ASTM D7751 Standard Test Method for Determination of Additive Elements in Lubricating Oils by EDXRF Analysis
10. Sampling and Test Specs and Units
10.1 Samples shall be taken in accordance with the instructions in Practices D4057 or D4177. For sample handling, also refer to Practice D7343.
10.2 When reusable sample cells are used, clean and dry cells before each use. Disposable sample cells shall not be reused. For each sample, an unused piece of sample film is required for the sample cell. Avoid touching the inside of the sample cell, the portion of the window film in the cell, or the instrument window that is exposed to X-rays. Oil from fingerprints can affect the reading when determining low levels of analytes. Wrinkles in the film will affect the intensity of the X-rays transmitted, therefore, it is essential that the film be taut and clean to ensure reliable results. When handling the window film, avoid touching the central part (the part that actually forms the optical window) as this can lead to contamination from sweat, grease or other petrochemical products. Discard film that has been exposed to the atmosphere (for example, hanging outside of the film roll dispensing box). The analyzer may need recalibration if the type or thickness of the window film is changed. Fill the sample cell to a consistent depth (sample cells typically have a fill mark), no lower than 4 mm. Refer to the manufacturer's instructions for the use of the sample cell if necessary. If the instrument is equipped with a replaceable secondary/safety window it must be replaced when damaged or contaminated. When determining low concentrations, it is recommended to replace the window prior to each measurement. When changing, it follows the precautions given in 10.2.
10.3 Impurities or thickness variations, which may affect the determination of low levels of analytes, have been found in polyester films and may vary from lot to lot. Therefore, the method shall be verified after starting each new roll or batch of film. (When opening a new roll of film, it may be recommended to discard the first meter as some films are packaged in plastic bags that contain sulfur.)
10.4 When connecting a new helium gas cylinder, always run a blank measurement to ensure the helium gas line is purged of air. When using QC samples, check the performance by running QC sample(s).
11. Preparation of Apparatus
11.1 Set up the apparatus in accordance with the manufacturer's instructions. Whenever possible, the instrument should remain energized to maintain optimum stability.
12. Calibration and Standardization
12.1 Preparation of Calibration Standards:
12.1.1 Precisely weigh the organometallic solutions and phosphorus and sulfur solutions with the diluent solvent along with the appropriate stabilizer.
12.1.2 Storage of Standards and QC Samples - Store all standards and QC samples in glass bottles either dark or wrapped in opaque material, closed with glass stoppers or inert plastic-lined screw caps, in a cool, dark place until required. As soon as any sediment or change of concentration is observed, discard the sample.
12.2 Establish calibration curve data by carefully measuring the intensity of the emitted radiation from each of the standards by the procedure described in Sections 12 and 13. The recommended X-ray lines for the elements' determination are listed in Table 3. Intensity for other regions of interest in the spectrum may also be measured in order to apply background corrections.
12.3 Construct a calibration model using the software and algorithms supplied by instrument manufacturer.
12.4 When using drift correction monitors, determine the intensity of the drift correction monitor sample(s) after the calibration procedure.
12.5 After completing the calibration, determine the concentrations of one or more of the QC samples (see 7.4). The measured value shall be within the control limits of the QC samples. See Practice D6299 for guidance to set up control limits. When this is not the case, the calibration or calibration standards are suspect and corrective measures should be taken and the calibration repeated.
