Spectrometric Oil Analysis (SOA) for Wear Monitoring

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The wearing of oil in a mechanical system is a problem that spans the automotive, aerospace and industrial sectors. Because oil is used in mechanical environments, metal particles can become detached from the surrounding components and wear down the oil, making it less efficient.

Frequent testing is required to maintain an optimized oil environment, and this can be done by various means. In this article, we look at an aspect of this oil analysis program – the use of different spectrometric methods.

The Importance of Measuring Oil Wear

Oils are used in various mechanical systems to minimize the wear when two metal components meet each other. Oils (and various lubricant oils) do this by providing a lubricating layer between the two rough surfaces that enable them to glide over each without damaging each other. Over time, there is eventually a small amount of damage to the mechanical parts, but the presence of the oil significantly reduces the extent of the damage and prolongs the lifetime of the mechanical parts.

However, because damage to components will eventually happen, metal particles can become detached from the components and enter the oil. The presence of metal particles not only disrupts the flow and lubricating properties of the oil, the metal particulate matter can further grind away the surrounding metal components. This can have an exponential effect where metal particles break off more metal particles, which then detach more metal particles and so forth. Hence ensuring that the oil is free of metal contaminants is of utmost importance.

Many oils also contain additives to boost the performance of the oil. These can also degrade or become depleted over time, and so the analysis of additives within oil can also be important to ensure that it is functioning optimally. Water and dust are also other potential contaminants.

Spectrometric Oil Analysis Methods


A spectrometric oil analysis program (SOAP) is a series of laboratory tests that are employed to determine the presence and extent of metal particles in oil systems. Of these, there are various spectroscopy methods that can be used. These analyses can be performed on the oil sample directly, or the oil can be filtered to remove the solid contaminants for a more accurate analysis.

Energy Dispersive X-ray Fluorescence (EDXRF) Spectroscopy

Energy dispersive X-ray fluorescence (EDXRF) spectroscopy has established itself as one of the go-to techniques for analyzing the constituents of an oil sample for metal particles that cause wear and any other wear debris in the oil system and to monitor the additives in a lubricating oil.

EDXRF works by analyzing multiple elements within a sample simultaneously. The sample is excited with X-rays which causes the molecule’s electrons to be promoted to an excited state. Upon its return to the ground state, the molecules emit fluorescence radiation. Each molecule has a characteristic wavelength that is emitted, and the detector collects the different wavelengths and compares them to known standards, thus, enabling the molecules in the sample to be identified.  The technique can be used to measure a sample in both two and three-dimensions.

EDXRF can be used to provide fast, accurate and repeatable monitoring of wear metals in oils (alongside lubricants and other machinery fluids). It is routinely used because of its simple sample process, high accuracy, excellent limits of detection (LoD), non-destructive process, high sensitivity to larger particles,  ability to measures in different shapes and forms (liquids, suspensions, solids and powders), low background noise, and ability to observe all the elements in the oil sample at once.

Infrared (IR) Spectroscopy

Both infrared (IR) spectroscopy and Fourier-transform infrared (FTIR) spectroscopy can also be used to measure the degradation and wear of an oil, although FTIR has been found to be more beneficial and so the focus will be on that technique. FTIR is used as a way of detecting any common contaminants in the sample, degradation by-products in the oil and additives present within an oil.

In an FTIR analysis, infrared radiation is passed through the oil sample and some of the radiation is absorbed, while some still passes through. All molecules absorb the radiation at different wavelengths, so each sample has an identifiable molecular fingerprint (no two molecules produce the same fingerprint). When the radiation which passes through the sample is detected, the radiation that was absorbed can be deduced and gives an interpretable spectrum that can be used to identify the elements and/or molecules in the oil sample.

It is thought that around 50% of people who analyze lubricants use this as their standard technique. This is because the oil mixtures can contain a vast array of components, and FTIR can be used to not only discover the presence of any contaminants but can also be used to determine if there has been any cross-contamination between oils systems and to detect significant changes in the overall chemistry of the oil, the composition of the oil and whether there has been any oxidation to the oil sample.

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