Determination of Pb, Cd, Cr and Ni in Grains Based on Four Chinese National Methods via Zeeman GFAAS

Significance of the Topic

The accurate measurement of lead (Pb), cadmium (Cd), chromium (Cr) and nickel (Ni) in grain products is critical to protect consumers from chronic exposure to toxic metals. Grain‐based foods such as wheat and rice are dietary staples worldwide, and regulatory bodies require sensitive, reliable and routine test methods to ensure that metal concentrations remain below maximum limits.

Objectives and Study Overview

This study evaluated four Chinese national protocols (GB 5009.12–2010 for Pb, GB 5009.15–2014 for Cd, GB 5009.123–2014 for Cr and GB 5009.138–2003 for Ni) for trace‐level determination of Pb, Cd, Cr and Ni in wheat flour. The work aimed to verify method performance using standard reference materials (SRMs) and Agilent’s Zeeman background‐corrected graphite furnace atomic absorption spectrometer (GFAAS) equipped with an autosampler.

Methodology and Instrumentation

Sample preparation was tailored to each element group:

• Pb and Cd: Microwave digestion of wheat flour SRM (NIST 1567b) in HNO₃ and H₂O₂ using a single‐reaction chamber (no vial seals), followed by dilution to 20 mL.
• Cr and Ni: Wet reflux digestion of durum wheat flour SRM (NRC‐CNRC DUWF‐1) with HNO₃ and H₂O₂ at controlled temperatures, then final volume adjustment.

Calibration standards were matrix‐matched to 20% HNO₃ and prepared automatically by the PSD 120 autosampler. Method detection limits (MDLs) were determined from seven replicates of low‐level standards and compared to GB requirements.

Used Instrumentation

• Agilent 280Z Zeeman GFAAS with longitudinal graphite tube and Constant Temperature Zone (CTZ).
• Agilent GTA 120 graphite tube atomizer for high sensitivity and uniform heating.
• Agilent PSD 120 programmable sample dispenser autosampler for automatic standard prep, modifier addition and dilution.
• Single‐element UltrAA hollow cathode lamps for Pb and Ni; hollow cathode lamps for Cd and Cr; coated partition tubes for all elements.

Main Results and Discussion

• Calibration linearity was excellent (r² > 0.999) over concentration ranges: Pb (0–100 µg/L), Cd (0–5 µg/L), Cr (0–20 µg/L) and Ni (0–50 µg/L).
• Measured recoveries in SRMs averaged 92–105%, indicating high accuracy relative to certified values.
• Calculated MDLs in solution (µg/L) were 0.13 (Pb), 0.019 (Cd), 0.038 (Cr) and 0.16 (Ni), corresponding to sample MDLs well below GB specified limits.
• Long‐term instrument stability was demonstrated by periodic analysis of a 5 ppb Pb standard over seven hours, yielding a 1.6% RSD and <4% drift.
• Automated sampling and rapid CTZ furnace ramps reduced analysis time and maintained graphite tube lifetime, supporting high throughput.

Benefits and Practical Applications

The combined GFAAS–Zeeman approach offers:

• Sub-regulatory detection limits for toxic metals.
• Robust matrix correction and precision suited for routine food testing labs.
• Lower running costs and reduced gas consumption.
• Automated sample handling to boost laboratory productivity.

Future Trends and Opportunities

Advances may include integration of ultrafast heating cycles, further miniaturization of furnace systems, coupling with real‐time speciation techniques, and expanding applications to other complex food matrices. Increased automation and data analytics will further streamline compliance testing and enhance data quality.

Conclusion

The Agilent 280Z Zeeman GFAAS, combined with the PSD 120 autosampler, reliably meets and exceeds Chinese national method requirements for Pb, Cd, Cr and Ni analysis in grain products. The method delivers high accuracy, low detection limits and operational efficiency, making it an ideal solution for food safety laboratories.

References

• GB 5009.12–2010: Determination of Lead in Foods.
• GB 5009.15–2014: Determination of Cadmium in Foods.
• GB 5009.123–2014: Determination of Chromium in Foods.
• GB 5009.138–2003: Determination of Nickel in Foods.
• NIST 1567b Wheat Flour Standard Reference Material.
• NRC‐CNRC DUWF-1 Durum Wheat Flour Reference Material.