ASTM D7212 Standard Test Method for Low Sulfur in Automotive Fuels by Energy-Dispersive X-ray Fluorescence Spectrometry Using a Low-Background Proportional Counter
9. Sampling
9.1 A sample shall be taken in accordance with the instructions in Practice D4057 or D4177, where appropriate. Do not shake samples, thus avoiding entrained air. Analyze samples immediately after pouring into a sample cell and allowing the escape of the air bubbles caused by mixing.
9.2 For the measurement of low sulfur contents disposable cells are recommended. Preparation of sample cells must be done with care and by following any advice from the supplier. A freshly prepared sample cell including film is required prior to analyzing the samples. Avoid touching the inside of the sample cell or portion of the window film in the cell or in the instrument window that is exposed to X rays. Oil from fingerprints affects the reading when analyzing for low level of sulfur. Ensure that the film is not scratched, especially by the tear-off bar as it is withdrawn from the container. Wrinkles in the film will affect the intensity of sulfur X rays transmitted. Therefore, it is essential that the film be taut and clean to ensure reliable results. Film may become electrostatically charged during handling so it is important that the cell window does not attract any dust, fiber, and hair. The analyzer will need recalibration if the type or thickness of the window film is changed.
9.3 If the instrument has a replaceable secondary/safety window, it must be changed for every sample to ensure there is no cross-contamination. When changing it, follow the precautions given in 9.2.
9.4 Impurities that may affect the measurement of low levels of sulfur have been found in polyester films. All films may vary in thickness from roll to roll. Therefore, the calibration shall be verified after starting each new roll of film.
9.5 Where laboratories use more than one XRF spectrometer or analyze different types of sample, a variety of cell window materials may be used. Always ensure that the correct film is clearly distinguished.
10. Preparation of Apparatus
10.1 Set up the apparatus in accordance with the manufacturer's instructions. Whenever possible, the instrument should remain energized to maintain optimum stability.
10.2 When changing the helium gas cylinder or after a system has been idle for a day or more ensure that the helium pipes are purged of air by performing a dummy measurement.
11. Calibration and Standardization
11.1 Preparation ofCalibration Standards:
11.1.1 Preparation of Stock Calibration Standard - Accurately weigh the nominal quantity of white mineral oil to the nearest 0.1 mg, as shown in Table 1, into a suitable narrow-necked container, then accurately weigh in the nominal quantity of di-n-butyl sulfide. Mix thoroughly (a PTFE-coated magnetic stirrer is advisable) at room temperature. Calculate the concentration of sulfur in the stock standard to 1 mg/kg using the following equation:
SStock = [DBS x SDBS/(DBS + MO)] x 10000
where:
SStock = mg/kg of sulfur in the stock standard,
DBS = actual mass of DBS, g,
SDBS = the mass % of sulfur in DBS, typically 21.91 %, and
MO = actual mass of white mineral oil, g.
11.1.2 Preparation of Calibration Standards - Accurately weigh the nominal quantity of white mineral oil to the nearest 0.1 mg, as shown in Table 2, into a suitable narrow-necked container, then accurately weigh in the nominal quantity of stock standard. Mix thoroughly (a PTFE-coated magnetic stirrer is advisable) at room temperature. Calculate the concentration of sulfur in the each standard to 0.1 mg/kg using the following equation:
SStd = STK x SStock/(STK + MO)
where:
SStd = mg/kg of sulfur in the calibration standard, and
STK = actual mass of stock standard, g.
11.2 Storage of Standards and Calibration Check Samples - Store all standards and check samples in glass bottles in a cool, dark place until required. The glass bottles shall be either dark or wrapped in opaque material and closed with glass stoppers, inert plastic lined screw caps, or other equally inert, impermeable enclosures. As soon as any sediment or change of concentration is observed, discard the standard.
11.3 Instrument Calibration - Calibrate the instrument following the manufacturer's instructions. Typically, the calibration procedure involves setting up the instrument for recording of fluorescent sulfur K series intensity, argon K series intensity, and two intensities to determine spectral overlap from chlorine. In effect, sulfur is measured simultaneously with three different regions of interest, which are the normal setting and two others, one narrower, and one wider. All three regions have the same lower limit. The normal setting is approximately symmetrical around the center of the sulfur peak (1.92 to 2.51 keV), the narrower setting has its upper limit in the center of the sulfur peak (1.92 to 2.31 keV), and the wider region has its upper limit in the center of the chlorine peak (1.92 to 2.62 keV). Obtain readings on a portion (see Section 9) of every standard, then repeat the procedure on freshly prepared sample cells and fresh portion of every standard. Once all the standards have been measured twice, follow the manufacturer's instructions for generating the optimum calibration line based on the sulfur and argon intensities with a correction for residual air.
11.3.1 There is no chlorine present in the calibration standards, and therefore there is no need to apply a correction for spectral overlap by chlorine during the calibration procedure. However, it is necessary to generate calibration lines for the narrower and wider regions ofinterest as well as the normal one. The normal region of interest is used for determination of sulfur content whereas the other two are used to test for the presence of chlorine and where necessary apply a correction (see 12.3).
11.3.2 Immediately on completion of the calibration, measure one or more calibration check samples (see 8.6). The measured values must be within with 3 % relative of the certified value, or within 2 mg/kg, whichever is the greater.
