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From: Food Quality & Safety magazine, February/March 2010

ICP-MS for Detecting Heavy Metals in Foodstuffs

The technology can analyze 50 samples in an hour

by Shona McSheehy Ducos, PhD; Meike Hamester, PhD; and Michal Godula, PhD

Heavy metals can be toxic for humans when they are not metabolized by the body and accumulate in the soft tissues. Depending on the heavy metal in question, toxicity can occur at levels just above naturally occurring background levels, meaning that consumption of food with a high heavy metal concentration can cause acute or chronic poisoning. Poisoning can result in damaged or reduced mental and central nervous function as well as damage to blood composition, lungs, kidneys, liver, and other vital organs. Long-term exposure to heavy metals may result in slowly progressing physical, muscular, and neurological degenerative conditions as well as cancer.

Arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb), and inorganic tin (Sn) are the most toxic heavy metals that account for most heavy metal poisoning cases. Poisoning is usually a result of environmental pollution or chronic intake of foods high in these metals. Levels of arsenic are usually high in fish and seafood because these organisms absorb and accumulate arsenic from the environment. Cadmium, found in soil because of insecticides, fungicides, sludge, and commercial fertilizers, can contaminate agricultural food products. Some foodstuffs are naturally rich in cadmium, such as liver, mushrooms, shellfish, mussels, cocoa powder, and dried seaweed. Mercury is generated naturally in the environment from volcanic emissions. It is then dispersed across the globe by winds, returning to the earth in rainfall and accumulating in aquatic food chains. Mercury can also contaminate crops sprayed with mercury-containing pesticides.

Typical sample preparation for metals in foodstuffs.
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Figure 1. Typical sample preparation for metals in foodstuffs.

Food Safety Legislation

Driven by consumer demand and quality, many food agencies have introduced directives that stipulate maximum allowable concentrations for heavy metals in foodstuffs. The European Commission directive 1881/2006 specifies maximum levels for Cd, inorganic Sn, Hg, and Pb in a variety of foodstuffs, with, for example, 0.02 mg Pb/kg allowable in milk products and up to 1.5 mg Pb/kg allowable in bivalve mollusks.

The U.S. Food and Drug Administration (FDA) enforces action levels for poisonous or deleterious substances in human food and animal feed, including cadmium, lead, mercury, and others.1 The FDA has also developed a comprehensive Food Protection Plan to address the challenges and changes occurring in food sources, production, and consumption.2 The plan builds upon advances in science and technology to protect the nation’s food supply from both unintentional contamination and deliberate attack.

An accurate, precise, and robust analytical method is required for measurements of heavy metals in foodstuffs to ensure regulatory compliance, maximum product safety and sustainability, and brand protection. The chosen technique must measure toxic elements, species, and micronutrients, and must also identify whether products have been contaminated during the production process or packaging or by the cooking utensils used to prepare them. Authenticity and origin determinations are also essential. Inductively coupled plasma mass spectrometry (ICP-MS) addresses all these requirements in the most efficient way.

Advantages of ICP-MS

To safeguard public health, global legislative bodies have introduced strict regulations that specify maximum allowable concentrations of heavy metals in foodstuffs.

ICP-MS is a high throughput, plasma-based technique with a single high-energy excitation source providing precise determination of heavy metals in foods. The multi-elemental and multi-isotopic nature of the method offers the potential to analyze a whole suite of elements in a single run, saving considerable time and money and allowing faster and more cost-effective decision-making. High linear dynamic range (LDR), another important feature, allows for the simultaneous detection of ultra trace and major elements in one run to obtain comprehensive information about the sample. Trace elements can be determined in a wide range of matrices from parts per trillion (ppt) to low percentage level.

The technology enables food safety laboratories to substantially enhance their productivity, with more than 50 samples analyzed per hour. Other important benefits of ICP-MS include increased sensitivity, a high signal-to-noise ratio, and the flexibility to analyze almost any element in the periodic table. The method offers much lower detection limits compared to graphite furnace atomic absorption (GFAA) and inductively coupled plasma optical emission spectrometry (ICP-OES) while generating less interference than ICP-OES.

Fully quantitative calibration curves for potassium (K) from 10 to 2,000 parts per million and mercury (Hg) from 0.05 to 10 parts per billion (ppb).
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Figure 2. Fully quantitative calibration curves for potassium (K) from 10 to 2,000 parts per million and mercury (Hg) from 0.05 to 10 parts per billion (ppb).

ICP-MS can easily be coupled with separation techniques like liquid chromatography (LC) and gas chromatography (GC), resulting in a literally matrix- independent method capable of performing dependable speciation analyses for toxicological or bioavailability studies. As a multi-isotopic technique, ICP-MS can also provide accurate and precise isotope ration (IR) information, important for authenticity studies or for pinpointing contamination, by verifying the origin of the foodstuff. Because of these benefits, ICP-MS is mandated in standard operating procedures for analyzing heavy metals in foodstuffs.

ICP-MS Regulations

The German Institute for Standardization (DIN) enforces the DIN EN 15765 norm, which specifies a process for the quantification of tin in foodstuffs and canned foods using ICP-MS after pressurized digestion.5 The institute has also introduced the DIN EN 15763 norm, mandating the use of ICP-MS after pressurized digestion for the quantification of arsenic, cadmium, mercury, and lead in foodstuffs.6 The collaborative study performed to develop this norm has included foodstuffs having an arsenic content ranging from 0.06 mg/kg to 21.5 mg/kg dry matter (DM), cadmium ranging from 0.03 mg/kg to 28.3 mg/kg DM, mercury from 0.04 mg/kg to 0.56 mg/kg DM, and lead from 0.01 mg/kg to 2.4 mg/kg DM.

Comparison of the measured and certified concentrations in mg/kg for the four certified reference materials. Meas: Measured value calculated from FQ calibration and back calculated to the solid material according to sample weight taken for digestion and final dilution volume. Cert: Certified value in mg/kg.
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Table 1. Comparison of the measured and certified concentrations in mg/kg for the four certified reference materials. Meas: Measured value calculated from FQ calibration and back calculated to the solid material according to sample weight taken for digestion and final dilution volume. Cert: Certified value in mg/kg.

Both norms address European Commission legislation EC 1881/2006; they describe the analytical procedure used for metal quantification and outline the sample preparation protocol based on microwave digestion according to the DIN EN 13805 norm.7 This standard specifies a method for the digestion of foodstuffs under pressure that is intended for use in the determination of trace elements. The method has been collaboratively tested in combination with atomic absorption techniques and ICP-MS.

A recent experiment demonstrated the unique capabilities of ICP-MS for measuring heavy metals in foodstuffs according to DIN EN 15765 and DIN EN 15763 standards.

A quadrupole ICP-MS system (XSERIES 2 ICP-MS, Thermo Fisher Scientific) was used for the quantification of heavy metals in foodstuffs. The instrument was operated in mixed-mode, namely with and without the use of the collision/reaction cell technology. It was configured with a sample handling system (SC4 PC3 FAST, Elemental Scientific Inc.). The DIN EN 15765 and DIN EN 15763 norm protocols were used for a number of foodstuffs purchased from a local supermarket and four food matrix certified reference materials (CRMs). A simplified analytical methodology was implemented (see Figure 1).

Comparison of the measured and certified concentrations in mg/kg and % (blue) for the two certified reference materials.
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Table 2. Comparison of the measured and certified concentrations in mg/kg and % (blue) for the two certified reference materials.

Triplicate microwave digests were prepared for each food, and triplicate procedural blanks were prepared for each microwave batch. External calibration was performed using multi-concentration, multi-elemental standards generated from single element certified stock solutions. Samples were blank subtracted, quantified against the fully quantitative calibrations, and internal standard corrected. The measured concentration was then used to calculate the amount of each element in the original solid foodstuff.

Table 1 presents the data generated for the elements outlined in the EC 1881/2006 regulation and for arsenic for the four CRMs. Due to the multi-elemental capacity of ICP-MS, a number of other elements were simultaneously quantified. The results obtained for the CRMs were in close agreement with the certified values, validating the method and the instrumentation for determining total elemental concentrations in foodstuffs.

A high LDR range was achieved, meaning that the major components of food such as phosphorous (P), sodium (Na), and potassium (K) could be determined in the same run as the minor elements such as Cd and Hg. A calibration of K from 10 parts per million (ppm) to 0.2% and a calibration of Hg from 50 ppt to 10 parts per billion (ppb), both acquired in the same experiment, demonstrate the advantage of the high LDR of ICP-MS (see Figure 2).

Comparison of the instrumental limits of detection and the limits of detection required by the DIN EN 15763 and 15765 norms. RLD: Required detection limit. CLD: Calculated detection limit of XSERIES 2 with SC4 FAST configuration. *The required detection limit of inorganic tin (Sn) has been estimated from a limit of quantification requirement of 1 ug/L Sn (no isotope specified) in DIN EN 15765.
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Comparison of the instrumental limits of detection and the limits of detection required by the DIN EN 15763 and 15765 norms. RLD: Required detection limit. CLD: Calculated detection limit of XSERIES 2 with SC4 FAST configuration. *The required detection limit of inorganic tin (Sn) has been estimated from a limit of quantification requirement of 1 ug/L Sn (no isotope specified) in DIN EN 15765.

Table 2 shows the quantitative data for a larger number of analytes and for two of the food CRMs. For all analytes, the measured and certified concentrations are in close agreement. This demonstrates the power of ICP-MS to measure simultaneously at sub-ppb level (Hg, silver, uranium) and percentage level (Na, P, K).

The limits of detection (LODs) required for the EU norms are shown in Table 3. The instrumental LODs, calculated from three blanks (3x standard deviation) are well below the required LODs, demonstrating the suitability of this instrumentation and analytical approach for the determination of trace elements in food according to the EN norms.

Food quality in relation to public safety is a primary concern that has led to the introduction of stringent legislation setting maximum levels of contaminants in foodstuffs. In particular, heavy metals are strictly regulated because their consumption in food is associated with a number of serious health conditions. ICP-MS is a multi-elemental technique, ideal for food safety analysis and the determination of heavy metals in foodstuffs, offering improved sensitivity, increased tolerance to matrix, high linear dynamic range, and high throughput. ■

Dr. Ducos is an ICP-MS application specialist, Dr. Hamester is an ICP-MS product manager, and Dr. Godula is a food safety specialist, all at Thermo Fisher Scientific. For more information, contact Dr. Ducos at shona.mcsheehy@thermofisher.com or +49 (0) 421 5493 227.

References

  1. U.S. Food and Drug Administration. Guidance for Industry: Action Levels for Poisonous or Deleterious Substances in Human Food and Animal Feed. Available at: www.fda.gov/Food/ GuidanceComplianceRegulatoryInformation/GuidanceDocuments/ChemicalContaminantsandPesticides/ucm077969.htm. Accessed January 21, 2010.
  2. U.S. Food and Drug Administration. Fact Sheet: Food Protection Plan. Available at: www.fda.gov/Food/FoodSafety/FoodSafetyPrograms/FoodProtectionPlan2007/ucm132705.htm. Accessed January 21, 2010.
  3. European Commission. Commission Regulation (EC) No 1881/2006 of 19 December 2006 setting maximum levels for certain contaminants in foodstuffs. Available at: eurlex.europa.eu/LexUriServ/site/en/oj/2006/l_364/l_36420061220en00050024.pdf. Accessed January 21, 2010.
  4. European Commission. Commission Regulation (EC) No 629/2008 of 2 July 2008 amending Regulation (EC) No 1881/2006 setting maximum levels for certain contaminants in foodstuffs. Available at: www.fsai.ie/uploadedFiles/Commission_Regulation_EC_No_629_2008.pdf. Accessed January 21, 2010.
  5. German Institute for Standardization. Foodstuffs - determination of trace elements - determination of tin by inductively coupled plasma mass spectrometry (ICP-MS) after pressure digestion; German version EN 15765:2009. DIN. Berlin; 2009.
  6. German Institute for Standardization. Foodstuffs - determination of trace elements - determination of arsenic, cadmium, mercury, and lead in foodstuffs by inductively coupled plasma mass spectrometry (ICP-MS) after pressure digestion; German version EN 15763:2009. DIN. Berlin; 2009.
  7. German Institute for Standardization. Foodstuffs - determination of trace elements - pressure digestion, DIN EN 13805:2002. DIN. Berlin; 2002.

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