Measurement of Grease Using FTIR

Significance of the Topic

Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR) is widely used for rapid, contact-based identification of base oils and degradation products in greases. Because greases are semi-solid dispersions of thickeners in base oils and are used extensively for lubrication of bearings and shafts, reliable compositional analysis supports lubricant selection, troubleshooting, failure analysis and condition monitoring in industrial maintenance and quality control.

Objectives and Study Overview

This application study compares single-reflection ATR measurements to transmission (liquid film/KBr) measurements for three commercial greases with different base oils (silicone, fluorine, mineral oil). The aims were to: (1) evaluate spectral differences between ATR and transmission modes, (2) identify measurement/analysis strategies to obtain transmission-equivalent spectra from ATR data for strongly absorbing greases, and (3) demonstrate practical workflows for routine grease analysis.

Instrumentation Used

The experiments used Shimadzu IRSpirit-TX FTIR with a Microm ATR accessory and liquid cell. Key measurement parameters included:

• ATR crystals: diamond (Dia) and germanium (Ge)
• Liquid cell window: KBr (for thin liquid films/transmission)
• Wavenumber ranges: 4000–400 cm-1 (standard), 4000–600 cm-1 (Ge measurements)
• Resolution: 4 cm-1
• Accumulations: 40 scans
• Detector: DLTGS

Methodology

Three commercial greases (silicone-based, fluorine-based, and mineral oil-based) were measured by:

1. Single-reflection ATR (spreading grease directly on the prism surface)
2. Transmission measurement using a thin liquid/film on a KBr window

Because ATR sampling depth depends on prism material, incidence angle and sample refractive index, the study investigated two approaches to reduce ATR-related spectral distortions for strongly absorbing greases:

• Apply an extremely thin layer of grease to the diamond ATR crystal to make the layer thickness much smaller than the ATR penetration depth.
• Substitute the diamond ATR crystal with germanium (Ge) to reduce penetration depth (Ge has a higher refractive index; under comparable geometry penetration in Ge is roughly one-third that in diamond).

Advanced ATR correction (software-based transform to approximate transmission spectra) was also applied to selected ATR spectra to evaluate equivalence to transmission measurements.

Main Results and Discussion

Key experimental observations and interpretations were:

• Mineral oil grease: ATR and transmission spectra showed matching peak positions and comparable intensity patterns. Spectral search identified the mineral oil component as predominantly paraffinic.
• Silicone and fluorine greases: ATR spectra exhibited pronounced differences from transmission spectra—characteristic peak shifts toward lower wavenumbers (notably in the 1300–700 cm-1 region) and altered peak intensity ratios across the spectrum. These effects are typical for samples with strong absorption measured by ATR because of refractive index contrast and wavelength-dependent penetration depth.
• Mitigation strategies: Applying a very thin grease layer on a diamond ATR crystal or using a Ge prism reduced penetration depth and attenuated peak shifts and intensity-ratio changes; normalized spectra approached the transmission results, particularly in the 1300–900 cm-1 region.
• Advanced ATR correction: Software-based ATR→transmission correction further harmonized ATR spectra with transmission spectra, restoring both peak positions and relative intensities for strongly absorbing silicone and fluorine greases. For the fluorine-based grease, spectral database searching (KnowItAll) suggested the presence of PTFE and perfluoropolyether (PFP) components in the sample.
• Quantitative note on penetration depth: With a diamond crystal, sample refractive index ~1.5 and 45° incidence give an estimated penetration depth of ~5 μm at 400 cm-1 and ~0.5 μm at 4000 cm-1; Ge reduces the penetration depth substantially (roughly one-third in the example geometry), explaining its efficacy for highly absorbing samples.

Benefits and Practical Applications

The study provides practical guidance for FTIR laboratories performing grease analysis:

• ATR is a convenient, rapid method for direct analysis of greases without sample preparation—suitable for routine identification and screening, especially for mineral oil-based greases.
• For strongly absorbing greases (silicone, fluorinated), straightforward mitigations—creating a very thin sample film, switching to Ge ATR, or applying advanced ATR correction—enable ATR workflows to produce spectra comparable to transmission measurements, preserving the convenience of ATR while maintaining analytical accuracy.
• Combining ATR measurements with spectral-search software allows identification of base-oil types and detection of specialty components (e.g., PTFE, PFP), useful for contamination analysis, product verification and failure investigations.

Future Trends and Applications

Anticipated directions and opportunities include:

• Improved ATR correction algorithms integrated into instrument software to more reliably convert ATR spectra of strongly absorbing, heterogeneous samples into transmission-equivalent spectra.
• Standardized sample-application protocols (controlled thin-film methods) for semi-solid materials to reduce operator variability and improve inter-lab comparability.
• Automation of prism selection and measurement parameter presets (e.g., Diamond vs Ge, incidence-angle-aware corrections) based on predicted sample absorption and refractive index.
• Integration with spectral libraries and machine-learning approaches for automated identification of complex lubricant formulations, additive packages and degradation products.

Conclusion

ATR-FTIR is a practical, low-preparation approach for grease analysis, but care is required for greases with strong infrared absorption. Mineral oil greases yield comparable ATR and transmission spectra under routine conditions. For silicone and fluorine greases, ATR spectra can show lower-wavenumber peak shifts and altered intensity ratios due to limited and wavelength-dependent penetration depth and refractive index contrasts. These artifacts can be mitigated by applying very thin sample layers, using a Ge ATR crystal to reduce penetration depth, or by applying advanced ATR correction to produce transmission-like spectra. Selecting the appropriate measurement and data-correction strategy is essential when comparing spectra acquired by different sampling modes.

References

1. Shimadzu Corporation. Measurement of Grease Using FTIR. Application News (IRSpirit-TX / Microm ATR). First Edition May 2026.
2. Shimadzu Application Note A476. Advanced ATR Correction to Convert ATR Spectra to Transmission Spectra.
3. Shimadzu Application Note A603. Degradation Analysis of Lubricants Based on ASTM E2412 by FTIR.
4. KnowItAll Spectral Search Database, John Wiley & Sons, Inc. (used for spectral matching of fluorinated components).