Fourier transform infrared (FTIR) spectroscopy is the testing technique identified in the ASTM E2412 standard. It is used to review the methods for trending various molecular signatures produced on a spectrum. These molecules can be contamination related, additive related or fluid property related. The key with this technique is understanding what the baseline spectrum looks like and then through spectral subtraction identifying any changes in the parameters when compared to the used oil spectrum produced by FTIR.
When analyzing contamination, this method is limited to only distinguishing certain types of contaminants such as glycol, water, soot and fuel, which are not as common as other contaminants like ambient dust or dirt that can ingress through any airborne source. Therefore, if you use this method for contamination monitoring, you must realize that it is not comprehensive.
Depending on the molecules being measured, the technique may not be as accurate or as precise as other methods that are designed for a specific type of contaminant. Of course, other methods can often cost more when compared to FTIR.
The advantage of FTIR is in the ability to review several parameters at once with trending techniques. This allows you to quickly see if something specific has changed at an unusual rate in comparison to another parameter or if everything has changed drastically, which could indicate cross-contamination (when the wrong lubricant has been added to a machine).
FTIR is often part of a typical oil analysis test slate. Specific values generally are reported to represent a change in the absorption range number correlated to a concentration or percentage of a certain parameter. The spectrum may also be displayed in the oil analysis report. This is particularly useful in detecting cross-contamination.
Trending FTIR data using the ASTM E2412 standard method can be quite beneficial if done correctly and when combined with a well-implemented oil analysis program that includes quality oil sampling and interpretation techniques. Just keep in mind that the majority of measured contaminants, like glycol, soot and fuel, will be more specific to engine oil applications. When trending contaminants for other applications, including most industrial applications, the benefits of FTIR will be more focused on the fluid properties. Additive depletion and oxidation are two examples of when this would be the case.
