EXPLANATION
This test method determines total sulfur in gasolines and diesels with minimal sample preparation and analyst involvement. A typical time for each analysis is 2 to 3 min. The scope of the method ranges from 2 to 500 mg/kg sulfur. The pooled limit of quantitation is estimated to be 4 mg/kg. Gasoline samples containing oxygenates may be analyzed with this test method provided the matrix of the calibration standards is either matched to the sample matrices or a matrix correction is applied to the results. Gasolines and diesels containing sulfur concentration above 50 mg/kg can be analyzed after dilution with appropriate solvent.
TEST SUMMARY
A monochromatic X-ray beam with a wavelength suitable to excite the K-shell electrons of sulfur is focused onto a test specimen contained in a sample cell. The fluorescent K alpha radiation at 0.5373 nm emitted by sulfur is collected by a fixed monochromator analyzer. The intensity (counts per second) of the sulfur X-rays is measured using a suitable detector and converted to the concentration of sulfur (mg/kg) in a test specimen using a calibration equation. Excitation by monochromatic X-rays reduces background, simplifies matrix correction, and increases the signal to background ratio compared to polychromatic excitation used in conventional WDXRF techniques.
INTERFERENCES - Differences between the elemental composition of the test samples and the calibration standards can result in biased sulfur determinations. For diesels and gasolines, the only important element contributing to bias resulting from differences in the matrices of the calibrants and test samples are hydrogen, carbon, and oxygen. A matrix correction factor can be used to correct this bias. For practical purposes, the matrices of the test samples and the calibrants are considered as matched when the calculated correction factor is within 0.98 to 1.04. No matrix correction is required within this range. For most testing, matrix correction can be avoided with a proper choice of calibrants. To minimize any bias in the results, calibration standards prepared from sulfur-free base materials of the same or similar elemental composition as the test samples should be used. When diluting samples, a diluent with an elemental composition the same or similar to the base material used for preparing the calibration standards should be used. Fuels containing oxygenates may be analyzed using calibration standards containing the same amount of the same oxygenate in the test fuel.
TEST PRECISION
Test method reproducibility data or bias are not yet available.