ASTM D4927 Method for Elemental Analysis of Lubricant and Additive Components
ASTM D4927 Standard Test Methods for Elemental Analysis of Lubricant and Additive Components - Barium, Calcium, Phosphorus, Sulfur, and Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy
3. Summary of the Test Methods
3.1 A sample specimen is placed in the X-ray beam and the intensity of the appropriate fluorescence lines of barium, calcium, phosphorus, sulfur, and zinc are measured. Instrument response factors related to the concentration of standards enable the determination of the concentration of elements in the tested sample specimens. Enhancement or depression of the X-ray fluorescence of a given element by an interfering element in the sample may occur. Two test methods (A and B) are described for compensating any interference effect.
3.2 Test Method A (Internal Standard Procedure) - Internal standards are used with the standards and sample specimens to compensate for the potential interelement effects.
3.2.1 Barium, Calcium, Phosphorus, and Zinc - A sample specimen that has been blended with a single internal standard solution (containing tin or titanium for barium and calcium, zirconium for phosphorus, and nickel for zinc) is poured into an X-ray cell. Total net counts (peak intensity - background) for each element and its respective internal standard are collected at their appropriate wavelengths. The ratios between elemental and internal standard counts are calculated and converted into barium, calcium, phosphorus, or zinc concentrations, or a combination thereof, from calibration curves.
3.2.2 Sulfur - A sample specimen is mixed with a lead internal standard solution and analyzed as described in 3.2.1.
3.3 Test Method B (Mathematical Correction Procedure) - The measured intensity for a given element is mathematically corrected for the interference from other elements in the sample specimen. This requires that intensities from all elements in the specimen be obtained.
3.3.1 The sample specimen is placed in the X-ray beam and the intensities of the fluorescence lines of barium, calcium, phosphorus, sulfur, and zinc are measured. A similar measurement is made away from the fluorescence lines in order to obtain a background correction. Concentrations of the elements of interest are determined by comparison of net signals against appropriate interelement correction factors developed from responses of calibration standards.
3.3.2 The X-ray fluorescence spectrometer is initially calibrated with a suite of standards in order to determine by regression analysis, interelement correction factors and instrument response factors.
3.3.3 Subsequent calibration is achieved using a smaller number of standards since only the instrument response factors need to be redetermined. One of these standards (or an optional synthetic pellet) can be used to monitor instrumental drift when performing a high volume of analyses.
3.4 Additives and additive packages can be determined after dilution with base oil to place the elemental concentrations in the range described in 1.1.
4. Significance and Use
4.1 Some oils are formulated with organo-metallic additives which act as detergents, antioxidants, antiwear agents, and so forth. Some of these additives contain one or more of these elements: barium, calcium, phosphorus, sulfur, and zinc. These test methods provide a means of determining the concentration of these elements which in turn provides an indication of the additive content of these oils.
5. Interferences
5.1 The additive elements found in lubricating oils will affect the measured intensities from the elements of interest to a varying degree. In general for lubricating oils, the X-radiation emitted by the element of interest is absorbed by the other elements in the sample matrix. Also, the X-radiation emitted from one element can further excite another element. These effects are significant at concentrations varying from 0.03 mass % due to the heavier elements to 1 mass % for the lighter elements. The measured intensity for a given element can be mathematically corrected for the absorption of the emitted radiation by the other elements present in the sample specimen. Suitable internal standards can also compensate for X-ray inter-element effects. If an element is present at significant concentrations and an interelement correction for that element is not employed, the results can be low due to absorption or high due to enhancement.
