Determination of Wear Metals in Lubricating Oil on The Liberty Series II ICP-OES With the Axially-Viewed Plasma

Significance of the Topic

The accurate determination of wear metals in lubricating oils is critical for predictive maintenance, machinery health monitoring, and quality control in industrial applications. Trace metal analysis offers insights into component wear, contamination sources, and lubrication performance over time.

Objectives and Study Overview

This study evaluates the performance of an axially-viewed Agilent Liberty Series II ICP-OES combined with an AGM-1 oxygen accessory for quantifying 13 wear metals in a standard reference lubricating oil (NIST SRM 1084a). Key goals include assessing detection limits, precision, long-term stability, and agreement with certified values.

Methodology and Instrumentation

Sample Preparation:

• Dilution of NIST SRM 1084a in kerosene (1:100 w/v) to achieve target analyte levels.
• Working standards prepared in kerosene with multi-element organometallic stock and neutral base oil added to maintain consistent viscosity.

Instrumentation:

• Agilent Liberty Series II ICP-OES with axially-viewed plasma.
• 40 MHz RF generator with Direct Serial Coupling (DISC) and Direct Power Control for enhanced plasma stability.
• Auxiliary Gas Module 1 (AGM-1) oxygen accessory to minimize molecular emissions and carbon buildup.
• 0.75 m Czerny-Turner monochromator, 1800 grooves/mm, resolution 0.018 nm (1st order) to 0.006 nm (4th order).
• Sturman-Masters double-pass spray chamber compatible with organic solvents.
• Glass concentric nebulizer at 230 kPa and viton/PVC pump tubing at 10 rpm.

Operating Conditions:

• Plasma power: 1.5 kW; plasma gas flow: 19.5 L/min; auxiliary flow: 1.5 L/min.
• Sample uptake: 0.6 mL/min; integration time: 3 s; sample delay: 60 s; stabilization: 15 s.
• Background correction: polynomial plotted; PMT voltage: 650 V.

Main Results and Discussion

Certified vs. Measured Concentrations:
All 13 elements (Ag, Al, Cr, Cu, Fe, Mg, Mo, Ni, Pb, Si, Sn, Ti, V) were quantified with agreement within ±5% of certified values. Precision ranged from 0.1 to 1.5% RSD across replicate measurements.

Long-Term Stability:
Continuous analysis over two hours demonstrated excellent reproducibility (0.2–1.3% RSD) for Ni, Cr, and Cu, underscoring the robustness of the axial plasma configuration.

Detection Limits:
Axial ICP-OES detection limits in kerosene were improved by up to tenfold compared to radial viewing, with some elements (Pb, Ni) showing 20–25 fold enhancement. Slightly higher limits for Al at 396.152 nm were offset by using the less interference-prone Al 308.215 nm line.

Benefits and Practical Applications

• Enhanced sensitivity and lower detection limits enable early detection of wear and contamination.
• Improved plasma stability reduces downtime due to cleaning and maintenance.
• Axial viewing combined with oxygen addition simplifies analysis of organic matrices without sacrificing performance.

Future Trends and Opportunities

Emerging developments may include automation of oil dilution and spiking protocols, advanced data processing algorithms for multi-element fingerprinting, and integration of ICP-OES with machine learning for predictive maintenance models. Further miniaturization and field-deployable variants could expand on-site oil monitoring capabilities.

Conclusion

The axially-viewed Liberty Series II ICP-OES, augmented by an AGM-1 oxygen accessory, delivers accurate, precise, and stable quantification of wear metals in lubricating oils. Its superior detection limits and robust performance make it a valuable tool for industrial lubrication analysis and maintenance programs.

References

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