How to read a Lube oil FTIR Trace

FTIR is a commonly performed test on lubricating oils. It is usually used on engine oils to determine parameters such as soot, oxidation, glycol, fuel etc. Further, FTIR is an exceptionally powerful and its benefits go much beyond those traditional routine parameter tests. Much as the elemental analysis can tell you much about the inorganic chemistry of metals such as Calcium, Phosphorus and Zinc in the oil, the FTIR analysis analyses the organic part of the fluid to identify sulphonates, fuels and esters which are important components of lubricant chemistry. More importantly, comparison of both the elemental and FTIR trace data of two products can be used to confirm if two products are indeed the same, useful for confirming the source of contamination in a system or confirming the correct fluid was used.

FTIR works on the principle of passing infra-red light of various wavelengths through a cell containing the sample and measuring the amount of light that passes through (transmittance) or the absorbance (that amount of infra-red light absorbed by the Lubricant molecules at that wavelength). Traditionally values from FTIR are expressed in Absorbance when looking at data points. For instance an OEM limit of 20 Abs/cm for oxidation would be based on Absorbance, but when viewing FTIR Spectra they are typically showed in transmittance. This may seem a bit confusing at first, but in simplicity this means an absorbance peak is a transmittance trough on the graph. So note when referring to a peak in this guide the spectra actually points down.

FTIR analysis is a Chemistry discipline that does take time to master and an in-depth analysis of the subject goes far beyond the realm of this guide, but the basic principles required to understand your lubricant analysis report are fairly straight forward, especially when the trace is labelled and have a reference for comparison. Some example FTIR traces are shown below.

FTIR New Lubricant – To left is an FTIR Trace of a new lubricant. Note x-axis goes from large to small. To the left are the higher wavelengths (>1500 cm-1) consistent with basic functional groups such as OH coming from Water, Glycol or phenol anti-oxidant additives, and carbonyl peak consistent with oxidation, ester additives, biofuel contamination etc. The big broad peak C-H Stretch between 2850 and 3000cm-1 can largely be ignored as this is simply telling the reader that the sample being analysed has a lot of carbon to hydrogen bonds, which is expected in any hydrocarbon based lubricant. These regions are roughly the same in most products and are mainly used to identify if the oil looks relatively new (see the phenol antioxidant peak), and look for e.g. oxidation or nitration peaks. To the right (500 to 1500cm-1) is the fingerprint region which is used for more specific identification of the lubricant and is used to identify one product from another.

FTIR Used Lubricant – The trace below has a lot of absorbance with very little transmittance, which is consistent with a spent, dark and sooty oil.

FTIR with Water contamination – The trace below is of a relatively new lubricant (note the phenol peak), which has developed a water ingress issue. This sample was taken from a relatively low oil hour piece of equipment with a coolant leak (note the OH broad peak)

Traces comparison – Traces can be overlaid onto each other to compare unused reference vs used oil. This can be provided if the client provides unused reference oil for comparison. The trace below is an oxidised oil vs the new oil.

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