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Heavy metal analysis in food
Learn how heavy metals are analyzed in food, from sampling and microwave acid digestion to ICP-MS, ICP-OES, method validation and quality control.
Key stages of the analytical workflow used
Heavy metal analysis is an essential part of food safety testing. Accurate determination of trace concentrations requires more than sensitive instrumentation alone. Representative sampling, proper sample preparation, contamination control, validated analytical methods, and robust quality assurance all contribute to reliable results. This page explains the key stages of the analytical workflow used to determine heavy metals in food, from sample collection and digestion to instrumental analysis and quality control.
How are heavy metals analysed in food?
Heavy metal analysis in food typically involves several stages, each of which can influence the accuracy and reliability of the final result.
Sampling: collecting representative samples from a food batch or lot.
Homogenization: preparing a uniform sample for laboratory subsampling.
Digestion: breaking down the food matrix to release target elements into a solution suitable for analysis.
Instrumental analysis: determining elemental concentrations using techniques such as ICP-MS, ICP-OES or atomic absorption spectroscopy (AAS).
Quality control: verifying analytical performance using blanks, reference materials, calibration checks and other quality control procedures.
Result evaluation: calculating concentrations, assessing measurement uncertainty and interpreting results against applicable specifications or regulatory limits.
The analytical procedure depends on the food matrix, target elements, required detection limits and applicable analytical standards. For example, determining cadmium in cereals may require different sample preparation and analytical performance than measuring mercury in fish or inorganic arsenic in rice.
Sampling for heavy metal analysis in food
Representative sampling is one of the most important steps in heavy metal analysis. Regardless of how accurate or sensitive the analytical method is, reliable results can only be obtained if the collected sample accurately represents the batch or lot being tested.
Heavy metals can be distributed unevenly within food products, particularly in raw agricultural commodities, where local variations in soil composition, irrigation practices, or processing conditions can result in significant differences in heavy metal concentrations between individual units of the same batch.
Sampling requirements and EU regulations
To ensure representative sampling, the European Union specifies harmonised sampling procedures for official controls of certain contaminants in food.
Commission Regulation (EC) No 333/2007, as amended, establishes sampling and analytical requirements for the control of specified trace elements and processing contaminants in foodstuffs, including lead, cadmium, mercury and inorganic arsenic.
The regulation specifies requirements for sample collection, preparation and analytical performance. The applicable sampling procedure depends on factors such as the food product, lot size and contaminant being investigated.
Sampling procedures for different food matrices
Sampling procedures vary depending on the food matrix.
For granular or powdered products such as rice, cocoa powder and spices, multiple incremental samples may be collected from different locations within the lot and combined into an aggregate sample before laboratory subsampling.
For liquid products, including edible oils and beverages, appropriate mixing or homogenization may be required before an aliquot is collected. The sampling procedure must account for the characteristics of the product and the possibility of non-uniform contaminant distribution.
Chain of custody documentation is another essential part of the sampling process. Recording who collected the sample, when it was collected, and how it was stored and transported helps ensure traceability and supports the legal defensibility of analytical results used for regulatory compliance or dispute resolution.
Commission Regulation (EU) 2023/915 establishes maximum levels for certain contaminants in food. These maximum levels must be considered alongside the relevant sampling and analytical requirements when evaluating regulatory compliance.
Homogenization of food samples
Homogenization is the process of reducing a food sample to a uniform, finely divided consistency before subsampling for digestion. Because only a small portion of the laboratory sample is ultimately analyzed, effective homogenization is essential to ensure that the test portion accurately represents the composition of the sample.
Inadequate homogenisation is a recognized source of analytical variability in heavy metal testing, particularly for heterogeneous food matrices. For example, heavy metal concentrations in whole fish can differ considerably between muscle tissue, liver, and other organs. The tissue selected for analysis must therefore correspond to the intended analytical objective and applicable regulatory requirements.
Homogenization techniques
The homogenization method depends on the food matrix. Cryogenic milling, in which samples are ground under suitable cooling conditions, can be used for materials that may undergo physical or chemical changes during conventional grinding. Standard blade mills or ball mills may be suitable for more robust, dry food matrices such as grains and spices, provided that the equipment is appropriate for trace element analysis.
Preventing cross-contamination during homogenization is equally important. Grinding equipment can itself introduce trace metal contamination, particularly when metal components are used.
For this reason, laboratories performing heavy metal analysis may use ceramic or other suitable qualified grinding equipment, together with validated cleaning procedures between samples to minimize the risk of contamination.
Sample digestion for heavy metal analysis in food
Before heavy metals can be measured by analytical techniques such as ICP-MS, ICP-OES, or atomic absorption spectroscopy (AAS), many solid and semi-solid food samples must undergo acid digestion.
During this process, the organic food matrix is broken down, releasing the target elements into a aqueous solution suitable for instrumental analysis.
Digestion serves two important purposes:
Element recovery: Releasing target elements from the food matrix so they can be accurately determined.
Matrix decomposition: Reducing organic material that could interfere with instrumental detection or affect sample introduction systems.
Two commonly used digestion approaches are microwave-assisted acid digestion and conventional open-vessel acid digestion. Both techniques use oxidizing acids, most commonly nitric acid (HNO₃), sometimes combined with hydrogen peroxide (H₂O₂) to improve the oxidation of organic matter.
The appropriate digestion procedure depends on the food matrix, target elements, required analytical performance and method validation.
Microwave digestion for heavy metal analysis
Microwave-assisted digestion is widely used for preparing food samples for trace element analysis.
Compared with conventional open-vessel acid digestion, closed-vessel microwave digestion can offer shorter digestion times, controlled reaction conditions and reduced risks of external contamination and loss of volatile analytes.
The technique uses sealed digestion vessels made from chemically resistant materials, such as suitable fluoropolymers or quartz, allowing samples to be digested at elevated temperatures and pressures.
These closed-vessel conditions accelerate the breakdown of the food matrix while helping minimize the loss of volatile elements. This is particularly important for mercury, where inappropriate open-vessel heating conditions may result in analyte losses and inaccurate analytical results.
Closed-vessel microwave digestion also reduces the risk of external contamination because the sample remains isolated from the laboratory environment during digestion. This is especially important in trace-level heavy metal analysis, where contamination from laboratory air, dust, reagents or sample handling can significantly affect analytical results.
Digestion programs are developed according to the characteristics of the food matrix and typically specify parameters such as temperature ramp rate, maximum temperature, hold time and applicable pressure limits. These conditions must be verified or validated for the intended analytical method to demonstrate adequate recovery of the target elements.
Acid digestion using nitric acid and hydrogen peroxide
Nitric acid (HNO₃) is one of the most commonly used reagents for heavy metal digestion in food analysis. Its strong oxidizing properties enable efficient decomposition of many organic food matrices, and nitric acid-based digests are widely used with ICP-MS, ICP-OES and AAS.
Because heavy metals are often measured at trace concentrations, laboratories use trace-metal-grade or ultrapure nitric acid to minimize contamination. Even small amounts of metal impurities present in lower-purity reagents can influence analytical results, particularly when testing against low regulatory maximum levels.
Hydrogen peroxide (H₂O₂) is frequently combined with nitric acid to increase the oxidation of difficult food matrices. It can be particularly useful for samples with a high fat or organic content, such as edible oils, cocoa products, and certain dairy products, where nitric acid alone may not achieve sufficient matrix decomposition within a practical timefrime.
Together, nitric acid and hydrogen peroxide can improve digestion efficiency for many food applications.
Other acids used in sample digestion
Some analytical methods also use hydrochloric acid (HCl) or hydrofluoric acid (HF) for specific sample types. Hydrochloric acid may be incorporated into certain digestion or analytical solution preparation procedures, depending on the target elements and method requirements.
When dissolution of silica or silicate materials is required, hydrofluoric acid may be used to release elements associated with these components. However, HF is highly hazardous and requires specialized laboratory equipment, compatible digestion vessels and strict safety precautions.
The acid mixture should always be selected according to the target elements, sample matrix, digestion vessel compatibility and instrumental analysis requirements.
Validation of sample digestion and analytical methods
Validation of sample digestion and analytical methods demonstrates that the selected procedure can produce reliable results for the target elements and intended food matrices.
Even when an analytical instrument provides excellent sensitivity and precision, incomplete digestion, analyte losses or contamination during sample preparation can lead to inaccurate results. It is important to distinguish between evaluating digestion performance and validating the complete analytical method.
Digestion performance primarily concerns the ability of the sample preparation procedure to recover target elements without unacceptable contamination or losses.
Analytical method validation evaluates the performance of the complete procedure, including sample preparation, instrumental measurement and result calculation.
Key parameters in method validation
| Parameter | Purpose |
|---|---|
| Accuracy and trueness | Evaluate agreement between analytical results and accepted reference values. |
| Recovery | Assess the proportion of the target analyte recovered through the analytical procedure. |
| Repeatability | Assess variation between replicate measurements under repeatability conditions. |
| Intermediate precision | Evaluate variability under defined changes in laboratory conditions. |
| Limit of detection (LOD) | Establish the lowest concentration that can be reliably detected under specified conditions. |
| Limit of quantification (LOQ) | Establish the lowest concentration that can be quantified with acceptable performance. |
| Blank contamination | Identify contamination introduced by reagents, vessels or sample preparation. |
| Matrix effects | Evaluate whether sample composition influences recovery or instrumental response. |
| Measurement uncertainty | Quantify uncertainty associated with the reported analytical result. |
LOD, LOQ and measurement uncertainty generally characterize the overall analytical procedure rather than digestion efficiency alone.
Contamination prevention and blanks
Because heavy may be measured at concentrations in the low microgram-per-kilogram range, even small amounts of contamination can significantly affect analytical results. Preventing contamination is therefore an essential part of heavy metal analysis and is just as important as the performance of the analytical instrument itself.
Potential sources of contamination exist throughout the analytical workflow, including sample collection, homogenization, laboratory equipment, glassware, reagents, and even airborne dust within the laboratory.
To minimize these risks, laboratories apply contamination control procedures and use suitable materials, high-purity reagents, and validated cleaning protocols throughout sample preparation and analysis.
Procedural blanks and reagent blanks
One of the most important quality control measures is the use of blanks.
Procedural or method blanks undergo the complete sample preparation and digestion procedure without containing a food sample. They help laboratories detect contamination introduced during the analytical process.
Reagent blanks assess contamination associated with the chemicals used during sample preparation.
Blank results are evaluated against predefined acceptance criteria as part of analytical quality control. Where permitted by the validated method, blank correction may be applied during data processing.
If blank contamination exceeds established acceptance limits, the laboratory must investigate the source and take appropriate corrective action. Depending on the method and quality control requirements, affected samples may need to be prepared and analysed again.
Inductively coupled plasma optical emission spectroscopy (ICP-OES)
Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a widely used analytical technique for determining heavy metals and other elements in food. It offers good sensitivity, a wide linear dynamic range, and the ability to measure multiple elements simultaneously, making it well suited for routine food testing laboratories.
During analysis, the digested sample solution is introduced into a high-temperature argon plasma. The plasma excites atoms and ions, causing them to emit light at characteristic wavelengths. By measuring the intensity of this emitted light and comparing it with suitable calibration standards, the concentration of each element can be determined.
ICP-OES is particularly suitable for elements present at moderate to relatively high concentrations and is widely used for both essential trace elements and heavy metals where regulatory limits fall within its analytical detection range. However, for applications requiring the measurement of extremely low concentrations, such as cadmium and lead in infant foods, the sensitivity of ICP-OES can approach its practical limits. For these demanding applications, inductively coupled plasma mass spectrometry (ICP-MS) is generally the preferred analytical technique because it provides substantially lower detection limits.
Inductively coupled plasma mass spectrometry (ICP-MS)
Inductively coupled plasma mass spectrometry (ICP-MS) is widely used for trace element determination in food, particularly when low detection limits and multi-element analysis are required. Compared with ICP-OES and conventional AAS, ICP-MS generally offers substantially lower detection limits while retaining the ability to measure multiple elements in a single analytical run.
Like ICP-OES, ICP-MS introduces the prepared sample into a high-temperature argon plasma. In ICP-MS, the elements are ionized, and the resulting ions are separated according to their mass-to-charge ratio in a mass spectrometer.
This enables highly sensitive determination of elements such as arsenic, cadmium, lead and mercury, provided that the method is suitable for the sample matrix and required concentration range. Modern ICP-MS systems may incorporate collision/reaction cells or triple-quadrupole configurations to reduce spectral interferences.
For example, argon chloride polyatomic ions can interfere with arsenic determination at mass-to-charge ratio 75. Appropriate interference control is therefore important for reliable results. ICP-MS is particularly useful for regulatory food testing when low quantification limits are required.
Atomic absorption spectroscopy (AAS)
Atomic absorption spectroscopy (AAS) is a well-established technique for determining heavy metals in food and remains useful in many analytical laboratories.
Two common variants are flame atomic absorption spectroscopy (FAAS) and graphite furnace atomic absorption spectroscopy (GFAAS). Of the two, GFAAS generally provides higher sensitivity and can meet analytical requirements for selected trace elements, depending on the food matrix and concentration range.
Unlike ICP-OES and ICP-MS, conventional AAS generally determines one element at a time. As a result, analyzing a sample for several heavy metals requires sequential measurements, making AAS less efficient for high-throughput, multi-element analysis.
For laboratories analyzing large numbers of samples or requiring very low detection limits across multiple elements, ICP-MS is often preferred. Nevertheless, AAS remains a reliable option for many applications, particularly where sample throughput is lower or where the required detection limits can be achieved with an element-specific method.
ICP-MS vs ICP-OES vs AAS: which method should you choose?
The appropriate analytical technique depends on the target elements, expected concentrations, sample throughput and required analytical performance.
| Technique | Main advantages | Main limitations | Typical suitability |
|---|---|---|---|
| ICP-MS | Very low detection limits; multi-element analysis | Higher instrument complexity and potential spectral/matrix interferences | Trace-level multi-element testing |
| ICP-OES | Multi-element analysis; wide working range | Generally higher detection limits than ICP-MS | Routine elemental analysis at suitable concentrations |
| FAAS | Established technique; relatively straightforward operation | Single-element analysis; lower sensitivity | Selected elements at relatively higher concentrations |
| GFAAS | High sensitivity for selected elements | Lower throughput; generally single-element analysis | Trace analysis of individual elements |
No single technique is optimal for every food matrix or analytical requirement. For regulatory testing, laboratories must demonstrate that the selected method achieves the necessary analytical performance, including appropriate limits of quantification, accuracy and precision.
Quality assurance (QA), quality control (QC), and proficiency testing
Quality assurance (QA) and quality control (QC) form the foundation of reliable heavy metal analysis in food.
Together, they encompass the procedures used throughout the analytical workflow, from sampling and sample preparation to instrumental analysis and result reporting, to ensure that reported concentrations are accurate, precise and traceable.
Routine QA/QC measures include:
Analysis of certified reference materials (CRMs)
Procedural and reagent blanks
Recovery studies
Calibration verification
Internal standard monitoring, where applicable
Replicate sample analysis
Assessment of measurement uncertainty
Evaluation of method detection and quantification limits
These activities are complemented by laboratory accreditation under internationally recognized standards such as ISO/IEC 17025, which specifies general requirements for the competence, impartiality and consistent operation of testing and calibration laboratories.
Proficiency testing
Proficiency testing provides an additional level of external quality assurance. During a proficiency test, participating laboratories analyze samples with assigned values that are not disclosed to participants in advance.
The reported results are evaluated against assigned values using an established proficiency assessment procedure. Regular participation in suitable proficiency testing schemes provides independent evidence of laboratory performance.
Together with internal QA/QC procedures, proficiency testing supports confidence in the data used for regulatory compliance, supplier verification, food safety monitoring and product certification.
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FAQ
What is the best method for heavy metal analysis in food?
ICP-MS is often preferred for trace-level multi-element analysis because it offers high sensitivity and low detection limits. However, ICP-OES and AAS may also be suitable depending on the target elements, food matrix, expected concentrations and analytical requirements.
Why is microwave digestion used for heavy metal analysis?
Microwave digestion breaks down food matrices using controlled heating and acid reagents. Closed-vessel microwave digestion enables elevated temperatures and pressures while reducing external contamination and the risk of losing volatile analytes.
What acids are commonly used for heavy metal digestion?
Nitric acid (HNO₃) is the most commonly used acid for heavy metal digestion in food analysis. Hydrogen peroxide (H₂O₂) is frequently added to improve oxidation and digestion efficiency, particularly for food samples with high fat or organic content.
Why is hydrogen peroxide added during digestion?
Hydrogen peroxide increases the oxidation of organic matter and helps achieve complete digestion of difficult matrices such as dairy products, edible oils, cocoa products, and high-fat foods.
What is homogenization in heavy metal analysis?
Homogenization is the process of reducing a food sample to a uniform consistency before analysis. It ensures that the portion analyzed is representative of the entire sample.
How is heavy metal digestion validated?
Digestion performance can be evaluated through suitable reference materials, recovery studies, replicate preparations, procedural blanks and other method validation experiments. These assessments help demonstrate that the procedure provides reliable recovery of target elements from the intended food matrices.
What is the difference between ICP-MS and ICP-OES for food analysis?
ICP-MS measures ions according to their mass-to-charge ratio and generally provides lower detection limits. ICP-OES measures characteristic optical emissions from excited atoms and ions and is well suited for multi-element analysis where the required concentration range is within its capabilities.
Can ICP-MS determine inorganic arsenic in food?
Yes, ICP-MS can be used to determine inorganic arsenic when combined with suitable sample extraction and chemical separation, such as HPLC. Conventional digestion followed by ICP-MS does not distinguish inorganic arsenic from organic arsenic species.
