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
5. Apparatus
5.1 Energy Dispersive X-ray Fluorescent Analyzer - Any energy dispersive X-ray fluorescent analyzer can be used if its design incorporates at least the following features.
5.1.1 Source of X-ray Excitation, X-ray tube with palladium, silver, or rhodium target programmable between 4 and at least 25 keV for preferential excitation to simplify the sample spectra. (Warning - Operation of an analyzer using an X-ray tube source is to be conducted in accordance with the manufacturer's safety instructions and federal, state, and local regulations)
5.1.2 X-ray Detector, gas filled proportional counter with high sensitivity and a resolution value not to exceed 1300 eV at 5.9 keV.
NOTE 1 - The limited data from instruments with solid state detectors in the inter-laboratory precision study did not support their inclusion in the method.
5.1.3 Primary Beam Filters, to make the excitation more selective.
5.1.4 Secondary Beam Filters - When a proportional counter is used, these are necessary as a means of discriminating between an analyte's X-rays and other analytes and the spectrum from the X-ray tube.
5.1.5 Multi-channel Analyzer, for discrimination between an analyte's X rays and background X rays.
5.1.6 Optional Helium Purgeable Optical Path.
5.2 Sample Cells, providing a depth of at least 6 mm and equipped with replaceable X-ray transparent film. Suitable films include polypropylene and polycarbonate with thickness from 3.5 to 8 µm.
5.3 Instrument Setting Up Samples (Elemental Reference Samples), to quantify spectral overlaps. These are required when the instrument's software does not include reference spectra to deconvolute spectra.
5.4 Drift Correction Monitors, to correct for instrumental drift. At least two samples are necessary to correct both sensitivity and baseline drifts. For each element and scatter region, there shall be one providing a count rate similar to samples from the upper end of the calibration and another providing a count rate as if from a blank. This last can be a blank oil. For the high concentration of each element, a glass disk. XRF fusion bead, or pressed pellet have all been found to be satisfactory. They can be the same samples as in 5.3.
6. Reagents and Materials
6.1 Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades can be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
6.2 Helium, at least 99.5 % purity, for the optical path of the spectrometer.
6.3 Diluent Solvent, a suitable solvent free of metals, phosphorus, and chlorine and containing less than 10 ppm of sulfur (for example, deodorized kerosene, white oil, or mineral oil).
6.4 Calibration Standard Materials:
6.4.1 Certified concentration solutions of liquid organometallic salts, each containing calcium or zinc, or both. The solutions shall be sulfur free or the certificate shall state the concentration of sulfur. Alternatively, the following standard materials can be used.
6.4.1.1 Calcium 2-Ethylhexanoate, approximately 12.3 mass % calcium, with a certified value.
6.4.1.2 Zinc Cyclohexanebutyrate, approximately 16.2 mass % zinc, with a certified value.
6.4.2 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity (9.62 mass % phorphorus).
6.4.3 Di-n-butyl Sulfide, 97 % purity, (21.9 mass % sulfur).
6.4.4 Stabilizers, 2-ethylhexanoic acid, 2-ethylamine. Also, proprietary stabilizer/chelating solutions are available commercially. Stabilizers shall be free of the additive element.
NOTE 2 - In addition to the calibration standard materials identified in 6.4.1-6.4.3, single or multielement calibration standards can also be prepared from materials similar to the samples being analyzed, provided the calibration standards to be used have previously been characterized by independent primary (for example, gravimetric or volumetric) analytical techniques to establish the elemental concentration mass % levels.
