Lubricating Oil & Grease Testing

Zheng, Lijuan, et al. Spectrochimica Acta Part B: Atomic Spectroscopy 99 (2014): 1-8.

Lubricating oils play a crucial role in reducing friction and wear in machinery, making the accurate determination of metal contaminants essential for maintaining performance and longevity. Laser-induced breakdown spectroscopy (LIBS) offers a direct and efficient technique for the elemental analysis of viscous liquids, including lubricating oils.

In a recent study, LIBS was employed to analyze Fe, Cr, and Ni in various lubricating oils, including synthetic, semi-synthetic, mineral, and used oils. A thin oil layer was applied to a pure aluminum target, and emission spectra were recorded under controlled conditions. Calibration curves for the three elements were established using spiked laboratory reference samples prepared with certified blank oil and four virgin lubricating oils. The emission intensities of the target elements were normalized using the C I 247.9 nm line to account for shot-to-shot laser fluctuations.

The study demonstrated that generalized calibration curves could be constructed by merging data from different oil matrices, indicating minimal matrix effects. The accuracy and precision of the method were validated by analyzing spiked virgin oils and cooking oils, with relative standard deviations (RSDs) between 3% and 8% for concentrations above 50 ppm, increasing to 7%–15% for concentrations between 20 ppm and 50 ppm. The method was further applied to five used lubricating oils, successfully quantifying metal content.

This study highlights the reliability and efficiency of LIBS for rapid and accurate elemental analysis of lubricating oils, making it a valuable tool for monitoring wear and contamination in industrial applications.

Braga, Jez Willian B., Araci Araujo dos Santos Junior, and Ingrid S. Martins. Fuel 120 (2014): 171-178.

Monitoring the viscosity index (VI) of lubricant oils is critical for ensuring performance consistency and compliance with regulatory standards. A robust and accurate method for VI determination is essential, particularly when dealing with a diverse range of lubricants from different manufacturers and origins. This study presents the development and validation of an infrared spectroscopy-based multivariate method for the rapid determination of VI in 1085 lubricant oil samples from 81 producers across Brazil.

After eliminating outliers, the final calibration model was developed using 473 samples and 6 latent variables, followed by validation with 231 samples. The reference versus estimated VI values exhibited high correlation coefficients (>0.95), with slopes and intercepts within the expected 99% confidence intervals, indicating an excellent fit. The method demonstrated strong predictive power despite variations in base oil origin (mineral or synthetic) and different API and SAE classifications.

Compared to conventional methods, this infrared-based technique is faster, requires smaller sample volumes, and generates fewer chemical residues, making it more environmentally friendly and cost-effective. The simplified approach enhances its applicability for routine quality control monitoring in the lubricant industry.

This study underscores the reliability and efficiency of infrared spectroscopy for VI determination in lubricant oils, offering a scalable solution for regulatory agencies and manufacturers to monitor quality across a wide range of products.

Amais, Renata S., et al. Analytical Methods 6.13 (2014): 4516-4520.

Accurate determination of trace elements like phosphorus (P), sulfur (S), and silicon (Si) in lubricating oils is essential for ensuring product performance and regulatory compliance. This study demonstrates the feasibility of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) in overcoming polyatomic interferences that typically affect isotope analysis. The system employs an octopole reaction/collision cell (ORS) between two quadrupole mass analyzers, allowing precise mass selection and interference removal through a mass shift operation mode.

Lubricating oil samples were digested using nitric acid and hydrogen peroxide under microwave-assisted conditions. Oxygen gas was introduced into the ORS, and an optimized flow rate of 0.75 mL/min was identified based on detection limits and accuracy. The limits of detection for ²⁸Si¹⁶O⁺, ²⁹Si¹⁶O⁺, ³¹P¹⁶O⁺, ³²S¹⁶O⁺, and ³⁴S¹⁶O⁺ were 1.2, 0.49, 0.31, 0.33, and 0.78 μg/L, respectively. High recoveries ranging from 95.0% to 113% were achieved for biodiesel, diesel, and lubricating oil samples, confirming the method's precision and accuracy at a 95% confidence level.

This ICP-MS/MS-based method provides a reliable and sensitive approach for trace element analysis in lubricating oils, enabling more accurate quality control and regulatory compliance. Its ability to mitigate polyatomic interferences ensures consistent results, making it a valuable tool for both research and industrial applications.