Trace element determination in dairy products is fundamental to verifying nutritional fortification, monitoring heavy metal contamination, and demonstrating compliance with international food safety regulations. Accurate measurement of both essential micronutrients and toxic elements in complex milk matrices depends on validated digestion procedures and high-sensitivity techniques such as ICP-MS, ICP-OES, and atomic absorption spectrometry. This article reviews the complete analytical workflow, the applicable ISO, AOAC, EN, and GB standards, and the quality control practices applied by accredited third-party laboratories for trace element testing of milk and dairy products.
Technical Overview
Dairy products are among the most widely consumed foods in the world and represent a significant dietary source of minerals and trace elements. Trace elements in milk play a dual role: essential micronutrients such as iron, zinc, copper, manganese, selenium, iodine, chromium, cobalt, and molybdenum support critical physiological functions, while toxic elements including lead, cadmium, arsenic, and mercury pose health risks even at very low concentrations. Reliable determination of both categories is therefore a core requirement for food safety surveillance, nutrition labeling, and regulatory compliance.
The dairy matrix presents distinct analytical challenges. Whole milk typically contains 3.5% to 4.5% fat, approximately 3.2% protein dominated by casein micelles, and around 4.7% lactose, all of which complicate digestion and can cause matrix effects, spectral interferences, and contamination-related bias. Modern practice therefore favors closed-vessel microwave digestion coupled with inductively coupled plasma mass spectrometry (ICP-MS) for multi-element analysis at trace and ultratrace levels, supported by complementary techniques for specific elements and concentration ranges.
Regulatory frameworks worldwide impose strict controls on both contaminants and declared nutrients. Maximum residue and contaminant limits for dairy are established under standards such as GB 2762 in China, Codex CXS 193-1995, and European Union regulations, while infant formula is additionally governed by GB 10765 and Codex STAN 72-1981. Nutrition labeling requirements, including GB 28050 and Codex labeling guidelines, further drive demand for accurate quantitative data on essential minerals.
Testing Samples
Liquid Milk Products
Raw bovine milk, pasteurized and UHT whole or skim milk, flavored and modified liquid milk, cream, and reconstituted milk are the most common liquid matrices. Because trace elements partition between the aqueous phase, fat globule membrane, and protein fraction, thorough homogenization and representative subsampling are essential. Liquid samples are typically analyzed on a fresh-weight basis and results may be reported as mg/kg or converted to mg per 100 mL.
Milk Powders and Infant Formula
Whole milk powder, skim milk powder, formulated milk powders, infant formula, follow-up formula, and growing-up milk powders are analyzed on a dry or as-is basis depending on the regulatory requirement. Powdered matrices concentrate both nutrients and contaminants relative to liquid milk, and infant formula demands the most stringent control because of the heightened vulnerability of the target consumer group. Fortified powders require verification that added iron, zinc, and selenium fall within permitted ranges.
Fermented and Ripened Dairy Products
Yogurt, fermented milk drinks, kefir, cheese, and processed cheese present higher protein and fat contents and, in the case of ripened cheese, concentrated mineral levels. Acidification during fermentation can also leach elements such as zinc and copper from processing equipment, making these products relevant for contamination monitoring as well as nutritional assessment.
Dairy Ingredients and By-Products
Whey powder, permeate, lactose, whey protein concentrates and isolates, milk protein concentrates, casein, and caseinates are tested both as finished ingredients for food manufacturers and as trade commodities subject to contractual specifications. Trace element profiles in these ingredients directly influence downstream product formulation and compliance.
Testing Items
Trace element testing of dairy products generally covers two complementary groups of analytes:
- Essential and nutritional trace elements: iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), selenium (Se), iodine (I), chromium (Cr), cobalt (Co), molybdenum (Mo), and nickel (Ni)
- Macro minerals frequently determined in the same analytical run: calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), and sodium (Na)
- Toxic and contaminant elements: lead (Pb), cadmium (Cd), total arsenic (As), inorganic arsenic, total mercury (Hg), aluminum (Al), tin (Sn), antimony (Sb), thallium (Tl), barium (Ba), and beryllium (Be)
- Speciation-related determinations where required: inorganic arsenic for refined risk assessment and organotin compounds for canned or foil-packaged dairy products
Typical natural levels in bovine milk include approximately 0.3 to 0.6 mg/kg of zinc, 0.02 to 0.05 mg/kg of iron, and selenium in the range of 10 to 30 µg/kg, although values vary considerably with feed, geography, season, and processing. Contaminant levels are normally far lower; for example, maximum levels for lead in liquid milk are typically set at 0.02 to 0.05 mg/kg under European and Chinese regulations, requiring methods with detection limits well below these thresholds.
Testing Methods
Sample Preparation and Digestion
Complete decomposition of the organic matrix without loss or contamination is the foundation of accurate trace element analysis. Microwave-assisted acid digestion is the preferred approach: 0.3 to 0.5 g of homogenized sample is treated with 5 to 8 mL of nitric acid and 1 to 2 mL of hydrogen peroxide in sealed PTFE vessels, ramped to 180 to 200 °C. This method is suitable for nearly all elements described in EN 13805 and GB 5009 series procedures.
Hot-plate wet digestion with nitric acid and perchloric acid mixtures remains in use for specific legacy methods, while dry ashing at 450 to 550 °C is only appropriate for refractory elements because volatile species of mercury, arsenic, selenium, and iodine are lost at elevated temperatures. For iodine specifically, alkaline extraction with tetramethylammonium hydroxide (TMAH) followed by ICP-MS determination, as specified in ISO 20647, avoids the volatilization losses associated with oxidative digestion.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is the technique of choice for multi-element determination at trace and ultratrace levels, offering detection limits from 0.001 to 0.05 mg/kg for most elements and simultaneous quantification of more than 20 analytes in a single run. Key methodological controls include internal standardization with elements such as scandium, germanium, rhodium, indium, terbium, and bismuth to correct for matrix suppression and drift, and the use of collision or reaction cell gases such as helium or hydrogen to resolve polyatomic interferences, including argon oxide on iron-56, argon chloride on arsenic-75, and argon dimers on selenium isotopes. Gold is commonly added to stabilize mercury solutions and reduce memory effects. GB 5009.268 and AOAC 2015.01 describe validated ICP-MS procedures applicable to dairy matrices.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
ICP-OES provides robust, high-throughput determination of macro minerals and higher-concentration trace elements such as zinc, iron, and copper in fortified dairy products, with typical working ranges from 0.01 to several hundred mg/kg. Its wider linear dynamic range and greater tolerance of total dissolved solids make it particularly suitable for milk powders and mineral concentrates, and it is often run in parallel with ICP-MS to cover both major and trace analytes efficiently.
Atomic Absorption and Atomic Fluorescence Techniques
Flame atomic absorption spectrometry (FAAS) remains a cost-effective, well-standardized option for iron, zinc, copper, and manganese, supported by dedicated international methods such as ISO 8070 for calcium, magnesium, sodium, and potassium, ISO 11813 for zinc, and ISO 5738 for copper. Graphite furnace AAS (GFAAS) provides the sensitivity required for lead and cadmium under GB 5009.12 and GB 5009.15, with characteristic detection limits at the sub-µg/kg level. Hydride generation and cold vapor techniques, together with atomic fluorescence spectrometry (AFS), are widely applied to arsenic, mercury, and selenium in accordance with GB 5009.11, GB 5009.17, and GB 5009.93.
Quality Control and Method Validation
Defensible data depend on a comprehensive quality system extending throughout the workflow:
- Use of certified reference materials such as skim milk powder or whole milk powder CRMs to verify trueness of the full digestion and measurement procedure
- Procedural blanks, duplicate analyses with relative deviations typically below 20%, and matrix spike recoveries within 80% to 120%
- Multi-point external calibration with correlation coefficients of at least 0.995, verified with independent calibration check standards
- Acid-cleaned high-purity labware, ultrapure acids, and Class 100 work areas to minimize contamination at ultratrace levels
- Regular participation in international proficiency testing programs for milk and milk powder matrices
- Measurement uncertainty budgets that account for homogenization, digestion recovery, calibration, and instrumental repeatability
Applications
Trace element data generated by accredited third-party laboratories support a broad range of practical objectives across the dairy value chain:
- Regulatory compliance testing against contaminant limits in GB 2762, Codex CXS 193-1995, and EU regulations for domestic marketing and import-export inspection
- Verification of nutritional fortification and labeling compliance for iron, zinc, calcium, and selenium under GB 28050, EU food information regulations, and Codex labeling guidelines
- Infant formula conformity assessment against GB 10765 and Codex STAN 72-1981, where both minimum nutrient levels and maximum contaminant levels must be demonstrated
- Monitoring of process-derived contamination, including nickel and chromium from stainless steel equipment contact, tin from cans, and aluminum from additives or packaging
- Environmental surveillance linking milk composition to pasture, feed, water, and soil conditions in dairy-producing regions
- Shelf-life and stability studies confirming that mineral fortification remains homogeneous and within specification through the product life cycle
FAQ
Which technique is best suited for trace element analysis of dairy products?
ICP-MS is generally the preferred platform because it quantifies more than twenty essential and toxic elements simultaneously with detection limits in the ng/kg to µg/kg range. ICP-OES is more appropriate for macro minerals and heavily fortified products, while flame and graphite furnace AAS remain excellent targeted options supported by long-standing ISO and GB standard methods. Most laboratories combine techniques to balance sensitivity, throughput, and cost.
How should dairy samples be stored before trace element analysis?
Liquid samples should be homogenized and held at 2 to 6 °C in acid-washed polyethylene containers and analyzed promptly, while powders and cheeses should be stored in airtight containers protected from moisture and cross-contamination. Freezing is acceptable for longer storage, provided the sample is thoroughly re-homogenized after thawing. Contact with glass should be minimized for iodine and boron determinations.
Why is iodine determination in dairy products particularly challenging?
Iodine is volatile and can be lost during oxidative acid digestion, and it is prone to memory effects in ICP-MS. The validated approach described in ISO 20647 uses alkaline TMAH extraction followed by ICP-MS with tellurium as an internal standard, which preserves iodine integrity and delivers reliable results in milk, whey, and infant formula matrices.
What detection limits can be achieved for toxic elements such as lead and cadmium?
With microwave digestion and ICP-MS or GFAAS determination, method detection limits of 0.001 to 0.01 mg/kg are routinely achieved for lead and cadmium in dairy matrices, comfortably below the 0.02 to 0.05 mg/kg regulatory maximum levels that apply to liquid milk in major jurisdictions.
How are spectral and matrix interferences controlled in ICP-MS analysis of milk?
High salt and protein digests cause matrix suppression that is corrected using internal standards, while polyatomic interferences are managed through kinetic energy discrimination with helium collision gas or reaction cell chemistry. Dilution of digests, matrix-matched calibration, and interference-free isotope selection further improve accuracy for elements such as iron, arsenic, selenium, and vanadium.
Are infant formula trace element requirements different from those for general dairy products?
Yes. Infant formula is subject to both minimum mandatory levels for essential elements such as iron, zinc, copper, manganese, selenium, and iodine, and stricter maximum contaminant limits, reflecting the sensitivity of infants to both deficiency and excess. Compliance is assessed against GB 10765, Codex STAN 72-1981, and equivalent national regulations, usually with tighter analytical uncertainty requirements.