Infrared Analysis of New and Used Motor Oils Using a Liquid Transmission FTIR Accessory

Table of Contents

Equipment and Samples


FTIR absorption spectroscopy is an appropriate method to analyze and characterize new, used, and diluted motor oils, and other samples.

Service oils, such as the Castrol Magnatec engine oil, are prepared in a way that reduces the wear and tear that occurrs in engines. In the long run the oils tend to absorb soot, elemental metals and other contaminants. This transforms the yellow motor oil into a dark, worn black oil.

Infrared (IR) spectral analysis can be employed to analyze the type of contaminants present, and the type of chemical degradation occurring in the oil.

Standard conventional methods involve using a vertically mounted cell to load the viscous or sticky oil samples. This method led to certain hitches during the stages of sample introduction and clean-up.

The Specac Pearl™ Liquid Transmission Accessory (Figure 1) overcomes these drawbacks, and enables easy, quick, and repeatable IR spectral measurements of fluid samples.

Figure 1. The Specac Pearl™ Liquid Transmission Accessory

Equipment and Samples

The Pearl™ Liquid Transmission Accessory equipped with an Oyster Cell, including wedged ZnSe windows, was used to record the IR transmission spectra. The different pathlengths used and the corresponding part numbers are listed in Table 1.

Table 1. Pathlengths and part numbers used for recording the IR transmission spectra

Pathlength Part number
25 µm GS31226
50 µm GS31221
100 µm GS31222

The analysis was performed by testing two oil samples: a fresh Castrol Magnatec engine oil sample, and a sample of oil used for 18 months in a petrol engine. The test was also carried out on fresh Renolin CL37 and 68 oils. The Pearl™ was coupled with an FTIR spectrometer to record the IR transmission spectra at a resolution of 1 cm- 1.


The three spectra were recorded at three different pathlengths (25, 50 and 100 µm) for the fresh Castrol Magnatec engine oil sample, the results are depicted in Figure 2.

As anticipated when the pathlengths in the Oyster Cell increased, the absorbance of IR radiation increased. Although the oil samples had the maximum concentration as they were not diluted, the maximum absorbance peak of 2.5 was recorded in the fingerprint region of 500–1500 cm-1 for the 100 µm pathlength.

This suggests that there is no necessity for sample preparation or dilution to record the IR transmission spectra of oil samples at the 25, 50 or 100 µm pathlengths.

Figure 2. IR spectra of fresh Castrol Magnatec.

A comparison of the spectra obtained from the fresh and the used Castrol Magnatec engine oil samples at the 25 µm pathlength is illustrated in Figure 3.

Less than 5 ml of the samples was required to rapidly differentiate between the oils. The analysis of the used engine oil sample demonstrates the chemical degradation of the oil, and the absorption of contaminants. The distinctive indications of sulfation are evident at the region of 1150 cm-1. In addition, nitration and oxidation of the oil are observed at the region of 1600– 1700 cm-1.

There is a loss of relative signal intensity at the region between 1500 and 1460 cm-1, this suggests that the amount of the original oil in the used oil sample is lower. The blue circles in Figure 3 indicate this phenomenon.

Figure 3. A comparison of new & used Castrol Magnatecs.

Figure 4 illustrates the comparison of spectra obtained from three different fresh oil samples recorded at the 25 µm pathlength. The blue, red, and black lines in the figure respectively represent the Renolin CL 68 hydraulic oil, the Renolin CL 37 hydraulic oil and the Castrol Magnatec engine oil. It is distinctly evident that all of the three oils share a similar aliphatic band at the region of 1470 cm-1. However, other molecular signatures of the three oils differ.

Figure 4. A comparison of the three oils.


FTIR absorption spectroscopy is a proven technique for the analysis and characterization of different samples.

This article has demonstrated that the Pearl™ Liquid Transmission Accessory can be conveniently employed to analyze samples of viscous oils at three different pathlengths in a quick, simple and reliable manner. The sample volume required to differentiate between the two oil samples was very low, and the introduction of contaminants in the oil, due to prolonged usage and contact with machinery, was evident in the used oil sample.

The small differences in the spectral signatures of different fresh and used oils enabled the chemical composition of the samples to be readily identified.

The chemical degradation of the oil sample upon prolonged use was indicated by a loss of signal intensity at specific frequencies. The maximum absorbance peak was observed at 2.5 in the fingerprint region of 500–1500 cm-1 for the 100 µm pathlength, indicating that a very low amount of the pure sample is adequate to record high-quality IR transmission spectra.

The Specac Pearl™ allows the performance of in-depth analysis to determine the rate of degradation of the oils.

This information has been sourced, reviewed and adapted from materials provided by Specac.

For more information on this source, please visit Specac.

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