ASTM D7751 Standard Test Method for Determination of Additive Elements in Lubricating Oils by EDXRF Analysis
7. Apparatus
7.1 Energy Dispersive X-ray Fluorescent Spectrometer - Any energy dispersive X-ray fluorescence spectrometer can be used if its design incorporates at least the following features:
7.1.1 Source of X-ray Excitation - X-ray tube with palladium, silver, rhodium, or tungsten target. Other targets may be suitable as well. The voltage of the X-ray tube shall be programmable between 4 kV and at least 30 kV for preferential excitation of elements or groups of elements.
7.1.2 X-ray Detector - Semiconductor detector with high sensitivity and a spectral resolution value not to exceed 175 eV at 5.9 keV.
7.1.3 Primary Beam Filters (Optional) - To make the excitation more selective and to reduce the intensity of background radiation.
7.1.4 Secondary or Polarization Targets, or Both (Optional) - To make the excitation more selective and to improve peak-to-background ratio.
7.1.5 Signal Conditioning and Data Handling Electronics - That include the functions of X-ray intensity counting, spectra handling by background variation correction, overlap corrections, inter-elements effects corrections, and conversion of X-ray intensity into concentration.
7.1.6 Helium Purgeable Optical Path (Optional) - Helium purge improves the sensitivity of low energy X-rays emitted from low atomic number elements (Z< 22).
7.1.7 Sample Cells - Providing a depth of at least 6 mm and equipped with replaceable X-ray transparent film.
7.1.8 Sample Film - Suitable films include polypropylene, polyester, and polycarbonate with thickness from 3.5 µm to 8 µm. A thick film may limit the performance for low atomic numbers (for example, Magnesium).
7.2 Instrument Setting-Up Samples (Elemental Reference Samples) (Optional) - To quantify spectral overlaps. These are not required when the instrument's software does include software to deconvolute spectra.
7.3 Drift Correction Monitors (Optional) - To correct for instrumental drift. At least two samples are necessary to correct both sensitivity and possible changes in the background. 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. Elemental reference samples (7.2) may also be used.
7.3.1 Drift correction is usually implemented automatically in software, although the calculation can readily be done manually. For X-ray instruments that are highly stable, the magnitude of the drift correction factor may not differ significantly from unity.
7.4 Quality Control (QC) Samples - Samples for use in establishing and monitoring the stability and precision of an analytical measurement system. Use homogeneous materials, similar to samples of interest and available in sufficient quantity to be analyzed regularly for a long period of time.
7.5 For additional information, also refer to Practice D7343.
8. Reagents and Materials
8.1 Purity of Reagents - 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 may 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.
8.2 Diluent Solvent - A suitable solvent containing less than 10 mg/kg of sulfur and containing less than 1 mg/kg of metals as well as of all other elements of interest (for example, base oil). If diluted samples are analyzed at low levels of sulfur, a lower sulfur content of the diluent solvent should be used and must be corrected for when recalculating the concentrations for the original, not-diluted sample. The precision stated in this test method does not apply to diluted samples.
8.3 Helium Gas - Minimum purity 99.9 %.
8.4 Calibration Standard Materials:
8.4.1 Commercially available calibration solutions.
8.4.2 Certified concentration solutions, of liquid organometallic salts, the following standard materials can be used:
8.4.2.1 Calcium 2-Ethylhexanoate, approximately 12.3 mass % calcium.
8.4.2.2 Zinc Cyclohexanebutyrate, approximately 16.2 mass % zinc.
8.4.2.3 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity (approximately 9.62 mass % phosphorus).
8.4.2.4 Di-n-butyl Sulfide, 97 % purity (approximately 21.9 mass % sulfur).
8.4.2.5 Magnesium-2-ethylhexoate, (2.99 % magnesium).
8.4.2.6 1-Chlorooctane, 98 % purity, (23.9 mass % chlorine).
8.4.2.7 Commercially available single element standard for molybdenum based on molybdenumsulfonate.
8.4.2.8 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 8.4, 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.
9. Hazards
9.1 Occupational health and safety standards for X-rays and ionizing radiation shall be observed. It is also recommended that proper practices be followed as presented by most manufactures documentation or described in Guide E1621.
