ASTM D6481 Method for Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils
ASTM D6481 Standard Test Method for Determination of Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray Fluorescence Spectroscopy
7. Preparation of Calibration Standards
7.1 To ensure complete solution of all components, prepare calibration standards by precisely weighing the organo-metallic solutions and phosphorus and sulfur solutions with the diluent solvent along with the appropriate stabilizer. Table 2 lists suggested concentrations when determining empirical influence coefficients. Complete sets of standards based on Table 2 are commercially available.
8. Calibration
8.1 Spectrometer Settings - Follow the manufacturer's recommendations and set up a series of measurement conditions, (X-ray tube voltage, X-ray tube current, primary beam filter, secondary beam filter, measurement time, and multichannel analyzer region of interest) to measure the •K spectrum of phosphorus, sulfur, calcium, and zinc. Include at least one region of backscatter.
8.2 Fill respective sample cells nearly full with the calibration standard solutions. Follow the manufacturer's recommendation to ensure that the cell is full enough such that X-ray count rate does not depend on the amount of sample in the cell. Make sure that no wrinkles or bulges are present in the film. The film must be flat, and the cell shall be vented.
8.3 Place the sample cell in the X-ray beam to measure and record the intensity for each element and scatter region in each calibration standard in accordance with the conditions set by following 8.1. Measure each standard two times, using a freshly prepared cell for each measurement.
8.4 Measure any instrument setting up samples necessary to quantify spectral overlaps. Use at least the measurement time of the calibration standards.
8.5 Measure and record the intensity for each element and scatter region of the drift correction monitors. Use at least the measurement time of the calibration standards.
NOTE 3 - An instrument setting up sample used in 8.4 can also be used as a drift correction monitor in 8.5 if its elemental composition is suitable.
8.6 Use the instrument's regression software to generate the optimum calibration curve for each element by applying the appropriate corrections. Suggested corrections are given in Table 3. The variable sulfur content results in a change in background for phosphorus because of filter fiuorescence. (If a filter is not used, there will be a spectral overlap of sulfur on phosphorus and an overlap correction will be necessary). Sulfur also affects the sensitivity for calcium because of its high absorption for that element. Because phosphorus X rays have an energy below the absorption edge of the sulfur filter, it overlaps sulfur. (If a filter is not used, there will be also be a spectral overlap of phosphorus on sulfur). Zinc is the heaviest element in lubricating oil, and therefore, all matrix effects can in practice be corrected by taking a ratio to the high energy backscatter.
NOTE 4 - No instruments in the interlaboratory precision study that established the precision statements used theoretical correction coefficients for mass absorption effects.