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
4. Summary of Test Method
4.1 The sample is placed in the polarized X-ray beam, and the peak area of the sulfur Ka line at 2.307 keV is measured. 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. The resultant net counting rate is then compared to a previously prepared calibration curve or equation to obtain the concentration of sulfur in mg/kg. (Warning - Exposure to excessive quantities of X-radiation is injurious to health. The operator needs to take appropriate actions to avoid exposing any part of their body, not only to primary X-rays, but also to secondary or scattered radiation that might be present. The X-ray spectrometer should be operated in accordance with the regulations governing the use of ionizing radiation.)
5. Significance and Use
5.1 This test method provides measurement of total sulfur in automotive fuels with a minimum of sample preparation. A typical analysis time is 200 to 300 s per sample.
5.2 The quality of automotive fuel can be related to the amount of sulfur present. Knowledge of sulfur concentration is necessary for processing purposes. There are also regulations promulgated in federal, state, and local agencies that restrict the amount of sulfur present in some fuel.
5.3 If this test method is applied to petroleum materials with matrices significantly different from the calibration materials specified in this test method, the cautions and recommendations in Section 6 should be observed when interpreting the results.
6. Interferences
6.1 When the elemental composition (excluding sulfur) of samples differs significantly from the standards, errors in the sulfur determination can result. For example, differences in the carbon-hydrogen ratio of sample and calibration standards introduce errors in the determination.
6.2 M-85 and M-100 are fuels containing 85 and 100 % methanol, respectively. They have a high oxygen content leading to significant absorption of sulfur Ka radiation. Such fuels can, however, be analyzed using this test method provided either that correction factors are applied to the results (when calibrating with white oils) or that the calibration standards are prepared to match the matrix of the sample.
6.3 In general, petroleum materials with compositions that vary from the calibration samples as specified in Section 11 can be analyzed with standards made from base materials that are of the same or similar composition. Thus a gasoline may be simulated by mixing isooctane and toluene in a ratio that approximates the expected aromatic content of the samples to be analyzed. Standards made from this simulated gasoline can produce results that are more accurate than results obtained using white oil standards.
