Since the 1990s international legislation shows a continuous trend of ever lower permissible levels of sulfur in petroleum products. The latest and most stringent is the EU Directive 2003/17/EC, effective since January 2009, which limits the total sulfur content in petrol and diesel fuel to 10 mg/kg.
In response, ASTM and ISO have addressed this with revisions of their standard test methods, D2622-16 and 20884, respectively. This report demonstrates that Zetium can readily satisfy all criteria of D2622-16, including the repeatability and sensitivity requirements.
This study was performed using a Zetium wavelength dispersive XRF spectrometer. It was configured with a 1 kW Rh anode SST R Tube and is a fully integrated system with a X-Y sample handler and state-of-the-art SuperQ software, featuring an advanced Fundamental Parameters algorithm. Zetium is engineered for excellence in terms of both analytical accuracy, precision and operational performance.
Sample preparation and measurement details Commercially available standards and diesel road fuel were prepared according to the D2622-16 test method. For each measurement, ten grams of sample were poured into a 38 mm disposable liquid cell prepared with 3.6 μm Mylar film.
Precision and instrument stability
The precision and repeatability of Zetium is excellent. A summary of the test results are listed in Table 1.
Table 1. Analytical precision for sulfur analysis (20 consecutive measurements).
The repeatability requirements of D2622-16 state that successive test results should not exceed the given limits for more than one case in twenty. The test method limit specified in ASTM D2622-16 can be derived using the equation:
Repeatability (r) = 0.1462* mean (mg/kg)0.8015
The repeatability of measurements for diesel road fuel sample, which contained 6.4 mg/kg sulfur are illustrated in Figure 1, together with the repeatability limits for ASTM D2622-16.
Figure 1. Repeatability measurements of sulfur measurements according to ASTM D2622-16. Dashed lines indicate the precision set in the method.
The accuracy of the calibration is illustrated in Figure 2. The calibration RMS value (0.87 mg/kg) corresponds to a comparison of the certified chemical concentrations of the standards with the concentrations calculated from the regression.
Figure 2. Calibration plot for sulfur in oil analyzed according to ASTM D2622-16
The accuracy of the calibration was verified using an independent check sample of diesel fuel, known to contain 12 mg/kg sulfur. The reported concentration of sulfur was equal to the expected 12 mg/kg.
The lower limits of detection (LLD) for sulfur in diesel fuel are given in Table 2.
Table 2. Detection limits of sulfur in diesel fuel.
The analysis of sulfur in road fuels by XRF is a fast, relatively simple analytical technique. As demonstrated, Zetium is capable of accurate and precise analysis of sulfur in petroleum products, easily fulfilling the requirements of stringent international test methods, like ASTM D2622-16. Sub-mg/kg detection limits ensure that Zetium is able to meet the requirements of industry, even as more stringent government regulations come into force.