EXPLANATION
Some oils are formulated with organo-metallic additives, which act, for example, as detergents, antioxidants, and antiwear agents. Some of these additives contain one or more of these elements: calcium, phosphorus, sulfur, and zinc. This test method provides a means of determining the concentrations of these elements, which in turn provide an indication of the additive content of these oils. This test method uses interelement correction factors calculated from empirical calibration data. This test method is not suitable for the determination of magnesium and copper at the concentrations present in lubricating oils. The method also does not cover lubricating oils that contain chlorine or barium as an additive element. This test method can be used by persons who are not skilled in X-ray spectrometry. It is intended to be used as a routine test method for production control analysis.
TEST SUMMARY
A specimen is placed in the X-ray beam, and the appropriate regions of the spectrum are measured to give the fluorescent intensities of elements of interest. Other regions of the spectrum are measured to compensate for varying background. If the detector does not completely resolve all the elements in a single measurement, then to improve selectivity, there is a combination of sequential and simultaneous measurements employing primary and secondary beam filters. There can be correction of measured intensities for spectral overlap. Concentrations of elements of interest are determined by comparison of these intensities against a calibration curve using empirical interelement correction factors and ratio to backscatter. The EDXRF spectrometer is initially calibrated using a set of prepared standards to collect the necessary intensity data. Each calibration line and any correction coefficient are obtained by a regression of these data, using the program supplied with the spectrometer.
INTERFERENCES - 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 elements 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 interelement effects are significant at concentrations varying from 0.03 mass % 5, due to the heavier elements, to 1 mass %, for the lighter elements. Enhancement effects can be minimized by selective excitation. The measured concentration for a given element can be mathematically corrected for self-absorption and for interelement effects by other elements present in the sample matrix. 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.
If a sample containing barium as an additive above 0.03 mass % is measured against a calibration derived from standards without barium, then results will be low. If a sample containing chlorine as an impurity above 0.03 mass % is measured against a calibration derived from standards without chlorine, then the results can be affected. There can be spectral overlap of one elements onto another, especially for phosphorus on sulfur, and the instrument must include correction procedures for any such overlaps.
TEST PRECISION
Where X is the concentration of element in mass %.
No information can be presented on the bias of this procedure since no material having an accepted reference value is available.