Atomic Emission Spectroscopy (AES)
The determination of elements is rightly called the backbone of oil analysis. Twenty to thirty metals are measured simultaneously in the oil. These provide a wealth of information about wear, additives, and contaminants. Therefore, every lubricant sample is meticulously analyzed for metallic elements in the OELCHECK-laboratory.
A strong energy source excites the atoms of a sample. Such an energy source can be, for example, a spark, an electric arc, or a plasma. Due to the high energy density, electrons are raised to a higher energy level, meaning they move away from the atomic nucleus. This excited state can only be maintained for a short time; afterward, the electrons fall back to their original energy level. During this process, they release exactly the energy they absorbed during excitation, in the form of visible light or ultraviolet radiation. The wavelength of the emitted radiation is specific to an element, and the intensity is directly proportional to the concentration of that element. Therefore, the total emitted radiation is first spectrally dispersed.
This can be done using a prism or a grating. Then, for each element, the radiation intensity at its characteristic wavelength is measured. By using appropriate detectors, this measurement can be performed simultaneously for all elements to be determined. However, so-called sequential instruments, where the radiation intensity at individual wavelengths is measured one after the other, are also common.
By measuring reference solutions with known element concentrations, a calibration function between radiation intensities and element concentrations is established before measuring the samples. Using this calibration function, the radiation intensities measured in unknown samples can then be converted into concentrations.
The results are given in mg/kg (mg of metal per kg of oil sample). Put more clearly: 1,000 mg/kg corresponds to a concentration of only 0.1%. Depending on the type of lubricant, application, and machine component, the amount of each metal can vary greatly. For wear metals, concentrations below 10 mg/kg can already be critical. In contrast, for additive metals, several thousand mg/kg may be permissible and necessary constituents in the sample.
