Analysis of inorganic impurities in lubricating oils by ICP-MS

Significance of the Topic

Monitoring inorganic impurities in lubricating oils is essential for maintaining machinery performance, extending equipment life and preventing unexpected failures. Low levels of wear metals, additives and contaminants can reveal early signs of component degradation, lubrication breakdown or contamination. By achieving lower detection limits and broader dynamic range, modern analytical methods support more precise maintenance schedules and quality control in petrochemical and industrial applications.

Objectives and Study Overview

The primary goal of this work was to evaluate inductively coupled plasma mass spectrometry (ICP-MS) for routine quantification of inorganic elements in lubricating oils. The study compared detection limits, stability and robustness against traditional techniques such as atomic absorption spectrometry (AAS) and ICP-OES. A series of calibration strategies, solvent choices and instrument parameters were tested to develop a reliable method suitable for trace and ultratrace analysis in complex oil matrices.

Methodology and Instrumentation

Sample Preparation

• Oil samples were diluted in analytical grade kerosene (oil-free of trace metals) to minimize background interferences.
• A yttrium internal standard (300 ppb) was added to each vial before analysis.
• Standard Reference Material NIST SRM 1848 was used to validate quantitative accuracy.

Instrumentation

• Agilent 7700x ICP-MS equipped with solid-state RF generator and 1.5 mm injector torch to sustain plasma in the presence of organic solvents and added O₂.
• Glass concentric nebulizer, cooled quartz spray chamber (–5 °C) and Pt-tipped sampler/skimmer cones for enhanced durability.
• Auto-tuning function and dual mode cell gases (H₂ reaction, He collision) were employed to optimize interferences.

Operating Conditions

• RF power 1600 W, carrier gas 0.6 L/min, makeup gas off, O₂ mixed at 0.4 L/min.
• Calibration range covered from sub-ppb to ppm level using commercial multi-element standards and single-element spikes.

Main Results and Discussion

Detection Limits and Background Equivalent Concentrations

• ICP-MS provided 1–2 orders of magnitude lower limits of detection (DL) and background equivalent concentrations (BEC) compared to ICP-OES.
• Achieved DLs as low as 0.013 ppb for V and BECs below 0.1 ppb for most transition metals, despite solvent contributions.

Recovery and Accuracy

• Spike recovery tests on diluted synthetic oil showed 85–123 % recovery across 19 elements, confirming method accuracy.
• NIST SRM results in kerosene and xylene agreed within certified uncertainties, supporting kerosene as a reliable diluent.

Long-Term Stability

• A 5-hour sequence on a 10× diluted oil spiked with 100 ppb of each metal yielded RSDs below 6 % for all elements, demonstrating excellent signal stability in high organic matrix.

Analysis of Commercial Oils

• Minor and trace elements (Na, Mg, Al, P, Ca, Fe, Zn, Mo, Cd) were quantified across 15 commercial mineral, synthetic and semi-synthetic lubricants in the range 0.1 ppm to 5000 ppm.
• Ultratrace metals (Ti, V, Cr, Mn, Ni, Cu, Ag, Sn, Ba, Pb) were determined between single-digit ppb and sub-ppm levels, revealing wide variability among product types.

Benefits and Practical Applications

• Extends quantification range from sub-ppb to thousands of ppm in a single run, reducing the need for multiple dilutions or techniques.
• Improves detection of early wear indicators and contaminants, enabling predictive maintenance and reducing unplanned downtime.
• Compatible with automated sample introduction, offering high throughput for routine laboratories.
• Enhances quality control of base oils and additive packages by verifying metal concentrations at trace levels.

Future Trends and Potential Applications

Advances in collision/reaction cell chemistry and high-resolution ICP-MS may further suppress interferences from organic matrices, pushing detection limits into ultratrace regimes. Miniaturized sample handling and on-line dilution systems could streamline workflows in mobile or plant-side labs. Integration with chemometric tools and predictive analytics will improve interpretation of multi-element profiles for condition monitoring and process optimization.

Conclusion

This study demonstrates that ICP-MS, when equipped with robust RF generators and appropriate cell gas modes, is a powerful tool for comprehensive metal analysis in lubricating oils. It delivers superior sensitivity, wide dynamic range and long-term stability, making it a practical replacement for AAS and ICP-OES in both trace and ultratrace applications. By adopting this method, laboratories can achieve more reliable monitoring of wear metals and contaminants, supporting proactive maintenance strategies and ensuring product quality.

References

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