FTIR oil analysis: what it detects and where it can mislead
Infrared spectra give a fast screening of oil condition: water, oxidation, nitration, soot, glycol, additive depletion. Know the limits before trusting a single number.
What FTIR measures
Infrared light is absorbed by the chemical bonds of molecules, not by atoms, so the spectrum is a fingerprint of the chemistry of the oil. One scan of a thin film of oil can report several things at once: water, glycol, soot, oxidation, nitration, sulphation, additive level and contamination with other oils. It needs little sample preparation and has a low cost per test once the instrument is bought. It tells you about the molecules, so it complements elemental analysis (wear metals) rather than replacing it.
Qualitative and quantitative
Qualitative use compares peaks with a reference: better or worse. Quantitative use calibrates absorbance against known concentrations to give ppm or percent. Some results are semi-quantitative because calibration samples are hard to obtain, soot particle size varies, and fuel composition changes between locations.
| Practical point | Why it matters |
|---|---|
| Use a standard sample cell path (100 micrometre for the standard method) | Different path lengths change the absorbance and the range |
| Filter and shake the sample | Large particles scatter the beam and the sample must be representative |
| Subtract the new oil spectrum | Removes base oil and additive peaks so contaminants and degradation stand out |
| Keep the right reference | Batch changes, top-ups and a different brand make the reference invalid |
| Watch interferences | Water and glycol absorb in the same region and can mask each other |
Standards
Practice standards for FTIR trend analysis and for oxidation, nitration, phosphate anti-wear and sulphation by FTIR exist in the ASTM family. Using them makes results comparable between instruments and laboratories.
When to use it
Use FTIR as a screening tool to find samples that need further work: viscosity, water content by a dedicated method, elemental analysis or particle counts. It is also useful for spotting an unplanned mix of oil types, such as a polyol ester oil mixed with a synthetic hydrocarbon oil.