Used Lubcricating Oil Analysis by FT-IR: An Overview

Significance of the Topic

Fourier-transform infrared spectroscopy (FT-IR) has emerged as a powerful tool for monitoring the condition of lubricating oils in engines and industrial machinery. By detecting dilution, degradation and contamination in-service, FT-IR supports proactive maintenance, extends equipment life and prevents unexpected failures.

Objectives and Overview

This application note presents an overview of FT-IR-based oil analysis, illustrating how the technique identifies key molecular markers for oil degradation, fuel dilution, water contamination and soot accumulation. The aim is to highlight workflow simplicity, analytical precision and practical benefits over traditional wet-chemistry methods.

Methodology

FT-IR spectroscopy measures the absorption of infrared radiation by molecules, providing characteristic “fingerprints” for specific functional groups. In oil analysis, a differential approach subtracts the spectrum of a fresh reference oil from that of an in-service sample, isolating signals arising from oxidation products, nitration, sulfation, water, glycol and fuel residues. Stored reference spectra enable trend monitoring over successive sampling intervals.

Used Instrumentation

• Fourier-Transform Infrared spectrometers (e.g. ALPHA II, INVENIO platforms)
• Liquid transmission cells and sampling accessories
• Software for spectral collection, subtraction and trend analysis

Main Results and Discussion

• Oxidation and Nitration: Carbonyl bands near 1700 cm⁻¹ and nitrogen-oxide bands around 1630 cm⁻¹ increase with oil aging.
• Sulfation: Sulfate by-products produce absorption features near 1150 cm⁻¹, correlating with sulfur content in fuel and oil.
• Fuel Dilution: Calibration of gasoline in engine oil (1 %–6 %) shows linear absorbance response in the 800–1000 cm⁻¹ region, enabling fast quantification.
• Water Contamination: OH stretching bands around 3500 cm⁻¹ reveal low-level water ingress, alerting to coolant leaks and risk of corrosion.
• Soot Quantification: Carbonaceous particulates exhibit broad C–H and C=C features, assessing dispersant capacity and engine combustion efficiency.

Benefits and Practical Applications

• Rapid, reagent-free analysis with minimal sample preparation.
• Cost savings by replacing time-consuming wet-chemistry tests (TAN, TBN, viscosity).
• Digital data storage for retrospective comparisons and trend analysis.
• Applicability across automotive, drilling, legal and motorsport environments for routine condition monitoring.

Future Trends and Applications

• Integration of automated sampling systems for online monitoring and real-time diagnostics.
• Utilization of machine learning to predict failure modes from complex spectral patterns.
• Miniaturized, portable FT-IR devices for field-based oil analysis in remote locations.
• Expansion of spectral libraries to cover emerging bio-based lubricants and advanced additive packages.

Conclusion

FT-IR spectroscopy offers a versatile, high-throughput approach to lubricating oil analysis, delivering rapid insights into oil health and engine performance. Its ease of use, low operating cost and capacity for trend monitoring make it an effective alternative to traditional wet-chemical techniques.

References

• ASTM E2412 Standard Practice for Condition Monitoring of Used Lubricants by Trend Analysis Using FT-IR Spectrometry
• ASTM D2896 Standard Test Method for Neutralization Number (TAN) of Petroleum Products by Potentiometric Titration
• ASTM D4739 Standard Test Method for Base Number (TBN) of Petroleum Products by Potentiometric Perchloric Acid Titration