Analysis of unused lubricating oils per ASTM D4927 and ASTM D6443

Significance of the topic

High‐performance lubricants rely on carefully balanced additive packages to protect engines and machinery against wear, corrosion and thermal degradation. Accurate quantification of key metallic elements in unused lubricating oils is critical for quality control in blending operations, compliance with industry standards and development of new formulations.

Objectives and study overview

This application note demonstrates a cost‐effective workflow for determining elemental concentrations in fresh lubricants according to ASTM D4927 and D6443. Using a Thermo Scientific ARL OPTIM’X wavelength dispersive X-ray fluorescence (WDXRF) spectrometer, the study aims to:

• Establish calibration curves for Mg, P, S, Cl, Ca, Zn, Cu and Ba in oil matrices.
• Verify detection limits, accuracy and precision against ASTM requirements.
• Assess long‐term analytical stability of the instrument.

Instrumentation used

The ARL OPTIM’X WDXRF system employs a 50 W Rh target tube with Ultra Closely Coupled Optics, delivering sensitivity equivalent to larger 200 W instruments. Key features include:

• SmartGonio sequential analysis covering elements from Mg (Z=12) to Ba (Z=56).
• Optional Multichromator™ channels for enhanced throughput or sensitivity.
• PetroilQuant™ pre‐calibration software package for ASTM D4927 and D6443 methods.

Methodology

Calibration standards were prepared by spiking high‐purity mineral oil with single‐element solutions of Zn, P, S, Mg, Cl, Cu and Ca to cover concentration ranges defined in ASTM tables. The ARL OPTIM’X channels were optimized for each analyte’s K‐spectral line. Finished lubricant samples required no dilution in most cases and were measured directly in analysis cups under helium purge. Typical counting times of 120 s per element balanced throughput with analytical sensitivity.

Main results and discussion

Limits of detection achieved ranged from sub‐ppm to tens of ppm, meeting or exceeding ASTM D4927/D6443 specifications. Standard errors of estimate between 1–20 ppm demonstrated excellent calibration linearity. Short‐term precision tests on a marine additive blend yielded relative standard deviations below 0.5 % for sulfur at ~1 % concentration. Over a two‐month stability trial, the instrument maintained better than 0.45 % relative deviation for ~2 000 ppm sulfur without recalibration.

Benefits and practical applications

The WDXRF approach offers:

• Rapid multi‐element measurement with minimal sample preparation.
• Wide dynamic range from low ppm to several percent.
• Cost‐effective operation with low maintenance and auxiliary cooling requirements.
• Compliance with leading industry standards for lubricant quality control and R&D.

Future trends and potential uses

Advancements in X‐ray optics and detector technologies will further improve sensitivity and throughput. Integration of automated sample handling and real‐time data feedback into blending processes offers opportunities for inline quality assurance. Expanded element coverage and machine‐learning‐driven corrections may enhance the analysis of complex additive chemistries and emerging bio‐based lubricants.

Conclusion

The Thermo Scientific ARL OPTIM’X WDXRF spectrometer, equipped with PetroilQuant pre‐calibration, provides a robust, accurate and stable solution for elemental analysis of unused lubricating oils per ASTM D4927 and D6443. Its streamlined workflow and long‐term reproducibility make it an ideal tool for lubricant manufacturers, formulators and quality control laboratories seeking reliable multi‐element data.