Determination of Lead in Unleaded Gasoline on the Liberty Series II ICP-OES with the Axially-Viewed Plasma

Importance of the Topic

Unleaded gasoline analysis for trace lead content remains critical for environmental protection and regulatory compliance. Combustion of leaded fuels historically contributed the majority of atmospheric lead, prompting stringent limits on residual lead in modern petrol. High-sensitivity analytical techniques are required to verify that unleaded gasoline meets health and safety standards.

Objectives and Overview of the Study

This work aimed to demonstrate a robust method for quantifying low levels of lead in unleaded gasoline using an axial-viewing ICP-OES system. The study evaluated analytical performance, detection limits, precision, accuracy against a certified reference material, and compared axial versus radial plasma viewing configurations.

Methodology and Instrumentation

A standard additions calibration was applied to account for the complex hydrocarbon matrix. Samples and standards were prepared by spiking unleaded gasoline with defined amounts of lead, followed by addition of iodine to form stable iodo-lead alkyl complexes and a quaternary ammonium salt (Aliquot 336) in dekalin to maintain analyte integrity. A cooled glass spray chamber minimized solvent vapor load to stabilize the plasma. An AGM-1 oxygen accessory introduced oxygen into the auxiliary gas stream, reducing molecular interferences and carbon deposition.

Instrumentation Used

• Agilent Liberty Series II ICP-OES with axial-viewing plasma
• AGM-1 oxygen accessory
• Cooled glass spray chamber (–10 °C)
• Standard concentric glass nebulizer
• Digital Equipment Corporation Celebris computer with Plasma 96 software

Main Results and Discussion

Wavelengths of 261.418 nm and 283.306 nm were selected for lead determination based on minimal spectral overlap and optimal detection limits. Detection limits in iso-octane were 11 µg/L and 12 µg/L for axial viewing, representing a five- to sixfold improvement over radial viewing. Analysis of a NIST SRM 2712 reference fuel produced values (7.7 ± 0.2 mg/L and 7.8 ± 0.1 mg/L) in excellent agreement with the certified 7.9 ± 0.3 mg/L. Four replicate measurements of a commercial unleaded gasoline yielded 0.507 ± 0.004 mg/L (261.418 nm) and 0.525 ± 0.003 mg/L (283.306 nm), within regulatory limits. Long-term stability tests over one hour spiked with multiple elements showed reproducibility of 0.9–1.1 %RSD.

Benefits and Practical Applications

The described axial ICP-OES method offers enhanced sensitivity and stability for volatile organic matrices, enabling reliable quality control in fuel production and environmental monitoring. Improved detection limits facilitate compliance with tightening regulatory thresholds for lead content.

Future Trends and Potential Uses

Further advancements in plasma gas optimization and accessory design may extend axial ICP-OES applications to other challenging organic solvents. Integration with automated sample handling and advanced background correction could yield even lower detection limits and higher throughput.

Conclusion

Axial-viewing ICP-OES with oxygen addition and a cooled spray chamber provides a precise, accurate, and sensitive approach for lead determination in unleaded gasoline. The method meets regulatory requirements and demonstrates excellent agreement with certified standards.

References

1. Air Quality Criteria for Lead. USEPA, EPA-600/8-83/028, vols I–IV, US EPA, 1986.
2. Demers DR. Evaluation of the Axially-viewed (End-on) Inductively Coupled Argon Plasma Source for Atomic Emission Spectroscopy. Applied Spectroscopy. 1979;33:584–591.
3. Nham T. Determination of Lead in Unleaded Gasoline by ICP-AES with the Use of Oxygen and a Cooled Spray Chamber. Varian Australia Pty. Ltd. ICP-AES At Work No. 15, 1993.