ICP-MS for Detecting Heavy Metals in Foodstuffs

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.

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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.

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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).

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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.