Measurement of Grease Using FTIR

Measurement of Grease Using FTIR — Concise Expert Summary

Importance of the topic

• Greases are widely used lubricants composed of base oils thickened to form semi-solid matrices; correct identification and monitoring of their composition and degradation state are critical for equipment reliability, preventive maintenance and failure analysis.
• Fourier transform infrared spectroscopy (FTIR) provides rapid, chemically specific information on base oils and thickener chemistry. The attenuated total reflection (ATR) accessory enables minimal‑prep measurements of semi‑solid greases, making FTIR/ATR attractive for routine laboratory and field screening.

Study aims and overview

• Compare single‑reflection ATR and transmission FTIR measurements for commercially available greases with different base oils (mineral oil, silicone, fluorinated).
• Identify measurement artefacts that arise in ATR spectra for strongly absorbing greases and evaluate practical countermeasures: thin sample layers, swapping diamond (Dia) ATR prism for germanium (Ge), and applying advanced ATR correction algorithms to convert ATR spectra toward transmission equivalence.

Methodology

• Samples: three commercial greases differing by base oil class (mineral/paraffinic, silicone, fluorinated/perfluoropolyether).
• Measurement modes: single‑reflection ATR (Microm ATR with diamond and subsequently Ge prisms) and transmission using a thin liquid film cell with KBr windows.
• Instrument and general acquisition parameters: IRSpirit‑TX FTIR, spectral range typically 4,000–400 cm⁻¹ (limited to 600 cm⁻¹ when using Ge), resolution 4 cm⁻¹, 40 scans, DLTGS detector.
• Analysis steps: direct spectral comparison, normalization to inspect peak shifts/intensity ratio changes, library searching (KnowItAll) to support component identification, and advanced ATR correction to transform ATR spectra toward transmission‑like spectra.

Instrumentation used

• Shimadzu IRSpirit‑TX FTIR system.
• Microm ATR accessory with diamond (Dia) prism and alternative germanium (Ge) prism.
• Liquid film transmission cell with KBr windows.
• DLTGS detector; typical acquisition: 4 cm⁻¹ resolution, 40 co‑added scans.

Main results and discussion

• Mineral oil (paraffinic) grease: ATR and transmission spectra showed consistent peak positions and comparable intensity patterns, permitting straightforward identification (paraffinic signatures confirmed by spectral search).
• Silicone and fluorinated greases: ATR spectra acquired with a diamond prism showed notable peak shifts (toward lower wavenumbers) and altered intensity ratios, especially in the 1,300–700 cm⁻¹ region. These differences are attributed to strong sample absorption combined with refractive index mismatch between the prism and the grease and the wavelength‑dependent ATR penetration depth.
• Mitigation strategies:
  • Applying an extremely thin grease layer on the ATR prism reduces effective absorption (sample thickness ≪ penetration depth), which decreases peak shifts and brings ATR peaks closer to transmission positions, albeit with reduced overall absorbance.
  • Using a Ge prism (higher refractive index) reduces ATR penetration depth (approx. one‑third of diamond under typical geometry), similarly suppressing ATR‑induced spectral distortions for strongly absorbing samples; note that usable low‑wavenumber range is reduced with Ge.
  • Advanced ATR correction processing can mathematically convert ATR spectra (from Dia or Ge) into spectra comparable to transmission mode, correcting both peak positions and intensity ratios. For silicone and fluorinated greases, this produced spectra equivalent to the transmission reference, including characteristic bands in the 1,200–1,000 cm⁻¹ region for silicones and fluorinated moieties.
• Library search (KnowItAll) identified fluorinated polymer signatures (e.g., PTFE, perfluoropolyether) in the fluorine‑base grease, supporting component-level interpretation when appropriate spectral features are preserved or corrected.

Practical benefits and applications

• ATR provides a fast, low‑prep method to analyze greases directly on a prism surface—valuable for routine screening, QC checks, contamination assessment and in‑house troubleshooting.
• For mineral‑based greases ATR spectra are typically directly interpretable and suitable for library matching and degradation monitoring workflows (e.g., ASTM‑based lubricant analysis).
• For strongly absorbing greases (silicone, fluorinated) awareness of ATR artefacts is essential; recommended practice is to use thin‑film application, Ge prisms or post‑measurement advanced ATR correction to obtain transmission‑equivalent spectra for accurate identification and comparative analysis.
• Advanced ATR correction enables laboratories to maintain the practical advantages of ATR while producing spectra compatible with existing transmission spectral libraries and historical datasets, simplifying continuity of analyses and automated matching routines.

Future trends and potential uses

• Further refinement of ATR correction algorithms and integration with chemometric models to automate conversion and improve quantitative performance for strongly absorbing, heterogeneous lubricant matrices.
• Hardware advances: variable‑angle ATR accessories, optimized high‑index materials, and micro‑deposition tools to reproducibly control thin film thickness on ATR crystals for more consistent spectra.
• Coupling ATR FTIR with surface mapping/imaging or hyphenated techniques (e.g., Raman) to discriminate mixed or layered components within complex grease formulations and contamination layers.
• Adoption of standardized procedures for ATR measurement of greases (sample thickness control, prism selection, correction workflows) to improve interlaboratory comparability and support predictive maintenance databases.

Conclusion

• Single‑reflection ATR FTIR is a convenient tool for grease analysis, but care is required for samples with strong mid‑IR absorption: spectral shifts and intensity distortions occur due to penetration depth and refractive index effects.
• Simple experimental measures (thin film deposition, Ge prism) together with software‑based advanced ATR correction reliably reconcile ATR results with transmission spectra, enabling accurate identification and continuity with library databases.
• Choosing the appropriate measurement geometry and analysis workflow is essential when comparing results across methods or building robust QA/QC and degradation‑monitoring programs for lubricants.

References

• Shimadzu Corporation. Measurement of Grease Using FTIR. Application News (IRSpirit‑TX), First Edition May 2026.
• Shimadzu Corporation. Advanced ATR Correction to Convert ATR Spectra to Transmission Spectra. Application News No. A476.
• Shimadzu Corporation. Degradation Analysis of Lubricants Based on ASTM E2412 by Fourier Transform Infrared Spectrophotometer FTIR. Application News No. A603.