ASTM D7757 Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
6. Apparatus
6.1 Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) Spectrometer, equipped for X-ray detection at 0.713 nm (7.13 Å). Any spectrometer of this type may be used if it includes the following features, and the precision and bias of test results are in accordance with the values described in Section 16.
6.1.1 X-ray Source, capable of producing X-rays to excite silicon. X-ray tubes with a power >20 W capable of producing Rh Lα, Pd Lα, Ag Lα, Ti Kα, Sc Kα, or Cr Kα radiation are recommended for this purpose.
6.1.2 Incident-beam Monochromator, capable of focusing with an effective collection solid angle greater than 0.05 sr and selecting a single wavelength of characteristic X-rays from the source onto the specimen. A monochromatic beam is considered to be monochromatic when it has an energy bandwidth (Full Width Half Maximum) less than +/- 1.5 % relative to the selected energy and containing more than 98 % flux of the spectrum of the excitation beam which is incident on the sample.
6.1.3 Optical Path, designed to minimize the absorption along the path of the excitation and fluorescent beams using a helium or vacuum atmosphere.
6.1.4 Fixed-Channel Monochromator, suitable for dispersing silicon Kα X-ray photons with an effective collection solid angle greater than 0.3 sr.
6.1.5 Detector, designed for efficient detection of silicon Kα X-ray photons.
6.1.6 Single-Channel Analyzer, an energy discriminator to monitor only silicon radiation.
6.1.7 Removable Sample Cell, compatible with the sample and the geometry of the MWDXRF spectrometer. A disposable cell is recommended.
6.1.8 X-Ray Transparent Film, for containing and supporting the test specimen in the sample cell (see 6.1.7) while providing a low-absorption window for X-rays to pass to and from the sample. Use an X-ray transparent film resistant to chemical attack that does not contain a listed silicon impurity. Follow manufacturer's recommendations for appropriate film types.
7. Reagents and Materials
7.1 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.
7.2 Calibration-Check Samples, for verifying the accuracy of a calibration. The check samples shall have known silicon content and not be used in determining the calibration curve. A standard from the same reliable and consistent source of calibration standards used to determine the calibration curve is convenient to validate the calibration.
7.3 Octamethylcyclotetrasiloxane (D4), a high-purity material (typical purity 98 %), is known to be suitable for making silicon calibration standards. Use the known silicon concentration and the material purity when calculating the exact concentrations of silicon in calibration standards. (Warning - Octamethylcyclotetrasiloxane is harmful if swallowed or absorbed through skin. It is an eye irritant and may cause skin irritation.)
7.4 Drift Correction Monitor(s) (Optional), to determine and correct instrument drift over time (see 11.4, 12.1, and 13.1). Various forms of stable (with respect to repeated exposure to X-rays) silicon-containing materials are suitable drift correction monitors; for example, liquid petroleum, solid, pressed powder, metal alloy, and fused glass. The count rate displayed by the monitor sample, in combination with a convenient count time (T), shall be sufficient to give a relative standard deviation (RSD) of less than 1 % (see Appendix X1).
7.4.1 Calibration standards may be used as drift correction monitors. Because it is desirable to discard test specimens after each determination, a lower cost material is suggested for use over time. Any stable material meeting the recommendations of 7.4 or 7.4.1 may be used for monitoring of drift on a given day when samples are being analyzed.
NOTE 4 - The effect of drift correction on the precision and bias of this test method has not been studied.
7.4.2 Drift correction may be done automatically if the instrument embodies this option, although the calculation may be readily done by conventional methods of data reduction and processing.
7.5 Quality-Control (QC) Samples, for use in establishing and monitoring the stability and precision of an analytical measurement system (see Section 15). Use homogeneous materials, similar to samples of interest and available in sufficient quantity to be analyzed regularly for a long period of time.
7.5.1 Verification of system control through the use of QC samples and control charting is highly recommended.
NOTE 5 - Suitable QC samples can be prepared by combining retains of typical samples.
7.6 Ethanol, use a high purity grade and account for its silicon content when calculating the silicon concentrations of the calibration standards. (Warning - Ethanol is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)
7.7 2,2,4-Trimethylpentane (Isooctane), use a high purity grade and account for its silicon content when calculating the silicon concentration of the calibration standards. (Warning - Isooctane is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)
7.8 Toluene, use a high purity grade and account for its silicon content when calculating the silicon concentration of the calibration standards. (Warning - Toluene is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)
7.9 Helium, (for units that require helium per manufacturer), minimum purity 99.9 %, for use as an optical path.
8. Hazards
8.1 (Warning - Exposure to excessive quantities of X-ray radiation is injurious to health. The operator needs to take appropriate actions to avoid exposing any part of his/her body, not only to primary X-rays, but also to secondary or scattered radiation that might be present. The X-ray spectrometer should be operated in accordance with local and national regulations governing the use of ionizing radiation.)
8.2 Consult current health and safety regulations such as OSHA, suppliers' Material Safety Data Sheets and local regulations for all materials used in this test method.
