ISO 20884 Petroleum products - Determination of sulfur content of automotive fuels - Wavelength-dispersive X-ray fluorescence spectrometry
10 Procedure
CAUTION - Some samples such as process rundown streams can contain finely dispersed iron sulfide which can drop onto the cell window, giving erroneously high results. In such cases, it is recommended that the samples be filtered through a 0.2 micron membrane filter prior to analysis.

10.1 Samples with sulfur contents between 5 mg/kg and 60 mg/kg
Fill a sample cup with a sufficient quantity of the sample under analysis so that the minimum volume determined in 9.2 is achieved. Expose the sample to the radiation of the X-ray tube. Measure consecutively the count rate, IS, of the SK-L2,3 X-ray fluorescence radiation at 0.5373 nm and the count rate, IB, of the background radiation at 0.545 nm. For instruments utilizing monochromatic excitation, measurement of the background radiation is not required (see 5.1).

Calculate the net count rate, R0, according to Equation (1) or (2). Read the sulfur content, in milligrams per kilogram, from the calibration curve (see 9.3) for the measuring range of 5 mg/kg to 60 mg/kg sulfur. If the sulfur content is higher than 60 mg/kg, a new sample in a new cup shall be measured using the calibration curve for the measuring range from 60 mg/kg to 500 mg/kg.

10.2 Samples with sulfur contents between 60 mg/kg and 500 mg/kg
Fill a sample cup with a sufficient quantity of the sample under analysis so that the minimum volume determined in 9.2 is achieved. Expose the sample to the radiation of the X-ray tube. Measure consecutively the count rate, Is, of the SK-L2,3 X-ray fluorescence radiation at 0,537 3 nm and the count rate, IB, of the background radiation at 0.545 nm. For instruments utilizing monochromatic excitation, measurement of the background radiation is not required (see 5.1).

Calculate the net count rate, R0, according to Equation (1) or (2). Read the sulfur content, in milligrams per kilogram, from the calibration curve (see 9.3) for the measuring range of 60 mg/kg to 500 mg/kg sulfur. If the sulfur content is higher than 500 mg/kg, the sample shall be measured using another suitable method (ISO 14596).

NOTE Sulfur contents higher than 500 mg/kg can be determined after sample dilution. However, the precision was not established for diluted samples.