EXPLANATION
This test method measures total sulfur in the concentration range 7 to 50 mg/kg automotive fuels with a typical analysis time around 10 min per sample. This test method uses an energy dispersive X-ray fluorescence spectrometer (EDXRF). The pooled limit of quantitation of the method is 7 mg/kg.
TEST SUMMARY
The sample is placed in the beam emitted from an X-ray source with titanium target and primary filtration so that excitation is by essentially monochromatic radiation of 4.51 keV. A low background proportional counter measures the intensity of the fluorescent sulfur K series intensity and argon K series intensity (from residual air) and the accumulated counts are compared with counts from previously prepared calibration standards to obtain the sulfur concentration in mg/kg. If chlorine is expected to be present in some samples then other regions of the spectrum must be measured to provide compensation for spectral overlap.
INTERFERENCES - Spectral interferences result when sample component element or elements emit X-rays that the detector cannot resolve from sulfur X-ray emission. This results in overlapping of peak lines. Such effects may result from the presence of lead alkyls, silicon, phosphorus, calcium, potassium, and halides in the sample if their aggregate concentration is more than 10 mg/kg. The most likely interference is chlorine that has been found in biodiesel derived from recycled waster vegetable oil. The presence of oxygenates or water may alter the sensitivity for sulfur. Follow the manufacturers' operating guide to compensate for these interferences.
TEST PRECISION
Where X is the average of two results.
Based on the analysis of a diesel fuel certified standard no significant bias was found for this method.