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degradation and contamination is often of interest, and also the determination of the presence and depletion of additives.
FTIR works by passing infrared light through a sample and measuring how different wavelengths are absorbed, creating a spectrum that reveals the sample’s molecular composition.
In this context, chemometrics refers to the use of mathematical and statistical methods to extract meaningful chemical information from FTIR spectra for analysing lubricant properties and conditions. This principle has already proven to be applicable for prediction of Total Acid Number (TAN) and Total Base Number (TBN).
In a pilot project, it was possible to show that the aniline point of base oils can be estimated using FTIR with the support of a suitable chemometric model.
Proof of concept with FTIR infrared spectroscopy As part of the proof of concept, a total of 30 base oil samples in a range of kinematic viscosities at 40°C between 8 mm²/s and 115 mm²/s were used. This corresponded to aniline points between 70°C and 105°C. The measurements were performed with ERASPEC OIL, the FTIR Spectral Lubricant Analyzer from eralytics.
In Figure 1 all measured infrared spectra are plotted for comparison. It is obvious that no straightforward approach is applicable, but it is possible to see certain variations, for example in the range between wavenumbers 650 cm-1 sections.
to 950 cm-1
Figure 2: Cross plot showing measured aniline point versus the predicted value
This number is compared to the reproducibility of 0.5°C for clear and light-colored samples according to ASTM D611 and gives a very reasonable outcome. It must be taken into account that all samples were used across a wide viscosity range. It can be assumed that the significance can be further improved in further studies. It is also possible to consider the density of the respective sample in the model, as this is usually also determined.
Conclusion , but also in other
Knowledge of the aniline point of base oils is of great importance in the lubricating oil and process oil industry, as it poses challenges and opens technical opportunities. In contrast to the classic determination via the solubility of the extremely questionable substance aniline, an alternative approach could be demonstrated with FTIR infrared spectroscopy. This measurement takes just a few minutes and also provides deep insights into the composition and condition of the base oils.
For the present case, a chemometric multilinear regression model (MLR) was developed to combine information from different areas. In Figure 2 the cross plot between calculated values and given reference values for the aniline point is shown. The calculated Standard Error of Calibration (SEC) was 1.0°C.
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Figure 1: Transmission spectra of all 30 samples
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LUBE MAGAZINE NO.187 JUNE 2025
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