X-ray fluorescence spectrometry (XRF) is widely used for the analysis of additives and wear metals in fresh and used lubrication oils. It provides fast, cost-effective, accurate and precise results with a minimum of sample preparation. On top of fundamental XRF benefits, Oil-Trace provides improved accuracy, savings on setup time, standards and calibration maintenance. Traditionally, different calibrations have to be prepared for different types of fresh and used lubrication oils. The difference in oxygen and hydrocarbon composition between fresh and used lubrication oils influences the sensitivity, especially for the lighter elements from magnesium up to chlorine, which leads to inaccurate analysis. Moreover, different sample amounts between calibration standards and samples can also lead to inaccurate results. A single Oil-Trace calibration removes the time-consuming maintenance of many calibrations. It takes into account the varying amounts of oxygen and hydrocarbon composition of different petrochemicals as well as variations in sample volumes.
Instrumentation and software
Measurements were performed using an Epsilon 4 EDXRF spectrometer, equipped with a 10 W, 50 kV silver anode X-ray tube, 6 filters, helium purge facility, high-resolution SDD silicon drift detector, spinner and a 10-position removable sample changer. Automatic data processing was performed using the Oil-Trace software.
A series of commercially available wear metal in lubrication oils standards from VHG Labs Inc. (U.S.) were used to set up calibrations for twenty-one elements: Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Mo, Cd, Sn, Sb, Ba and Pb. Eight grams of each oil were poured into a 15 ml volume liquid cell, assembled with a 3.6 µm Mylar® supporting foil. Used oil samples were simulated by adding laboratory-grade diethylene-glycol-dibutyl-ether to wear metal standards. The total amount of oxygen added to wear metal standards was approximately 10 wt%.
Table 1. Measurement conditions on Epsilon 4
Four measuring conditions were used for the analysis, resulting in a total measurement time of 15 minutes per sample (Table 1).
Increased accuracy with Oil-Trace
As lubrication oils oxidize and degrade over time, a matrix mismatch between the fresh calibration standards and the used oil samples can occur. Oil-Trace uses an innovative algorithm to correct changes in oxygen and hydrocarbon composition to improve the accuracy. To demonstrate this, 2 standards with additional oxygen were analyzed as unknown samples 1 and 2 (Table 2). Note the improved accuracy for the light elements Mg, Al and P in sample 2.
Calibrations and detection limits
The high sensitivity of Epsilon 4 in combination with Oil-Trace is excellent for the quantification of additives and wear metals in lubrication oils. This is illustrated by the RMS values of the calibrations and the lower limits of detection (LLD), for all twenty- one elements (Table 3).
Table 2. Comparison of certified vs. calculated elemental concentrations (with and without the Oil-Trace corrections) for wear metals and additives
Table 3. Calibration results and detection limits
Robust calibrations by advanced mass and density corrections
The analysis of light-matrix fuel samples is sensitive to density and mass variations between routine sample and standards. Traditionally this was overcome by analyzing a constant mass. Using integrated fluorescence volume geometry, finite thickness and multi-scatter phenomena corrections, Oil-Trace effectively handles variations both in mass and density. Without the corrections, 8 g and 4 g of standard material define two distinct calibration curves for tin in lubrication oil (Figure 1). However, using advanced Oil-Trace algorithms, results in a single calibration curve, which is largely independent of the amount of sample analyzed (Figure 2).
Table 4 shows the results when analyzing 4 g of sample using an Oil-Trace calibration that was set up with 8 g oil standards.
Figure 1. Traditional two distinct calibration lines for Sn using 4 g and 8 g samples
Figure 2. Oil-Trace single calibration line for Sn using 4 g and 8 g samples
Table 4. Comparison of certified vs. measured concentration, with and without the Oil-Trace corrections
Epsilon 4 in combination with Oil-Trace brings XRF lubrication oil analysis to the next level. With a simple mineral oil calibration, it is possible to analyze additive and wear metal elements accurately and precisely in fresh and used lubrication oils. Moreover, Oil-Trace can correct for differences in sample amount and density. Therefore, users will benefit from more robust routine analysis and fewer calibrations to maintain.