ASTM D7212 Standard Test Method for Low Sulfur in Automotive Fuels by Energy-Dispersive X-ray Fluorescence Spectrometry Using a Low-Background Proportional Counter
4. Summary of Test Method
4.1 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 and virtually no background at 2.3 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. One group ofcalibration standards is required to span the concentration range from 0 to 150 mg/kg sulfur.
NOTE 1 - Operation of analyzers using X-ray tubes is to be conducted in accordance with the manufacturer's safety instructions and federal, state, and local regulations governing the use of ionizing radiation.
4.2 Practice D7343 should be consulted regarding standard operating protocols in XRF analysis.
5. Significance and Use
5.1 This test method determines total sulfur in automotive fuels with a typical analysis time around 10 min per sample.
5.2 The quality of automotive fuel is related to the amount of sulfur present. Knowledge of sulfur level is necessary for processing purposes.
5.3 Sulfur level in automotive fuels affects performance characteristics and air quality. Federal, state, and local agencies regulate the level of sulfur in fuel delivered at the pump.
5.4 This test method can be referenced in specification documents to determine ifthe material meets the desired sulfur content.
5.5 If this test method is applied to petroleum matrices with significantly different composition to those used in the interlaboratory precision study, then the caution and recommendations in Section 6 should be observed when interpreting the results.
6. Interferences
6.1 Spectral interferences result when some sample component element or elements emit X rays that the detector cannot resolve from sulfur X-ray emission. Overlapping peak lines are the result ofthis. This overlapping effect may be by lead alkyls, silicon, phosphorus, calcium, potassium, and halides 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 waste vegetable oil.
6.2 The presence of oxygenates or water may alter the sensitivity for sulfur.
6.3 Follow the manufacturer's operating guide to compensate for the interferences.
