ASTM D7751 Standard Test Method for Determination of Additive Elements in Lubricating Oils by EDXRF Analysis
4. Summary of Test Method
4.1 A specimen is placed in the X-ray beam, and the appropriate regions of its spectrum are measured to give the fluorescent intensities of magnesium, phosphorus, sulfur, chlorine, calcium, zinc, and molybdenum. Other regions of the spectrum are measured to compensate for matrix variation. To optimize the sensitivity for each element or group of elements, a combination of optimized excitation and detection conditions (for example, different primary beam filters (7.1.3), secondary or polarization targets (7.1.4), and so forth) may be used. The measuring time should be kept as short as possible, typically under 10 min per specimen. Avoid using different measurement conditions that yield only marginally better results for a specific analyte. There may be a correction of measured intensities for spectral overlap.
4.1.1 Concentrations of the elements of interest are determined by comparison of these intensities against a calibration curve using a fundamental parameters (FP) approach, possibly combined with corrections from backscatter. The FP approach uses the physical processes forming the basis of X-ray fluorescence emission in order to provide a theoretical model for the correction of matrix effects. The correction term is calculated from first principle expressions derived from basic physical principles and contain physical constants and parameters that include absorption coefficients, fluorescence yield, primary spectral distribution and spectrometry geometry. The calculation of concentrations in samples is based on making successively better estimates of composition by an iteration procedure.
NOTE 1 - The algorithm used for the procedure is usually implemented in the instrument manufacturer's software.
4.2 The EDXRF spectrometer is initially calibrated using a set of standards to collect the necessary intensity data. Each calibration line and any correction coefficient are obtained by a regression of this data, using the program supplied with the spectrometer. (Warning - Exposure to excessive quantities of X-radiation is injurious to health. The operator needs to take appropriate actions to avoid exposing any part of their body, not only to primary X-rays, but also to secondary or scattered radiation that might be present. The X-ray spectrometer should be operated in accordance with the regulations governing the use of ionizing radiation.)
5. Significance and Use
5.1 Lubricating oils are formulated with organo-metallic additives, which act, for example, as detergents, antioxidants, antifoaming, or antiwear agents, or a combination thereof. Some of these additives contain one or more of the following elements: magnesium, phosphorus, sulfur, chlorine, calcium, zinc, and molybdenum. This test method provides a means of determining the concentrations of these elements, which in turn provides an indication of the additive content of these oils.
5.2 Several additive elements and their compounds are added to the lubricating oils to give beneficial performance (Table 2).
5.3 Additive packages are the concentrates that are used to blend lubricating oils.
5.4 This test method is primarily intended to be used for the monitoring of additive elements in lubricating oils.
5.5 If this test method is applied to lubricating oils with matrices significantly different from the calibration materials specified in this test method, the cautions and recommendations in Section 6 should be observed when interpreting the results.
6. Interferences
6.1 The additive elements found in lubricating oils will affect the measured intensities from the elements of interest to a varying degree. In general the X-radiation emitted by the element of interest can be absorbed by itself (self-absorption) or by the other elements present in the sample matrix. Also the X-radiation emitted from one element can further excite (enhance) another element. These inter-element effects are significant at concentrations varying from 0.03 mass %, due to the higher atomic number elements (for example, molybdenum), to 1 mass %, for the lower atomic number elements (for example, sulfur). If an element is present at significant concentrations and an inter-element correction for that element is not employed, the results can be low due to absorption or high due to enhancement.
6.2 Absorption and enhancements effects will be corrected by corrections from the FP approach or by other matrix correction models.
6.3 There can be spectral overlap of one element onto another, and the instrument must include correction procedures for any such overlaps.
