Onsite FTIR quantitative analysis of water in mineral-based oils using a novel water stabilization technique

Importance of the Topic

Monitoring water in mineral-based turbine and hydraulic oils is vital to maintain optimal lubrication, prevent corrosion, additive depletion, and avoid machinery failure.
Onsite analysis minimizes sample alteration due to storage, transport, and de-emulsification processes.

Study Objectives and Overview

This application note introduces an FTIR-based quantitative method employing a novel surfactant-based water stabilization technique.
The aim is to achieve Karl Fischer-equivalent accuracy and range for water determination while reducing analysis time and eliminating hazardous reagents.

Methodology and Instrumentation

• Instrumentation Used: Agilent 5500t FTIR spectrometer series.
• Reagents: Non-ionic polyethylene oxide surfactant blend (water stabilizer).

Sample Preparation and Measurement:

• Gravimetric preparation of oil standards with water levels from 5 to 5300 ppm in both new and aged/in-service oils.
• Add ~3 % surfactant, mix gently to form a stable micro-emulsion, minimizing droplet scattering.
• Acquire FTIR spectra (100 μm pathlength), subtract pure oil baseline, apply mean centering, thickness and baseline correction.
• Develop a partial least squares (PLS) calibration model (R2 ≈ 0.9985, SECV = 85 ppm), correcting for surfactant’s inherent water contribution (~25.5 ppm).

Key Results and Discussion

The surfactant stabilizer transforms free water droplets into uniformly dispersed micelles, tripling the OH absorbance at ~3350 cm⁻¹ and virtually eliminating light scattering.
Validation with independent standards (500–5000 ppm) showed an average prediction error of 5 %, RSD <2 % above 1000 ppm, and <5 % at 500 ppm.
Comparisons against coulometric Karl Fischer titration demonstrated agreement within 4 % error for typical water levels.
Surfactant-treated samples exhibited water retention within 10 ppm over 24 hours versus ~100 ppm loss in untreated oils, enhancing reproducibility.

Benefits and Practical Applications

Advantages of the stabilized FTIR method include:

• Accuracy on par with Karl Fischer titration but without toxic reagents.
• Rapid analysis (~2 minutes per sample) and immediate readiness for successive measurements.
• Portable and user-friendly for onsite oil condition monitoring.

Applicable to turbine oils, hydraulic fluids, gear oils, transformer oils, compressor oils, and crude/diesel oils.

Future Trends and Potential Applications

Anticipated developments include:

• Portable FTIR systems integrated with real-time diagnostics for industrial field service.
• Extension of water stabilization techniques to other challenging matrices (highly viscous or multiphase fluids).
• Advanced chemometric models for simultaneous monitoring of additives, oxidation products, and water content.

Conclusion

Agilent’s surfactant-stabilized FTIR method delivers a fast, accurate, and safe alternative to Karl Fischer titration for water quantification in mineral-based oils.
This approach ensures reliable onsite measurements critical for equipment maintenance and asset protection.