The Determination of the Priority Pollutant Metals Using the CRA-90 Carbon Rod Atomizer

Significance of the Topic

The accurate determination of trace levels of priority pollutant metals in water and waste streams is critical for environmental protection, regulatory compliance, and public health. Legislation such as the U.S. EPA Primary Drinking Water Standards and the National Pollutant Discharge Elimination System (NPDES) mandates monitoring of metals including As, Cd, Cr, Pb, Hg, and others. The Extraction Procedure (EP) Toxicity Test further extends control to hazardous waste leachates. Graphite furnace atomic absorption spectrometry (GFAAS) with a carbon rod atomizer (CRA-90) enhances sensitivity by two orders of magnitude compared to flame AA, enabling sub-ppb measurements in challenging matrices.

Objectives and Study Overview

This application note evaluates the performance of the CRA-90 carbon rod atomizer for measuring twelve priority pollutant metals (Ag, As, Be, Cd, Cr, Cu, Ni, Pb, Sb, Se, Tl, Zn) in various matrices:

• Pure aqueous standards
• EPA acetic acid extraction simulating landfill leachate
• Certified wastewater reference materials (ERA WasteWatR)
• Municipal drinking water
• High-dissolved-solids industrial effluent

Analytical goals include establishing sensitivity, detection limits, precision, accuracy (recoveries), and addressing chemical interferences through matrix modifiers and background correction.

Methodology

Sample preparation varied by matrix:

• Acidification with dilute HNO3 or acetic acid (pH 5 EP extraction)
• Spiking for recovery studies
• Dilutions to minimize high-salt effects

Calibration approaches:

• External aqueous calibration
• Matrix-matched calibration in acetic acid or dilute HNO3
• Standard additions for high-salt or interfering matrices

Matrix modifiers tested per element included nickel nitrate, silver nitrate, ammonium salts, and sulfuric acid to stabilize analytes and reduce chloride or sulfate interferences.

Instrumentation

Graphite furnace AA system equipped with CRA-90 carbon rod atomizer and element-specific hollow cathode lamps. Key operating steps:

• Drying: 80–105 °C
• Ashing: 400–1500 °C (matrix-dependent)
• Atomization: 1800–2400 °C
• Ramp rates: 300–800 °C/sec
• Injection volumes: 2–20 µL
• Sheath gas: nitrogen or argon
• Background correction: Zeeman or continuum source as needed

Main Results and Discussion

Reported sensitivities (characteristic concentration) ranged from 0.16×10⁻¹² g (Cd) to 19×10⁻¹² g (Se). Detection limits spanned 0.3×10⁻¹² g (Ag) to 10 µg/L in flame AA for Cu, Zn, Ni when applicable. Representative findings:

• Silver: Linear calibration to 0.8 absorbance; DL ≈0.3×10⁻¹² g; recoveries 92–104% with 25% HNO₃ modifier
• Arsenic: DL ≈6.5×10⁻¹² g; nickel nitrate modifier essential; standard additions yielded accurate results in effluent
• Beryllium: Low recoveries (<75%) without H₂SO₄; 1% H₂SO₄ improved precision and recoveries (~99%)
• Cadmium, Lead, Chromium: Excellent agreement with ERA standards; matrix modifiers and peak area measurements addressed high-salt interferences
• Antimony, Selenium: Required high-level modifiers (Ni, Ag) or standard additions for accurate recoveries
• Thallium: Severe chloride interference overcome by 1–2% H₂SO₄; recoveries ~95% in complex matrix
• Cu, Zn, Ni: Flame AA sufficed for mg/L levels; GFAAS used for sub-ppb Ni in high-salt effluent

Overall, a combination of appropriate modifiers, careful furnace programming, and background correction yielded precision typically 0.3–2.5% RSD and accurate recoveries across matrices.

Benefits and Practical Applications

The CRA-90 carbon rod atomizer provides:

• Enhanced sensitivity for low-level metal monitoring (sub-ppb)
• Flexibility to analyze diverse matrices with minimal sample volume
• Reduced reagent consumption and faster analysis than wet chemical methods
• Capability to comply with EPA and state regulations for drinking water, NPDES effluents, and hazardous waste

This method supports environmental laboratories, QA/QC in industry, and research applications requiring trace metal quantification.

Future Trends and Potential Applications

Expected developments include:

• Automated sample preparation and online matrix removal
• Integration with rapid scan spectrometers and continuum lamps for multi-element analysis
• Coupling with chemometric or machine-learning approaches for interference correction
• Miniaturized graphite furnaces for field or onboard environmental monitoring
• Expansion to additional emerging contaminants (e.g., rare earth elements)

Conclusion

Graphite furnace AA using the CRA-90 carbon rod atomizer delivers reliable, high-sensitivity determination of priority pollutant metals across a range of challenging matrices. Proper selection of matrix modifiers, calibration strategy, and furnace parameters ensures accurate and precise results necessary for regulatory compliance and environmental protection.

References

1. T. N. McKenzie et al., “The Reduction of Chemical Interferences in High Salt Matrices using Chemical Modification in Graphite Furnace AAS,” Varian Techtron (1982). Presented at 1982 Federation of Analytical Chemistry and Spectroscopy Societies.
2. C. W. Fuller, “The Effect of Acids on the Determination of Thallium by Atomic Absorption Spectrometry with a Graphite Furnace,” Anal. Chim. Acta, 81, 199 (1976).