An In Depth Look At Infrared Spectral Used Oil Analysis... Part One

By DAVE WOOTON
Wooten-Consulting

Many of the used oil analysis programs today include a test defined as an infrared spectral method (commonly called FT-IR based the technique "Fourier Transform Infrared").

An infrared analysis was added to the oil analysts' toolbox in the early-mid 1980's and has grown in its use considerably since then. The early 1980's was the time when major instrumental improvements were occurring. These improvements included the advent of the Fourier Transform technique for acquiring the infrared spectrum. The Fourier Transform technique has allowed considerably faster spectrum acquisition speed and spectral sensitivities.

What used to take 10-15 minutes for a single scan of a sample in 1980, now takes less than a minute and there are also at the same time numerous scans that are averaged into one spectrum. This instrumental change, along with the advent of computerization for automation and calculations, has made the FT-IR analysis a very useful tool for used oil analysis.

The FT-IR analysis is measured by putting the sample into a "cell" with a known light pathlength. Infrared light is then passed through the sample in this cell. The parameter is measured by comparing the absorption (or lack of light transmission) through the "sample-cell" with respect to an empty cell.

The analysis is reported as abs/cm (absorbance/centimeter). Abs/cm is the amount of light that is absorbed per pathlength of the measuring cell in centimeters. This abs/cm is related to the concentration of the chemistry being measured.

Each chemistry type has a different "response" to the infrared and thus a different abs/cm relationship to the concentration. One can therefore only compare results within a given chemistry type. That is, 10 abs/cm of oxidation is not the same concentration as 10 abs/cm of nitration. Calibration of the abs/cm value to the chemistry's concentration is usually not done. However, with known standards, one could determine the concentration of the component rather than just its abs/cm value currently reported.

The infrared spectrum is essentially a graph of the amount of absorbance versus wavelength. The wavelength-axis is the horizontal axis (the x-axis). Each different type of chemistry in a lubricant "absorbs" in a different frequency of the infrared wavelength region. This allows the x-axis to become an identification tool for the different chemistries within the lubricant.

Since the FT-IR spectrum becomes a relation of concentration to a chemistry graph, the real value of FT-IR analysis is… the technique can study the chemical concentrations in the used oil.

The common analytical parameters measured by FT-IR include oxidation, nitration and sulfation. In addition some programs will include fuel dilution (either gasoline of diesel fuel), glycol, water and soot. These analysis parameters are based on easily observed spectral changes in the used oil. For most used oil programs, these parameters were designed around combustion engine (gasoline or diesel) lubricant degradation or contamination.

Although the methodology appears to be simple, it is not as simple in reality. Slight differences between sampling techniques, measurement techniques, instruments or calculation methods can yield property analysis differences.

Unfortunately, there isn't any defined industry standard or standard method for measuring these components. Therefore, most results, even though they tend to be consistent and repeatable, can vary greatly from lab to lab. When one attempts to compare FT-IR results, staying within a particular lab's results will be best.

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