once the sample is registered, including total selenium confirmation.