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

Importance of the topic

Water contamination in mineral-based lubricating oils critically affects machinery performance and longevity. Even small amounts of water can alter physical properties such as viscosity, density, and surface tension, accelerate additive depletion, and promote chemical degradation (oxidation, nitration, varnish formation). Traditional Karl Fischer (KF) titration provides accurate water measurement but is time-consuming, relies on hazardous reagents, and is not ideally suited for onsite analysis. A rapid, accurate, and field-deployable alternative is highly desirable to capture true in-service water levels before sample handling alters results.

Study objectives and overview

This application note describes development and validation of a novel FTIR-based method for quantitative analysis of water in turbine and other mineral oils. The goals were to overcome errors caused by water droplet scattering in conventional FTIR approaches and achieve accuracy comparable to the KF gold standard, while enabling fast, onsite measurements with the Agilent 5500t spectrometer.

Methodology and Instrumentation

• Instrumentation: Agilent 5500t FTIR spectrometer; Karl Fischer coulometric titrator for reference measurements.
• Sample preparation: Mineral turbine oils (Chevron GST 32, Mobil DTE 797), aged or in-service, spiked gravimetrically with water (5–5300 ppm).
• Water stabilization: Addition of ~3% non-ionic polyethylene oxide surfactant mixture to form a stable emulsion with uniform droplet size and minimize light scattering.
• Spectral acquisition: Transmission mode with 100 µm pathlength cell; spectra collected immediately after gentle mixing.
• Data processing: Baseline and thickness correction, mean centering; partial least squares (PLS) calibration with cross-validation (four–fold, three factors).

Main results and discussion

• Surfactant effect: Water OH absorbance near 3350 cm⁻¹ increased threefold and baselines became uniform, indicating minimal scattering.
• Calibration performance: R² = 0.9985, standard error of cross-validation (SECV) ~85 ppm after subtracting 25.5 ppm surfactant contribution.
• Validation set (500–5000 ppm): Average prediction error ~5%; relative standard deviation <5% at 500 ppm and <2% at higher concentrations.
• Comparison to KF: Differences of 0.8–9.2% across validation levels; one higher deviation likely due to KF bias rather than FTIR error.
• Onsite stability: Unstabilized samples lost ~100 ppm water over 24 h; stabilized samples lost <10 ppm under similar conditions.

Benefits and practical applications

• Rapid analysis: FTIR measurement in ~2 minutes vs. 10–15 minutes for KF titration plus instrument equilibration.
• Safety and convenience: No hazardous reagents; non-volatile, non-corrosive, non-flammable surfactant; simple sample cleanup.
• Onsite readiness: Immediate reusability of instrument; kits available with premixed stabilizer, dispensers, and vials for field use.
• Broader applicability: Method extends to hydraulic, gear, transformer, compressor, diesel, crude, and other mineral oils.

Future trends and potential applications

• Integration with field-portable FTIR systems for real-time condition monitoring in industrial and power-generation settings.
• Extension to multifunctional analysis: simultaneous monitoring of oxidation, nitration, and additive depletion alongside water content.
• Development of automated sample introduction and mixing modules to further reduce handling variability.
• Exploration of tailored surfactants for specialty fluids with challenging additive packages or high water affinity.

Conclusion

The Agilent water stabilization FTIR method delivers rapid, accurate, and reproducible quantification of water in mineral-based oils, matching KF accuracy without its drawbacks. By employing a non-hazardous surfactant to stabilize water droplets, the technique eliminates scattering-induced errors and supports reliable onsite measurements, improving maintenance decisions and equipment reliability.

References

• Higgins F., Seelenbinder J. Onsite FTIR quantitative analysis of water in mineral-based oils using a novel water stabilization technique. Application note, Agilent Technologies, 2013.