For a long time infrared spectroscopy was the only method for measuring oil oxidation. In the case of mineral oils, which were mainly in use about 25 years ago, the spectrum showed oxidation so clearly that the values determined with the IR according to DIN 51453 were specified by gas engine manufacturers. This takes advantage of the fact that during oil aging, oxygen accumulates on the molecular chains of oils consisting of hydrocarbons and new molecular chains are formed. When they are illuminated with infrared light, they absorb it differently than fresh oil. A clear peak is visible in the IR spectrum of an oxidized mineral oil at a wave number of about 1,710 cm-1. After the fresh oil spectrum is subtracted from the used oil spectrum in this region, the oxidation is stated as the absorption of IR raditation referenced to a one-centimeter oil layer thickness (A/cm).
However, this logical procedure does not work with some modern gas engine oils. In the region of wave number 1,710 cm-1, the IR spectrum also indicates the double bonds typical of synthetic oils containing esters or some oil additives. Oxidation products and esters cause peaks in the same region, with a distinct overlapping of the ester peaks and the oxidation peak.
For gas engine oils based on petroleum, IR spectroscopy still provides reliable information on oil ageing and the limits specified by the DIN standard are still valid. But since many modern gas engine oils contain synthetic group I or group II base oils and most of them also contain low-ash additives, IR spectroscopy in accordance with the DIN standard cannot provide any useful oil oxidation values for these oils. Base oils and additives may contain thermally stable ester-based synthetic oils that distinctly overlap the oxidation peak at the specified wave number of 1,710 cm-1. If such a peak is already present in the fresh oil, subtraction often does not provide a meaningful oxidation value. Even heavily oxidised oils will yield a value of 1 A/cm. The value at 1,710 cm-1 often lies on a shoulder of the peak, giving rise to what appears to be strongly varying oxidation values. For example, a value of 1 A/ cm, 18 A/ cm or even 27 A/ cm might be calculated for oils with the same degree of oxidation using the standard-compliant method.