ASTM D7220 standard test method for sulfur in automotive fuels
ASTM D7220 Standard Test Method for Sulfur in Automotive Fuels by Polarization X-ray Fluorescence Spectrometry
12. Procedure
12.1 When using drift correction monitors, before analyzing samples on a given day, analyze the drift correction monitor(s) and determine the counting rate, using the same material as used at the time of calibration. The value determined corresponds to the factor B in 13.1.
12.2 Place the sample in an appropriate cell using techniques consistent with good practice for the particular instrument being used. Although sulfur radiation will penetrate only a small distance into the sample, scatter from the sample cell and the sample can vary. Laboratory personnel have to ensure that the sample cell is filled above a minimum depth; beyond which additional sample does not significantly affect the count rate. Generally, fill the sample cell to a minimum of three-fourths of the cell's capacity. Make a small vent hole in the lid of the sample cell.
12.3 Place the sample in the instrument, and perform measurement according to instrument manufacturer's instructions.
12.4 Determine the peak area of the sulfur Ka radiation at 2.307 keV. The net count-rate will be calculated by subtracting the background. The background spectrum, measured with a sulfur-free white oil or other matrix matching blank sample (see 8.4) is adapted to the measured spectrum using adjustment regions following the instrument manufacturer's instructions and then subtracted from the measured spectrum.
NOTE 5 - When using a Pd target X-ray tube, background adjustment regions at 2.86-2.92 keV and 2.95-3.04keV have been found suitable.
12.5 Determine the corrected counting rate and calculate the concentration of the sample as described in Section 13.
NOTE 6 - It is recommended to prepare and analyze each sample directly. Also, it is recommended to take care not to leave the test sample in the instrument after the measurement process has finished. If the spectrometer is equipped with an autosampler, it is recommended to use only one position of the autosampler. This minimizes the risks of sample cell leakage, or deformation, or both.
13. Calculation or Interpretation of Results
13.1 When using the drift correction monitor described in 8.3, calculate a correction factor for changes in daily instrument sensitivity as follows:
F = A/B
where:
A = counting rate of the drift correction monitor as determined at the time of calibration (see 11.4), and
B = counting rate of the drift correction monitor as determined at the time of analysis (see 12.1).
13.2 The net counting rate is given by the spectral deconvolution of the sulfur Ka line after subtracting the background spectrum.
13.2.1 The use of the factor F to correct the counting rate of sulfur is optional.
13.3 Calculate the sulfur content of the sample by inserting the corrected net counting rate in the chosen calibration model from Section 11. In many cases the instrument vendor will provide software or the required calculations.
