International standard test methods used for the determination of Mg, P, S, Cl, Ca, Cu, Zn and Ba in unused lubricating oil samples by means of WDXRF include ASTM D4927 and ASTM D6443. In this study we have set up calibrations in accordance with the methods stipulated in both ASTM norms.
XRF is an excellent method for the multi-elemental analysis of new lubricating oils for the purposes of quality control, product development and product performance classification. It produces fast, cost-effective, precise and accurate data with a minimum of operator dependence.
No single factor more greatly affects the life of a gear drive and other moving parts in a machine than lubricating oil. It prevents metal-to-metal contact between all sliding and rolling surfaces in a machine, thereby reducing friction and extending its life. Additionally, lubrication oils protect surfaces from corrosive substances, absorb and transfer heat, transport wear particles and contaminants to filters and transmit force and motion in hydraulics.
Nowadays, almost all commercial lubricants contain chemical additives to enhance their performance for a particular application. For instance, Zn, P and Cu are common as anti-wear additives, S and P are common components of extreme pressure additives, Ca, Ba and Mg are components of dispersants and detergents, and Ca and Mg are frequently components of engine oil alkalinity improvers.
X-ray fluorescence spectrometry (XRF) is an excellent method for the multi- elemental analysis of new lubricating oils for the purposes of quality control, product development and product performance classification. It produces fast, cost-effective, precise and accurate data with a minimum of operator dependence.
International standard test methods used for the determination of Mg, P, S, Cl, Ca, Cu, Zn and Ba in unused lubricating oil samples by means of WDXRF include ASTM D4927-10 and ASTM D6443-04 (2010). These methods are broadly analogous, employing mathematical matrix correction procedures. In this study we have set up calibrations in accordance with the methods stipulated in both ASTM norms.
A series of commercially available LOE23 standards from Analytical Services (Texas, U.S.) were used to set up the calibrations according to ASTM D4927 and ASTM D6443 for the determination of Mg, P, S, Cl, Ca, Cu, Zn and Ba in
The power settings of the X-ray tube were between 24 kV / 100 mA (for Mg, P, S, Cl and Ca) and 60 kV / 40 mA (for Cu and Zn). Ba was analyzed using the Ba Lα line (in accordance with ASTM D4927) and was excited using
50 kV / 48 mA X-ray tube setting and detected using a flow counter. Collimators of 700 and 300 mm spacing were used for Mg, P, S, Cl, Ca, and for unused lubricating oils. The concentration ranges of the elements are listed in Table 3. 20 ml of each lubricating oil standard was poured into a P2 ‘de Kat’ liquid cell, assembled using a 6 µm polyester (Mylar) supporting foil.
Cu, Zn, Ba, respectively. To enhance the sensitivity and resolution of P, S and Cl, the Petro edition of the Zetium spectrometer was configured with a special curved Ge111 crystal. The total measurement times on peak and background positions for each element are listed in Table 1. The total acquisition time per sample was only 6 minutes.
Table 1. Measurement time on peak and background positions
Measurement repeatability is an important requirement of quantitative analysis, particularly in production control. To illustrate the precision of the Petro edition of the Zetium spectrometer, the mean, maximum and minimum concentrations for twenty repeated measurements are listed in Table 2, and illustrated graphically for P in Figure 1. The two solid lines in this figure represent the repeatability limits for P at this concentration allowed by ASTM D4927. For comparison, the difference in minimum and maximum concentrations allowed by the ASTM D4927 and D6443 norms are shown for all elements in Table 2.
Comparison of the precision measurements with the ASTM norms emphasizes the outstanding analytical precision of the Petro edition of the Zetium spectrometer.
Table 2. Analytical precision
Figure 1. Precision measurements of P in an unused lubricating oil standard. The two solid lines represent the repeatability limits for P at this concentration allowed by ASTM D4927.
The accuracy of the calibration is presented in Table 3. The calibration RMS value is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by regression in the calibration procedure.
A calibration plot for S gives a graphic illustration of the accuracy of the method (Figure 2).
Table 3. Calibration quality
Figure 2. Calibration graph for S in unused lubricating oil
Detection limits for the analytes of interest in typical unused lubricating oil matrices are given in Table 4. The lower limit of detection (LLD) is calculated from:
Table 4. Detection limits. The LLD values quoted, depend on the type of foil used and are typical for the prepared lubricating oil samples. LLD values for individual samples vary according to sample matrix composition.
Components typeset in bold were present in the spectrometer used to obtain the data in this note.
The results clearly demonstrate that the Petro edition of the Zetium spectrometer is capable of analyzing additives in typical unused lubricating oil samples down to 1 ppm or even less. Measurements are accurate and precise and the method benefits from simple sample preparation. The stability of the Petro edition of the Zetium spectrometer is such that individual calibrations can be used for months. Time-consuming re-standardizations are unnecessary and the resulting data are highly consistent over time. For all elements in lubricating oil samples, this results in 20 repeated concentrations situated well within the limits of the ASTM D4927 and ASTM D6443 norms.
