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Read our primer articles on Oil Analysis and Tribology

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GC is the absolute method for the determination of fuel concentration in oil. This method however requires a certain amount of expertise, knowledge and investments.

As for FTIR for fuel detection, the method relies on the presence of aromatic molecules basically coming from the sulfur additive (arround 3050, 1605, 874, 811, 748 cm-1). If low sulfur fuel is analyzed, FTIR won't tell anything. The method was working OK with 5000 ppm sulfur fuel in the past. It is not the same game with 50 ppm and 15 ppm sulfur content. Still, depending on the regions or on fuel batches, sulfur level weren't always be the same thus, method was not precise on a quantitative level.

Flash point can give you an idea, particularly if it is compared to viscosity. That is what I do in my lab but I must say that I analyze oil from our units weekly (close follow-up) and oil is always from the same type.

This method is however not fully reliable due to several possible interference: oil contamination (water, glycol, soot and other), hydrocarbon molecules shear down...etc. There was an interesting article talking about this subject in Practicing oil analysis a couple of months ago.
There is an ASTM method for FUEL Dilution by FTIR E2412
But the problem with this method is it considers the the Aromatic Sulphur peak for the detection and quantification.Now days the fuel is low and ultra low sulfur fuels,this makes it difficult to detect using FTIR.
Also the standards are required to be prepared using a aromatic free base oil using the Actual Fuel used in that engine.This method is to be used as a trend analysis and GC is the best option.
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