Determination of Pb and Cd in Porphyra tenera using Microwave Digestion and Graphite Furnace Atomic Absorption Spectrometry

Significance of the Topic

The rapid expansion of offshore chemical, petroleum, and mining activities has increased the discharge of heavy metals into marine environments. Porphyra tenera, a widely consumed edible seaweed rich in micronutrients, is at risk of accumulating toxic elements such as lead (Pb) and cadmium (Cd). Monitoring these contaminants is essential to ensure food safety, protect public health, and assess coastal ecosystem quality.

Objectives and Study Overview

This study evaluates the capability of graphite furnace atomic absorption spectrometry (GFAAS), coupled with microwave digestion, to determine Pb and Cd concentrations in dehydrated Porphyra tenera samples. The primary goals were to establish a robust sample preparation protocol, optimize instrumental conditions for complex matrices, and verify accuracy and precision using certified reference material.

Methodology and Instrumentation

Sample Preparation:

• Accurately weigh 0.500 g of homogenized seaweed.
• Add 5 mL concentrated HNO₃ in a microwave digestion vessel.
• Program digestion: ramp to 120 °C (5 min), hold 5 min; to 160 °C (5 min), hold 10 min; to 180 °C (5 min), hold 10 min.
• Evaporate residual acid on a hotplate at 140–160 °C until near dryness.
• Dilute the digest to 50 mL with ultrapure water; prepare a reagent blank in parallel.

Instrumental Analysis:

• Instrument: Agilent AA 280Z Graphite Furnace Atomic Absorption Spectrometer with Zeeman background correction.
• Matrix Modifier: Online addition of 5 µL palladium nitrate to each sample.
• Pb measurement: wavelength 283.3 nm, slit 0.5 nm, lamp current 10 mA; ashing at 600 °C, atomization at 2300 °C; peak area integration.
• Cd measurement: wavelength 228.8 nm, slit 0.5 nm, lamp current 5 mA; ashing at 500 °C, atomization at 2000 °C; peak area integration.

Main Results and Discussion

Calibration and Linearity:

• Pb standards: 5.0–50 µg/L; correlation coefficient R² = 0.9998.
• Cd standards: 0.5–5.0 µg/L; R² = 0.9993.

Precision and Accuracy:

• Six replicates of certified reference (GBW08521) yielded mean recoveries of 96.3% for Pb and 98.0% for Cd.
• Relative standard deviations were 1.35% (Pb) and 0.48% (Cd), confirming excellent repeatability.

Signal Stability:
The absorbance profiles of seaweed samples closely matched those of standards, indicating minimal interference from the complex organic matrix thanks to Zeeman correction and matrix modifier use.

Benefits and Practical Applications of the Method

This approach offers:

• High sensitivity and low detection limits for trace Pb and Cd in food matrices.
• Reliable background correction for complex samples, reducing spectral interferences.
• Relatively low operational cost compared to ICP-MS.
• Applicability in food safety testing, quality control of seaweed products, and marine environmental monitoring.

Future Trends and Potential Applications

Emerging directions include:

• Automation of on-line matrix modifier delivery to further improve throughput.
• Speciation analysis coupling GFAAS with separation techniques to differentiate metal chemical forms.
• Integration with portable GFAAS units for in-field screening of coastal waters and aquaculture sites.
• Application to other dehydrated or high-matrix biological samples such as herbs and powders.

Conclusion

The combined microwave digestion and GFAAS method demonstrates robust performance for trace Pb and Cd determination in Porphyra tenera. High linearity, precision, and recovery validate its utility for routine analysis in environmental and food safety laboratories.

References

1. Agilent Technologies. Application Brief: Determination of Pb and Cd in Porphyra tenera using Microwave Digestion and Graphite Furnace Atomic Absorption Spectrometry. 5994-3171EN, May 22, 2021.