FINA Lubricants Infrared Analysis

Infrared Analysis (IR)

Glossary Description: A form of absorption spectroscopy (IR) confined to the infrared spectral wavelength region, which primarily addresses identification and quantification of organic functional groups.

How is the test performed? A thin layer (approx. 0.1 mm) of lube is spread on a zinc selenide crystal. Infrared radiation is directed at the sample (though the crystal’s bottom side) and a detector measures the amount of energy absorbed on a per-wavelength basis. Specific classes of compounds absorb specific infrared wavelengths, allowing detection and quantification. A computer subtracts the reference from the sample, providing results as a difference, with the exception of the hydrocarbon value, which is reported without a reference subtraction.

Reporting units: Absorption units (AU) per centimeter (cm) in the form, x.xxxx (or some variation of multiple thereof; we use xxxx).

Expected repeatability: +/- 3 AU or 5%, whichever is greater.

Technical overview: IR deals mainly with the chemistry of a lubricant’s base stock and additive packages (if any). For lube analysis purposes it might be considered the chemistry of Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus and Sulfur, even more specifically, the configurations and combinations in which these elements are linked (referred to as “functional groups”), but IR is certainly not limited to these elements. “FTIR” (Fourier Transforms IR, after the French mathematician), is the current technical standard for IR. Fourier Transforms allow the generation of complex curves from digitally represented data. An on-board computer allows data acquisition/storage, as well as provides the number crunching power to achieve the transforms rapidly, thus, Fast FTIR (FFTIR), a very powerful technique.

Interpretation: There are two basic types of results available: 1) Individual absorption readings at specific wavelengths (most common) to investigate specific characteristics; 2) Complete wavelength scanning (or curving) for product matching and investigation of unknowns.

Used lube analysis often utilizes specific wavelengths for repetitive logging of data on an individual sump. By so doing one may be able to identify and develop trends as with other test procedures.

Wavelength	Functional Group	Range
2.9 microns	Water	0 – 5
3.5 microns	Hydrocarbon	500 – 1000
5.8 microns	Oxidation	0 – 24
6.1 microns	Nitration	0 – 24
9.6 microns	Glycol	0 – 50

The above ranges are for orientation purposes only. As with most used lube testing it is often trends, not absolute number, which determine when to conclude, for instance, that the lube has “oxidized” to a significant extent. Baseline data from new lube references are very important if correct inferences are to be made. The client’s onsite supply should be the baseline source, since batches of lube may significantly vary in IR absorption properties, thus semi-annual lube storage sampling is advised in order to remain current.

Suggested application: IR is amenable to any lube system, offering its fullest possibilities with reciprocating engines and compressors. Hydraulic systems and gearboxes are also applicable.

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